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Displacement or plane hull?

7K views 34 replies 11 participants last post by  Jeff_H 
#1 ·
I know that I'm going to get an earful on this: Which is better for offshore cruising? A displacement hull or a planing hull?

I'm drawing up sketches for a potential build and currently the prismatic coef is 0.65 -- right between a displacement and planing hull. It would be nice to be able to get the hull up on plane, but I'm concerned that it will be a pounder in rougher water and slow in light winds. On the other hand, the potential planing speed sure looks nice when facing a long trip. I suspect that having the pc at 0.65 will yield a hull that sucks at both.

Comments?

stats: LWL: 33.5', LOA: 37', Displacement: 13500, Beam (waterline): 9.7', Beam:11.5', Sloop rigged.
 
#31 · (Edited)
fast or slower?

As an example , take a look at these two similar displacement boats:
Santa Cruz 40 - from the stable started by Bill Lee who coined the phrase "Fast Is Fun"
40' loa
36' lwl
12' beam
7' draft
10,500 disp
5,000 ballast
760 sq ft sail area

and contrast this with:
Cape Dory 30
30'2" loa
22'10" lwl
10,000 disp
4,000 ballast
4'2" draft
 
#32 ·
continued...

As you can see, the Santa Cruz 40 has every spare ounce pared from her construction - this requires very good engineering - no extra weight anywhere - the bulkheads are 3/8" ply without edge trim. It's the price you pay for travelling faster. The Cape Dory 30 (sorry no picture, but easy to find)
is much more traditional and an extra pound here or there is much less of an issue. Space is larger below on the Santa Cruz, you just can't fill it up and have the same performance. As boats get larger, it's easier to have speed and more amenities.
 
#33 ·
A couple things that may be a little misleading in the Santa Cruz vs Cape Dory discussion is that the Santa Cruz is a much earlier generation semi-displacement design, and the newer semi-displacement designs are much more tollerant of weight and are constructed with more modern interior components that look a lot more finished than the 'scan' interiors of the 1970's. It is also my sense is that the Santa Cruz would actually tollerate more payload weight than the Cape Dory as a percentage of their displacement. What is often forgotten is that a boat like the SC40 would be raced with a crew of 8 or 10 in the Transpac race and that this represents as much as 3,000- 4000 lb of crew, gear, and consumables. I have a tough time thinking of a Cape Dory 30 carrying anything like that.

Jeff
 
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