Some bad advice from VIC Dive Shop continues to put local divers at risk

1 month 3 weeks ago - 1 week 11 hours ago #303540 by packo
packo created the topic: Some bad advice from VIC Dive Shop continues to put local divers at risk
Sadly after a two year battle against some very stubborn stupidity, my Local Dive Shop (LDS) at the southern end of Port Phillip Bay in Victoria, still offers on its website this incorrect advice to local divers:-

I had to add the red warning lines in case "mr google" hoovers-up this dangerous image and adds it to his image archives.

At the "false slack water times" advised under the red cross, the current at those locations will have already reversed and built up close to the maximum flow in the reverse direction. Current levels may then be in the 1 to 2 knot range. Definitely not slack water, and definitely a dangerous time to dive.

This bad advice is based on a decades old myth that apparently some still believe today (and very strongly!). The areas concerned are all within around 10km of the dive shop's own front door, and this potentially dangerous advice is contained in a "Guide to Diving Safely.." and is touted as local knowledge.

Also galling is that the very same article is dripping with platitudes like "Most of all, stay safe..", and yet the author simply refuses to properly check these slack water timing claims in the face of abundant contrary evidence and statements from the Port Authorities (quote:"what would they know!").

Even if the bad advice does not cause a serious incident, it will lead to abandoned dives, risky dives, wasting of time and money, general annoyance, and leave both newbie divers and those new to the area confused about tidal currents in this area. This bad advice needs to be removed or amended.

My initial complaint to the shop was made in August 2015 after I happened to stumble on that web page. In any SANE WORLD this is how it SHOULD have played out:-

Step #1 Customer claims LDS website has some untrue, unhelpful, and UNSAFE advice.
Step #2 Dive Shop investigates customers evidence to see if complaint has any basis.
Step #3 Dive Shop gathers its own evidence to verify the complaint is justified.
Step #4 Dive Shop amends bad advice, and apologises to its website readers.
Step #5 Done and dusted! Customer & LDS live happily ever after, and nobody drowns!

The REALITY: After two whole years the LDS can't seem to properly complete step #2, despite a MOUNTAIN of evidence and a TON of logic against their advice. Although I begged numerous times for the LDS to simply move on to step #3 and go out and look for themselves, they just couldn't seem to do that. Excuses for not doing so ranged from initially "I'm too busy" to finally "I don't care".


I am aware this disappointing dive shop behaviour is not entirely deliberate, and probably stems from a fanatical religious belief in their "very delayed slacks MYTH". This myth was more common in much earlier times. The percentage of divers still afflicted today is also a mysterious unknown. Among the staff connected with this dive shop and service centre it is 100%.

A personality clash with both the dive shop owner and service centre owner has probably also played some part in prolonging this dispute. However stripping away all excuses, it is very disappointing that where lives can be put at risk, or dives are forced to be abandoned and divers seriously misinformed about tidal streams, the dive shop continues to stick to its claim despite:-

Other advice:-
a) The Port of Melbourne Corporation has repeatedly said this advice is untrue.
b) The Port Phillips Sea Pilots say this advice is untrue.
c) All currently operating dive charter and fishing charter businessess in this area say the advice is untrue.

Simple water volume/area/height calculations from: Volume (gigaLitres) = Area (square km) x Height change (metres):-
The dive shop's claim that locations roughly 10km inside the Heads, such as Sorrento, Swan Island Sub, etc, don't experience a new flood stream until 2 hours after ebb slack at the Rip is utterly ludicrous considering at this 2 hour later time:-

d) All seven tide gauges in the vast central, northern and western reaches of the Bay record strongly rising levels at this time and have been recording rising levels for at least 100 minutes. How does the dive shop think all this extra water "magically appears" in the central and northern parts of the Bay? - please explain!

e) The first 2 hours of inflow at the Heads is about 300 gigalitres on a big tide day. If it isn't "allowed" to push any water more than 10km in from the Heads, then it would build up in this area (about 60km2) to a height of 300gL/60km2 = 5m. This figure isn't even allowing for a similar volume of water the dive shop claims is ebbing across the 10km line in the other direction during those 2 hours. This gives a total height rise for this area of around 10m to accommodate all the water the dive shop thinks accumulates there. - ludicrous!

f) At the 2 hour point the ocean level is rising at around 30 cm/hr and the Heads inflow rate at that time is about 250gL/hr. If no "onflow" was allowed beyond this 60km2 area near the Heads, it would require the height in that area to be rising at a rate of 416 cm/hr. That is around 14 times faster than the rate the ocean itself is rising. - ludicrous!

Real life observations:-
Unfortunately the dive shop has consistently refused to perform their own observations even though the journey to the water's edge is less than 2km. I have tried to bring a view of the tidal streams to the shop through photography:-

g) Dozens of "time-lapse" photo sequences of buoy "switch overs" in the Sorrento - Rye area all show the buoys reverse their positions within 5 to 15 minutes of the tide reversal at the Rip.

h) A recently available 3rd party shoreline webcam at Sorrento confirms this result. It is impossible for the shop to claim this live streaming video has been "doctored", so instead they simply refuse to watch it!

The last straw came when the LDS refused point blank to use the live streaming camera system to check their claims. This would require very little time or effort. When I arrived in the shop with the live streaming video running on my tablet, I was pushed out the door with the comment "Go away and don't come back. You are obsessed with this issue!"

