Leeway

I'm better at this in dutch, but here it goes: wind in the sails will create a force. This force can be divided in 2 components: a forward pushing force and a sideways pushing force (drift). The attachment point of these forces is the sail point (the theoretical centre of all sails carried).

Whether or not drift will ove your boat sideways or slew it, depends on the position of the sail point vs the lateral point. The lateral point is the point around which the boat pivots. This lateral point is determined by the appendages under water. It is easy to find: stand on the pontoon and push on the boat at the bow: the boat will turn its nose away from the pontoon. Stand at the end of the boat and push, the boat will turn the end away from the pontoon. Stand right at the lateral point and push the boat, and the boat will move parallel from the pontoon.
If the lateral point is in the same perpendicular line of the sail point, drift will cause your boat to move sideways.
Usually the lateral point is ahead of the sail point (keel is in front of main sail), so the drift forces will maket the boat luff.
http://nl.wikipedia.org/wiki/Bestand:Zeiltheorie1.png

BTW: don't mind the rude people on this forum. Neanderthals are everywhere. Just ignore them, at some point Darwinism will resolve the issue.
 
... and that\'s not true either

the slot effect (genoa less than ~135%) can maximise the main 'lift' at the luff. It doesn't accelerate the air.

I haven't a clue what happens underwater except that it must be worse as water is thicker than air.
 
Re: ... and that\'s not true either

It seems to me that this long and painful discussion of leeway with assorted mud-slinging on the way can be summed up by the following quotes from the physicist Richard Feynman. The first seems applicable to the analysis of leeway, the second to those who dismiss as too trivial any discussion of this issue, and the third to those prepared to read and learn.

"I think it is safe to say that no one understands quantum mechanics."

“The first principle is that you must not fool yourself and you are the easiest person to fool.”

“I'm smart enough to know that I'm dumb.”
 
Re: Leeway: Here\'s the only answer

Leeway happens whenever you sail.

You have to work out for yourself what the variation for your boat is according to sail set .................. etc etc etc etc etc.

love and kisses to you all because some of you are right and most are wrong.

M
 
Re: ... and that\'s not true either

[ QUOTE ]
the slot effect (genoa less than ~135%) can maximise the main 'lift' at the luff. It doesn't accelerate the air.


[/ QUOTE ]

But the air travelling through the slot does so at a speed which is faster than the ambient air speed, so something somewhere must be speeding it up /forums/images/graemlins/wink.gif
 
Re: Leeway: Here\'s the only answer

No leeway if going dead downwind. On all other points there will be some.
Monohulls do not need to heel to sail, it just happens as a result ofthe wind force.
The Winged keel shown has quite complex water flow and the big end plate (wing) stops the waterflow `short cutting´round the bottom of the keel. This makes the short foils more efficient and keeps the draft low.
In general: water and wind are fluids, just different densitys.They behave in much the same way on foils (keels wings or sails). My .02 centimes
A
 
[ QUOTE ]
I have not made a cock up, you have misread what I am saying. That part of my post was stating that there can be other factors in addition to the sideways movement caused by the wind that can add up to the overall effect of leeway. You can make a boat want to turn to leeward, or indeed windward by not having it balanced. Although this can be counteracted by lee helm and weather helm. I am simply saying that the original poster was not entirely incorrect in his question about whether some leeway effect can be caused by other means.
Why do some people on this forum insist on getting personal and dish out insults instead of being constructive.

[/ QUOTE ]

If you think I insulted you - which I did not - then you aint had replies from some characters on here !

Your post implied that by balancing a boat you would steer straight and not have leeway. Sorry but your post does not clear that preception away until your second reply in retort to mine.

As to OP and why or what ... ever since dawn of time and mans venture upon water - leeway has always been estimated as too many factors natural and boats / object affect it to be calculable .. only after the fact can you calculate.
 
Sorry so many people feel the need to be rude and i agree with those that say if you don't know ask!

I think a simple explanation is in a dinghy and say wind is from 45deg off the stern (broad reach). If simplistically the centre board is up and dinghy floats on the surface it will go in the direction that the wind blows it.

Now put centre board down and the sideways motion is resisted but some water will still flow round the centre board (and boat to some extent) leaving the boat to appear to only sail forwards but actually still slip a small amount sideways through the water.

The centreboard does act as an aerofoil (wing) and with enough water flow over it - it will act as a wing and create an amount of lift thereby reducing the amount of leeway.

I would be interested in others comments are there boats that are so perfectly designed that given a flat sea and adequate speed can make little or no leeway?
 
kinetic energy ...

and you can't get more energy out of nothing ... it's not E = mc^2 but a plain balance of e=mV^2.

Sailing is sailing is sailing. Even JJ agrees - so let's put it to bed and agree that leeway is a physical function of steering a sailboat with a bit of wind (whether methane or not) sufficient to fill the sails.

xxx

M
 
Re: Leeway: Here\'s the only answer

[ QUOTE ]
That my friend, isnt Leeway, that is Tidal Set /forums/images/graemlins/confused.gif /forums/images/graemlins/blush.gif

[/ QUOTE ]

Have to agree as well ...

