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From: C. Marin Faure (no email)
Date: Sat Mar 01 2008 - 15:49:14 EST
>Bob, the question therefore is are semi-displacement hulls actually
planing hulls or are they half way in between? I know most of the boats
you mention are not full displacement.
The term "semi-displacement" is rather misleading, and in fact a
number of boat designers like Tom Fexas (sp?) use the term "semi-
planing" to describe the same hull form because that's what the hull
really is. A displacement hull is a hull that cannot be pushed
faster than its hull speed, even with a large application of power.
Excess power simply causes it to dig a deeper hole in the water
although it may go a bit faster in the process. So a hull is either
a displacement hull or it isn't.
If it isn't, then it's either a fully planing hull or a hull that
enough power can force part-way onto a full plane, hence "semi-
planing." I don't know that a hull like a Grand Banks can be
considered a true planing hull because the rounded forebody and full-
length keel will prevent the hull from achieving a full plane unless
the entire boat was an engine. But with enough power, a GB can be
pushed relatively efficiently part-way toward a full plane, say 15 to
18 knots. So the GB hull is a "semi-planing" hull.
I suspect the term "semi-displacement" was coined and came into favor
because saying a boat is "semi-displacement" implies that it is
closer to a true displacement hull with its economy, seakeeping
traits, heavy-duty "workboat" characteristics, etc. It's a good
image to promote in a boat. It's sort of like calling GM's gussied-
up Tahoe a Hummer H2. Tahoe implies soccer mom, Hummer implies
rugged, go-anywhere capabilities. Even though they use the same
platform, if rugged image was an important buying criteria, which one
would you buy? Calling a boat "semi-planing" implies high fuel
consumption, lighter weight construction, not so good in the open
ocean, etc. I think it's largely about image rather than reality.
After reading stuff by Tom Fexas and others, I now use the term "semi-
planing" to describe the hull on our GB or other similar boats
because that's what I think they really are. A Willard, on the other
hand, is a true displacement boat.
____________________
C. Marin Faure
GB36-403 "La Perouse"
Bellingham, Washington
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