Well yes, that is true. It has grown into an obsession, but only because the dive shop has shown "less than zero" interest in investigating it. Had they made any proper attempt to check out the real situation for themselves, all could have been done and dusted in late 2015 with no obsessive behaviour needed. Instead they blindly continue to put local divers lives at increased risk, or at least cause divers to waste time, money, and effort, as well as misinforming all and sundry (via the www) about tidal streams within the Bay. All up it is not a good look.

Are these men stupid? Hard to say but the more likely explanation is that they are just over-confident and arrogant pricks who display an appallingly poor attitude to investigating this perfectly legitimate safety concern about one aspect of their business. I can't tell if the evidence and arguments against their ideas were understood or even read. If they actually took the time to think about what they are claiming then maybe things would be different. I have received almost no feedback on why they think the Bay works "their way" it is just always presented as "we know".

With the possibility of heavy handed government intervention in recreational diving hanging overhead perhaps "just 1 or 2 accidents away", now is not the time for any industry player to be showing itself as both foolish and reckless in regard to any safety issue. Nor is it the time for their industry body "DIVA" to be showing itself as weak and incapable of getting a member to behave responsibly in regard to this safety matter.

With the recent spectacular collapse of Victoria's biggest dive charter operator, some government bodies must be wondering whether the current self-regulation regime of the recreational diving industry is sustainable. We all need to be trying very hard to minimise excuses for intervention.

So I have been defeated in my modest aim of slightly improved diver safety in my local area. This was because my endeavours were met by a toxic mix of ignorance, arrogance, and negligence. These are strong words but all of them perfectly appropriate in this case.

As I have said before, simply believing in "The Myth" is no cause for shame or embarrassment. Plenty of others have done so in the past and no doubt some will do so into the future. It is after all a seductively simple piece of . . . . nonsense.

Rather my angst is directed only at those who can't bring themselves to objectively consider the evidence against it, or are unwilling to properly test their belief in this myth. It is "double angst" for those who are also in a position of responsibility, or for those who promote this view to others without first verifying it for themselves. That is why my LDS is now on the receiving end of "angst times four".


I have raised this issue a number of times in this forum and on one or two occasions asked concerned divers to contact the dive shop and support my complaint. Unfortunately the dive shop owner claims no one has done so. My request for help seems to have backfired and convinced the dive shop staff to hold even more tightly to their myth because no one has bothered to back me up with a call to the dive shop.

A part of that problem is because the area concerned is well inside the Heads where the currents are not as strong. Many divers may see badly timed dives here as not such a big deal. However to put it into a more easily appreciated perspective as regards current strength, the bad advice offered is roughly the equivalent of saying that divers should dive at the Heads around 45 minutes AFTER slack water.

I'm sure if my LDS was making that claim on his website, then hundreds of safety conscious local divers and diving organisations would be attacking the dive shop from all sides and demanding such advice be removed.


In early August 2017 I emailed around a "please help" message to a number of prominent Melbourne "diving elders" and the diving organisations: DAN, DIVA, and SDFV. MSV got a copy too. That email gave details of the camera system access and asked the recipients if they could view the streaming video and then perhaps nudge the dive shop towards also viewing it.

So far there has been no change in attitude from the dive shop and they are simply refusing to use the camera system. Not sure whether any of the email recipients took up the issue or whether the dive shop owner has rebuffed them too. I was perhaps foolish to include a few known "Myth Men" in my distribution list, but was confident the evidence I presented to them was sufficiently strong to turn them away from "the dark side".


In normal retailing situations, a complaint about a retailer's conduct in regard to a customer complaint might be taken up with their industry association. In this case "DIVA" (Dive Industry of Victoria Association), seemed like the appropriate body. Among the bullet points within their "mission statement" are these relevant ones:-

* Acting on concerns raised by the public or by those involved with the diving industry that may affect the sport of diving in Victoria.

* Providing a source of information and assistance to those within the Victorian diving industry.

During a somewhat difficult phone conversation with the President of DIVA (damn! - a die-hard "Myth Man"), the following points were established:-

a) If I could provide evidence of a death or injury due to the dive shop's advice, then yes DIVA would follow it up.

b) My evidence against "The Myth" was read, but in the President's view was no more than an opinion, and no stronger than the dive shop's opinion (for which no real evidence is ever presented!)

c) My LDS has recently signed up to become a DIVA member but was previously not a member. (In an effort to re-invigorate this association, the membership fees for dive shops were reduced to nil for this financial year.)

d) DIVA members had the right to say whatever they liked on their own websites, and DIVA was not able to direct this in any way.

e) If an "idiot diver" chose to dive (and die) in unsuitable conditions, then the consequences of that choice rested with them.

On that last point I tried to point out there were consequences for all divers. While an industry association may accept that "dills may die", governments usually do not. A coroner will always pontificate, and new legislative restrictions often follow.

I didn't quite get an answer as to whether DIVA thought an "idiot dive shop" who gave an "idiot diver" some "idiotic advice" had any culpability at all until an "accident" occurred.

The dive shop, and to some extent the DIVA president, have expressed the view that if the advice is wrong and a diver gets into trouble, then because the diver wasn't astute enough to see the advice was wrong before entering the water, then he is to blame.

Yet another diver (me), who is astute enough to see the advice is wrong, also gets blamed - this time for being a "deplorable whacko". Seems the dive shop wants to blame everyone except the real culprit. They have taken "shooting the messenger" to a high art form.

