Designing a fast open deck catamaran

When considering loads from the rig middle beam going all the way across would help stiffen the structure, mast press down stays pull up at outer hull sides. If middle beam just sit in some notches at inner hull sides then hulls take all the twisting load and transfer it to forward and rear beam. I see more chance for bending and loosing rig tension. I just don't see much of an inconvenience to bend down a head to pass the bulkhead while gaining stiffer structure and a bit of structural redundancy in case of some damage to front beam for very little additional work. What's really annoying is lack of headroom and elbow room in entire hull like in my current boat.

That sounds plausible. I think up to a certain wave size wave piercing hulls would have less drag and pitching, but once waves get big enough to really submarine leeward hull then chance of capsize may increase. Racing boats with wave piercing hulls generally come also with lifting foils which may compensate for reduced reserve buoyancy at the bows. Not sure if your average performance cruiser cat would benefit much from wave piercing bows vs normal bows. There are plenty of old performance cats that sail really well with normal flared bows. Maybe designers thought that most of the sailing is done in light to medium sea state anyway so wave piercing bow has a little advantage overall. Would be nice to sea identical performance cruisers one with normal bows, other with wave piercing bows raced against each other.

 

Carlos - I really like to "see" such a pressure distribution and the nearfield wave pattern around a hull - to calculate the trim moment(s) - or at least getting a impression of them.

Unfortunately, the only CFD code I'm familiar with, is michlet - explicitly NOT giving this values - while claiming to give a good overall net result in terms of ware resistance, by integration over the farfield wave pattern - in the constraints of completely symmetric hulls with no side moving.

So - if somebody knows a beginner-friendly tutorial, how to setup a proper simulation (steady state), in respect of the free surface condition - preferably on a opensource cfd-package like openfoam - a link would be greatly appreciated.

 

So the question remains - how do you want to achieve this?

If you extend the hulls - while maintaining their height and width - the weight increases at least quadratically with their length (if you want to maintain the overall strength of the structure)

The area of the bridge deck increases quadratically with the length when simply scaled - but it also needed stronger support - so you get a weight increase in the third power of the length. The same applies to the rigging.

So - where do you want to start saving weight?

 

Ropf
If I’m not mistaken the weight gains will be made through better use of materials and engineering that production boats rule out due to “economics”
It will be precision built not lowest common denominator built.

 

Ok - if that works, then a beachcat with the interieur of a luxus cruiser ist precisely what i want. ;-)

 

So have you built a few boats ?

 

By not having fat bridgedeck cabin, lavish interior like most charter cats, 5 toilets AC, big diesel generator and what not, but minimalistic and functional. For example mass produced boats often have hull structure built and then interior is inserted as separate part that add tons of weight. I would have in living hull outer plywood with fiberglass cover - 50 mm insulation - thin painted inner plywood. Much lighter. Other hull would have only structure, big work table and storage spaces. Outboard motors instead of heavy diesels. It's certainly possible to build around 15m 5 - 6 ton empty weight plywood boat. Wharram catamarans of same size are one example. Multi 50 trimarans weigh around 3.5 to 4 tons. That is a pure racer. A similar size more cruise friendly boat, but 2 tons heavier with smaller rig seems realistic.

 

So your idea - getting the same living space and loading capacity as a 12m boat on longer hulls, without increasing the weight - is not about scaling up a basically similar concept, but refers to a boat with a full bridgedeck cabin... that's something different than I've understood so far.

Minimalism is of course an option. Since you mention Wharram - his Tiki 46 should be about what you expect in terms of size and weight (and minimalism too). Why not take this design (or another one that you like) as a basis - and specify what you like to be different? I think, this will be more clear and understandable for all of us.

Exactly one, sir. That was nearly 40 years ago, and maybe one can't seriously call it a boat, but it brings a smile to my lips when I think about it.

 

I think I have mentioned it somewhere here, maybe you missed it, but yeah my idea is basically improved version of Wharram Tiki 46. Keep the basic KISS principles, but improve where possible without adding too much complications like better hull shape, longer waterline, better crossbeam design.

There are too much changes that I want, like get rid of prone to pitching deep V hull shape, install kick up centerboards, transom hung kick up rudders, different crossbeam layout, single mast. It's essentially different design more in common with SIG 45 cruiser racer than original Tiki 46 .

 

I added more detail to my model showing possible arrangement of internal structure and interior of one hull used for living. From the back. Under cockpit seat would be 2 bunks, below them sealed floatation compartments. Just forward of cabin entrance a shower and toilet which would also possibly function as wet clothes locker. Then comes galley area with large table containing stove and sink located on top of a kick up centerboard box. In front of mast beam small saloon area with some kind of diesel or wood stove for heating. Then finally a double bunk. Forward of front crossbeam would be floatation compartment with inspection hatch in crossbeam bulkhead.

Does this arrangement seem sensible?

 

Which one of these performed best to windward?

 

Do you find that rot happens from latent moisture in the wood interacting with the screws, or only when extra moisture makes it's way in?

 

Dustman. The later Wharrams are better to windward but still are not up to cats that have deep keels or daggerboards. You choose what you want. Wharrams are seaworthy boats that will not be to uncomfortable upwind but the trade off is high pointing ability with minimum leeway. The really high pointing cats like HH or Outremers can point high and sail fast but at speed (10 knots plus) upwind can throw you around and a lot of owners limit their speed to about 8 knots to get a more comfortable ride. Take your choice.

 

Added main crossbeams, deck, rudders, centerboards, mast, sails, net, bowsprit. Mast 20 m long, total height 22.2 m. After giving every part realistic weight total empty weight around 5.4 tons. Seems to align pretty well to similar size plywood Wharram catamarans. I wonder if I could get away with one centrally mounted around 40 hp outboard, similar arrangement like my current boat have or cat this size would become unmanageable in tight spaces with only one motor. One motor is less weight, less maintenance, only one motor mount to make and cheaper.

 

Nice boat . Are you ever consider double diesel setup ? Is 40hp enough power for it size ?