fivepointfive - April 2023

between maximum and minimum attainable coefficients for
each true windspeed, until the best VMG is obtained for that
particular windspeed.
SAIL AREA
Sail area, as such, is not equal boat to boat. Because, in most
classes, the 5.5 Metre being no exception, rig height is limited
and fixed no matter what the size of the sail area. A boat with
a smaller sail area has a higher aspect ratio which, upwind,
means that the area, although smaller, is more efficient and
can thus produce more power per square metre. Because, due
to the better aspect ratio, the lift induced drag is less it can,
righting moment allowing, be powered up more than the boat
with more sail area on the same span in fact, if separation was
not too great and therefore a limiting factor the smaller sail area
could produce the same power as the larger rig. Another way
of looking at it is that induced drag is equal to (lift/span)^2. As
span in this case is the same for both rigs, the rig with less sail
area can produce the same lift as a boat with more sail area and
induced drag will be the same.
However, because the smaller rig is producing more power
per unit area and is thus running at a higher lift coefficient (Cl),
separation and thus profile drag (all aerodynamic drags less lift
induced drag) will be greater As it is the rise in all aerodynamic
drags, not just induced drag, that pulls the aerodynamic force
vector backwards, and as it is the angle of this force vector that
determines how much of its force is available as driving force,
this rise in drag limits the attainable power from the rig upwind.
Our VPP rig model takes all this into account and arrives at
the best Cl for that particular rig on that particular boat at that
particular windspeed which will be different depending upon
the boat that the rig is attached to. In other words, that rig on
another hull may well produce a different Cl, true wind angle
etc. It will all depend upon that particular hull’s characteristics.
The main point is that the VPP gives a very reliable speed
comparison between boats with very different rigs and hulls and
it did indicate a definite advantage in most windspeeds for the
boat with the smaller but more efficient sail area.
It is comforting that logic would also suggest this result. If the
rig can produce the same or almost the same power and it has
less weight to propel through the water, it must surely be faster?
HULL SHAPE
Secondly, the hull shape is completely different. Instead of
the flat U section of the majority of the fleet, this hull has a V
midsection morphing into a U at the bow and a flat section
with a very hard bilge at the stern. After many years of research
we have found this to be the best shape for low resistance
over a wide range of Froude numbers and it has many other
advantages, which I will let you discover for yourselves. Suffice
it to say that it has proved very successful in a range of boats
from our International 14, which planes sooner than its U section
brethren and stays on the plane longer, through the 5.5 Metre to
the Twelve Metre.
DESIGN PHILOSOPHY
Thirdly, our design philosophy is different. Existing boats seem
to have fairly steep overhang angles to give, presumably, as
long a waterline length as possible which reduces the volume
coefficient (Cv) (a measure of how heavy or light a boat is
compared to its length). In theory this should make the lines
slimmer with a smaller frontal area, which should reduce drag.
However, the downside is that, the steep overhangs necessary
to achieve this, are not very effective at extending the sailing
length once the boat is heeled and/or moving. On the other
hand, a boat with a shorter waterline length and thus a higher
volume coefficient has, for the same measured length, naturally
lower pitched overhangs. Not only do these, because they are
APRIL 2023 • fivepointfive • 29