A second DIVA member I contacted (also an office bearer), gave me a better hearing but still revealed "semi-myth leanings" by insisting that slack water at Portsea Hole was delayed by 30 to 45 minutes behind slack at the Heads.

So here we have three dive shop owners and DIVA members who just seem to be "making stuff up". That kind of talk is both silly and potentially dangerous. When so many aspects of scuba diving are dependent on science, physics, and maths for staying safe, why do these dive shops then seem to toss it all out the window when it comes to "tide talk"?

Take the supposed "30 minute delay" for Portsea. On a big tide day, at 30 minutes after the ebb slack at the Heads, the inward current through the Heads will be up around 1.5 knots (or 2.8 kph). The acceleration rate of the water around slack is roughly constant so the average current over the 30 minutes is 1.4 kph or 1,4000 m/hr.

A reasonable estimate for the cross section at the Heads is: (2.4km x 15m) = 36,000m2. So the volume of water coming in through the Heads for the 30 minutes we are talking about is around: 36,000 x 1,400/2 = 25,200,000m3 or 25.2 gigalitres (gL) .

Where does all this water go? Those claiming a 30 min delayed slack for Portsea (and say Pope's Eye) would want us to believe that somehow it all accumulates between those locations and the Heads. The surface area of this area is around 27km2 or 27,000,000m2.

To accumulate all the 25.2gL in this area would required a level rise of: 25,200,000/27,000,000 = 0.9m, and this isn't even accounting for a roughly similar rise due to the additional water that is supposed to be ebbing past Portsea and Pope's Eye in that same 30 minutes and adding to the water in the 27km2 zone. This would give a total rise of around 1.8m

The true 30 minute level rise as observed by tide gauges in the region is only around 0.15m, indicating only 4 gL is stored locally with the remaining 21 gL being pushed beyond Portsea and Pope's Eye during that 30 minutes. This indicates the delay in new flood stream reaching Portsea must be way less than 30 minutes.

Most experts put the Portsea or Pope's Eye slack water delay in the order of 5 minutes. The most tide savvy of the dive charter operators (Neal Walker) also says around 5 minutes. The other charter operators are also close to this number. Even the charter schedules show that when a Portsea Hole dive is scheduled, the boat always aims to depart the pier for the short run to the site well before slack is due at the Heads. Why do the dive shop owners have to go off and invent their own nonsense?

So the DIVA consensus so far is that they see my LDS's advice as unlikely to cause a diver any real harm. However the website is factually incorrect and anyone who uses the information exactly as given would put themselves at quite some risk.

My point is simply that this risk can be entirely eliminated by replacing the "false delay times" with more truthful ones. The added benefit is a better informed diving population that will reduced the spread of such unhelpful myths. They will also avoid the waste of time and money that occurs if you turn up at these dive sites at the wrong time.

I understand perfectly that in some cases a degree of risk has to be accepted where the cost of eliminating that risk is judged way too high in the context of a low event probability. However in this case the "cost of a fix" is roughly somewhere between zero and ten bucks. The dive shop cannot guarantee that EVERY diver's common sense will ALWAYS override the shop's own dangerous advice when they arrive at the actual dive site. Making the fix is really the only moral course.

From my perspective the dive shop manufactures and distributes a "product" (ie. dive timing advice). That product has been shown to be defective in hundreds of tests, and will be shown as defective in any real test the dive shop bothers to perform.

The responsible course of action should be to immediately withdraw the product from "sale" (ie. remove the disputed paragraphs), conduct an investigation (ie. bother to go to the shoreline and take a look), then issue a "product recall" (ie. draw customers attention to the faulty product), and finally perform a "product replacement" (ie. put up the correct slack water timing advice for those areas). That is how the dive shop should be behaving.

***** DAN's (Diver Alert Network) PERSPECTIVE *****

It was explained to me that DAN's role is really to provide the best possible treatment for a diver after an accident. That primary aim is best served if they remain strictly politically neutral in any dispute among divers, dive clubs, or dive industry players.

Only in this way can they be guaranteed of full cooperation form all parties in the case of a diving incident. I do see the logic and do have to respect that position.

They did notice that the dive shop had eventually made the concession of including a comment in their website that the Port Authorities and the Sea Pilots both disagreed with the dive shop's advice. It took 9 months of prodding to get to that point!

***** THE SDFV (Scuba Divers Federation of Victoria) PERSPECTIVE *****

The SDFV seemed to be saying that although my claim did seem to be a valid one, they are unable to take it on at this time because they are flat-out with other issues. Unfortunately my email hit their inbox a day or two after the Harbour Master made sweeping Port Phillip Heads diving bans for private boats. No wonder they are busy!

Their reply also included some text about different sources of tide info, all based on different models with different levels of (in)accuracy, and besides "nothing beats assessment at the site".

However the dive shop's advice is way outside the scope of that statement. It is in fact almost the perfect "anti-model", with its slack water predictions being almost perfect "maximum current times" in the Rye to Rosebud stretch.

As to the likelihood that a diver will "bail out" before entry should a sizeable current be evident, I'm not so sure. If you put a lot of effort and some money in getting there at the dive shop's advised time, there will be a tendency to "push on".

Interestingly my LDS owner got himself caught in exactly that scenario while attempting a Hurricane wreck dive in 2013. The group decided to "push on" despite the shot line becoming tightly entangle around the prop - a sure sign of a current.

