tom_t
new user
Reged: 02/07/2008
Posts: 2
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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.
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MoodySabre
regular
Reged: 24/10/2006
Posts: 3438
Loc: Bradwell and Leigh-on-Sea
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There is only one response to a question like that - WHY?
There are so many variables that there can be no magic number.
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Don't believe everything you think.
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BrendanS
regular
Reged: 11/06/2002
Posts: 35459
Loc: Me: Wilts. Boat: Lymington
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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.
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See http://www.frappr.com/ybw to add yourself to the forum map
and as everyone else is doing it
www.plaintalkconsulting.co.uk
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Kawasaki
regular
Reged: 21/06/2004
Posts: 4389
Loc: Anglesey Wales
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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.
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Jerk of all trades. Expert in none.
Plus Don,t do serious, lifes too short.
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whipper_snapper
regular
Reged: 09/08/2006
Posts: 1266
Loc: Kenya
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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.
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lw395
regular
Reged: 16/05/2007
Posts: 610
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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,
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Kawasaki
regular
Reged: 21/06/2004
Posts: 4389
Loc: Anglesey Wales
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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?  
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Jerk of all trades. Expert in none.
Plus Don,t do serious, lifes too short.
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cardinal_mark
regular
Reged: 08/10/2004
Posts: 481
Loc: UK
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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
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chrishelen
regular
Reged: 07/01/2007
Posts: 1047
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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.
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morgandlm
regular
Reged: 03/03/2005
Posts: 211
Loc: Essex
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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
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