Leeway

jrt

jrt

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How does leeway act on a boat? Does it push the bow round making the boat point a few degrees to one side or the other. Or does it shift the whole boat to the side like the tide. /forums/images/graemlins/confused.gif
 
Leeway isn't anything working ON the boat at all - it's what the boat can't cope with so it doesn't go in the straight line you want.
The reasons are many - wrong sails, wrong keel, poor helming, rough seas - the result is slipping sideways.
Yes, it's just like the tide pushing you, but varies on different points of sailing. and you can control it to some extent.

Why are you asking when your Profile sez you teach sailing?
Telling porkies are we?
 
If you consider the wind (especially on a beat) acting on the sails, the biggest force component is sideways.

To try to stop the boat actually going sideways is the keel. However, it doesn't stop all of the sideways movement, and what is left is called leeway. It varies with heading; boat design; sea state etc.
 
Well I don't know the 'real' answer, but if there is a difference bewteen tide induced leeway and sailing induced leeway, I'd suspect that in the latter the boat crabs more as the keel/hull is probably stalling.

Now someone will come along and tell me how wrong I am, but its just what occurred to me as I read your post.

Tim
 
Consider the keel as a sail as well

and then consider the rudder as a keel leach.

All you have to think about is moments and couples and their centres of effort etc ..............


Are you really an engineer? Do you have any applied maths background? If so you will know exactly what the other posters have posted and why they have posted what they have posted.

I can't believe that anybody who teaches sailing doesn't know that knowledge of leeway is fundamental to both sailing and navigation. (I hope I'm not being arrogant in that assumption ... but it's just that I feel that anybody who says that they "teach sailing" should have at least read Glenans.)

Monohull leeway = hull and keel/daggerboard/rudder sideslip caused by windage and heel.

Your next question should be "What is Lee-Bowing /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif"


Yours aye,


M (Rockham Bay)
 
[ QUOTE ]
tide induced leeway

[/ QUOTE ]

Leeway is a difference in your heading through the water to your compass heading - the effect of the tide is a difference between heading/speed through the water, and heading/speed over the land. Completely different things.
 
"it's much more on a catamaran than a mono."
Wrong - it's much more on all shallow draft boats. Anyway, that's what I've worked out after owning a Prout, a bilgekeeler and several deep fins - and come back from Cherbourg finding myself the wrong side of Selsey Bill!

And thanks Mortehoe. Are you going to ask for his RYA qualifications or shall I?
 
Re: Consider the keel as a sail as well ...

is good advice. Even better is to think of it as a wing, though in this case the 'lift' is horizontal.

On every point of sailing, except a dead run, the wind will generate a side force on the boat. To resist this force the boat must generate an equal and opposite force (remember Newton's third law: to every force there is an equal but opposite reaction). It generates this force by setting the keel at a slight angle to the water, so that it acts like a wing with a small angle of attack. This angle is the leeway angle.

How big an angle depends on several factors, including the keel shape and size, the water density and the boat speed. For all practical purposes, other than on centreboard boats, the keel shape and size and the water density are constant. For a constant boat speed the side force generated will increase proportionally to the keel angle untill the angle becomes so high (usually about 15 degrees) that the keel stalls; beyond this point the force will fall. For a constant keel angle the side force will depend not just on the boat speed, but on the square of the boat speed.

So if you think of a condition such that the side force from the wind is roughly constant, then doubling the boat speed will give only a quarter of the leeway; conversely, halving the boat speed will multiply the leeway by four. If you are pinching up to windward and allowing the speed to drop, then the apparent gain in pointing angle may well be negated by the increase in leeway. Coming alongside a windward berth, as the speed drops the leeway will increase, you'll point further in, slow down, and further increase the leeway until finally the keel stalls and the boat moves uncontrollably away from the pontoon. Sound familiar?
 
I\'ve got no qualifications apart from the paper bits and EW & NS ....

... 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)
 
I can see what jrt is thinking and I think some of the responses have been a little unfair.
If you really do teach sailing then that is best done in a dinghy and part of the RYA instructors course (and RYA level 5) is to take the rudder off and steer the boat just by its balance, using the main sheet and your weight. A perfectly balanced sailing boat of any size will go straight ahead, but you can control it and make it go in either direction by putting it “off balance”.
In addition to being a sailing instructor I am also a Physics graduate so I do understand the maths and the forces that make a boat move to leeward. However surely when sailing it is the angle that the boat actually males good compared to the one that is steered that is important and this could also be affected by the balance of the boat and not just the windage on the boat pushing it sideways.
 
How the *$%k can you claim to teach sailing if you don't know what leeway is?! Sorry to be so blunt, but a teacher should at least know the basics of how a boat sails.
 
I know what leeway is, I was asking how it acted on the hull. Does it yaw the hull or does it push it side ways. And I teach in the classroom, not on the water. Why don't you read the question. /forums/images/graemlins/mad.gif
 
[ QUOTE ]
I can see what jrt is thinking and I think some of the responses have been a little unfair.
If you really do teach sailing then that is best done in a dinghy and part of the RYA instructors course (and RYA level 5) is to take the rudder off and steer the boat just by its balance, using the main sheet and your weight. A perfectly balanced sailing boat of any size will go straight ahead, but you can control it and make it go in either direction by putting it “off balance”.
In addition to being a sailing instructor I am also a Physics graduate so I do understand the maths and the forces that make a boat move to leeward. However surely when sailing it is the angle that the boat actually males good compared to the one that is steered that is important and this could also be affected by the balance of the boat and not just the windage on the boat pushing it sideways.

[/ QUOTE ]

Oh Dear ... you may be a Physics Grad etc. - but you have made a fundamental cock-up in your statement ... balance ANY boat and it will steer itself etc. - sorry BUT it WILL make leeway. In fact all objects on the water will make leeway stopped, high, low speed or whatever .. it's a sad fact of boating.
 
Ouch Ouch Ouch ....

Leeway doesn't act on the hulll. YAW is completely different.

Are your REALLY an RYA approved instructor????

Shalll we ask the RYA?
 
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 ]
How does leeway act on a boat? Does it push the bow round making the boat point a few degrees to one side or the other. Or does it shift the whole boat to the side like the tide. /forums/images/graemlins/confused.gif

[/ QUOTE ]Golly there are some rude people on these forums nowadays. Ignoring the comments from others:

Leeway is not a question of the bow being pushed round, its how much the boat 'slips sideways' in the water instead of going exactly along the track it is steered along. Leeway is at a maximum when the boat is close hauled and drops to somewhere apporaching zero on a run or broad reach.

Leeway can be exacerbated by poor sail trim and poor helming. Further you get more sideways spillage in big seas through wave action on the boat.
 
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