From: thompson@atlas.socsci.umn.edu (T. Scott Thompson)
Subject: Re: Daggerboard design question
Reply-To: thompson@atlas.socsci.umn.edu
Organization: Economics Department, University of Minnesota
Date: Tue, 26 Jan 1993 00:51:02 GMT

EPLUS17@vmd.cso.uiuc.edu (Richard Engelbrecht-Wiggans) writes:

[much stuff about problems with weather helm deleted]

If you are getting LOTS of weather helm in less than 10 knots of wind
then I suspect that your Banshee is not set up properly, or else you
are not sailing it flat enough.  

1.  Dinghies of this size must not be allowed to heel even a little
bit, except in light airs.  (In light airs a little bit of heel
reduces wetted surface area and aids steering by increasing weather
helm.)  Their hull shape causes them to generate a lot of weather helm
when they heel.  I believe this is because the center of effort shifts
to leeward, giving the sails a longer lever-arm for turning the boat
to windward.  The effect is magnified if the sails are poorly trimmed
to begin with (e.g. overtrimmed, over-vanged, or with the center of
effort too far aft).

2.  Does your boat have a kick-up rudder like a Laser does?  If so,
are you sailing with it ALL the way down?  I have found that allowing
the rudder to come up even a little bit in small dinghies tends to
dramatically increase the pressure on the tiller.  (This is _not_ the
same as weather helm.  See below.)  In an Olympic 470, for example,
having the leading edge of the rudder as much as 1/2 inch away from
the stop can make the boat difficult to steer in any kind of wind.

3.  Lots of weather helm in light winds can also be caused by having a
little bit too much mast rake.  Yours probably isn't adjustable, but
has the mast step ever been repaired?

4.  Another possible cause is a really blown out sail that can't be
brought into proper trim using the usual methods (downhaul, etc.).

>Could I improve the helm by moving the leading edge of the
>rudder forward with respect to its axis of rotation?

Moving the blade forward relative to the pivot will tend to reduce the
pressure that you feel on the tiller simply because the water pressure
on the part of the rudder forward of the pivot is now pushing the
tiller to windward instead of back towards the middle of the boat.  So
it counteracts the usual pressure coming from the part of the blade
aft of the pivot.

But this is not the same as reducing weather helm.  Weather helm is
the amount that the rudder must be turned (to weather) in order to
keep the boat tracking in a straight line.  Weather helm reduces
performance because having the rudder out of alignment with the
direction of travel through the water creates drag.  This is true no
matter what tricks you have used to get a "feather" touch on the helm.
A perfectly symmetrical (fore and aft) rudder will have a neutral feel
at almost any rudder angle, for example.

To see that weather helm and tiller pressure are not the same,
consider what would happen if you made the rudder unstable by moving
the pivot all the way to the trailing edge.  Suppose further that you
needed to have the tiller at about 10 degrees to weather of the center
line to track straight through the water.  Obviously you would have to
apply pressure on the tiller towards the lee side in order to keep it
in that position since it would want to go all the way to weather.
But you wouldn't want to call this situation lee helm.

Note: The idea that a boat has weather helm if it turns to weather
when the helm is released is not technically correct.  A better
definition is to say that a boat has weather helm if it turns to
weather when the rudder is held amidships.  The wierd rudder I
described above shows why.
--
T. Scott Thompson              email:  thompson@atlas.socsci.umn.edu
Department of Economics        phone:  (612) 625-0119
University of Minnesota        fax:    (612) 624-0209
Newsgroups: rec.boats