How sailboat leeway depends on a boat speed?

[dokondr]

Originally Posted by Wotname

Well, I guess I don't understand your original question.
If you have two identical boats, sailing in identical conditions with all aspects of the operation of the boat identical, then (by definition) the speed of both boats will be identical, the leeway will be identical and thus both will reach the destination at the same time.

Change any one aspect, then most of the other parameters will change and one boat will reach the destination before the other i.e. the speed is now different.

So what is it you wish to know?

Ok, thanks this answers my second question and confirms my understanding that boats with different speed are NOT identical.
Now back to the first question: which of two similar boats on the same course will have less leeway - the faster one or a slower one? Common answer here: faster one will have less leeway because higher speed creates grater lifting force of the keel, which results in less heel and because of that less leeway. Correct?

Now why greater speed creates greater lifting force of the keel?

[StuM]

Originally Posted by dokondr

Now why greater speed creates greater lifting force of the keel?

Same reason greater windspeed increases driving force on a sail.

[dokondr]

Originally Posted by StuM

Same reason greater windspeed increases driving force on a sail.

Yes, I agree, but there other forces also affecting the keel. When boat speed increases apparent wind also increases. Will wind component that creates heel also increase? Will it counteract the keel lifting force? How lifting force depends on the shape and size of the keel?

Any good book on the subject with comprehensive diagrams? Thanks!

[HappySeagull]

That diagram makes no sense. Firstly, all sailboats MUST make leeway to create the "lift" you are trying to show. The waterflow of a sailboat keel is a vector "sum" of leeway and forward motion.

A keel is symetrical ( unlike a bird or airplane which are designed to lift in one direction very well) but still, at an ideal angle of attack will "lift"...and yet the angle of attack depends on the leeway...they must have leeway or no "lift" is possible. Note that more leeway moves the vector direction towards the beam, more speed moves the vector forward. Which of these vectors produces eddies? Which separation and drag on a foil? That must depend on the design of the foil..perhaps, fat (slower boats) thin(faster boats).... Also,fat foil design will have excessive drag from its keel's frontal area but were a thinner keel attached,while easier to drive ahead, flow will be much more sensitive to a "critical angle" ie, a particular vector....

Next, just for fun, imagine a sailboat moving very slowly in a lightlightair. Leeway is near zero. His chum in an identical boat gets a gust and goes! Which of these makes more leeway? I'd say the faster boat....

Overall, I'd say a sailboat's keel has an ideal speed and attack. Depending on the foil, it might even be a slower speed than the fastest achievable where only leeway is the issue.

[Sailmonkey]

Originally Posted by dokondr

With what keel - modern, fin or spade?
Is such a thing possible: "two identical boats sailing exactly the same course, in exactly the same wind and wave conditions, but one of these boats goes faster then another" in real life?

Any keel will develop more lift with greater flow over it....even if by accident.

If the trimmer and helmsman are clones and the boat are able to occupy the same space at the same time....the boat speeds will be identical.

However these types of things evolve/devolve quite rapidly. Ever hear about a butterfly flapping it’s wings and changing the weather on the other side of the world? It’s the little things that make all the difference. Moving the rudder 1/2 degree vs 1 degree compounded over an hour adds up....it’s the little things!

[Wotname]

Originally Posted by dokondr

.......

Now why greater speed creates greater lifting force of the keel?

The same reason why aircraft have to accelerate and reach 'take off' speed before the wings generate enough lift to overcome the aircraft's weight.

More correctly aircraft first reach V1 (velocity 1) and then rotate etc etc.

[Lemsteraak]

My buddy used to hang behind me and wait for a strong puff to blow my boat over creating a ton of leeway. He then would sail right through. Identical boats, mine wood, his glass but the main difference was I had a heavy stiff Douglas Fir mast and he had a whipy Spruce mast.

We race in high winds, big waves and current. One race I decided to go easy on my boat and crew, and ease the boat through the waves, no pounding. I would bear off slightly at the crests so I wouldn't bash into it and opened up more sail twist, feathering the top of the sail. To my astonishment I passed a bunch of boats. Now my upwind technique.

