Looking for the magic number in wind vs tide...

[tom_t]

I am trying to find a 'magic number' - a factor by which a boat is slowed down by the waves and choppy sea that are created when the wind and the tidal stream are coming from different directions.
Say my boat is sailing at a speed of 5 knots through the water. If the wind is going with the tidal stream, the sea is calmer than if it's going against the tidal stream. It is different again if the two are moving at 90 degrees to each other.
I am interested in peoples opinions and experiences about how much certain combinations of wind and tide slow them down - would you say if you have tide against the wind, it will slow you down by 10%? 20%
I am not interested in the difference of boat speed through water vs speed over land - only in the difference that is caused by a choppier sea because the tide is going against or with the wind.

 

[MoodySabre]

There is only one response to a question like that - WHY?

There are so many variables that there can be no magic number.

 

[BrendanS]

Even if there were, it wouldn't be a magic number, it would be a large complex equation, as no rule of thumb that I can think of for this type of thing.

 

[Kawasaki]

tom_t that's one heck of a first post.
Welcome anyway!
Tidal streams are relatively predictable.
In general.
Local conditions, peculiararities--back eddies etc etc are not.
Wind?
Changes continuously as we all know.

Your interest.


'I,m not interested in the difference of boat speed through the water vs speed over the land - only the difference that is caused by a choppier sea because the tide is going against or with the wind'

As MoodySabre says, Why?

'the difference that is caused by a choppier sea' etc etc.

Boat speed through the water ? who cares, it tells a tale ok.
Boat speed over the Land as you put it, more interesting.

'only in the difference caused by a choppier sea' is your interest you say

You say you are not interested in boat speed but want a magic number.
Please explain the result and purpose of this magic number.
Then praps the Forum can enlarge.

 

[whipper_snapper]

Seems a perfectly reasonable question. He seems to be simply saying please don't spin off onto stuff about the effect of the tide per se because that is obvious (or in danger of repeating the dreaded lee-bow threads). I believe he is interested in a) how wind interacts with current to make 'choppiness' and b) how choppiness slows down a boat.

I would have thought that the answer to b would be quite answerable - but I have no idea how to do it. It would depend on boat size, shape, weight, speed, heel, method of propulsion, but it would be interesting to hear guesstimates for given scenarios. As for how much (and why) wind vs tide kicks up a nasty sea; I have never really understood the magnitude of the effect. It seems much greater than you would expect from a few knots increase in velocity at the air-water interface.

 

[lw395]

The only numerical work I am aware of is Frank Bethwaite's book 'High Performance Sailing'. This has some experimental work on dinghies.
The book Elements of Yacht Design, by Larrson and Elliasen ( I might not have either author or title exact), would probably interest you also. I read a library copy first, so it can be borrowed.
I must read them again!
Cheers,

 

[Kawasaki]

Fair enough whipper_snapper.
But b?
how choppiness slows down a boat.
But our new poster says 'I am not interested in boat speed' etc.
However He is looking for a magic number/ factor to calculate such.

What am I missing here? /forums/images/graemlins/confused.gif /forums/images/graemlins/grin.gif

 

[cardinal_mark]

A couple of weekends ago we were beating into strong wind over tide which was creating the inevitable Solent short chop. At times we were slowed from 5 kts through the water to just over 2. Our speed rose to 6 once we were able to bear away onto a reach. This was in an HR 31, bigger boats didn't seem to be suffering quite so much but there didn't appear to be that much in it.

Mark

 

[chrishelen]

Hi Tom,welcome, i presume you have a sailing boat?
wind over tide can be a pain and slow you down a bit due to the chop it kicks up,here in the BC in can become quite dangerous in places,but i suppose some of the factors will depend on your type of boat,if your boat slams into waves then you can be in trouble but if she cuts through it will be easier if slightly wetter,i spend a lot of time in wind over tide and my Eventide does ok.
hope this helps.

 

[morgandlm]

I don't think there can realistically be a magic number because there are so many variables. I do however fully agree that choppiness can have a major slowing effect. I think it affects both the hull and the sail efficiency. I noticed two days ago when beating to windward with the tide, that the VMG was improved by bearing away a little, and so changing the angle of the waves to the hull. The ride was more comfortable and the boat was faster. I suppose that these sort of judgements are one of the things that differentiate the best racers from the rest. For myself, I am not generally worried too much about speed but certainly like to keep SWMBO as happy as possible - that might mean getting home quickly or without too much bouncing around.
Morgan

 

[AliM]

Last week, beating into ~20kt app wind with the tide against us, in one area where the wavelength was very short (therefore steep), we were really struggling to stem the tide at all. A few hundred metres further on, where the wavelength was only a little longer (tide, wind and angle of course all the same), we were sailing 2 kts faster. Of course in that case the wind was with the tide.

