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Leeway can occur on both sail and power boats.

One form of leeway comes simply from the wind acting on a vessel's topsides
(I think all of us who own boats have noticed the tendency of the bows of
our boats to weathercock).

An additional form of leeway occurs in sailboats because the force
generated by the sails is not directly fore-and-aft.  The non-fore-and-aft
part of this force tends to push the vessel sideways.  While keels are a
way to get weight down low to counteract the heeling force generated by
sails, the greater reason for a sailboat to have a keel (or centerboard or
daggerboard) is as a way to provide an appendage which presents a broad
area to sideways movement of the boat.  Moreover, keels are actually
hydrodynamic wings, with anti-leeway lift generated by their leading
edges.  This is why a deep keel of the same area as a shallow keel will
allow less leeway.

I once saw a formula for estimating the leeway of a sailboat; it had terms
in it for keel area, keel depth, etc, etc; unfortunately I can't locate it
else I would have shared it with the group.  As I recollect, the numbers
for a modern sloop with a medium to high aspect ratio keel were on the
order of 4 to 6 degrees close-hauled, a bit less than Dave quotes here (on
the other hand, the formula did not include leeway due to that same wind
acting on the vessel's topsides).

Lu Abel

At 07:45 AM 12/6/2001 -0500, daveweilacher@earthlink.net wrote:
>These are my notions regarding leeway.
>
>All sailboats have it.
>Read in one of the standard how-to books that a boat on a beam reach has
>circa 4 degrees and same boat close hauled circa 8 degrees.  Obviously
>different for different boats.
>This isn't a problem for an experienced helmsman coastal cruising. They
>just allow for it by visual ranges.
>It is an issue when navigating by compass bearing or chart planning though.
>I figure that I can't steer closer than 5 degrees on the compass; I can't
>do arithmetic in my head at all (especially if both hands are in use); and
>it is better on a sail boat to sail high of your mark than right at it.
>So...  I allow 5 degrees for leeway if I'm beem reaching and 10 if close
>hauled.
>
>Oh...  This was only meant to address leeway caused by windage; not set
>and drift.
>
>
>Original Message:
>-----------------
>From- Peter ffive@TPG.COM.AU
>Date: Thu, 6 Dec 2001 10:35:51 +1100
>To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
>Subject: [NAV-L] leeway
>
>
>'I gave myself a headache once trying to account for leeway on a trip,
>and gave up.'
>
>This is just a rough rule of thumb. Look at the wake your boat is
>leaving behind. If the leeway is significant you will see it stream out
>not dead astern, but at an angle of, say, 10 degrees. I have heard of
>people putting marks on their pushpit indicating 5, 10, etc degrees so
>they can factor it in to their calculations. My experience is that the
>effect is generally mild  EXCEPT for when you are carrying too much sail
>for the conditions and are consequently being excessively heeled. The
>keel can no longer work efectively and then you do make lots of leeway.
>Another reason for reefing early.
>
>Peter Fogg
>
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