Hi,
Part of a project I'm submitting to the Port of Melbourne Harbour Master was to include a quick quiz of small vessel skippers who operate near Port Phillip Heads. The quiz was to discover how folk think the about the way tidal currents change with time both before and after slack water.

This plan was scuttled by a long closure (for big rebuild) of the only trailer boat launching ramp close to the Heads.

Even when it reopens, with Covid-19 off and running strongly again, I am reluctant at my age to be hanging around these areas handing out quiz forms and collecting responses. So I'm posting on-line here to hopefully catch some sea-breezers who sail (or motor) through Port Phillip Heads and would normally aim to catch slack water time.

The picture below shows my original quiz questionnaire but the original red "answer boxes" have already been filled in by either a letter or number (in purple).


If you would like to help by answering this quiz, you just need to reply with just one letter, followed by one number.

Even folks who haven't even been there are catered for by answering as "An", where "n" is your choice from 1 to 4.
Although such and answer might be just a perception of yours, all views are helpful for the purpose of the project.

If you have friends who read this forum but don't have their own sea-breeze account, you can post on their behalf by posting their answer followed by "(for friend)". You can be a "friend of yourself" if you would rather not hang out your own answer in public.

Alternatively you can send it to me via a PM. The more answers I get the better the statistics will be.

**** PLEASE NOTE:- We don't what any discussion about answers here, as this might taint the survey.
So please don't "wax lyrical" - just a letter from "A" to "F" followed by a number from "1" to "4", or don't respond at all!

I won't be providing any commentary about the outcome for several months, or no response at all if there are too few answers to get any meaningful statistics.

thanks in advance,
stay safe on and off the water,
Jake

A 3

C 4

A 4

A3

C 4

C4

I was thinking A3 myself, then I note some C's say 4. Looking again at the graph in particular the x axis in graph 3, the period where the tide is slack or nearly so is about 1/3 of the total of the x axis, that's way too much as that would be 2 hours of slack. I would have thought 20 -30 minute of slack time but never been on Port Philips much less less the heads. In away since the axis are not labeled with any units you might not be getting an accurate response to your question.

A3

Hello its Jake again,
I said I wasn't going to comment or enter into any discussions, but "John24's" comment about the lack of timing or current calibrations does need some attention. I had expected mainly locals to be responding, and am surprised by the number of non-locals chipping in. I see now that without the expected local knowledge it might have been hard for others to get the right context.

Lets just clarify that the total time span represented is perhaps 2 hours, and certainly not meant to cover the entire 5-7 hours of a tidal stream from max +ve to max -ve.

As for the range of +ve and -ve currents depicted lets say around +/- 1.5 knots. Peak currents at Port Phillip Heads run up to about 6 knots at springs, with the average max values around 4 knots.

I hope that helps John24, but I would rather not give any more detail. I'm new to the rules of this forum and can't be sure posters can re-edit their post after a later reply has come in. If you have a change of mind and can't re-edit, then just send a new post.

Thanks for the all the replies so far,
Jake

IMHO a somewhat inscrutable post which makes answering difficult without associated context. Realise your don't "wax lyrical" directive still stands but certainly agree with john24's comment which is why I gave it a thumbs up. So a good response Jake. I am not a local but have some knowledge of Port Phillip from skiff sailing there and also quoting for the UKC studies conducted over many years by the authorities. These are now DUKC studies. So your explanation is welcome. Is your question a sub-set of such UKC or DUKC studies? Probably not able to be answered. If you want me to delete my post I will.

C3

Hello r13,
How could I be upset by commentary from you? I find your posts are always insightful, technical, and helpful. While I live in the nerdy side of the population, I do realise many don't. On the topic of graphical plots I am aware some won't even want to try to interpret a fully fledged plot with detailed scales and calibrations.

I used the "keep it simple" approach - knowing that the original questionnaire artwork was to be presented face-to-face, and any questions arising could be dealt with on the spot. I didn't really see the difference before going the online route.

