fivepointfive - April 2023

closer to the water, extend the sailing length better with heel and
forward motion, they extend the length better when the boat
sinks in sailing trim with crew and stores aboard. The result is
that although the steep overhang boat has a longer waterline
length in measurement trim it is shorter in sailing trim and it is the
length in sailing trim that determines the speed, not the length in
measurement trim. Our design philosophy is to make the boat as
short as possible in measurement trim but as long as possible
in sailing trim. True, it will have a higher Cv in measurement trim
but it will have a lower Cv in sailing trim and once again, it is the
Cv in sailing trim that matters. Additional to the longer waterline
length and thus lower Cv in sailing trim, as already noted, the
lower overhangs extend the boats sailing length better once it is
heeled and moving
KEEL
Finally, the keel is very different. During our research for Graham
Walkers 86/87 America’s Cup Challenge we obviously tested
the inverse taper keel otherwise known as the upside down
keel and found that hydrodynamically it was very inefficient
for reasons that are beyond the scope of this article but, if you
are interested, are explained in detail in some of my writings in
Seahorse Magazine. It’s saving grace, however, was that it got
the lead down low and thus increased the righting moment on
a lighter shorter Twelve Metre that would otherwise have lacked
power. Most of the 5.5 Metre fleet do appear to have either an
inverse taper keel or a keel with very little or no taper at all, both
of which are still hydrodynamically inefficient. To make matters
worse they also carry a large percentage of their ballast in the
bilge as internal ballast where it adds little to righting moment
and thus power. For a number of reasons this did not make a lot
of sense. True, it produces a smaller fin with less wetted area but
because the smaller fin displaces less the canoe body volume
has to increase to keep the total displacement the same. This has
two effects.
Although the wetted area of the fin goes down, because
more displacement has to be carried in the canoe body, the
canoe body wetted surface area goes up so there is little if any
gain in that area. Additionally, because displacement that would
have been carried in the fin, a long way from the water surface
where its effect on wave drag is small is now, as mentioned,
accommodated in the canoe body where it is closer to the water
surface and where its effect on wave drag is greater, wave drag
rises so it is altogether not a good idea.
The fin on Jean Genie is thus large enough to accommodate
all but a small amount of trimming ballast which, as mentioned,
allows a slimmer canoe body containing less volume and
with thus smaller wave drag. Additionally, because the lateral
area of the fin is greater a trim tab is not required. The whole
idea of a trim tab is to allow a given lift requirement to be
produced efficiently from a smaller fin. If you consider that a
well designed foil section will probably have an area, either side
of zero lift, where drag is low but outside that area the drag is,
comparatively, very high, as long as the foil is working within that
low drag bucket, which will be a function of side force speed and
foil area, drag will be low. However, when sailing to windward
only half of that drag bucket is available, the other half will work
on the other tack and is thus, in a sense, wasted until the boat
tacks. A trim tab or flap is nothing other than a camber changing
devise and the idea of camber is to move that low drag bucket
bodily sideways so that more of it is working on the particular
tack that the boat is on. This has the effect of making the drag
bucket wider on each tack and if the drag bucket is wider the foil
30 • fivepointfive • APRIL 2023