34 ft asymmetrical catamaran/tacking outrigger/cataproa

A few comments. The design is 34' LOA 14' BOA and is intended to be a coastal cruiser. The main hull has the overnight accommodations, the mast, the leeboard (daggerboard) and the rudder. The bridgedeck is built on the smaller hull.

The rationale is to lower the cost and build time of a catamaran with sufficient accommodations for a 2-4 people, while maintaining the speed and stability that catamarans are known for. Based on the reading I've done, pointing ability should be sufficiently maintained with a single large daggerboard as opposed to two small ones.

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You will have much lower build time (and thus cost) by building two identical or mirror image hulls than asymmetrical. Esp. if using a build style that requires a mold. You don't actually get any benefit from a small hull until you get down to a true outrigger float like on a proa or trimaran.

 

Thanks. I don't think that I'm terribly worried about that sense of the hulls being asymmetrical. If it's quicker to build two identical hulls, that's fine, to a point. Most of the asymmetry, as I'm sure you noticed, is really above the freeboard (or perhaps more accurately, a foot or two above the waterline). The parts of the hulls that are regularly in contact with the water can be mirror images of one another, or maybe about 90% of the volume for the smaller one. The important parts of the asymmetry are in the distribution of the interior volume, with one hull being dedicated to living quarters and the other hull serving more as a platform for the bridgedeck. Obviously, there's nothing preventing use of the bridgedeck for living space. The goal here is to avoid fitting out two hulls with living space simply for the sake of symmetry. For most people, the majority of their time is spent with just one person/couple, and so the other hull is basically finished "just because".

The other relevant sense of asymmetry is in the control surfaces and the sail. Rather than a biplane rig or a conventional center rig, the idea is to have a single sail (or set of sails) mounted on one hull. Especially in the case where a hull needs to be buried, it can be done where the structure is already substantial, and in a way that would minimize invasiveness in the living quarters. In principle, this should allow the structure to be lighter. But even if it's not lighter per se, it ought to simplify the rigging. A similar situation holds for the daggerboard (or leeboard) and the rudder. A large rudder and large daggerboard (or leeboard) replace the two smaller conventional ones to reduce the number of mount points and hardware that need to be constructed.

 

Neither of those is a particularly valid justification for the costs and effort needed to produce asymetric hulls. Trust me when I say to you that you'll be better off with some "wasted" volume in one hull or the other (you will figure a way to use it). Material amount/costs are also relatively minor and shouldn't be a consideration. Even a minor deviation in scale or one dimension brings you all of the headaches of building 2 boats with very little of the benefits of a symmetric cat.

If you want to build an asymmetric cat, fine. But just do it because there is no real "reason" than you want to. I would even go further and suggest you seriously consider a Harryproa. Or at least study the concept. It seems to be what you are asking for, and they have a claimed simplified build system that compensates for the fact that you are building a "boat and a half".

 

I'm familiar with the harryproa, and it's actually in doing the research between the differences between atlantic proas and pacific proas (of which harryproas are a subset) that I became convinced that the idea would work.

As far as the claims to benefits: I think it's something of an empirical claim as to whether there would be real benefits (at least that would outweigh the costs). So, the reason to build it might simply be to test the theory of whether it would indeed simplify construction and reduce cost.

To address what seems to be a misunderstanding: I'm not claiming that the difference in the cost of plywood (for example) of building the above design versus something like a wharram 32' would be substantially (or at all) different. What I'm arguing is that instead of two mediocre living quarters and a bridgedeck squished between them, you'd get one good set of living quarters and a good sized bridgedeck. And rather than thinking of it merely in terms of hull construction, think of it in overall build time: hull, interior and rigging. Let's say that that hull construction is a wash or even slightly longer. Then think of the interior as simply leaving the starboard hull unfinished (no windows, no interior, no hatches), and moving the bridgedeck over to sit on top of it, but leaving it otherwise the same. And finally, rigging. I think the rigging may also be a wash, except that I do think that one rudder and daggerboard is going to be easier and simpler than two. There's an Aussie who even converted his twin daggerboard to a single larger daggerboard because of the added simplicity and better pointing upwind.

Here's the link to his description of his research into daggerboards while trying to fix his cat's ability to go to windward:

 

Yeah, I figured what you are after. But it won't simply construction or costs. It will increase it, not the least because you aren't using someone else's plans or even the conventional solutions to the problems of boat design. There is a bit of engineering and marine architecture to this question beyond just laying out on a sheet of paper. Drag ratios and balance etc. etc. What you are asking to do is not simple.

