What angle between wind and tide is the threshold for trouble?

[shmoo]

The background to the question is this: I have written a program to do passage planning in tidal waters (like Neptune, Sea Pro sort of thing). It answers the question "what time should we leave" for multi-leg trips in tidal waters. It also incorporates some project management/OR type constraints such as "it needs to be between HW -1 and HW +2 at some port when we are at such and such" and/or "it must be between dawn and dusk when we pass here", and/or "we need at least 6 but not more than 12 hours layover at this waypoint". So far it works fine (and thanks to forumites who have helped with testing)

Now I am thinking about overlaying GRIBs to give an estimate of wind over tide conditions along the route. The program already knows the tidal streams along the route. Hence the question.

Clearly 180 degrees between wind and tide is worst and 90 or 270 neutral, but at what angle coming up from 90, or down from 270 would the panel say is the onset of "wind over tide" condition.

I guess "it all depends" is an acceptable answer... But worth asking anyway.

 

[wotayottie]

I'd be inclined to use the vector component of the wind in the opposite direction to the tide flow. That said there are far more variables - perhaps the two most important being water depth and the fetch / time the wind has been blowing.

You're into a very complicated area here.

 

[25931]

It also depends on strength of wind and/or tide.
Jim

 

[shmoo]

Thanks for useful start. I am not after absolute values of waviness, just relative best and worst for the tidal cycle for particular winds.

If I explained a bit more how I will use it that may help.

Consider the passage from Suffolk rivers to Dover. A straight tidal flow analysis of the route comes out with a couple of hours between fastest and slowest times. However if I could add a constraint that avoided the times when I could expect a F5-6 against strongest flow of the spring tide round the Forelands, I would happily take the longest predicted tidal stream passage. Would be better than getting there and having to anchor off Margate for hours waiting!

Anything written on this that anyone is aware of?

 

[dt4134]

I'm far from convinced that you can calculate it with any accuracy from first principles.

In theory the wind over tide effect occurs because the wavelength of the waves is shortened without the amplitude being reduced, thereby making the waves steeper.

However it is quite easy to think of a number of practical difficulties with this simple model.

The wave train might not be aligned with the wind, for various reasons.

It doesn't take into account local circumstances, such as the Needles Channel where the disturbance is partly due to a fast flowing tidal stream meeting a slower moving body of water so that incoming waves are held up at the boundary so concentrate in quite a small area.

It also doesn't take into account seabed topology which is a major factor in some cases.

I suspect you might well have to map it all empirically, which could be a lot of work. However, you could probably derive a lot of info from charts, alamanc, pilot books etc. As people do in practice.

 

[shmoo]

Ok, Ok. You've all convinced me its a mad scheme, but thanks for the responses anyway.

I will get back in my box now...

 

[Rivers & creeks]

What's the program called and where do I get it?

Simon

 

[shmoo]

Its called (for now) yapp (Yet Another Passage Planner) and you can't get it yet 'cause I need to finish it, and then decide if its got enough commercial attraction to sell it (unlikely) or to give it away (my preference for several reasons). I will need to do some negotiation with UKHO over use of the tidal stream (tidal diamonds) info. There is provision for users to enter their own, copying from charts, but it's grunt work I have very many in southern N Sea, Thames Estuary and Channel already done and saved in xml files.

The status at the moment is that it works for me and gives good agreement with e.g. Neptune and Sea Pro but is a bit fragile in places. It takes as input a Seaclear route file, you enter the various constraints and a starting date and time and it iterates over the next 12 hours to offer the shortest passage time meeting all (or breaching fewest) of your constraints.

Will likely be ready for beta test some time in next few months. I will offer free beta test copies on here, however I finally decide to distribute it.
<edit>
Now you've got me started...
Here is the output from our plan for the East Coast Forum run from Orwell to Oostende at end of May

(Constraint passed... means its light in Oostende at the time you get there)

Clicking any of the + signs gets you the full skinny on each leg.

The "tides" bit shows which diamonds were used in the tidal vector calcs, so you can assign confidence to the result.

 

[dt4134]

Best of luck with it.

There's quite a few established business models for giving away software yet still making some income from it. Maybe worth looking into.

 

[misterg]

[ QUOTE ]
Clearly 180 degrees between wind and tide is worst and 90 or 270 neutral, but at what angle coming up from 90, or down from 270 would the panel say is the onset of "wind over tide" condition.

[/ QUOTE ]

Allow the user to specify an angle? Which can then be tuned (or ignored) based on actual experience?

Interesting project.

0.02p

Andy

 

[wotayottie]

have some data on times and waves based on a 10 mile fetch: 10kn wind for 6 hours = 10cm waves, 20kn gives 100cm waves and 30kn gives 300cm. The wind speed I would use is the relative one ie add tide vector to wind vector.

got a bit more data for different fetches if I can find it.

but as I said earlier, its really complicated with lots of variables so I dont give much for your chances of reducing it to a simple algorithm you can write into a program.

 

[shmoo]

Thanks. I am thinking of combining misterg's suggestion of a user parameter and yours of the vector component of the wind 180 to the tide. The constraint would be something like "report that the constraint fails if the component of the wind 180 degees to the tide exceeds [user choice of speed, default 15kt] at the time you expect to be at this waypoint". Users would be expected to enter such constraints for racy spots on the route.