His on-line dive log (since removed) revealed the dive was aborted after just 12 minutes with comments like "the current was stronger than expected" and "we think we dragged the shot off the wreck". This is the type of outcome I'm trying to eliminate as well as any more serious ones.

Recently when I reminded him of this episode he replied "but I didn't have anything to do with organising that dive!" My point again, he accepted bad advice from others, and then chose to "push on" despite all the glaring signs of an unsuitable current.

It is just human nature that if you have spent some effort to get to a site and yet the assessment of conditions is not the best, then we all often decide "I will manage" and continue with the dive. Some day someone won't manage, and it could be my LDS that has put them at that site at an unsafe time.

***** THE MSV (Maritime Safety Victoria) PERSPECTIVE *****

Unfortunately the flu has been sweeping through the MSV folks over the last couple of weeks and I haven't been able to get feedback from them on this issue. In the past they have said a confusing mix of "yes we will act", and then "no we can't act". I will update this section once the most recent "yes we can" guy is back from sick leave.

I'm not holding my breath of anything useful happening, but don't want to go any higher up the chain of command. It seems MSV also has some "diver issues" they are having trouble sorting out. They pointed out that other recreational sectors like "wakeboard riders" do have active, coherent, and united "peak body" groups with which MSV can successfully negotiate safety issues and waterway rules with.

Later note: I'm really reluctant to push further down this path because it does reflect badly on the dive industry in the eyes of government at a time that I sniff out as being "sensitive". I'll only go back on this tack if DIVA fails to intervene. It is much better if we can be seen to get our own house in order.

***** TIME TO BOW OUT *****

I'm now going to bow out of this seemingly hopeless deadlock. This post and rant below will be my final blast against "The Myth" and my local dive shop, except that every 6 months or so I will keep re-elevating it to "recent" status whilst the dangerous advice remains.

I'll also update it if any new style of evidence surfaces. Any dive-oz members who would like to see a copy of the "please help" email with camera details can request to do so by dive-oz's PM (Private Message) facility.


Over the two years I have battled through many conversations with numerous "Myth Men". I've listened as all my documented findings are dismissed by a wave of the hand to be replaced by their various pieces of voodoo and witchcraft on how the Bay's tides work - all without reference to any evidence at all, except perhaps some tired and not strictly relevant anecdote.

I have dived in, power boated on, and sailed across the waters of the Bay between the Heads and the northeast edge of the Great Sands for nearly 50 years and have simply observed nature's story as she chooses to unfold it. How divers claiming similar lengthy experience in this area can see a totally different story is quite bewildering.

Hell let's forget all this "my experience is bigger than yours" stuff, as even an observant "newbie diver" with just a dozen southern Bay pier dives under their belt could acquire better "tidal stream wisdom" of the area than the local dive shop seems to possess.

To all "Myth Men & Myth Women", and to any divers still in doubt on this issue, I simply say go out and look for yourselves! (and then pass on your findings to the dive shop folks - they need to hear it from someone other than me!)

It is sad that I have to throw so much mud at this business in order to try to force a change in attitude. In the end they forced my hand. On my final visit to see if the problem could be rectified in a less damaging way, I got told by both the owners of this 2-premises operation to go way and they wouldn't discuss it any further. Their parting comment was "we are very comfortable with the advice we are offering." Mine was a big long sigh! (plus some under my breath mutterings I will not repeat.)

It is also sad that calling on "diving elders" and the various "diver organisations" did not bring about a safer and less hurtful outcome. Perhaps some of the "calls for help" were heard but maybe the numbers responding were just not enough. The term "immovable object" was used in a number of conversations with various people. Although I thought I was reasonably close to applying "an irresistible force", in the end the "immovable object" remains in exactly the same position as 2 years ago, and this tired old battler is now spent.

Perhaps the mistake was applying intense pressure on the dive shop from one individual rather than using a much smaller level of pressure but from a multitude of individuals. Remember that I did ask for help but either none was forthcoming, or the dive shop owner also buried his head in the sand to others.

Lesley and other readers who wanted me to end this campaign more than a year ago, now have their wish! Not sure what moral compass they use, but I did what I thought was right. It was never a "battle of opinions" as some may have thought, but instead a case of "unsafe untruths" versus "provable facts".

I am more than happy to be judged by history on this. However if you are up against an opponent completely unwilling to examine evidence or investigate the situation, the mighty tools of science, maths, and reason are rendered powerless.

It is also sad that almost without exception, those I contacted on this issue chose to steer the discussion sideways into the question: "Shouldn't the diver take responsibility for decisions they make on the day if conditions are unsuitable?".

The answer is obviously "YES", but wrong choices will sometimes be made. The return question for all Victorian divers, and DIVA folk in particular is:
"Shouldn't a dive shop owner and DIVA member take responsibility for his decision to publish and promote untrue and unsafe advice to (potentially newbie) divers?" (especially in an article that begins with "Guide to diving safely . . ." and ends with "Most of all, stay safe . .".)

I never asked anyone to pontificate on how sensible Victorian divers are as that is a whole other debate. I simply asked people to examine the strong evidence that "The Myth" is untrue, and then pass on their recommendations to the dive shop concerned. Hopefully DIVA may still respond to that request despite its apparent prejudices on the topic.

***** ANY POSITIVES? *****

On the positive side, all the "Myth Men" I have met are 55 or more years in age. That's not to say younger ones don't exist, but I haven't found any yet. (I did meet a "Myth Woman" who would probably qualify, but it is hard to tell for sure, and a very dangerous topic!)

