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Old 07-10-2017, 08:25 AM   #21
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Travel lift scales are notoriously inaccurate. Based on hydraulic pressure on winch motors. How tension in cable relates to pressure depends on how much cable is on spool, affecting radius of the pull.

Also, anchor load has more to do with windage and less to do with weight. Load a boat full of stuff and anchor load probably does not change much compared to same boat light.

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Old 07-10-2017, 08:39 AM   #22
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IMO boat weight is important when waves or shearing forces are involved

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Old 07-10-2017, 10:46 AM   #23
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Originally Posted by makobuilders View Post
This shouldn't be too difficult on a modern design. Find out the displacement at DWL. The designer will also be able to tell you the disp at each inch above or below DWL. You'll need to confirm if your waterline is drawn (painted) correctly.

Regarding the boat equaling the water it displaces... that may work if the king wants to know if his crown is pure gold, but it's a bit tough to do with a full size boat, unless you have a really big bathtub .
Had an interesting discussion on Archimedes Law with my kids after going through the Peterborough Lift Lock. Amazing piece of engineering, built in the 18th century and self powered. Basically two big bathtubs with the descending one containing more water. They were puzzling over how it could work when the tub going up was full of boats but the tub coming down was empty of boats. Took a lot of convincing to satisfy them that the weight of the filled tubs didn't change by adding boats :-)

Also agree that it can be determined by using float marks. Many racing sailboats that have strictly prescribed weight limits are not weighed for compliance, but rather have float marks installed bow and stern, allowing everyone to essentially see their weight. They are typically installed by measuring the distance from the sheer.

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