Foiled

An Ask the
Designer question, answered by Doug Schickler of Schickler Tagliapietra
Yacht Engineering.

Q: am looking to produce a plug to fabricate a daggerboard or maybe 2 if
time,assymetric is the needed shape,need a few directions in terms of
profile. 30 feet open bridgedeck cat is the rough spec.the d.board approx.dims.
are 2440*450*45millimetres.Body of article here. Use this for pieces
with with mostly text and small articles.

A: First off,
if you are creating daggerboards for a bridgedeck cat, it seems you
need to make two. Or you are going to be running back an forth a lot
with what sounds like a fairly hefty piece of kit. If you are debating
making one pair of plugs or two, that is another story. You can make
the plug pairs over-length with tips at each end and take the moulds
only from a part of them. That should save a bit of material and milling
time.

Next, to select a foil, particularly an asymmetric foil, you will want
to do more than look at 2D section performance from the seminal work
Abbott & von Doenhoff Theory of Wing Sections. Or should I say you
want to have more done. This job requires some careful consideration,
but can be done quickly by an experienced designer with the right tools.
(End shameless self-promotion.)

Selecting the "best" foil family, which describes the distribution
of thickness along chord and that of camber along chord, is dependent
on a number of variables, which you haven’t volunteered. Speed of the
boat being the most important, but also a consideration of the leeway
angle. Of course, the boat will gain leeway to generate the necessary
sideforce imposed by the sails, so it may be more useful to describe
a sailing condition with a generated sail vector instead of an assumed
leeway angle. Are the foils going to be toed-out, as the asymmetric
daggers are on a VO70? Further complicating matters, there may not be
one best condition. You may want to have all-around performance in a
range of speeds and leeway angles, or one may consider the upwind and
downwind cases, for example. The compromises must be evaluated one to
another.

To your case study. First thing that jumps off of the page is that you
have an aggressive t/c ratio of 10% for a foil that is quite long. What
you have not made clear is the amount of the foil that will remain inside
the hull. 9.5 m boat, probably hulls of over 1 m in depth, so a bit
more than half of the blade in the free stream, right? This sounds like
a structurally challenging foil, even if the span outside the boat is
just 1.3 m, giving an aspect ratio of 1.73. Straight from the gut, with
boatspeeds over 20 knots and potentially huge lift generated as the
boat gets pushed by ocean waves, thinking more like 13+% t/c ratio.
It is relatively simple to compute the structural requirements for the
blade, once the speed of the boat is known.

We are going to refrain from naming any specific NACA cambered foil
shapes. We simply need more information and would not want to lead you
astray. It may not be a standard NACA foil that is best suited either.
In house, we generate and evaluate our own foil shapes for each finite
foil under development. There are tools to theoretically test a foil
created arbitrarily or by blending two existed foils for example. (OK,
maybe the shameless self-promotion lingers.)

Give us a call!

Doug Schickler
Schickler Tagliapietra Yacht Engineering
www.styacht.com