These are normally distributed 3 months at a time to allow for any revisions or "tweaks" that come to light.
The 2021 April, May, and June tables can be downloaded via these links:
April 2021 predictions
May 2021 predictions
June 2021 predictions
The updates for July --> September 2021 are scheduled to be released here around the 22nd of June.
******** What Are These Tables? ********
The tables are based on computer modelling of how the tidal volume of water in Port Phillip Bay varies during each 6 minute data interval over the entire year. The rate at which the Bay water volume changes with time is then used to calculate the corresponding inflow or outflow rate through the Heads.
Using an estimate of the effective cross-section at the Rip, the volume flow rate at each 6 minute interval is converted into a current value in knots. Using interpolation, the times at which the current value passes through zero as the flow reverses can be obtained. These are the predicted slack water times.
The predictions are most accurate when there is no additional "weather current" component flowing in or out of the Heads at that time. The usual practice is to arrive at the dive site around 30 minutes before the predicted time and to then monitor the current. This time allowance is needed firstly to allow for any weather effects, and secondly so that divers can "drop-in" somewhat before true slack to maximise their dive time.
The main weather items that can produce these "weather currents" and so alter slack water times are barometric air pressure changes over Bass Strait, and strong winds from certain directions. Another "Packo post" on this forum discusses these effects in some detail. With this knowledge it is often then possible to refine the time allowance value to a more appropriate value.
Various other pieces of information related to each slack water event are also listed in the tables. There is a legend in the tables that gives a slightly cryptic message about what each table column means. The names of the various columns are replicated roughly every seven days and particularly at weekends.
The main feature over and above the normal "official tables" is they have all the information you may need for planning dives at or near the Heads in one place. This includes calculations of the "current reversal rate" which I consider to be a significant safety advance and so am trying to convince the "official table" producers to also include these numbers sometime in the future.
From the current vs time predictions, the likely "dive duration" in minutes for each slack is calculated. These are based on times when the initial stream drops below 0.7 knots and the reversed stream rises above 0.7 knots. The chosen current limit of 0.7 knots corresponds to about 22 metres per minute which, depending on gear configuration, drag and fitness, may be difficult to make headway against on a sustained basis.
While there is no guarantee of high absolute accuracy in the "current change rate" or "slack length" calculations, accurate comparisons with the numbers from other slacks can be made to help formulate a good dive plan on which slacks to dive.
Other useful info includes: Slack water times with respect to sunrise or sunset, Max stream rate, Times when the current exceeds 75% of the maximum figure, Duration of the Flood or Ebb stream, Point Lonsdale tide times and heights, Estimated delays in slack water reaching other locations, and Slack Time Differences from the "official tables".
******** Tables Value Differences with the "Official Tables" ********
Some table values vary a little from the official "Rip Predictions" computed by Cardno Pty Ltd, and published by BoM, VRCA and Australian Hydro (as ANTT). In general I believe these differences are for the better. However due to weather variations, which can also alter current - time profiles, it is a difficult case to prove to the port authorities.
There are also some unresolved issues around the fact that there can be slight differences in slack water times between the Point Nepean side and the Point Lonsdale side. I'm working to try to clarify this but that will take some time with lots of direct current measurements in the field needed.
In a theoretical sense it is easy to prove the official ebb slack times are in general slightly too late. Arriving late at a Port Phillip Heads dive site does raise the risk associated with the dive. The official ebb slack times are often very close to the Williamstown low tide time. However while the vast "main body" of the Bay has its low tide (or minimum volume) close to low tide at Williamstown, it is a fact that at the Heads and within 15km of them, the water levels are rising quite rapidly at this time.
It is logical that the "total bay minimum volume point" must occur somewhat before the low tide at Williamstown, when the still falling tides there can counteract the rising tides inside the Heads. Slack Water following an ebb tide is more or less defined as "the time when no more water is draining out of the Bay". That is the minimum water volume time.
At that time outflow through the Heads has stopped, but the residual momentum of southbound water in the central/northern regions of the Bay is still delivering some water into the southern region. The volume changes in the two areas are then equal and opposite with the rate of loss of water from the north being just enough to supply the rising tide levels in the south without any extra spilling out through the Heads. The modelling shows the minimum water volume time occurs between 5 and 21 minutes before Williamstown low tide, depending on the particulars of each tide cycle.
On their "worst day" (22/09/2018), the official tables claim the late afternoon reversal from ebb to flood at the Heads does not occur until 21 minutes AFTER Williamstown low tide (20 mins AFTER Pt Richards low tide, and even 17 mins AFTER Geelong low tide.) It is difficult to see how it is possible to have slack water at the Heads when tides all across the Bay have been rising for all of this time.
The Packo tables claim slack occurs just 6 minutes BEFORE Williamstown low tide on that evening. At that time, the very slightly still falling water volume in the Bay's main body exactly offsets the rising water volume near the Heads to give zero net inflow or outflow at the Heads. It all makes sense to me but with weather induced timing variations of a similar order also possible, it is a hard case to prove to the authorities. In particular many days with the largest difference between "packo" and "official" slack water predictions are also the slacks most vulnerable to time shifts due to weather effects.
