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Old 10-03-2021, 13:05   #1
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How sailboat leeway depends on a boat speed?

Hi all!


All my life I was happy with the following explanation of what leeway depends on:
The amount of leeway depends most of all on the wind strength and the speed of the boat. Leeway differs from boat to boat depending on hull design, draught, windage of the superstructure and rigging. To great extent leeway depends on the point of sailing. Usually leeway is greatest when sailing close hauled. In particular when helmsman is ‘pinching’ which results in excessive heeling and reduction in lateral resistance of the keel. Leeway is at its least when running downwind as well as when you motor straight into the wind.

And a simple explanation how leeway depends on boat speed for me is this:
When sailing close hauled boat heels more, moves slower and has more leeway then on a beam reach when it heels less, moves faster and have less leeway.
Now in a discussion I had elsewhere the following question came up:
- We have two identical boats sailing exactly the same course, let's say close hauled, in exactly the same wind and wave conditions, but one of these boats goes faster then another. Which of these boats will have more leeway?
Any ideas? Does this question make sense at all?

I doubt that such thing as "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" may exist in real life. Am I wrong?
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Old 10-03-2021, 13:09   #2
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How sailboat leeway depends on a boat speed?

A faster boat will develop more lift from the keel and be more resistant to the the forces that cause leeway
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Old 10-03-2021, 13:14   #3
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Re: How sailboat leeway depends on a boat speed?

IMO, the slower boat will have more leeway.

1. The faster boat will be developing more 'lift' from the keel due to the higher STW.
2. The faster boat will reach the destination point sooner thus be less affected by the drift to leeward.

EDIT: cross posted with Sailmonkey
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Old 10-03-2021, 13:19   #4
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Re: How sailboat leeway depends on a boat speed?

Quote:
Originally Posted by Sailmonkey View Post
A faster boat will develop more lift from the keel and be more resistant to the the forces that cause leeway

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?
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Old 10-03-2021, 13:24   #5
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Re: How sailboat leeway depends on a boat speed?

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Originally Posted by dokondr View Post
With what keel - modern, fin or spade?
All keels work the same way, just that some are (far) more efficient in preventing leeway.

In your example, the keels are identical so the slower moving keel will always create more leeway.

However the difference will be greater if the keels are more efficient.
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Old 10-03-2021, 13:29   #6
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Re: How sailboat leeway depends on a boat speed?

Quote:
Originally Posted by dokondr View Post
.......
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?
No, if everything is identical, then the speed must be identical.

However it would be rare to have identical crew ability and if one crew can helm and trim better, it will be the faster boat and experience less leeway.
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Old 10-03-2021, 13:45   #7
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Re: How sailboat leeway depends on a boat speed?

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No, if everything is identical, then the speed must be identical.

However it would be rare to have identical crew ability and if one crew can helm and trim better, it will be the faster boat and experience less leeway.

Don't you think that when sail trim is different the wind driving force becomes different and as a result all other forces affecting the boat also change, not only speed?
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Old 10-03-2021, 13:55   #8
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Re: How sailboat leeway depends on a boat speed?

Quote:
Usually leeway is greatest when sailing close hauled. In particular when helmsman is ‘pinching’ which results in excessive heeling and reduction in lateral resistance of the keel.
Pinching does not result in excessive heeling... rather the contrary, where one pinches up to get a boat back on her feet. Leeway increase is due to reduction in STW when pinching.

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Old 10-03-2021, 14:05   #9
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Re: How sailboat leeway depends on a boat speed?

Quote:
Originally Posted by dokondr View Post
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?

The boats may be identical but are the skills of the helmsperson and crew identical? Is the sail trim identical?


One class racing comes to mind
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Old 10-03-2021, 14:27   #10
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Re: How sailboat leeway depends on a boat speed?

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Originally Posted by dokondr View Post
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?
This happens most every Saturday during the racing season in one design racing.

Identical boats start at the exact same time, and one boat goes ahead of all the rest.

This is due to lots of reasons including but not limited to mast rake, mast prebend, sail age/selection for the conditions, batten selection, sheeting tension for the conditions, skipper' steering ability, skipper's ability to read the course, boat's bottom condition, traveler position, outhaul tension, mast rotation (if the boat has that capability) etc.
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Old 10-03-2021, 14:32   #11
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Re: How sailboat leeway depends on a boat speed?

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Originally Posted by thomm225 View Post
This happens most every Saturday during the racing season in one design racing.

Identical boats start at the exact same time, and one boat goes ahead of all the rest.

This is due to lots of reasons including but not limited to mast rake, mast prebend, sail age/selection for the conditions, batten selection, sheeting tension for the conditions, skipper' steering ability, skipper's ability to read the course, boat's bottom condition, traveler position, outhaul tension, mast rotation (if the boat has that capability) etc.

But all these things (mast rake, mast prebend, sail age/selection for the conditions, batten selection, ...) make boats completely different from each other, not identical!
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Old 10-03-2021, 14:40   #12
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Re: How sailboat leeway depends on a boat speed?

with our (numerous) race boats, it was well known and accepted that in order to point you needed firstly SPEED

if the boat is not pointing, foot off fractionally, pick up your speed, and then you can start pointing

it's all about getting the keel working

racetracks are full of bunnies going slow, but with their boat jammed up high...

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Old 10-03-2021, 14:43   #13
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Re: How sailboat leeway depends on a boat speed?

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?
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Old 10-03-2021, 14:45   #14
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Re: How sailboat leeway depends on a boat speed?

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Originally Posted by dokondr View Post
But all these factors (mast rake, mast prebend, sail age/selection for the conditions, batten selection, ...) make boats completely different from each other, not identical!
The boats are identical. Same masts, same sail size, same boom length, same keel depth, etc

The adjustments and Skipper's knowledge are different.

It's call one design racing.
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Old 10-03-2021, 14:49   #15
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Re: How sailboat leeway depends on a boat speed?

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The boats are identical. Same masts, same sail size, same boom length, same keel depth, etc

The adjustments and Skipper's knowledge are different.

It's call one design racing.
Plus local knowledge of the course. One skipper can chose a slightly different part of the course and pick wind and current advantages (or disadvantages ).
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