The point of that it to say that there are far too many variables for a magic number. We've found wavelength, angle of heel, angle to waves, wind speed, tide direction and of course hull shape, displacement and skill of the helmsman are all major factors.

The best answer is to go out in all sorts of conditions and sail - at least you can have some control over the last factor!

 

[Chrissie]

I have a boat which cuts through the water, but at 26ft she is slowed down considerably by the short chop, which bounces up and soaks us, I would imagine by 40% but that includes the effects of the strong tide against us.
When the short chop isnt there, we do much better through the same tide against us.

 

[Twister_Ken]

0.734

 

[CPD]

I made it 0.733 ?????????????/

 

[cliff]

[ QUOTE ]
0.734

[/ QUOTE ]No, No, No... That is the old fart's number which did not take into account the effect of foaming nor compensate for other esoteric factors - I believe the erudite authorities now take the magic number as being somewhere between 0.83141286 and 0.83141288 assuming WNA conditions /forums/images/graemlins/wink.gif
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Artificial Intelligence is no match for Natural Stupidity

 

[Saddletramp]

The answer must be 42 surely

 

[Twister_Ken]

The 0.83 estimate takes no account of turbulent airflow over the sails caused by the chop. It's applicable only to moronboats, sorry, motorboats.

 

[tom_t]

Thanks for all your replies so far. Let me explain a little where I'm coming from.
I'm trying to write a software which will assist in passage planning. This is part of a final year computer science project.

When planning a journey, I would use the following information:
- a chart
- a tide timetable
- a tidal stream atlas or if not available, the information from tidal diamonds on the chart
- I would also estimate how fast my boat will move, from experience and by checking the wind forecast.

I would then pick a convenient departure time and check what the tidal streams are like at that time. I work out if the tide pushes me along or not and so determine how far I would effectively move (over ground) in an hour.

Then check where I am in an hour, take the tide at that time and place, and again work out how fast I'm moving under these new conditions.

Keep doing that until your at your destination and you have a rough travel time.

Then the fun part starts... would it be better to leave an hour or two earlier, or later? Would the tide be more favourable then?

As everyone knows, this is a time consuming process and can make your head spin pretty quickly. I have worked out a way of doing all of this using software and pretty much instantly (after you have entered the known tidal stream information into the software).

To me, the obvious problem is that the suggested route I'm getting is only valid if the sea is calm and I'm motoring - I have a constant speed and I can move in any direction I want.

So the next steps would be to take into account
- what direction of travel is possible (considering possible angles to the wind)
- predict if sea is calm or not, considering wind vs tide direction
- how all of these affect the speed through the water

Maybe I should also add that although I have a keen interest in sailing and have been out plenty of times, I would not consider myself to be an experienced sailor... and some of the terms I am using might not be the correct ones...

Any opinions are greatly appreciated. I'm trying to get a feel for people's experiences to see if I can make my software more realistic, and all your replies so far have been very interesting and encouraging.

 

[cliff]

[ QUOTE ]
The 0.83 estimate takes no account of turbulent airflow over the sails caused by the chop. It's applicable only to moronboats, sorry, motorboats.

[/ QUOTE ]Yes, quite agree but there again real "sailors" would have taken that as read after all, all a sail boat has to do is tighten the sails a little more to compensate for turbulence and in doing so reduce the afore mentioned turbulence to negligible values using the age old "tell tales". Of course one may have to play with one's Cunningham a little to achieve the correct angle of attack which is much easier achieved if one has the proper tool for the job. Having a dangling tell tale or even worse a sloppy Cunningham is not conducive to achieving the proper angle of attack and can lead to one going off course which could result in the first mate becoming rather irate. /forums/images/graemlins/ooo.gif
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Artificial Intelligence is no match for Natural Stupidity

 

[chrishelen]

Tom go into Cardiff Yacht clubs website and scroll down to "Geoffs Web pages,i think he has something on this,i sure he wouldnt mind you pinching some data.