Designing a fast open deck catamaran

Currently I have 9.5 m long open deck sailing catamaran that I built myself 8 years ago. I have sailed it extensively around Baltic sea and a bit North sea. However hulls are just too cramped for long term living aboard and payload capacity is only around 500 - 600 kg before performance start to suffer. So I have decided I want something similar, but larger that would solve those shortcomings. Basically my existing boat scaled by factor of around 1.5 . Then I get hulls big enough for comfortable living, load capacity of around 3 tons and at the same time great waterline length. Basically a 15 m cat with cabin space and weight of typical 11 - 12 meter fat bridgedeck cabin cat.

One of the main goals would be to keep it simple to minimize build time and cost. Wharram catamarans are fairly close, but I want better faster hull shape with less wetted area and improved structure. Essentially use the same materials one would use to build Wharram cat, but make a better boat. One hull would be living space, other for storage. That arrangement makes things simpler since only one hull needs insulation , heating, wiring, plumbing and livable interrior.

Thoughts of this general concept?

 

Mulkari. A well drawn shape, do you have any displacement, PC over all and PC for forward and aft sections, center of buoyancy etc This will give a better feel for what may be possible in the build process. EG if the displacement is high enough you may be able to build in foam e-glass, if the displacement is small carbon fibre epoxy may be required.

 

Here are parameters for single hull. I'm targeting full displacement of around 9 tons so if I manage 6 tons total empty boat mass I have 3 tons of load capacity. I'd prefer building in plywood if possible since here I can buy marine quality plywood directly from factory at very good prices and wood and plywood are such nice materials to work with.
Comparable Wharram catamarans of similar size and internal volume Pahi 52 and Tiki 46 have listed empty mass of around 5 tons so it should be possible to stay within 6 tons for wood and plywood build.

 

Mulkari. Are you using Hullform as your hull design software? If so there is a second page which shows for and aft PC and the % from the bow of the Centre of Buoyancy. It will help me understand if the hull has its center of buoyancy in from of the center line. To the structure. Plywood of good quality good. 9 mm skin, 12 mm bulkheads backed by 19 x 65 mm framing with stringers. Slight lateral curve in ply will help stiffness. Flat bottom keel line ply 12 mm. Internal keel and external keel will depend on how you do lateral resistance EG fixed keels, daggerboards or lee boards. The cross beams can be box sections with ply fore and aft faces and timber top and bottom flanges but the design will depend on your rigging arrangement and intended beam of the complete cat. This is a viable concept and a possible build to a 5 ton build but it will all depend on the detail design of the structure.

 

12.2m Woodwind Catamaran : Boatcraft Pacific, the home of wooden boat building. https://boatcraft.com.au/Shop/index.php?main_page=product_info&cPath=32_33&products_id=60

 

I use Freeship, I have tried Hullform, but it crashes on startup, apparently there is some compatibility issue with Windows 10. Freeship shows my centre of bouyancy at 6.4 meters from stern. Would 12 mm ply be enough for main bulkheads where crossbeams connect? For lateral resistance I plan to use kick up centerboards. I'm not yet sure if a single kick up board in the middle below mast like my current cat have or 2 boards inside hulls. On boat this big a single board would have very high loads since it is not as well supported as boards inside hulls that are laterally supported at hull bottom and have unsupported span below maybe 1.5 meters. Rudders would be kick up transom hung tiller steered like on most beachcats. That way whole steering system is simple and everything is accessible from above water. No expensive lifting out because some rudder issue. With all boards lifted draft would be around 0,5 - 0,55 m

It would be too small to have all living space only in one hull for long term living aboard. Since I plan to sail a lot in fairly cold places I feel it would be much easier to heat only one hull than hull, bridgedeck cabin and other hull. Also less complexity and build time if only one hull is fitted out for living other is only for storage.

 

John Hitch designed a 50 foot 2 hull cat called "Wired" part of his Hitchhiker range in the 1980's. It is 50 x 36 foot wide with a series of tube crossbeams and a central swinging centre board. (page 35 Multihull Structure Thread). It originally displaced 11,000 lbs but several owners added there own "modifications" including new crossbeams, central cabin and a lot of cruising gear result about 22,000 lbs displacement. John choose to build in standard plywood construction. The idea was to build a cheap light boat that was diagonally very stiff. Hitch hiker hulls and decks are 9mm marine ply, with 50 x 25 mm stringers at 225 mm centrelines on 12 mm plywood bulkheads. The hulls and decks are epoxy saturated, sheathed externally with 6oz above the waterline and 12 oz below. The hull bottom is 37 mm thick. When I spoke to John onboard the cat he said the central swing centre board under the mast worked OK but he was going to have more control over the top of the board as it board could twist a few degrees in either direction when going upwind which meant the cat didn't point as high as it should. This cat could average 10 knots and peak at over 20 knots in its original simple form. The jpegs are of Wired after being sold to a new owner and renamed. The next owner put new box cross beams to allow a deck cabin to be done.

