Now I haven't designed any C-class catamarans in the 80's but the numbers presented look funny.
For example in order to create 80 Kg lift (about 800 N lift) the drag of the board is only increased by (64-55) n = 9 N or about 0.9 kg. So the efficiency of such a foil is 80/0.9 = 800/9 = 89 = Cl/Cd (= lift/drag ratio)
This is the most efficient foil (either air of hydro foil) that I have ever seen ! With such foils a human powered vehical could lift a 350 kg from the ground or above the watersurface. Now many universities has tried really and human powered flight has only been barely reachable. They had to hire champion cyclists to peddle the craft and all weighted less than 150 kg including the pilot.
A good foil does a ratio between 10-30; so no where near 89 !
Also the mr Bergami (or somebody else when he didn't) makes a very deceptive statement :
With round daggerboard the overall drag is lower by 57 N or by 25% (57/227).
This is simply WRONG. Overall drag (the kind that determines final speed) encloses MORE than only the hull related drags. So any savings need to be devide by a number bigger than 227 N that was used here. Result the ratio will drop.
The next statement is obviously made without careful investigation :
The conclusion of Mr Bergamini is that a boat fitted with round boards has
a 25% lower (friction plus wave) drag than the same boat fitted with standard
parallel boards, the same advantage could be achieved by lengthening the
hull by 25% (from 16 to 20 foot) or reducing the overall weight by 25% (from
225 to 180Kg).
This is very wrong. This totally disregards all the higher (non-linear) dependencies than are linked to the designs. You can't just say that 25 % lower hull friction has the same result as making the boat 25 lighter. Same for the other statements.
Also there is another fenomenon at work that limits the amount of lift that can be generated by an inclined board. The lift is NOT unbounded. It comprised of two factors ; -1- cavitation, -2- total force balance around the whole craft.
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I wrote this so that readers know that these numbers are not accepted by all. I even doubt by many.
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On a different topic.
I'm quite amazed that they used a 16 foot, 105 kg boat with a 120 kg crew as example. Sounds like an F16 to me !
Of course the F16's allow both T-foil rudders (and have them even) as well as inward canted boards. HOWEVER, you may only cant them to a 6 degree angle to the vertical ! Rule 7.13 of the F16 class rules. Any designer better take that into account or risked getting banned from the F16 class. So the 15 degree angle in the example is not allowed in the F16 class. We decided to allow inward canting boards to not arrest a little development, but we did limit the possible benefits.
Curved foils are allowed but may never arc passed the 6 degree inclination. That is the official intepretation of rule 7.13
Wouter