My point here is the issue may "self-correct" over the next dozen years as my generation of "silly old farts" swap their scuba diving flippers for their nursing home slippers.

Of course this happy and safer outcome does assume that "silly old farts" can be prevented from handing down their "un-wisdom" to the newer generations of southern Port Phillip Bay divers.


Note the "Rant" below on false slack delays and real slack delays is spread over several separate posts because dive-oz limits the number of external links in a single post.

PS: Sadly I did very recently find a "semi-myth" person who was way below 55 years old. :(



The LDS's simplistic myth is along the lines: "if the tides are progressively delayed by up to 3 hours, then so must the tidal streams and slacks be delayed". There is no logic in this assertion as tides and streams are quite different things, and they can and do behave differently.

In some Bay areas the time at which the current (or horizontal flow) reverses will be in sync with the time the tide (or vertical flow) reverses. This occurs throughout most of the "main body" zone. However in the "devil's triangle" this is not the case and a current reversal there will have varying levels of "out of sync" with the time the local vertical tide movement reverses. (A better perspective is that the vertical tide reversal times will have varying levels of "out of sync" with the current reversal time.)

As a general guide to most harbours and estuaries, the "in sync" type of of behaviour occurs where the tide stream directions have a significant "onshore/offshore" component, and the "out of sync" behaviour occurs where the tide stream directions have a significant component parallel to the shoreline. In the latter case it is clear the flood and ebb streams are creating tide rises and falls in some other more distant areas, and most likely to some other timetable.

Those claiming a 3 hour delayed flood slack water for Rosebud and other locations near the St Leonards to Rosebud border line must concede the time of maximum stream flow at Rosebud should also be delayed by 3 hours. Since the max flow occurs midway between successive slacks, you can't delay the slacks without also delaying the mid-point max flow time.

The 3 diagrams below represent how they think the situation would be at the times shown:-

Although the above covers only 3 hours of a typical 12 hour tide cycle, the story is the same everywhere. The predicted changes in the "main body" tide levels according to the "very delayed slacks" myth is always at odds with the real observed and measured level changes. Why this fundamental Tide Table evidence against the myth seems insufficient to deter those who "want to believe" is difficult to answer.

Widely available tide tables for the northern section of the Bay clearly show that tides all across the "main body" zone begin to fall soon after flood slack water at the Heads. This clearly indicates the northern water must begin draining south soon after the ebb stream begins at the Heads. The myth's notion of water continuing to flow northwards for three more hours is simply unsupportable.

I note this myth has managed to stay alive with various levels of support for over 50 years. With a smarter and better educated population now is the time to banish it forever. That my LDS sings its virtues on the world wide web is very frustrating. As well as being dangerous, it runs counter to producing a well informed population of local divers who may want to dive safely at sites between 10km and 20km inside the Heads.

What does the myth say about tide level changes within the "devil's triangle"? The myth's proposed flows indicate this area is losing water equivalent to a 5 knot Heads current for the whole 3 hours. How far will the level in the "devil's triangle" drop during this time period?

We can estimated the Height change (in metres), that would occur when a Volume (in cubic metres) of water is removed from an Area (in square metres) by the simple formula: H = V/A

To avoid lots of zeros, we measure Volumes in gigalitres (gL) and surface Areas in (km2).

This is useful because 1 gL = 1,000,000 m3 and 1km2 = 1,000,000 m2 , so the same formula can be used to give the Height change (still in metres) ie H(m) = V(gL) / A(km2)

The surface areas of the "main body" and "devil's triangle" are 1720 km2 and 250 km2 respectively. Tide curves show that an ebb stream of 5 knots out through the Heads drops the average Bay level by around 15cm/hr or 0.15m/hr. The hourly volume loss through the Heads is then V(gL/hr) = H(m/hr) x A = 0.15 x (1720 + 250) = 296 gL/hr.

In the a), b) and c) diagrams above, the myth asserts roughly 300 gL is lost from the "devil's triangle" in each of the 3 hours, giving a total water loss of around 900 gL with some going north and some going south. The Height change in the "devil's triangle" over these 3 hours is then roughly: H(m) = -900/250 = -3.6m.

Obviously this does not happen as it would take the level several metres below the lowest ocean level. The true drop in level over these three hours as measured by the Queenscliff tide gauge is only around -0.4m. This indicates a net loss from the "devil's triangle" zone of more like : V(gL) = H(m) x A = -0.4 x 250 = -100 gL.

The explanation is simply that during the 3 hours about 550 gL is lost out through the Heads but this is mostly replaced by around 450 gL draining from the "main body" zone into the "devil's triangle". The 450 gL loss from the "main body" zone represents a level drop there of H(m) = -450/1720 = -0.26m which is consistent with typical "half-tide" falls in the north. So the ebb stream starting at nearly the same time throughout the Bay is consistent with the observed level changes whereas the water flows proposed under "The Myth" are not.

Although it is everyday experience that very large water flows do pass through the "devil's triangle" zone, the amount of water entering the zone at all times must be in rough balance with the amount leaving it at the other end. Its surface area is just far too small to support the grossly imbalanced flows proposed by the "very delayed slack idea" without impossibly large level changes occurring within the zone.

