From: EPLUS17@vmd.cso.uiuc.edu (Richard Engelbrecht-Wiggans)
Subject: Re: Daggerboard design question
Organization: C.C.S.O.
Date: Sat, 23 Jan 1993 16:24:43 GMT

Many of you kindly responded to my question on
daggerboard design.  I will try to summarize my
conclusions; I would appreciate hearing where
you agree and/or disagree.
 
Briefly, my Banshee (a 13' dinghy comparable to a
Laser) has a daggerboard that extends 34" below the
hull, measures 10.5" front-back at the hull, and
tapers to 7" at the tip.  It fits into a well that
measures 16" front to back.  I would like to make
an appropriate board for the shallow water and light
winds in which I often sail.  I am not gung-ho
race oriented here, but would prefer not to give
up performance that could be easily retained.
plam to keep my current board on board so I can
 
 
Dimensions:
 
1.  The lighter the wind, the larger the ideal board
will be, and the ideal aspect ratio may decrease a bit.
 
2.  Both theory and actual performance suggest that
the stock board is pretty well designed.  I recognise
the difference between "nearly optimal in performance"
and "nearly the same dimensions as something that is
optimal in performance."  None-the-less, I will accept
the stock dimensions as close to ideal for typical use.
 
3.  As draft decreases, front-to-back width must increase,
and the total area must also increase due to a less
efficient aspect ratio.
 
4.  In my case, reducing the draft by 6" to 9" suggests
a board about 16" front to back.  This would just fill
the centerboard well on the Banshee.
 
Tentative decision:  Go with the full 16" in width, and
go 9" shorter than the stock board.  The draft would then
equal that of the stock rudder; I can always draw the board
up further if necessary.  (I plan to take along the stock
board and switch to it when I will be in deeper water for
any length of time.)
 
One person suggested winglets.  I indeed had considered
the possibility.  But I know even less about the design
of such structures than about plain foils.  I would
like to keep the design relatively simple.  I would be
willing to dive under the boat to install/remove the
winglets in order to allow the board to pass through the
centerboard well; I could probably design a relatively
simple one-horizontal-piece-slides-up-over-the-tip
-of-the-daggerboard-and-then-snaps-in-place system.  But
I would need some concrete suggestions as to dimensions.
 
Foil shape:
 
Here I am lost.  At worst, I will just scale up the foil
shape on the current board.  Some recommended a specific,
"conservative" shape, but I don't know how to translate the
alphabet soup into drawings or dimensions.  I would
appreciate details or a source for such if my current shape
isn't a good one to use.
 
Trim:
 
Several people warned about moving the center of
lateral resitance as I widen the board.
 
Here we got a problem.  As I understand it, the Banshee
has such a long well to allow some trim adjustment.  I
usually (for my total 100 hours of experience) sail with
the board all the way back.  In very light winds, I can
trim the sail and position my weight so that we track a
straight(ish) path without my touching the tiller.  In winds
of 10 knots, there is a LOT of weather helm.  (Beyond that
is irrelevant...I am on a sailboard.)  The draft in the
sail seems to move back a lot.  How should I trim the
sail to adjust (I have a downhaul, outhaul and a vang, and
when I trim the sail so it looks like it has laminar flow
around it, all these seem to end up tighter in higher wind.)
How much could the age of the sail (14 years?) be limiting
my ability to eliminate the weather helm?
 
The helm is so great that moving the centerboard 4" forward
has a hardly noticable (to me) effect.  So I figure that
widening the centerboard to fill the well (and thus moving
its center forward only about 2") is only going to add a drop
to my current bucket of trim problems.  Could this be the
straw that breaks the camel's back?
 
Rudder redesign (oh, oh you say):
 
Could I improve the helm by moving the leading edge of the
rudder forward with respect to its axis of rotation?  If I
wanted to reduce the draft of the rudder also, would roughly
the same guidelines apply as for the board?
 
Thanks,
..Richard E+17