Join Date: Jul 2000
Thanked 184 Times in 173 Posts
Rep Power: 17
Fixed or swing keel, which is better?
Skimming dish hulls (scows) etc. are meant to be sailed fairly over on a heel (design heel @ approx. 25degrees) so that much of the flat **underwater** sections are raised clear of the water, greatly minimizing wetted surface area thus significantly reducing drag while simultaneously gaining waterline length and dynamic form stability at high speed (Imagine what would happen if one half of the wetted surface area drag factor magically disappeared from a keel boat !). When sailed correctly, the middle of the flat undersurface of the hull is virtually raised clear of the water (as well as everything else to windward), the boat balances essentially against the immersed leeward surface of the hull (imagine a high speed catamaran flying only one hull). If the boat had a centerboard (has 2 bilgeboards), with the middle of the flat bottom being raised out of the water, the centerboard would be partly out of the water also; therefore, the board(s) is relocated to a position under the remaining wetted area. The bilge boards are not parallel but angled (approx. 10 degrees) towards windward to gain hydrodynamic lift .... two boards down give LOTS of drag. These are PLANING hulls, even the 38ft. A-class scows are adept at planing (upwind and downwind); and, until the advent of windsurfers were THE fastest mono-hulls. Once a scow ''breaks free'' on a plane then crew weight can be shifted to windward to gain max-advantage of the sail plan .... but while keeping ''most'' the hull bottom relatively free of dragging in the water. These are NOT stable boats at all, ...until onto a full plane when they reach "dynamic" stability.
Down side of a scow is that they can quickly disappear entirely into the backside of a wave and instantly become a high speed submarine! Once you''ve raced scows, fixed keel boats seem very boring and slow ... just ask Buddy Melges. The difference in sensation is like the difference between a Ferarri and an Oldsmobile. When one considers that these variant designs were established 110 years ago (1890s) and are still raced with few "shape" changes, etc. .... than you''ve got to admit that they are a VERY successful design.
Take a scow, remove the bilge boards, add a keel and a sharp bow = Melges-24! ... of which the ''breadboard" was probably an old M20 scow refitted with a keel and a sharp bow.
For those who dont know what a scow is: www.ilya.org. Classes: A E C M20 (now Inland20), M16, MC ... ice boats with hulls!