I am now concentrating on days where my predicted current values at the official "slack time" are large (>0.5 knots), and hoping to catch some "no or very small weather effect" days in the data. This approach seems to be working but there still needs to be a lot of data collected to make a convincing case to the authorities. Unfortunately the Covid-19 lock-down has forced me to suspend further work. The image below shows one of my field work runs at the Rip:-
******* Other Tidal Stream Issues **************
The volume modelling's usual 6 - 20 minutes time range for "Rip Slack before Williamstown High/Low tide" is also the delay time between slack water at the Heads and slack water at the more distant locations of Rosebud or St Leonards which are both close to the Bay's "main body" (anywhere north of the "Great Sands" area).
Common "yachtie folklore" is that slack water at the Heads and anywhere in the south end of the Bay is at the same time as Hi/Low tide at Williamstown. These times are of course a little late but the time difference is unlikely to trouble most yachts. Divers however may be affected by timing errors should the "yachtie folklore" be relied on at the Heads.
Some still common "diver folklore" is much worse! Its claim, also carried on the Scuba Doctor website, is that a delay of around THREE HOURS occurs between slack water at the Heads and slack water at Rosebud or St Leonards. This is a dangerous claim and all divers are strongly urged to ignore it. (Sorry Scuba Doctor!).
In December 2017 a solo diver chose to dive well offshore from Rye at around three hours after slack at the Heads. He was swept over 10km away from his anchored boat and towards the Rip. He was exceedingly lucky to be found alive some 4 hours later during an extended helicopter search. The local peninsula newspaper report is at this link:- Diver Rescued.
I spoke to the local coppers, the police air wing, the southern peninsula rescue and Searoad Ferries staff to try and piece things together a bit more so we might learn from this incident. The coppers weren't that helpful and would not pass on my contact details to the diver. (Apparently his right to privacy overrode the fact his mistake had just cost the taxpayers over $5,000 in search expenses. So nobody got to find out why he chose to dive at that time.)
My best effort at piecing together the details of this incident (locations are a bit approximate):-
Date: Thursday 14th December 2017.
Timeframe: 2pm to 6pm.
High Tide at Point Lonsdale: = 9:04am.
BoM Rip Predictions: Slack at end of flood stream = 11:35am and following Max Ebb current -4.75 knots @ 2:21pm
Packo Predictions: Rip_Slack = 11:31am, Rye_Slack = Rip_Slack + 8 mins delay = 11:39am
Local Dive Shop's advice: Rye_Slack = BoM Rip_Slack + 2hrs 50mins = 2:25pm (cf. BoM's Max Ebb stream current prediction = 2:21pm)
Dive Entry: Approx 2pm about 4.5km north of Rye, about 1km short of the ship channel. (Scalloping?)
Alarm raised: Approx 3pm - 3:20pm by friend at Rye beach when diver failed to return.
First Sighting: Police Helicopter spots diver near channel marker #4 but then looses sight of him in "unpleasant choppy conditions" with 20 knot SSW wind and 0.5m waves.
Second Sighting: Several hours later Sorrento ferry captain on normal route to Queenscliff, but running slower and keeping a good lookout at police request, spots him a little west of #2 Channel marker about 6pm.
Recovery: Ferry diverts for attempted recovery but in the end shelters the scene to allow a police boat pickup around 6pm. Totally exhausted and cold after around 4 hours in the water. Conveyed to Rosebud Hospital for treatment for mild hypothermia.
Total Drift Distance: around 10.5 km (5.7 nautical miles), average drift speed around 1.4 knots. Note Ebb slack due around 6:30pm so peak drift speeds probably around 2 knots. Tide cycle would be classed as "medium to big".
There was a similar incident off Rye several years earlier. This time the diver was diving out of an attended boat but in the current he surfaced too far away from the boat to swim back or attract the attention of those in the boat. He was eventually rescued around 5km away along the shipping channel. Please note that slack water occurs at quite similar times throughout the southern end of the Bay.
Even the many divers who persist with the common notion that slack at Portsea Hole is 30 minutes behind slack at the Heads need to rethink things, or risk missing dives. If water has been coming in through the Heads for 30 minutes and you claim the flood tide hasn't reached Portsea yet, you need to think how all this water could be "stored" somewhere between the Heads and Portsea.
The area of that region is roughly around 25km2, and the amount of water coming in during the first 30 minutes of a strong flood tide is roughly 22 gigalitres. Even if this extra water happened to be spread uniformly between the Heads and Portsea a level rise of (22,000,000m3)/(25,000,000m2) = 0.88 metre would be required in that 30 minutes to store all that incoming water without letting any move beyond Portsea.