Although the dive shop was presented early on with these arguments that actual tide data directly contradicted their webpage advice, it had no effect. I'm not saying the arguments weren't understood, but more likely weren't even read. Their attitude was one of such confidence that any testing of their advice was deemed as unnecessary and a waste of time.
Last Edit: 1 week 11 hours ago by packo. Reason: color adjust

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1 month 3 weeks ago - 1 week 4 days ago #303541 by packo
packo replied the topic: Some bad advice from VIC Dive Shop continues to put local divers at risk

All the folk at the dive shop were asked numerous times to just do the short drive down to the shoreline and actually LOOK at the ebb and flow of the real tides off Rye or Sorrento. I don't think this simple testing was ever done.

In an attempt to "bring the mountain to Mohammed" I produced many time lapse photo sequences at several locations to show that the tide stream reversals could be observed by buoy "switch-overs", and all these sequences showed only small delays compared to the predicted tide stream reversal time at the Heads.

A photo session for the ebb slack was taken on 2/06/2017 at Pt King near Sorrento. The result is shown here:-

In contrast the dive shop was predicting that slack water should not reach Sorrento waters until 2 hours later at 16:20. A number of other photo sequences were also taken at the shop's home township of Rye. They showed a similar story that slack water occurs there only a dozen or so minutes after slack at the Rip.

All this evidence was dismissed without comment or discussion. Presumably this was on the basis that the dates and timestamps could have been faked. This was despite the sequences deliberately capturing some ships passing through the South Channel in the background. There are public records that track a ship's position and speed over time. The shop was urged to use these to confirm the dates and times were genuine. Some of these sequences can be inspected at:-

Flood Slack at Rye - Beach 21/5/17
Ebb Slack at Rye - Beach 21/5/17
Ebb Slack at Rye - Boat Ramp 14/6/17

The last one not only includes two passing ships but also shows that ebb slack offshore from Rye occurs around the time of low tide there. This is very conspicuous in the image and rather hard to fake. On the point of trustworthiness I implored the dive shop to go out and gather their own evidence. The response was always along the lines of "we are too busy to do that", or "it isn't necessary, we know our stuff".

At Rye about an hour is needed to observe the full tide reversal. An alternative I suggested for such busy men was a quick trip to nearby Sorrento at the dive shop's "false slack water" time of Rip_slack + 2 hrs to observe if it really was slack water then. Had they bothered taking this simple step they might have seen something like this:-

On two visits to Sorrento Pier at the website's "false slack time", I noted some weed drifting past in the current. Keeping abreast of the weed, I paced out along the pier decking and noted the distance covered in one minute.

The results on the different days were 21m/min (0.7 knots) & 37m/min (1.2 knots). Currents of 20 metres/minute will quickly exhaust a fully kitted up diver. Higher currents of 30 to 50 metres/min can sweep a diver away from his boat with little prospect of return.

In the same safety article the dive shop promotes both PLB and VHF style emergency beacons for a few hundred dollars that could "save your life should you find yourself swept away by a current that is just too strong".

Come on guys - kill the bad advice, not the divers! As the old saying goes:-
"An ounce of prevention is worth a pound of cure".
Last Edit: 1 week 4 days ago by packo. Reason: colour adjust

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1 month 3 weeks ago - 1 week 4 days ago #303542 by packo
packo replied the topic: Some bad advice from VIC Dive Shop continues to put local divers at risk

Ok it is now nearly two years on, with the wrong and dangerous advice having paraded across who knows how many more digital screens. The dive shop owner has still not visited the local shorelines to "see for himself". He claims this is unnecessary because the shop and its advisors "know they are right". Even in the winter periods when the shop wasn't exactly bustling with activity they would not take the time to go and look for themselves.

I thought a solution may have been found when I discovered a live video feed "webcam" that is both user steerable and zoomable, and has a good view over much of the waters off Sorrento. The camera shows buoy "switch-overs" and even "moored boat swing-arounds" that demonstrate the tide stream reverses in this area just a handful of minutes after the official reversal time at the Heads.

I gave the details on accessing this camera and some targeting information to the dive shop owner but he said he probably wouldn't be viewing it. He's about as pissed off with me as I am with him - but I would never close my eyes and ears to someone arguing against my claims.

I had hoped that an underwater "webcam" on the Pope's Eye Reef would be up and running by now. However it has been delayed by so many months I have given up asking what the latest "launch date" is. Particles in the water drifting past that camera are expected to show very clearly that the reversal of the current at Pope's Eye is also not delayed by 45 minutes as the dive shop claims. ( later note: Launch date is Aug 25th with a "do" in Geelong. Not long to wait now! )

The Sorrento webcam details having been in the hands of the dive shop since 14/7/2017 and no changes to its website seem to be forthcoming. The stills below, were captured from the live camera feed and some coloured markings were then added to highlight changes that occurred around the time a flood slack water was due in the "Rip" at Port Phillip Heads. It shows the sort of thing the dive shop could be viewing in real time if they chose to do so:-

The righthand sequence above covers the dive shop's "false slack water" prediction time of Rip_slack + 2hrs. There is no movement of either boat or buoy showing the current does not reverse at this time. During my final visit to the dive shop last week I was waived away from the service centre with the words: "we are very comfortable with the advice we are giving."

Earlier in this long journey I used to think of the old saying:-

"You can lead a horse to water, but you can't make it drink."

Now I realise this saying wasn't quite right. Firstly you have to replace the horse with the planet's most stubborn mule. Secondly, you can't lead the beast anywhere near the water. Thirdly, not only can't you make it drink, but as well you can't even make it look, or see, or learn.



