T&T: <Trawlers versus motor yachts

[C. Marin Faure]

 >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