[Rhodes22-list] Keel/Diamond Board Hydrodynamics

[Roger Pihlaja]

Roger In UK:

Good grief man!  Have you ever considered switching to decafe?  This isn't some breach of national security.  It's just a running production line change to the keel design on a little 22 foot trailerable sailboat produced by a small mom & pop company. 

If you look in the sailing literature; then, you will find plenty of examples of shoal draft keel with centerboard designs wherein the centerboard has a rather triangular shape, when viewed in side elevation.  Your hydrodynamic experts are correct when they assert that high aspect ratio foils are more efficient than low aspect ratio foils.  But, please remember the diamond board is not operating in isolation.  Rather, it is an extension of the shoal draft fixed keel, a very low aspect ratio foil.

I too was somewhat sceptical of GBI claims regarding their new diamond board.  The claims seem counterintuitive relative to hydrodynamic theory as I understand it.  However, the new diamond board boats may very well point better than the older high aspect ratio centerboard boats.  But, this is really an apples vs. oranges comparison because the diamond board boats have more total keel foil area than the older boats.  The diamond board also does not have the drag losses associated with the centerboard slot and the exposed centerboard pennant that the old high aspect ratio foil centerboard has.

Please remember that GBI is not attempting to conform the Rhodes 22 to any racing or handicapping rule.  I think what Stan at GBI has rediscovered is that the fixed shoal draft keel lends itself to the storage of the, new triangular shape, diamond board better than the older high aspect ratio centerboard.  If the design objective is to have the maximum possible amount of deployable keel area in the centerboard, which still retracts up into the shoal draft keel; then, the new diamond board achieves that objective.

As far as the foil shape of the new diamond board vs. a "real" NASA foil shape, who knows?  Remember, we're talking about low aspect ratio, rather low efficiency foils here, operating in concert with an even lower aspect ratio, lower efficiency, foil in the fixed shoal draft keel.  Without extensive computer simulation & tank testing, it would be very difficult to sort out the contributions of the greater diamond centerboard area & reduced parasitic losses (no centerboard slot & pennant) from the intrinsic efficiency of the foil shape itself.  I doubt that GBI has the resources to do this sort of development.

I took a look at the claims made on the GBI website.  They look pretty typical of the hyperbole commonly found in the marine industry.  Please remember you are looking at advertising, not some technical publication in a scientific journal.  I think all GBI really knows is that the new diamond board boats probably sail to weather somewhat better than the older boats.  The new diamond board fits up inside the fixed shoal draft keel.  Thus, the boat is still trailerable.  That's really all that GBI & most of their customers care about.

If you want a better explanation than that; then, you will have to schedule some time with Stan Spitzer.  If you will calm down a bit & improve your manners, he might take the time to respond to your questions.

Roger Pihlaja
S/V Dynamic Equilibrium (A 1976 Rhodes 22 with the old high aspect ratio foil centerboard)