A useful conceptual model is to consider the current at the heads as having two distinct components. One current component is due to tidal breathing within the "main body" zone. Because of the large surface area and the closely in-phase tides across this zone, this component is very large and many reach up to 5 knots at the Heads.

In contrast the minor component due to tidal breathing in the "choke zone" region will contribute only around 0.6 knots to the total. This minor component is smaller than might be expected just from the lower surface area ratio alone (7:1).

The reason is that despite a slightly higher tidal range within the "choke zone", the varying phase differences from 0 to 90 degrees across the "choke zone" mean the tides across this area are never rising at their maximum rates all at the same time as occurs within the "main body".

For example near the Heads at half tide when the level is rising at its fastest, there will be no rising water near the northeast border of the "choke zone" because it is low tide there, and the rise has yet to begin. Similarly when the tide height is changing at its maximum rate near this northeast border area, the tide height near the Heads is barely changing at all.

The times of maximum or minimum water volume within the "choke zone" occur roughly midway between the times for hi/lo tides in the ocean and hi/lo tides in the main body. This is roughly 1.5 hours after hi/lo tide at the Heads. The "choke zone" current component will be 0.0 knots at these times.

Because the relative water depths across the large shallow areas of the choke zone vary a lot over a tide cycle, the "choke zone" current component is not as "sinusoidal looking" as the "main body" component. There is also roughly a 45 degree phase difference between these components which is why at times they will have opposite signs.
Last Edit: 1 week 4 days ago by packo. Reason: colour adjust

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1 month 3 weeks ago - 1 week 4 days ago #303543 by packo
packo replied the topic: Some bad advice from VIC Dive Shop continues to put local divers at risk

CONCEPTUAL MODELS & REALITY: The next dozen paragraphs are only for "nerds" or "interested others". The "skimmers" should skip forward to the next '*****' heading.

The "two current component" model was introduced to help explain bay behaviour because there are indeed two Bay areas that do behave quite differently. Readers might be puzzled by the claim that at flood slack water there appears to be simultaneous -0.5 and +0.5 knot currents flowing through the Heads. Is this realistic? I'm afraid the answer is best summed up as firstly "NO", and then a slightly fainter "YES".

THE "NO" CASE: The model translates the rate of the loss or gain of water in a zone into a "heads equivalent" current component. The components are either +ve(in) or -ve(out) but the important point is their SUM represents the real current averaged across the Heads fairly well because all tidal water flows have been accounted for and there is no other way for water to enter or leave the Bay.

However although the model is mathematically sound, it contains the assumption that a net loss from the "choke zone" is through the Heads. It ignored the possibility of it either going out across the northern border, or being split across both borders. This does in fact happen when the components have opposite signs - roughly the last 1.5 hours of a stream.

Rather than the 0 knots at the Heads being a mathematical cancellation of two real and opposing currents at that point, no cancellation is needed when the -0.5 heads equivalent loss from the "choke zone" is correctly attributed to outflow across its northeastern border. In effect the -0.5 & +0.5 components the model identified as separate, are in fact actually one in the same thing, and this small flow is northward from the "choke zone" and into the "main body" zone. This is merely a redistribution of water already in the Bay and there is no flow through the Heads at this time.

Depending on the exact tide cycle, 10 to 20 minutes before slack at the Heads, the main body's appetite for water might have been stronger at +1.0 knots because its tide then was rising slightly faster. To make up this flow, the "main body" is cannibalising the "choke zone" to the tune of +0.5 knot, with the other +0.5 knots coming in through the Heads as the real inward dwindling last part of the flood stream. (divers should really be just about hitting the water)

At the moment of slack water at the Heads, no actual water is flowing in or out of the Bay. However inside the Bay water still creeps northwards. Why? - because it has momentum. While the average speed is very low, roughly a billion tonnes of water are still on the move. That is a lot of incentive to just keep going! While the total water volume is not changing at Heads slack water time, water is redistributed as the residual momentum "sucks out" water from the "choke zone" (creating its loss) and deposits it in the "main body" (creating an equal gain).

In the minutes following slack water at the Heads, the "main body" becomes less hungry for water as approaching its high tide the rise rate is shrinking. An increasing portion of the "choke zone" loss then begins to be diverted out of the Heads to become the beginning of the real ebb stream. The remainder of the "choke zone" loss continues northeast to satisfy the main body's diminishing needs.

By the time slack occurs along the St Leonards - Rosebud border, all of the choke zone's -0.5 knot loss is exiting through the Heads as the real "baby ebb stream" just 10 to 20 minutes old. After that, the ebb stream keeps growing due to the new -ve "main body" component that is developing as the tides in the north start to fall.

THE "YES" CASE: Although the "cancelling" -ve and +ve arrows as shown in the earlier diagram is now seen to be a product of an assumption in the modelling, there is evidence that small opposing currents do exist at the Heads around slack water. However these are mainly observed for the ebb slack rather than the flood slack case discussed here.

There are observations that show a new incoming current begins on the Pt Nepean side while a fading outgoing current is still running on the Pt Lonsdale side. It is rather like in the diagram except the "arrows" are smaller at perhaps 0.25 knots.

More work needs to be done but it is possible that the momentum still in the long tail of the ebb stream "jet" that runs passed the Pt Lonsdale reef and extends well out into Bass Strait continues to "suck out water" on that side for some minutes after the flood stream begins entering closer to the eastern headland of Pt Nepean.


