I hesitate to resurrect this discussion, since I've already gone into details at https://www.catsailor.com/forums/ub...ds/%2Bschrs/Search/true/schrs#Post282859, but:

Originally Posted by samc99us
Unfortunately SCHRS doesn't differentiate between pinhead or square top sails etc.

Really? Then what is LTM (length of top of mainsail) for?

They do have a sinking correction factor which I think you should use for the Isotope, and on the 2018 rating calculator this gives you a SCHRS number of 1.072, quite a bit slower than a 1 up F16 (1.041). Without this I get a number of 1.057, again relatively slower than a 1 up F16.

SH (sinking hull) is "only for HC14, HC16, Prindle 14 and 16". While I could get a fair number by claiming the SH adjustment, and omitting mast area from sail area, that would be illegal. SCHRS claims to produce fair handicaps without cheating.

I used a board depth below waterline of 0.56 m, the number for the Taipan 4.9 (which in stock configuration has very short and stubby boards). You didn't post a number from what I can tell, so that may be part of your ratings issue.

Sorry, I inadvertently omitted VLB when transcribing numbers in my previous post. I measured VLB = 0.560 for the Isotope.

Personally I feel the 1.057 number is a fair rating considering the following statement on the Isotope website:
"The Isotope Catamaran is a registered Formula 16 boat!
The hulls meet the 16ft LOA requirements of the Formula 16HP class. The strong points of the Isotope are that its balanced helm allows it to out point any other catamaran to windward. An Isotope meeting the One Design class specifications can be raced as a single handed Formula 16 boat. An Isotope with a spinnaker can be raced with crew in the Formula 16 one up class."

When the F16 class was first created, a number of existing 16 foot catamarans were grandfathered in, including the Isotope and the Hobie 16. Technically, both the Isotope and the Hobie 16 MAY race as F16 class boats. That doesn't mean that they actually are the same speed as a modern F16 design. If it did, then why does the Hobie 16 have a SCRHS rating of 1.193 rather than 1.050?

Does anybody really believe that 45 years of marine engineering, including the advent of computer modeling, CNC manufacturing, and the addition of spinnakers has yielded no difference whatsoever in boatspeed? I don't.

If the hull doesn't plane, maybe the 1.072 number is fair, maybe. How does the boat sail compared against a 1 up Blade, Falcon, Viper or Nacra F16?

I haven't raced directly against against a Blade, Falcon, Viper, or Nacra F16. They've always been in "A-Fleet" whereas I sailed in "B-Fleet". The times I have sailed on the same racecourse, the A-Fleet boats were definitely faster. I have raced several times against Hobie 16s (1- and 2-up), Hobie 17s, and Hobie 18s. The Isotope is definitely faster than a Hobie 16, perhaps a little more than the US Portsmouth handicap indicates. Although US Portsmouth rates the Hobie 18 faster than an Isotope, I was often able to beat some H-18s around the course. I'd be willing to race boat-for-boat against a H-18. To claim that a 1-up Isotope is as fast as a 1-up Goodall Viper (which carries a spinnaker larger than the Isotope's main and jib combined) however, is ridiculous.