The Hull Design Ideas Behind The SC15 and 17

[Bill Roberts]

This text came form a discussion with Carl Roberts.
Hi Carl,
I want to help you out a little with this SC15. The 15 plug was made from the 17 plug. Two feet was cut off the back end of the 17 plug and then the keel rocker in the aft end was increased, rolled up to a new transom at 15ft. This also added displacement in the aft end of the hull. The important thing for this discussion is that the front end of the hull was not changed. This means that the 15 like the 17 has the hull lifting surface, deep vee hull shape, in the front 6ft or so of the hull. The middle 5ft is transition shape from deep vee to round and the aft 4ft is a round hull bottom shape, therefore no lift here. Same 2x size rudder, balanced, is on 15 and 17. This was all done in 1979/1980 to produce a boardless beach cat that would TACK. The aft end of any hull must be free to slide sideways over the water for a boat to tack quickly and easily. The no lift hull aft end automatically puts the lift burden on the rudder which is a high aspect ratio foil, therefore low induced drag. The rudder is like a steering daggerboard. This boat design is another example of "shared lift" done in 1979/1980 to make a boardless beach cat TACK well.
Now here we are in 2004 and we want to put a spinnaker on this boat. Well how about that; aren't we lucky. This boat already has the hull CLR well forward, like 3 to 4ft back from the bow so adding the spin is "no problem". This boat will exhibit no lee helm with spin up because the sail plan CE will always be behind the hull CLR.
The motto of the best SC15 an 17 sailors is "drive those bows". By that they mean trim the boat out with the waterline halfway up the stem, especially sailing to windward. Always keep the waterline tape underwater at the bow.
This hull design scheme has also led to an increased boat speed hull shape sailing to windward for boardless beach cats. The normal boardless beach hull shape has its lifting hull shape for the full length of the hull. The underwater aspect ratio for this hull shape is the average hull depth divided by the waterline length, depth/chord. These are numbers like .75ft deep by 16ft long. The induced drag due to generating lift is inversely proportional to this number. This number in this example has a value of .75/16 = 0.0469. The inverse of this number, 21.3, is a multiplier in the induced hull drag calculation. In the case of the SC15 and 17 this multiplier is made up of two parts. One is the hull induced drag and the other part is the rudder induced drag with each lifting body carrying half the total lift force, side force from the sail. For the SC15 and 17 the hull induced drag aspect ratio is something like .75ft deep by 7ft long which leads to an induced drag multiplier like, .75/7 = 0.107, or a multiplier like 9.3. The rudder aspect ratio is like 2ft deep by .75 ft wide which leads to an aspect ratio of, 2/.75 = 2.67, or a multiplier factor of 1/2.67 = 0.375, acting on 50% of the lift force. So the induced drag coefficient for the hull plus rudder generating lift on a SC15 or 17 is equal to 0.5 x 9.3 plus 0.5 x 0.375 or a total induced drag multiplier of 4.84. This compares to 21.3 for a normal beach cat hull design. Needless to say, the hull drag on the SC15 and 17 is significantly less than the other beach cats. This shows up when comparing PNs between boats of similiar size. The SC17, which has a US Sailing PN base, and its PN is 4 to 5% lower than other beach cats of similiar size. The hull induced drag makes the difference. The SC15 never developed a PN base but its number should be something like 5 PN points slower than the 17. At the Tradewinds Regatta two weeks ago the 17 demonstrated a 70 PN sloop rigged with square top main and self tacking jib. The 15 might get as low as 75 with the same sail plan design. That puts it faster than the H16 or P16. Let's see what happens.
Good Sailing, Carl
Bill

[PolyMath]

Do the remarks concerning sailing a SC15 and SC17 (waterline strip below the water) apply to the SC19? I had one of the first SC17's (did not have vent holes in transom - heat and water pressure split hulls) - and managed to move up to an early SC19 (#14)... has no diamond stays... and it seems to sail best with hulls pushed down to waterline stripe... but you are FAR more knowledgable about these fantastic craft than I.

Also, the Tramp is not very tight... the design appears to have changed over the years and the replacement is not tight. Advice there would be interesting as the dealer (over 20 years ago - WOW) said it needed to be drum tight.

TNX