The other issue with specific calibrations is that they destroy generalisations. Tidal cycles are widely varying in both size and shape. This is reflected in the way the tidal streams behave including the the duration of what we know as the "slack water period". When the survey is over and discussions with the HM have the hoped for outcome I will put up a post that fully describes what is all been about.

I'm intrigued by r13's phrase "quoting for the UKC studies" many years ago. Were you trying to be involved in that work?

A DUKC (Dynamic Under Keel Clearance) has been in place for Port Phillip for nearly a decade now. It is mandatory for ships over a certain draft. It was developed by a local Melbourne based company OMC (Ocean Marine Corporation?). They now sell their technology to several Australian and Overseas ports.

In practice the system means the ship's pilot takes aboard a special small "ruggedized" computer that can model the ships rolling and pitching behaviour using certain ship specs that the ship owner must upload well before transit through the Heads. The computer also receives a secure web stream of real-time info on tide heights, winds, swell, and wave conditions.

It crunches all these numbers, along with the detailed depth surveys of the southern end of Port Phillip Bay to predict and help maintain the minimum clearance between the keel and the bottom. This applies through Heads and along the dredged South Channel. This can be as little as a metre!

The system is sold as a "safety system". While that is true to some extent, it is used mainly to save money by loading the ship a little deeper. Apparently every 0.1m of draft you can carry saves some really big bucks.

To finally answer the question r13 asked, the stuff I'm on about is mainly related to tidal dynamics for the recreational sector and not the "big guys". However that said, there is some overlap in that we need to make sure the tidal dynamics is really well understood. I don't think we are at that point yet and Port Phillip seems to have a few surprises up her sleeve. Some disagree.

The 2008 Chanel Deepening Project had an aim of allowing ships with a maximum 14.2m draft through "in all tides and all weathers". However, over the years a growing number of transit restrictions have been brought in. These include baring transits for large drafts while entrance currents are above certain levels. Swell and wave height limits also feature here. Also really big tankers are now being confined to transit only around slack water with a 1 knot current limit.

The entrance current is one thing that can't be directly measured as the Rip can chew up and spit out almost all forms of instrumentation. So instead they use a "computer modelling" approach to give a theoretical current. I'm not so happy with this for a number of reasons, one of which is that I've witnessed a very large tanker approaching from the southwest, while the Pilot Control room was reporting a dwindling ebb current of -1.2 knots, but I was experiencing the flood already underway at around +0.7 knot.

My take is that these creeping transit restrictions might reflect some small DUCK numbers that could indicate the experts may not understand the Rip as well as they thought. There are a few obvious hints at this that I am trying to resolve, but its been tough gig amid the pandemic, a stream of different HMs, and several reorganisations of the port bodies.

Gee, and who said "no waxing lyrical"? Sorry about that - perhaps I should have PMed r13 instead.

Regards Jake
(PS If you feel you can't answer due to an inadequately framed questionnaire, then don't. Let's not make a "federal case" of it.)

A3
(and thanks for the info Jake, very interesting!)

Yes ditto, thanks Jake for an interesting thread.
When you mention the advised tidal streams, are you referring to the values published by BoM in their tables? If so, these will of course be predictions with no way of allowing for short-term effects such as wind and barometric pressure. I wonder if the figures that VTS quote on any particular day are adjusted for these factors?
Like you, I have observed slack water times and flow rates significantly different from those in the tables. But then again, the BoM/VTS numbers probably relate to what's expected to be happening in the Eastern, Great Ship and Western channels, not off to the side in shallower water where we in our small vessels are most likely to be.

As the kids say, ymmv

Cheers, Graeme

Select to expand quote

JakeH5 said..
Hello r13,
How could I be upset by commentary from you? I find your posts are always insightful, technical, and helpful. While I live in the nerdy side of the population, I do realise many don't. On the topic of graphical plots I am aware some won't even want to try to interpret a fully fledged plot with detailed scales and calibrations.

I used the "keep it simple" approach - knowing that the original questionnaire artwork was to be presented face-to-face, and any questions arising could be dealt with on the spot. I didn't really see the difference before going the online route.