Also comment that any leeway will be disguised as well as it will be in addition to boats own movement. Leeway doesn't really have to be across the boat ... but we usually accept it as that.
 
Re: kinetic energy ...

[ QUOTE ]
and you can't get more energy out of nothing ... it's not E = mc^2 but a plain balance of e=mV^2.


[/ QUOTE ]

If it's kinetic energy you're after there's about 2x too much in your equation /forums/images/graemlins/ooo.gif
 
It\'s worse than that ....

E = 1/2 mv^2

but leeway, unless deliberate, is always a negative entity and scores minus brownie points when its forgotten on any dead reckoning exam ....


.... or ocean crossing


.... without GPS (forbidden)





"Cloudsley-Shovell nearly got it right" Harrison,



and there's still the question whether lee bowing is fact or fiction! Sofar, nobody has dared to answer.
 
My 2p's worth:

Leeway is the consequence of all the different forces working on the hull, through the rigging and in the water.

It's a sideways movement of the boat, which taken together with its forward motion, results in the boat moving through the water at an angle to the course steered.

That is, the boat is pointing one way but is actually travelling through the water a bit crabwise.

So what is happening as the boat crabs along like this? (Which I think is the question).

Firstly the keel or fin, which is pointed at an angle to the waterflow, is creating lift to offset the sideways pressure of the sails.

Secondly the effect of the lifting force on the keel, which is to windward, makes the boat heel to leeward. (The other half of the heeling couple is provided by the sideways pressure of the sails.)

Thirdly the heeling of the boat changes the shape of the part of the hull that's underwater; typically causing the boat to tend to round up to windward (weather helm). The water will be flowing across the hull to some extent, due to the leeway, my guess is that this may have a slight counterbalancing effect, ie a moment that would turn the boat to leeward, towards the track actually being made. (Think of a dart in flight)

Fourthly, sufficient helm is applied to maintain the course.

The leeway you get is what you need to maintain that track, given all the circumstances.

There's also some insights into leeway effects when a boat is hove-to, or when the keel stalls in very light winds; anyone else care to comment?
 
Re: Consider the keel as a sail as well ...

[ QUOTE ]
In response to Peterb: Very informative answer. Can you match that with an explanation of why race boats are now moving to T-keels with virtually no "wing" shape?

[/ QUOTE ] For the same reason that airplanes have little turned up wing tips. The flat plate at the bottom of the keel minimises the wing tip/keel tip vortex.

To get a bit more complex, a foil (keel) at an angle of attack (leeway) to an oncoming fluid (air/water) will have a higher pressure on one side of it, and a lower pressure on the other side. That's what creates the force that keeps the plane in the air/stops the boat going sideways.

However, with a plus pressure one side, and a minus the other, there's a snag at the end of the wing. Fluid tries to flow around the end, from plus to minus, creating a swirl (vortex). That rotating vortex has energy, borrowed from the boat . . . in the form of drag.

The vortex can't form so easily if there's a flat plate at the end. Less vortex=less drag=faster boat. Airplane - less fuel.
 
Re: I\'ve got no qualifications apart from the paper bits and EW & NS ....

[ QUOTE ]
... having sailed most west to east and north to south around our globe ...

(deleted ... rant!)

Do you remember Tom Cunliffe's article about arriving in Holyhead having sailed solo from Brazil to be examined by an RYA peg-leg with a pastie ......

There are 3 types of sailors: them that are natural; them that can be taught and them that are just plain dangerous.

M (Rockham Bay)

[/ QUOTE ]

How about a fourth - the really pompous!
 
Re: Leeway: Here\'s the only answer

And there was me thinking that Leeway was the accumulative effects that changed the boats COG from direction that you were pointing it to!

Just shows how wrong one can be.

<u>Note to me</u>: Carry on calling it Leeway and calculating it as I always have, but don't tell anyone /forums/images/graemlins/wink.gif
 
[ QUOTE ]

The leeway you get is what you need to maintain that track, given all the circumstances.


[/ QUOTE ]

I think you are making this much more complicated than it is.

A perfectly balanced boat sailing herself, with no fancy wing shaped keel etc., will suffer from leeway.

If a boat has lee or weather helm it will simply be going slower than one perfectly balanced, so may suffer a greater leeway 'angle' (which is how it is generally applied). It is simply all about vectors.

Leeway is as a result (mainly) of the wind blowing the boat sideways. Boat design may help to counteract leeway, but think of this as a bonus.
 
Re: I don\'t understand that keel

As an ex-windsurfer, we had slots in the fins many years ago. Main benifit was that they stopped "spin out", that is they prevented the fin from cavitating at high speeds. They also seemed to prevent stalling at lower speeds. However overall they were less efficient than solid fins, and the top speed was noticably less.
 
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