Thread: Displacement
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Old 01-29-2013, 12:31 PM   #7
Nomad Willy
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City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
Semi-planing

Yes I agree w that one Marin.

""Displacement" is like "dead." You either are or you aren't." Dead?

There actually isn't a sharp break-of point where FD ends and SD (or SP) takes over. The still slightly fuzzy point is most accurately defined by the QBBL (quarter beam buttock line) angle. The all important line is in-between midships and the transom, in-between the chine and the keel ... and runs fore and aft. The angle of this line relative to the WL at rest tells the story. I'll try to find the angle as a number.

An easier and less technical way to "usually" determine SP from FD is to observe the transom at rest. Basically if ant significant amount of transom is submerged the boat is SP. The important thing is that how fast or slow the boat goes has nothing to do what type it is. It's the shape of the bottom aft. With a steep QBBL some transom can be submerged on a FD hull and shallow angle QBBL hulls are all planing or semi-planing.

I know you all want to think your boats are full displacement because they are trawlers but it just isn't so. What marketing people call boats is in a word ... marketing.
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