Particularly within the "choke zone" where many scuba divers operate, the behaviour of tides & streams can at times seem very confusing. Much of this is due to the Bay's unusual size and geographic shape. Unfortunately many government bodies, the shipping industry, safety authorities, and even the BOM offer only "dumbed down" explanations of how the Bay works. These contain many inaccuracies and inconsistencies that just lead to further misunderstandings.

In essence, close to the Heads we have the local tides controlled largely by the nearby ocean tides, yet the streams at the Heads are controlled largely by water level changes in the "main body" zone because of its much larger surface area. Since this is well over the horizon to the north and out of sight, its unseen influence tends to be forgotten. That is a big mistake! Odd quirks of the tidal cycle near the Heads include:-

a) Three hours where incoming flows are accompanied by a falling tide level.

b) Another three hours where outgoing flows are accompanied by a rising tide level.

c) Interleaved between these times are two "normal" periods where the tide rises with the flood stream and falls with the ebb stream.

The duration of the strange "a)" and "b)" behaviours will also vary strongly from place to place. No wonder many get confused about tides and streams in the "choke zone"!

The situation is easily explained by the phase delay between tides at the Heads and in the "main body" zone. In case "a)" the local tides begin to fall soon after the ocean starts to fall. However in the "main body" zone it is just after half-tide with tides still rising strongly and there are 3 more hours to go before high tide is reached. The strong incoming stream at the Heads during this period is the water needed to raise levels in the "main body" zone.

Case "b)" is the reverse case with the tide at the Heads beginning its rise, yet the "main body" tide is falling fast at around half-tide with 3 more hours to fall. The outflow at the Heads is the result of the draining down of the "main body" zone.

In case "c)" we either have the "main body" tides just starting to rise, or just starting to fall. In the first situation it is a rising half-tide at the Heads so the local tides rise and the stream direction is inward. In the second situation the ebb stream is just starting and it is half-tide at the Heads with rapidly falling levels.

Although in case "c)" the tides and streams might seem to be more "normal", they are not in fact connected in the normal sense. This is evident on a closer inspection where we see the stream strength is weak at the start of this period when the local tides are changing most rapidly, and the stream is strongest 3 hours later when the tide level at the Heads is hardly changing at all. There is no "cause and effect" connection between the two.


d) Slack water does not occur "when the Bay and ocean levels become equal".

e) Momentum effects keep the flows going in the same direction for another 40-60 minutes beyond the "equal levels" point.

f) At slack water a reverse slope from 15-45 cm builds up during that extra run time.

g) It is the reverse slope that is mainly responsible for stopping and then reversing the flow.

h) Water acceleration rates are higher near slack water than at any other time in the cycle.

i) Regardless of whether it is a flood slack or an ebb slack, the ocean level at that time is about the same. It is the "main body" level that is either a "step upwards" for flood slacks, or a "step downwards" for ebb slacks.


Ok so now I am bowing out. I figure a good and safe outcome is probably more likely if I exit the scene. Since the dive shop won't check its own claims, I have asked a selection of prominent Melbourne divers and some relevant diving organisations to take over.

I know there is a strong aussie tradition of "we don't dob", but I hope the sentiment "for the good of Bay divers" may just trump the no-dob ethic. I suspect that provided it is coming from someone other than me, the mule just might be turned back into a horse, then led to the water's edge, and then be made to drink without so much fuss.


Well, for some unknown reason I have found myself "up close and personal" to a total of six drownings/near-drownings (so far) in my life. They seem to happen about every ten years. I saw the dangerous web advice in September 2015 and knew I had to act as "number 7" seemed a little overdue and might be expected anytime soon!

Half these incidents were fatal and in the other half the victims survived with various levels of medical treatment. Both outcomes affect the immediate family and an extended circle for life. One funeral, that of a young man in his prime, just about tore the heart out of all attendees.

These incidents spanned the whole gamut of "humans in water" situations including:-
under-supervised toddlers, adventurous boys, scuba divers, swimmers, paddle boarders and kayakers. All could have been prevented if only some small issue was properly attended to.

The scuba diver I really didn't know at all, but we chatted for a bit as I exited a Mt Gambier dive site and he entered it. That was his final dive - ever. He got lost in a silted tunnel and drowned.

The paddle boarder was in the Sorrento Channel, the same waters the camera now overlooks. He was knocked off his board by a very ugly wind against tide situation late one afternoon. I stumbled on him some 40 minutes later by pure fluke.

Rescue in the conditions was a difficult affair. Mostly I thought the chance of survival was way less than 50-50 and a swamping of my boat was a real danger. He was a solid man but nearly spent, barely conscious and with lots of inhaled water. However he fought back from the edge with a determination that only comes from seeing a slim chance at life arising out of what was previously a hopeless situation. It was a very close call indeed.

On his release from hospital ten days later we reconnected. I asked him how he got separated from his board. He explained that it got rough fairly quickly and the third time he got knocked off the board, his leg rope broke and the 25 knot wind whisked the board away. He was wearing only bathers and had no flotation device of any kind.

When I asked why was he out in such rough tide against wind conditions, he said "the tide turned against the wind much earlier than I had expected."

So you see, sending wrong advice on "delayed stream reversal times" out onto the web is not a good thing to do. You never quite know which water sports people you are endangering!


Those readers who might want a more extensive overview of how Tides and Currents work in the Bay can read the jpeg document at this link:-
Packos Port Phillip Bay Rant
Last Edit: 1 week 4 days ago by packo. Reason: color adjust

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