 

Of course the old, oft forgotten problem of insuring an amatuer designed large boat.
You can't go to marinas, dock at controlled jettys without a couple of million in third party insurance.
If you are brave, you will "self insure" all your hard work and expense. The harder thing to be brave about is your concern for passenger safety based on your "gut feel" engineering, A 34' ft overnighter is going to be exposed to potentially dangerous seas and winds.

Having an engineered design, especially one that has been proven in production, is worth a lot. Can you calculate righting moments and beam load s for example ?

Purely as an aside, I really don't see what benefits in performance you are going to get by this design. You certainly have double the work that a HarryProa would require.

 

Here's a boat that's really similar to the one I drew:

 

What gets me is why they have a weather proof cabin when you have to be out in the wind and spray to sail the thing.
Are you supposed to heave to in inclement weather, or suffer from exposure when you have to be out on the windward hull ?

It defies logic.

 

? How is that different from any other sailboat when you are on the helm or trimming sails? The only difference is in configuration, amidship centric and across the beams/ tramps vs. at the stern or scampering around fore and aft on a monohull.

 

The point is that it's not different in any bad way. The ways that it's different are that it was simpler to construct than a normal 30 ft. catamaran or trimaran (only one rudder, one keel, etc.), but more stable than a monohull, both while underway, and moored/at anchor. I'm not sure if his lady friend (wife?) is in this video, but there are multiple shots of her reading down below while underway, and of them grilling on the ama.

 

The obvious answer to your question is sleeping. If the idea is to have a boat you can sail around, then anchor and sleep, then not having a fully enclosed cockpit is fine. There's a big difference between wearing foul weather gear for a few hours in a pinch and sleeping in the rain. It also seems your critique to just about every other boat this size that isn't specifically designed to be sailed in foul weather. The Farrier F-32 and Wharram Tiki 31 are both open cockpit designs with overnight accommodations. People don't seem to complain too much about having to be "out in the wind and the spray" to sail them. I mean, I don't necessarily disagree. If I were building for myself, I think I'd include a harryproa style pod to use as a bridgedeck. But then, I think I'd like to take longer trips than Arpex was built for. From the videos he's posted, it seems perfectly functional for day and overnight trips.

 

Don't you have to shunt with a proa and need two bows? Or is this not that important if the second hull is big enough to not get submerged too much.

As a total beginner I really like the harry proa concept with the unstayed mast and the type of sail. It's just immediately obvious how all of this works. And fixing the mast on the vaka should be easier than for a beach cat. I think I'm going to build a small proa in stitch and glue as a beginner project (no coastal cruising though).

Regarding the mold for a proa, afaik the plan for intelligent infusion molding is to make one single quarter mold and use that eight times, for top bottom and both ends and ALSO for long and short hull by just adding a straight section to the longer ama. The hull shape doesn't make much difference in this case. Of course you could do the same thing for catamarans as well. But a proa doesn't have to be longer build time.

Also I think in fairness you should compare a proa with a somewhat smaller catamaran with same displacement and material cost. Then wave resistance should make the longer proa faster, correct?

 

Proas only make sense if they are symmetrical fore and aft. In that case you can shunt which is what makes them efficient. The small hull is always to windward and flies which reduces resistance. Otherwise it will have a somewhat good and a very bad tack. One of the main disadvantages of a proa is that it tends to round up because of the difference resistance of the hulls. In short, maybe dsigned wants a boat that looks different. If you are ready to put up with all the vices and disadvantages then go for it. However, if you ask for the opinion of most experienced people, it will not be in favor of the idea.

 

What you propose is called a tacking outrigger and it is related more to the trimaran not the catamaran. In a cat the displacement is distributed equally over the two hulls. An asymetric catamaran would have two identical hulls, only the rig offset to one hull. This brings no benefit that I know of other then the ability to use an unstayed rig, and most cats doing that are actually biplane rigged.

A tacking outrigger behaves like a tri regarding stability, it heels. On one tack it pushes the ama down, so you have to have enough displacement in the right places, on the other tack it lifts the ama, so you need enough weight to not go over.
Having the bridgedeck on the outrigger is a great ideea on one tack, and a PITA on the other.

The advantage of less material and workhours for building the hulls is relative. It only saves one outrigger and 1/3 to 1/2 of the beams. That is not very much in the overall time and budget for a vessel having systems and overnight accomodations. A small daysailer benefits more from it.

Like for anything else there is a design for sale even for big tacking outriggers:
The 50 feet one with accomodations: NG YACHT DESIGN | Nicolas GRUET Architecte Naval | Expert Maritime: Multicoques voile http://www.ngyachtdesign.com/multicoques_voile.php?id=56
The 30 feet daysailer:
NG YACHT DESIGN | Nicolas GRUET Architecte Naval | Expert Maritime: Multicoques voile http://www.ngyachtdesign.com/multicoques_voile.php?id=68