We never see these huge 1.76 metre/hour rises, but instead they are measured at more like 0.28 meter/hour, or around 6 times smaller. The real tide rises suggest that at the 30 minute mark only around 3.5 gigalitres of extra water has accumulated and is "stored" between the Heads and Portsea. So the other 18 or so gigalitres of Heads inflow volume has already been pushed beyond the Portsea/Queenscliff region, meaning the flood tide at Portsea is already well underway at the 30 minute mark. ( In theory the typical slack delay for Portsea is around 3 to 5 minutes, but in practice just assume it is zero.)
Note however that at this time, the flood stream at Portsea doesn't actually contain any "new ocean water" but only "old stuff" that is pushed towards Portsea by the "new stuff" that has entered the Heads. However in volume terms, a volume of water equal to around 84% of what has entered the Heads, has already "pushed past" Portsea meaning the flood tide is well underway. Note it is the "volume flow" that counts towards "giving current", regardless of whether it is "new" or "old" water. Its a tricky distinction but I hope it is understood.
******* Other Difficulties In Understanding Tides and Streams **************
Much of the misunderstanding comes from people confusing tides and tidal streams as more or less the same thing. They are not. This is particularly true in Port Phillip where at places like Portsea we can at times have a flood tide stream, but a falling tide level.
Yes the time of the max/min tide level is progressively delayed by around 3 hours between the Heads and places like Rosebud, but the delay in max/zero flow of the tidal stream is progressively delayed by only around 12 minutes on average.
In the southern part of the Bay, high and low tide times have nothing to do with slack water but instead mark only the time when, for that particular location, the flow rate on the Heads side of that location exactly matches the flow rate on the Melbourne side. At other times the two flow rates will be slightly different, (but both in the same direction), leading to either a slight net water gain and a rising tide height, or a slight net water loss and a falling tide height.
At Pt Lonsdale the INFLOW=ONFLOW point (ie high tide) occurs about halfway through the flood stream, while at Portsea this does not occur until about three quarters of the way through the flood stream. The wide deep waters beyond the Great Sands and north of places like Rosebud mean the ONFLOW speeds are always quite small. So the high tide at these points does not occur until right near the end of the flood stream when the INFLOW speed nears zero.
Further confusion is cause by various government bodies and the Port Phillip Pilots, publishing a grossly simplified version of how, why, and when the tide reverses at the Heads. It is incorrect, logically inconsistent, and generally unhelpful. Hopefully one day it can be changed!
Update 19/9/2018: Finally we might be making some progress here. The Port of Melbourne Harbour Master has finally been won over that the explanation on tidal streams offered by a whole raft of official players is not correct and needs a rewrite. I've been asked to draft some new paragraphs and I guess we will haggle it out from there. It will however take some time, maybe up to a year, before the better explanation replaces the BoM, AusHydro, PPSP, & VPCM wordings.
Update 21/9/2019:
Yes its now a year later without the result I want! The HM did not update the wording when the 2019 Port Information Guide was updated in March. I may have made a strategic error by pushing that it should include a note that slack water was typically 5 to 18 minutes before Williamstown hi/lo tide. (The official tables have a range from around -10 minutes earlier to +10 minutes after). I told the HM I expected to be able to deliver proof to his satisfaction sometime in Autumn. I couldn't meet that expectation due to delays getting my GPS current tracker drougue fully sorted. Gathering acceptable proof will also involve some remote logging gear. The prototype has been designed & tested but the operational setup is yet to be completed. Getting a bit jack of all this official indifference. I'll keep battling along as my spare time permits. However it seems pointless to speculate on timelines anymore.Update 22/3/2020:
Well I feel a bit silly now! In the update above I expressed disappointment that "VicPorts" had not amended its advice and also appeared to back away from some undertakings given previously. I now have come to learn that the Melbourne Harbour Master I was dealing with at the time has retired from work and left that post some months back! As a retiree myself I understand that in the last couple of months in the job it is rather easy to let "nuisance issues" slip away without proper attention.If I'm still standing after the covid-19 pandemic plays out, I will tackle the "new guy" with my usual persistence and some more field measurements. Not sure how I missed the personnel change, but probably because for the later part of 2019 I was battling out some other issues with the Geelong Harbour Master. He was also a "new guy" at that time, but gave me no joy. Simple faults in the VRCA tidal plots webpage remain unfixed. Some sensible safety messages removed from the Vic Tide Tables when the VRCA took over table publication from PoMC have also not been restored. It is all so disappointing but I will "soldier on" when it is safe to come out of the virus lock-down.
Update 10/4/2020: One tiny bit of joy is that after years of nagging the Geelong HM and various board members at the VRCA, one of the defects in their real-time tide & "surge" plots has recently been fixed. All the missing data lines in the image (about 1 in every 33) now display properly on their webpage. There are no longer those annoying "gaps" in the plots that appeared from time to time. Too small plot scales are still an issue. . . perhaps several more years for this fix?
In the meantime please do not try to reply to this post. If you have questions or counter agreements, reply instead to the "Tide Questions or Arguments" topic also listed under this general area. This topic needs to remain locked to allow it to be used again for future table updates. Thanks for this consideration.
cheers,
packo