My point is that there is more to leeway than just the boat's design. There are these pesky waves that you have to work through when the wind pipes up. A lighter rig helps but a lighter touch on the helm also pays big dividends.

[chrisr]

i know this is cruiser forum (and i love it), but some of this stuff takes me back to when i was seriously racing.

recall doing mainsheet on a farr 40 when those boats first came out...rounding bottom mark fairly close behind a 'warm' 50 footer. i could see we were in his dirty air n said to the owner 'this is going to take some work to get out of here'...'no wukin furries' he said 'watch this'...called to jib trimmers 'bring the barber hauler in 3 spots'

the boat literally jumped to directly to windward like a scalded cat...almost negative leeway. speed dropped abt 0.5k but the angle was fantastic. weren't going to maintain it for long of course, but it got us out of the big boats dirty air. we then went back into normal mode and sailed over the top of him...noice...

my point i guess is that everything on a sailing boat is related - speed / sail trim / pointing angle / leeway. when you're in a good boat and know what you're doing, magic can happen

btw, the story was that the L shaped keels on the F40s would twist slightly giving better lift (although i never saw hard data on this)

cheers,

[GordMay]

If leeway is the result of the vectoral addition of two forces, then the faster boat [longer u⃗ ] should have less.

[FabioC]

Originally Posted by GordMay

If leeway is the result of the vectoral addition of two forces, then the faster boat [longer u⃗ ] should have less.

+1
This is the correct explanation. It is as simple as this, as the speed increases, the vector moves forward, so the lift increases and the leeway decrease.

[jmschmidt]

Reminds me of the old "tail wind vs head wind" issue with airplanes. You are hindered for a longer period of time flying into a head wind of a given velocity than you can make up by turning around and flying back to your starting point with the same wind velocity as a tail wind.

Same with leeway, then add a current, or worst a cross current, and you have a major algebra equation to solve. Good at quadratics are you?

[zstine]

No the faster boat will not necessarily have less leeway. Leeway is a function of the L/D ratio of the Aerodynamics to the L/D of the Hydrodynamics. A boat with a high freeboard and large deck-house will create disproportionately more wind drag at higher speeds, which can lead to increased leeway at higher wind speed compared to low wind speeds (though the VMG may still be higher)... so the answer is "it depends on the boat".. Well, I guess in the OP's physically impossible scenario with 2 identical boats sailed identically going different speeds (I guess one boat has the motor on?), then yes the faster boat would likely have less leeway. i think. But you'd have to have an external force pushing the boat other than the wind.

[dokondr]

This video explains some things:

https://www.youtube.com/watch?v=LUDkrn8yADU

[chrisr]

speed is particularly important for the very narrow high aspect ratio fins on modern racing boats. you can stall these fins quite easily if your speed is low, and then the only way to get the foil working again is foot off, gain speed, and then point

i suspect this is not such a problem for low aspect ratio long keels (which in any case do not gain much speed by footing off)

cheers,

[Apollo19.5]

Originally Posted by zstine

No the faster boat will not necessarily have less leeway. Leeway is a function of the L/D ratio of the Aerodynamics to the L/D of the Hydrodynamics. A boat with a high freeboard and large deck-house will create disproportionately more wind drag at higher speeds, which can lead to increased leeway at higher wind speed compared to low wind speeds (though the VMG may still be higher)... so the answer is "it depends on the boat".. Well, I guess in the OP's physically impossible scenario with 2 identical boats sailed identically going different speeds (I guess one boat has the motor on?), then yes the faster boat would likely have less leeway. i think. But you'd have to have an external force pushing the boat other than the wind.

Much more to leeway reduction than simply increased speed. As your diagram shows, there are multiple forces at work. Identical boats in identical seas with different speeds can be caused by different sail combinations. A boat that is carrying more sail than its identical sister may actually be travelling quicker but due to increased heel may actually be carrying much more leeway than the boat with more appropriate sail combinations for the conditions. Over pressed may be quick through the water but slower VMG due to excessive leeway. We all see this everyday on the water. Certainly in sub hull speed conditions, speed does help with close hauled efficiencies.