Actually I investigated this very issue in 2002 and discussed with with various builders and designers. I also discussed it with Martin Fisher himself and talked with the owner of a test platform here in NL.
Personally I think the letter the BIM yard owner send around is full of non-sense. The numbers simply don't check out and some are even obviously wrong. I refer to a 88 ratio between the lift and drag of a hydrofoil. Very good foils only achieve a ratio of some 30. But even more importantly a canted board can NOT provide as much lift as you want. It is locked into a force balance with the sails and so its produced amount of lift is a direct result of the sail drive, the inward angle of the boards and the fact wether the luff board is set. There is a reason why patient lady 6 raised here windward board after every tack. So I disgard the BIM data all together.
What did I find out.
Well that, as I wrote, the produced lift is a direct result (and limtied) by sail drive, angle and the fact wether the luff board is set. I ran a physical model on the issue checked the results with the builders and implemented a limiting structure in the F16 class rule that was made official with the Nov 2002 general vote on the F16 class rules.
Rules to note in this issue are :
1.6.2 In addition to the rudders the platform may also be equiped with
a pair of daggerboards or centerboards.
(Note "a pair")
7.13 Daggerboard and Centreboards
All lift producing elements that have as the pre-dominant purpose to resist sideways movement of the craft while sailing and that aren't canted at a greater angle than 6 degrees of the vertical when the boat is level on the waterline.
A centreboard is only different from a daggerboard in the sense that it is folded or rotated away rather than slid away when in danger of hitting submerged objects.
The definition of a daggerboard limits the canting of daggerboards to 6 degrees outward or inward. This limit of 6 degrees limits the total amount of lift that can be generated relative to the conventional vertical boards.
There are two situations to consider.
-1- When both boards are down and the inward canted luff boards is perfectly vertical due to heeling of the platform this will angle both conventional boards also to 6 degrees.
-2- When only the leeward board is down and the platform is never angled more than a few degrees (between 3 and 6 degrees) (only upwind
In situation -1- the max gain = 1/2 * sin (6 degrees) = 0.052 * sideways component saildrive = 0.052 * about 150 kg = 7.84 kg = say 8 kg lift. (only upwind; downwind the sail drive in sideways direction is a lot less and so too the lift of the boards)
In situation -2- the max gain = 1 * sin (6 degrees) = 0.104 * sideways component saildrive = 0.104 * about 150 kg = 15.68 kg = say 16 kg lift. (only upwind; downwind the sail drive in sideways direction is a lot less and so too the lift of the boards)
You can not create more lift with straight canted boards than this and in the second case you will have to raise and lower the boards after each tack. This last necessity is a serious drawback especially when singlehanding. When doublehanding the amount of lift provided falls well within the limits of the normal spread between crew weights (about 20 kg). This limits its gain ones more also we don't think the last to be particulary unfair so why should these boards be ?
Of course we all need to realize that these are MAXIMA , meaning you have to create maximum sideways sail forces by double trapezing the achieve these lift levels. In conditions below 12 knots no-sailor can even come near these lift levels. So the biggest gain is always to be found in 12-25 knots when double-trapezing upwind. In all other situations the gains are seriously less.
Only curved boards can sort of pass the barrier that has been put up by limiting the canting angle and the force balance around the boat. However these boards have several drawbacks in themselfs. You'll need asymmetric crossection profile to mkae proper use of curved boards and you run into cavitation problems sooner than with a straight board. In general these boards have to be lowered and raised after every tack and gibe. And the same F16 rule limits any part of the board to be angle more than 6 degrees of the vertical. I confess that the rule is a little open to intepretation here but we can easily correct that by replacing
" ... that aren't canted at a greater angle than 6 degrees of the vertical ..."
By
" ... AND OF WHICH NO PART IS ANGLED at a greater angle than 6 degrees of the vertical ..."
This is the official intepretation of the rule anyway, Since the 6 degree angle limit was there to prevent more horizontal bearing surfaces that are arguably more efficient.
The end conclusion I reached and that was accepted by the builders of the time was that this was to small a advantage to ban. It was preferred to allow a little development in this area; mostly to stay in touch with the other classes in the future. One builder compared it to allow the square-top mainsail in the Taipan 4.9 OD class. At the time it was a controversial decision but it made the Taipan class an excellent choice during the 90's. They limited the amount of the square-top to a convervative amount to keep things in check but allowed some growth path to the future allowing the Taipans to compete with the expected new class that would be formed. As it turned out that new class was the F18 class and we all can see the wisedom of allowing this Taipan square-top now.
Personally I think the current limits in the F16 rules are sufficient to garantee level racing. Especially since racing in 12 to 25 knots is more determined by boat control and co-operation between the crew than absolute boatspeed. I really don't expect to see crews lower and raise their boards after each tack, you will loose any tacking duel and the conventional boarded boat will gain a few boatlengths on you with each tack that you can have to win back by additional boatspeed. The lift of canted boards looks to be insufficient for that.
Any more questions ?
Wouter
