Light weight big open deck catamaran

Our old TRT GT may be close but only 12m. The LWL:BWL is almost 14:1, lightweight is 6700lbs. with a working SA of about 930 sg ft. We lived aboard her for 5.5 years so she was loaded for cruising yet never passed by another sailboat. The construction to me is also quite impressive.

 

Rhea comes pretty close although I would have to modify it a lot. I want tiller steering, transom hung kick up rudders for simplicity and durability in case of collision and ease of access if something goes wrong. I really don't like vertical dagger boards, they are collision and breakdown waiting to happen. Even if hull survive you still end up with broken board maybe jammed firmly in the case and potentially have to make a new one in some middle of nowhere. My 9.5 m cat has single pivoting centerboard under mast beam that is almost impossible to break by hitting something, it just pivots up. At 15 m scale it may not be best solution because unsupported span may be too long. So I consider pivoting boards inside hulls, I think some Gunboats have such arrangement.

I more kind of think building long and simple instead of big. Empty hull length is fairly quick to make and extra waterline make better faster boat. At 15m length one hull has plenty of space, other can be left with no interior fitout and be used for storage saving on need to build lots of storage lockers on deck. Real time consumer is all the little bits wiring, plumbing, hatches, windows, fittings, yacht grade finish, interior fitout and so on. To save lot of time make only what is essential like that Ontong Java.

 

SMJ does look very fine indeed!
Would you like to start a new thread on this forum and tell us a bit more about her please, including some more photos?

 

Nice boat. I'm a bit curious why there are so few similar boats in a bit bigger size range. If you have accommodation in hulls only then longer boat works much better. My DIY 9.5m cat has fairly cramped hull interior space, if I fattened it up keeping existing length it would ruin performance. Scaling it by factor of 1.5 would solve the space issue and produce better faster boat although more expensive, but can't have everything.
Sailboats must be fun to sail and these heavy floating apartment style catamarans that there are plenty on second hand market are as exciting as cargo barge with mast planted on.

 

Daycharter boats is the niche you are looking for, not cruisers, and there are plenty of them, but few in plywood.

One example here, 15m, ply on stringers, 6t at max load, minimal interior.
Balta Patrick, Architecte naval, motor yacht, catamarans http://balta.fr/CATP.html
Ask the designer for centerboards and kick-up rudders, I am sure he can accommodate your wishes.

 

Mulkai. Another thought. The Alfresco 430. The 430 is 43 x 26 foot displacing 15400 lbs and carrying a 573 square foot main and 418 square foot genoa. The hull length to beam is 9.5:1. The Alfresco 430, Bossanova, built in NZ, gained a reputation as a fast cruiser but occasionally was wet, as water went over the bow, pass the main crossbeam and down into open entrance hatches. Tony Grainger has lost the plans for this boat but basically it is a WRC glass hulls with plywood bulkheads and under wing. The decks were ply foam ply which was the fashion in NZ in the late 90's. During the initial test sail in NZ in 1997 Bossanover did 17 knots reaching in 25 knots of wind and 8.5 knots upwind under a full rig with 9 people on board. The accommodation has a wrap around seat that can sit 10 people and the galley etc is large and practical. The Alfresco 430 then morphed into the G430 full bridge deck cabin cat that basically had the same hulls but more bridge accommodation, slightly more displacement and sail area.

Now if you are interested Tony Grainger may still be able to "find" the plans. There was a write up in a Boating NZ magazine years ago which may still be on the web. The jpegs give the idea.

 

Thanks, that's nice boat, will try to contact designer. With some modifications it could be developed into great minimalistic fast long range cruiser. It is also pretty close to what I came up after messing a bit with Freeship.

 

"LAR keels fitted which increases the size of the boat and means much wasted time climbing ladders "

"I want tiller steering, transom hung kick up rudders for simplicity and durability in case"

Ask. Rhea can by hung kick up rudders or LAR. 44' 6500 kg. ?

 

Ply and epoxy is too expensive in Australia now. Epoxy resin price is absolutely ridiculous. Ply is almost as expensive as foam but vynilester and polyester resins are far cheaper here. Per square metre the equation is now in favour of foam. The added benefit of no rot is your free bonus. Much better resale value as well. You could use a very low quality Ply full of voids to tip the price back in favour of Ply Epoxy construction but you then have to contend with structural problems as well as rot. Building is also faster and much easier with the esters. You can premix big tubs of bog, fillet mix etc. No peel ply or amine blush with unwaxed resins, flowcoat finish etc. I have used both systems for decades and the esters win hands down now with pricing, ease of build, maintenance and resale.

 

I think cost of plywood is very location dependent. Here I can buy locally made quality birch plywood suitable for marine use directly from factory for as little as 15 Euro per 1m2 for 9 mm sheet. It is also available in sheet sizes up to 2x4 m. 20 mm PVC foam are somewhere 60 - 80 Euro per m2 and it needs more fiberglass too although being rot proof is nice bonus.
RIGA Raw Birch Plywood https://www.finieris.com/en/products/plywood/raw-plywood
Epoxy is expensive, but in plywood boat you need relatively little compared to foam glass boat of similar size.

It is often said that hull cost is little fraction of total boat cost, but since I plan very minimal interior fit out and will use second hand mast, sails and deck hardware I think in my case hull cost will be much larger fraction compared to a similar boat made with everything brand new and finished to showroom standards.

 

Yes there are different prices depending on your location. 9mm ply to 20mm foam is a big difference in thickness. Is this because you do a lot of framing and shorter spans between bulkheads. From my experience in Australia the ply guys use pretty much the same laminates as the foam guys. I think the main reason for this behaviour is to prevent damage to the laminate which would allow water into the ply. Say a 300 gm2 double bias might do the job structurally it would be too light to withstand the rigours of boat life. On a fifteen metre boat I imagine we would be closer to 800gm2 biax etc

 

Yes, generally plywood boats have more frames and stringers than foam glass boats. I think it is because it is easy to have thicker foam glass panels with little weight penalty resulting in stiffer structure. I guess if you made plywood boat from 15 - 20 mm plywood sheets it also could get away with little framing, but would be very heavy. Ply epoxy boat could be made with no fiberglass covering at all, plywood is plenty strong but it would absorb water and start rotting fairly quickly.

My 9.5 m foam glass cat have 25 mm foam and it only have bulkheads where connecting beams attach, no stringers. I used 25 mm highest density I could get XPS foam and it works fine, 8 years and no delamination anywhere. However for larger heavier boat I think this type of foam would be too weak. Marine grade foams are around 80 kg/m3 or more while XPS foam is around 40 - 50 kg/m3 and have much less shear strength.
This is my DIY cat.
Here a video of it sailing.

 

European Birch ply is really nice stuff but it is very heavy, much heavier than Meranti or Douglas fir which are much heavier than Okoume/Gaboon. Not what i would want to use to build a lightweight multihull.

 

What makes it so nice?

 

To me the European made birch plywood is, as far as construction goes, is about the closest thing i have seen to BS 1088 marine plywood, Far, far better than the garbage made in North America. I have used Baltic Birch plywood on various non boat related projects , i like the light color and grain of birch a lot and really appreciate that it has lots of plies, exterior adhesive, lack of voids, same species all the way through. just nice quality stuff. Just too heavy for boat work imho.