I was fortunate that when I was a design student I was able to use the college test tank to run some model tests on a 5' (1.5m) model catamaran. I wanted to see how much extra drag was generated by having two hulls relatively close together rather than at infinite spacing. I quickly discovered that test-tanking models is not a very exact science (even using 5 ft models in the biggest educational test tank facility in the UK) A bit better than a guess, but not a lot.

Purely theoretical /computed tests are even more of a guess. There are far too many assumptions that have to be made. The monohull race boat rule, the IMS is a good example of such a "theoretical" rule.

I'm sure I'm not the only one who has a background of dinghy sailing. Thousands of dinghies race thousands of handicap races every year. So we "know" a Laser 1 rates 1077 and a Laser 2 1035 (using the UK's RYA data from 2000). That's what, 4% faster? Even my mother could look at the two boats sailing and say the Laser 2 is the faster boat. After all it is longer, and has a trapeze and spinnaker.

I believe the IMS is supposed to be accurate to +/-2% and people are happy with the results because the formulae look complex and results can only be calculated by computer so therefore must be right! Yet if a Laser 1 speed was calculable under IMS it could be 1050, say, ie within 2% of the real value. Similarly a Laser 2 could work out at 1050, also within 2%. So the super accurate, everyone is happy with the results, IMS would make the Laser 1 and Laser 2 the same speed!! As I said a second ago, my mother is a better handicapper than that.

Compare the complex measurements needed, the small speed range that monohulls use and the type forming of the IMS rule and I think you can see that no completely theoretical calculation can be much better than a guess dressed up by computers to convince the uninitiated.

Having said all that, about 15 years ago I developed a spread-sheet to try and predict the performance of my new designs. It seems to give reasonable results. As an example, the 35' Banshee has been tested full size under power with engines ranging from 4hp all the way up to twin 90hps. The predicted speeds were always close to the real thing. One reason for the accurate results is that my hull shapes are all fairly similar so I was able to "type form" the results. I also used the data I obtained from those early tank test results and also from work done by the US Navy in the Taylor tank. Unlike many I also make an allowance for windage (those who have tried to motor a large multihull under outboard in a strong wind will know how great windage drag can be!)

At its simplest, it seems that the best indicator of speed is the Texel Rating, developed by Nico Boon of Holland (Lilian's father). More details can be found on the website www.texelrating.knwv.nl for beach catamarans and on the website www.ctcnederland.nl for all cruising multihulls.

Incidentally, I think the nut on the end of the tiller has a bit to do with it as well. I once raced my Stealth dinghy one New Years Day. A number of Laser 1's also raced, including one from the UK Olympic squad. He was 30% faster than the second Laser and that's in identical boats!! I regret to say he also beat me and I was supposed to be in the  faster boat.