Join Date: Jul 2002
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Displaced weight vs Dry Weight
Mike, there are several useful discussions on the topic of displacement in Offshore Sailing by Bill Seifert. The essential points he makes are as follows:
1. There''s no commonly agreed to definition used by all boat manufacturers when citing ''displacement''. (Remember: Bill worked for many years in boat building, with Tartan, TPI (J Boats, etc.) and Alden).
2. The most common practice is to "list displacment at a datum waterline rather than at the actual floatation plane." (p. 170)Essentially, the designer has no absolute knowledge of how the workers construct the boat, only of his/her design...but selecting a datum waterline allows computation of displacement - and yes, that is the weight of the water the boat will displace.
3. The actual weight of the boat after construction can vary from one hull to the next considerably. Generator? A/C? What sail compliment? Capacity of the DC electrical system along with its many components? Moreover, builders change materials and construction methods over time. And since builders don''t print up brochures for each hull, the displacement figure of a given boat can''t be completely accurate even if the boat sat on its datum waterline.
4. Builders also don''t agree on the tankage being referred to at this ''design displacement''. "Remember that 60 gallons of diesel and 200 gallons of freshwatere add a ton of displacement and that''s before food, cltohing, spare parts and, oh yes, crew." (p. 170). FWIW, for extended cruising we estimate each permenent crew member brings 1000# aboard the boat.
For these reasons, numerous writes on this subject (not just Bill but also Nigel Calder & Beth Leonard come to mind) recommend purchasing a Performance Package from U.S. Sailing. This will include info typically available on a sistership''s IMS Certificate and provide a relatively accurate report on the displacement of that sistership for a known condition of loading. With adjustments, this is a good way to assess the weight of your own boat.