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InetRoadkill 12-30-2008 06:35 PM

Displacement or plane hull?
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.


stats: LWL: 33.5', LOA: 37', Displacement: 13500, Beam (waterline): 9.7', Beam:11.5', Sloop rigged.

sailingdog 12-30-2008 07:39 PM

Umm... just how much experience do you have designing boats??

Also, where are you getting the numbers for the displacement from??

On a long trip it is probably better to have a displacement hull that will get you there safely, than a planing hull that will pound and break under the stress... If speed is an issue, you should probably be looking at multihulls rather than a monohull.

BTW, I'd highly recommend you read the POST in my signature to help you get the most out of your time here. It has tips on searching sailnet, writing a good post, etc..

InetRoadkill 12-30-2008 09:16 PM

Boat design is new for me. But I do understand structures.

The displacement was calculated using software I wrote. It provides info about displacement, center of buoyancy, stability, etc..

I knew when I posted the question that it was going to be a trigger. Nevertheless, I was hoping to gain some practical insight from experienced sailors over the two types of hulls as opposed to just blindly relying on theory. Since this is the Sailboat Design and Construction forum, I thought that someone might be able to provide some discussion over the pros and cons of having a hybrid hull. I imagine it's been tried before. But I'd rather not re-invent the wheel if it doesn't work well.

Faster 12-30-2008 09:40 PM


Originally Posted by InetRoadkill (Post 424631)
stats: LWL: 33.5', LOA: 37', Displacement: 13500, Beam (waterline): 9.7', Beam:11.5', Sloop rigged.

These numbers indicate a relatively slender boat by today's standards, and clearly on the light side for a 37' cruising boat. Depending on the sections she could be a very nice upwind boat despite being perhaps a bit tender.. However without info on keel configuration, ballast ratio and hull form all of that is simply guessing.

For offshore work my fear would be the stress (mental and mechanical) of travelling rather quickly in large seas. Such boats require constant attention to steering through the waves to avoid piling into the back of a wave you're travelling faster than. Not a recipe for a comfortable ride in a storm. We have a fair bit of experience with a (smaller) planing hull and can assure you that no autohelm or windvane is going to keep such a boat upright in a seaway (I know - you're thinking "what about Open 60s?" - but you're not talking that kind of league)

A skinny lightweight boat such as you describe also will not have significant storage for stores, gear, safety equipment etc that a heavier vessel will.. another consideration for longer passages.

The fast vs slow debate has been going on for ages.. all have their proponents for all their own reasons.

But I do have to add that setting out on a self designed boat built through self designed software is a daring proposition - one I'd give a few years' trials before setting out for keeps.

Nevertheless, best of luck!

chucklesR 12-30-2008 10:09 PM

At 9.7 beam and 13,500 pounds, it's not going to plane with anything but say 1000 hp of motor (too lazy to do the math), and that's if it's a planing hull shape.
Getting up on a plane is both shape and sheer driving force. At 9.7 it can't carry the sail needed to provide the HP to get it up. At least not and stand up to the wind it takes to get the HP.

Catamaran's manage the extra sail a non-serious racing mono hull (read Volvo quality) simply can't stand up to. My little bitty 33.5 footer carries 780 sq ft, more than a lot of 40 footers and all down low with 14 ft of boom and only 47 off the water in a square top main and screecher on a traveler like track forward, riding on two hulls that are 9:1 length to width, pointy as get out with 4 ft max beam per hull back aft and only draw 18 inches. It's like two lasers with a cross beam.

No need to re-invent the wheel by writing software to show that, it's in many a good book. Try David Kerr, Charles Kanter explains it well but is lazy at math like me.
My D/L is up in the mid 20's depending on what sails I'm flying- you want to plane you have to get to at least that, and still have a flat fat bottom to ride it out on.

InetRoadkill 12-31-2008 12:15 AM

The displacement is just a first-order swag based on some similar light-weight designs. The original sketches where around 19000 displacement when I stumbled across an article about the Didi 38 which apparently uses something close to a semi-displacement hull. It supposedly achieved some pretty decent speed. Personally, I am really not all that interested in getting beat up by my boat or have to continually chase the helm, so I'll probably go back to a regular round bottom. I just wanted to explore all my options.

I would love to find some good books on practical design. But the local book stores don't stock any boating books beyond some picture albums. I found several interesting books on Amazon which I'll order in the near future.

I'm not worried about the software. Hydrostatics is rather straight-forward. I verified it using known hull shapes to check to see that the results matched. I'll continue to crosscheck it along the way. Hydrodynamics on the other hand is a little tougher.

Yes, I know I'm a newb to this at this stage, but I don't go into something like this without a lot of research before becoming committed. It's all on paper at this stage. FYI: You want pucker factor? Try test flying something you built.

Question: Is beam normally measured at the waterline? 99% of the definitions I've read state that beam is measured at the widest part of the boat's structure. The waterline beam measure makes much more engineering sense. Which is correct?

blt2ski 12-31-2008 12:35 AM

As far as beam goes, you will need and want both widths. Waterline for the displacement part, overall for the storage part.

If I had my druthers, I would go with a semi planing, but that is me. All have plus's and minus's in there abilities.


Faster 12-31-2008 12:48 AM


Originally Posted by InetRoadkill (Post 424897)
Question: Is beam normally measured at the waterline? 99% of the definitions I've read state that beam is measured at the widest part of the boat's structure. The waterline beam measure makes much more engineering sense. Which is correct?

Beam stated in brochures etc is pretty much always max beam.. waterline beam is useful to determine hydrodynamic properties of the hull form, and the difference between BWL and BMAX gives one an idea of the flare of the hull design.

Yours is an interesting project and I wish you the best.

InetRoadkill 12-31-2008 01:07 AM

Yep, this is going to be a fun project indeed. I mostly do this stuff for the challenge of doing something new.

I have the hull defined on a 3d modeling program using NURB splines so I can do all sorts of hull shapes rather quickly. The program I wrote can import the model and do the static stability stuff in less than a minute. Unfortunately, it can't do the math for modeling a boat on plane.

sailingdog 12-31-2008 02:06 AM

The problem I see is that the boat isn't really wide enough to take advantage of a planing design, which generally requires a wide flat hull form, at least aft... and at 13,500 lbs., the boat is going to be relatively light in terms of displacement. Unless the boat is made of fairly lightweight, high-tech composites, IMHO, the boat is going to be relatively tender too, but that really depends on the exact hull form, keel design, ballast percentage, etc.... so without more information, you can't really say how it will perform.

The Dudley Dix designs are fairly decent ones from all I've seen... and if he's at 19,000 for his 38' design, I think that a 37' design that is only 13,500 is pretty light.

Good luck, have fun, and keep us posted.

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