Fortunately, I don't have the "credentials" and I'm no engineer, so I'm not distracted by the facts -- though I've read a few articles meant for dummies like me.

First, I intentionally referred to the fishing trawlers of the Grand Banks as the "inspiration" for the Grand Banks brand pleasure boats because they are not a literal application of the fishing boats hull design.

I don't understand the distinction between "semi-planing" and "semi-displacement" unless it's a reflection of a design's emphasis.  In theory, so long as it doesn't nose-dive, any hull -- even a "full displacement" hull can be made to plane if enough power can be applied without adding too much weight.  "Displacement" type hull is one which is designed to operate entirely in the mode where it never develops any dynamic lift and always fills the hole it makes in the water, displacing it's weight in water and being kept afloat by the static lift of buoyancy.  The speed of a displacement type hull is limited by its waterline length and there's a well-known rule-of-thumb for estimating maximum displacement speed, or "hull speed" which is approximately 1.34 times the square root of the water line length.  The term "planing hull" is used to describe a hull which, with the application of sufficient power, is designed to operate in a mode where dynamic lift (generated by the water pressure on the bottom of the hull as the hull is powered through the water) raises the boat out of the hole in the water which it creates at rest or displacement speeds and the dynamic lift is close to, equal or greater than the vessel's displacement.  The terms "semi-displacement" or "semi-planing" seem to refer to a vessel which is designed to operate somewhere in between, where the dynamic lift is not sufficient to lift the vessel's entire weight.  If I have it straight a "semi-displacement" would refer to a vessel designed to operate in more of a displacement mode than planing and "semi-planing" would refer to a boat which generates more lift putting it closer to a full planing hull.

Originally, most Grand Banks were equipped with relatively small engines (single or twin 120 or 135 horsepower) and operated at speeds which only barely exceeded hull speed making them faster slightly faster than a full-displacement Krogen or Nordhavn of similar waterline length but substantially slower than your typical semi-displacement or semi-planing Viking, Pacemaker, Uniflite, etc.  More recently, Grand Banks have been equipped with far more powerful engines to place them more solidly in the semi-displacement family but I'm not sure that's something Ken Smith intended or anticipated.  Also, the keel, is only one factor.  The deep forefoot of a typical Grand Banks hull doesn't provide a lot of lift and especially for the vessel with the GB's Displacement to Length ratio.

I'm sure the real engineers will chime in and show me where I may have drifted off course.

Bob
GB 36-790
----- Original Message -----
The basic Grand Banks configuration-- the look
of the cabin, the sheer of the hull, etc.--. is taken from working
trawlers, but not their underwater hull shape.  The working trawlers
from which the Grand Banks aesthetic design was derived are
full-displacement boats...

...  Unlike a working trawler or fishing
boat, the Grand Banks has a "semi-displacement" hull, which is probably
a better hull for marketing to the recreational boater than a
full-displacement boat as it can be driven faster than hull speed.

I have read arguments by people with the credentials to back them up
that a semi-displacement hull is really not semi-displacement at all,
but semi-planing.

That's not correct. You can overpower a full displacement boat so much it
will basically submarine and sink. It will never plane; it's impossible.

Keith
__
A man usually feels better after a few winks, especially if she winks back.
----- Original Message -----
From: SailC34@aol.com
In theory, so long as it doesn't nose-dive, any hull -- even a "full
displacement" hull can be made to plane if enough power can be applied
without adding too much weight.