Performance Index

Understanding boat design by George Brewer uses sail area/displacement ratio in his design portfolio. A boat with a ratio of 12 would probably be a slow sailing boat. But that does not tell you how fast or how slow. The performance index gives an expected speed on a reach. Goerge's Morgane Le Fey is 17.1, no slouch. .5(618 x 29/13950)^.5 = 0.567 or 5.67kts in a 10kt wind(relative wind) so this boat could complete the fastnet race distance in about four and a half days. 5.67kts is SL 1.05, A racing yacht has a sail area/displacement ratio of about 20. So lets design Morgane as a striped out racer of 12500lbs and design a sail plan for SL = 1.2. The performance index gives a sail area of 720sqft. SA/D now equals 21.4.
Now my point is I don't know how George decided on the area, but using the performance index is a possibility. Surely it is a better method than picking a number between 15 - 17 for an average cruiser.

 

Where did you find the "Performance Index"?
In case you haven't run across it yet see Eric Sponbergs Design Ratios below:

 

His name is Ted Brewer.

The performance index is an old formula from Derek Kelsall, a variation on the Bruce Number and Richard Boehmer's Base Speed formula. Gerr reprinted them in the Westlawn Masthead.

Yacht designers do not use these, but they are of interest to amateurs. Today yacht designers use VPP programs to compare any and all speed related factors.

 

My boat is very close to those numbers. Using P*E/2 for mainsail and I*J/2 for jib sail area is 63.4 m2 (680 ft2) and with real main sail area 69.6 m2 (750 ft2). The measured displacement for an empty boat is 5700 kg (12550 lbs) thus it could be 6200 kg (13600 lbs) with crew, sails and some equipment. Which ones of these should be used when calculating ratios?

The measured waterline is 10.1 m (33.1 ft), thus SL=1.2 equals 6.9 knots. In 10 knots true wind it can do 7.4-7.5 knots with spinnaker at 90-120 TWA and with a jib 7.3-7.4 knots at 70-90 TWA, 6.9 knots at 110 TWA and 6.4 knots at 120 TWA.

My boat is certainly not a stripped out racer. It is a typical modern C/R boat with all the cruising comfort on board.

 

I estimated Morgane could be striped down to 12500lbs, I did not mean that would make her a stripped out racer. Use 13600lbs because as you use up fuel and food the boat gets lighter (and a little faster) the empty weight will seldom if ever be reached therefore any performance prediction using that weight will likely never be achieved.