The other issue with specific calibrations is that they destroy generalisations. Tidal cycles are widely varying in both size and shape. This is reflected in the way the tidal streams behave including the the duration of what we know as the "slack water period". When the survey is over and discussions with the HM have the hoped for outcome I will put up a post that fully describes what is all been about.

I'm intrigued by r13's phrase "quoting for the UKC studies" many years ago. Were you trying to be involved in that work?

A DUKC (Dynamic Under Keel Clearance) has been in place for Port Phillip for nearly a decade now. It is mandatory for ships over a certain draft. It was developed by a local Melbourne based company OMC (Ocean Marine Corporation?). They now sell their technology to several Australian and Overseas ports.

In practice the system means the ship's pilot takes aboard a special small "ruggedized" computer that can model the ships rolling and pitching behaviour using certain ship specs that the ship owner must upload well before transit through the Heads. The computer also receives a secure web stream of real-time info on tide heights, winds, swell, and wave conditions.

It crunches all these numbers, along with the detailed depth surveys of the southern end of Port Phillip Bay to predict and help maintain the minimum clearance between the keel and the bottom. This applies through Heads and along the dredged South Channel. This can be as little as a metre!

The system is sold as a "safety system". While that is true to some extent, it is used mainly to save money by loading the ship a little deeper. Apparently every 0.1m of draft you can carry saves some really big bucks.

To finally answer the question r13 asked, the stuff I'm on about is mainly related to tidal dynamics for the recreational sector and not the "big guys". However that said, there is some overlap in that we need to make sure the tidal dynamics is really well understood. I don't think we are at that point yet and Port Phillip seems to have a few surprises up her sleeve. Some disagree.

The 2008 Chanel Deepening Project had an aim of allowing ships with a maximum 14.2m draft through "in all tides and all weathers". However, over the years a growing number of transit restrictions have been brought in. These include baring transits for large drafts while entrance currents are above certain levels. Swell and wave height limits also feature here. Also really big tankers are now being confined to transit only around slack water with a 1 knot current limit.

The entrance current is one thing that can't be directly measured as the Rip can chew up and spit out almost all forms of instrumentation. So instead they use a "computer modelling" approach to give a theoretical current. I'm not so happy with this for a number of reasons, one of which is that I've witnessed a very large tanker approaching from the southwest, while the Pilot Control room was reporting a dwindling ebb current of -1.2 knots, but I was experiencing the flood already underway at around +0.7 knot.

My take is that these creeping transit restrictions might reflect some small DUCK numbers that could indicate the experts may not understand the Rip as well as they thought. There are a few obvious hints at this that I am trying to resolve, but its been tough gig amid the pandemic, a stream of different HMs, and several reorganisations of the port bodies.

Gee, and who said "no waxing lyrical"? Sorry about that - perhaps I should have PMed r13 instead.

Regards Jake
(PS If you feel you can't answer due to an inadequately framed questionnaire, then don't. Let's not make a "federal case" of it.)

Thanks Jake for the info. Good that you posted back here for others interest. Yes was part of a company unsuccessfully trying to be involved in the UKC work in Port Phillip in the 2000s. OMC Int is a true Aust success story as you say - a truly enormous effort getting the system off the ground and proven and thriving.

omcinternational.com/about/the-omc-story/

omcinternational.com/media-centre/

omcinternational.com/about/milestones/

C3

Haven't looked at the link yet but interesting. I spent a season racing in the Solent years ago. The computer programs work as long as the person using it knows what they are doing. You need to speed up or slow down the program according to what the tide is actually doing. If you don't it's useless. It can't be accurate every day!
What was more helpful was having a local navigator onboard who knew where to go and was watching the tide. You could watch the boats tacking and he would say" fools, tide hasn't changed yet!"
It's like weather forecasts, sometimes it's accurate and sometimes it isn't. And like a weather forecast you can't think it's going to be accurate. You have to adapt to what is actually happening.
Like a weather forecast there are so many variables. Water temp, salinity, moon phase, pressure, as others have said. The bottom must be changing with that much water flowing in and out?