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Hiracer · 25-11-2008, 12:18

OK, here is the math on how weight more negatively affects SA/D for light boats than heavy boats.

If you take two boats at 35' with the same empty SA/D ratio of 17, the one with an empty displacement of 17,000 pounds will have a better final SA/D ratio than the other boat with an empty displacement of 10,000 pounds after you load each of them up with 4,000 pounds of cruising stores. Assuming reasonable design quality for each boat, the heavier boat will be the better sailor fully loaded--period. Its SA/D after loading will be 14.8 compared to only 13.6 for the light boat. That is a meaningful difference and will be readily apparent in the real world.

All the talk in the world about Froude numbers is irrelevant when comparing a SA/D of 14.8 to 13.6. Horsepower rules. This in a nutshell is why a fully loaded steel boat, if properly designed, can keep the pace with plastic cruisers.

Also, bear in mind that the stability curve for the heavy boat will be less adversely impacted by this load than in the case of the light boat. Further, the heavy boat will have the room to store it, while the light boat might well lack the space.

On the race course, of course, it's different set of physics because racers don't carry heavy loads.

25-11-2008, 12:18	#34
Hiracer Registered User Join Date: Oct 2006 Location: The boat lives at Fidalgo Island, PNW Boat: 36' custom steel Posts: 602	OK, here is the math on how weight more negatively affects SA/D for light boats than heavy boats. If you take two boats at 35' with the same empty SA/D ratio of 17, the one with an empty displacement of 17,000 pounds will have a better final SA/D ratio than the other boat with an empty displacement of 10,000 pounds after you load each of them up with 4,000 pounds of cruising stores. Assuming reasonable design quality for each boat, the heavier boat will be the better sailor fully loaded--period. Its SA/D after loading will be 14.8 compared to only 13.6 for the light boat. That is a meaningful difference and will be readily apparent in the real world. All the talk in the world about Froude numbers is irrelevant when comparing a SA/D of 14.8 to 13.6. Horsepower rules. This in a nutshell is why a fully loaded steel boat, if properly designed, can keep the pace with plastic cruisers. Also, bear in mind that the stability curve for the heavy boat will be less adversely impacted by this load than in the case of the light boat. Further, the heavy boat will have the room to store it, while the light boat might well lack the space. On the race course, of course, it's different set of physics because racers don't carry heavy loads. __________________ John, sailing a custom 36' double-headed steel sloop--a 2001 derivation of a 1976 Ted Brewer design.