Your hull design is better, you are likely to have a better rig and therefore likely to have better performance. On the structure, I am not a designer and can give some idea of what is possible but detailed design needs to be done by yourself or a professional designer. There are some thread members who are capable of doing cross beam, rig calculations at a near professional level who may help. PS Storer boat design has a version of Hullforms that runs on Windows 10, look at: Hullform 9P Works with Win11 (sometimes) win10 - download links https://www.storerboatplans.com/boat-design/hullform-9p-now-working-with-windows-10-download-links/

 

Way back when I was selling System Three epoxy in Aus I ran a design competition for an innovative boat. From memory, there were 50+ entries, many of them clever and ahead of their time. John visited me with a cardboard model of Wired which was incredibly stiff in torsion. I was not a judge, but if I had been, it would have got my vote. The actual winner was Mi6, a proa designed by a couple of guys who sometimes post here. It would be interesting to run a similar competition now and see what came up.

 

Richard Woods has a design for a 13.7m open deck plywood cat called Ondina. It's design displacement is 7 tons, 4.5 tons empty, so clearly it is possible to build lighter than the 6 tons you were guessing. Ondina plans are either deep V or deep V with a rounded V, so not what you are wishing for.

Personally I would not design my own boat. In the past I would have thought it was a good idea, but I have since realized that in many engineering endeavors you won't even be aware of all the issues that need to be addressed, so there's no hope of designing something that is well optimized. Maybe you'll get lucky, but it's better to hire a designer or build an existing design. Just my 2 cents...

 

That's true, problem is there seemingly aren't stock designs available of what I want. Some are closer than others, but still would need extensive redesigning. For example there are racing catamarans that are similar, but they lack space in hulls and have very low load capacity. My design is kinda similar, but with wider hulls (about 2 meters wide at around shoulder height ). If I stretch to 15 m I get enough space in one hull for all living space to be there. Other hull have large work table and storage. Building hulls generally is quick part so may as well make them longer for better performance and load capacity. When time comes I will get help with calculating structure to get it right.

 

No luck with that Hullform version, it crashes on startup. I may try Delftship to see what it does.
Added more internal frames to my model. Currently I assume centerboards inside hulls, probably galley table on top of centerboard box. At the stern would be bunk space and near the bow another bunk.

 

Actually, Ondina might meet your needs, depending on the actual hull shape on the rounded V version. The hulls appear to be exactly 2m wide, and you could probably stretch them longitudinally by re-spacing the bulkheads. I would imagine the hulls are constructed like strider where the topsides are plywood and just the rounded bottom is cold molded ply (see attached for strider hull method). Seems worth contacting Woods to get more detail on the hull shape, whether it can be stretched, price of the plans etc. He no longer lists plans >40ft because he thinks it's best to stick with smaller boats, but I'm sure they are still for sale.

 

I'm wondering at what point it is better to adapt existing design vs designing from scratch. Looking at Ondina hull shape it appears bow and stern are fairly narrow which means it could suffer from excessive pitching. It looks like Woods have abandoned V shape in his later more modern designs. Currently it seems that the norm in performance cats are narrow bow with V shaped bottom, close to semicircle midsection and fairly wide sterns with relatively flat bottom to discourage pitching It looks like Ondina has rudders under hulls, I want transom hung rudders with tiller steering for simplicity, ease of maintenance and repair so modification needed. Not sure what kind of boards it have, centerboards, daggerboards. I want kick up centerboards to prevent damage when inevitable grounding happens so possibly another modification which could be fairly mayor because board cases often are integrated with load carrying parts. Interior arrangement also may have to be changed because I want everything necessary for living in one hull for 4 people. Depending how interior furniture is integrated with structure it could also lead to structural modifications.

Since I will be building 20 km from nearest suitable water for launch I need design that can be built on build site to maximum completion and then only minor final assembly at the waterside like joining crossbeams with hulls and installing mast.

 

Mulkari. Ondina is a reasonable design but it is over 30 years old. Hull shapes have advanced a lot over that time, even Wharram's have gone from deep V to rounded V to chine hull shapes for performance and ease of build issues. Your proposed hull shape appears to be good, it has the right numbers and a well done plywood build will hit the 11,000 lbs target weight. Do the design, design and draw the structure you think is required then go to a professional to review your work. This will be a lot cheaper than asking someone to do a complete new design. Have a good look at Multihull Structure Thoughts Page 1 index, there is a lot of information about cats around this size but it is spread across many items which will give you a serious guide. Mulkari your requirements are sensible, you have built a boat before, you have access to the build materials and your accommodation needed is simple to suit your needs. Go the next design step.

 

Mulkari. If you are going to a professional designer with a plan make sure you have done a weight study. That is the weight of all the materials in the structure and importantly the weight of the item distance from the centre of the design. EG 8 mm plywood fore deck 60 lbs 18 foot from the centre line. This is the way designers match the centre of bouyancy to the centre of gravity of the structure to know if the trim is correct. For most designers this is a time consuming task.

Just a reminder as I have posted it before. I am not suggesting the Ondina design but the construction specs may be of use. Page 163 Multihull Structure Thoughts thread. Ondina has a 10 mm ply skin with 37 x 25 mm stringers. The keels are multiple layers of 25 mm timber to the depth you require to provide a rounded shape. The secondary bulkheads a 9 mm plywood with 12 mm plywood crossbeam bulkheads. The decks are 8 mm plywood on 50 x 25 mm deck stringers and 75 x 25 mm deck beams. The interior is 6 mm and 8 mm plywood with 8 mm plywood floors and bunks. The cross beams are wooden box beams with a 15 mm plywood web 50 x 50 mm top bottom flange, 15 mm ply web with 75 x 50 mm top and bottom flange, 15 mm ply web with 50 x 50 mm top and bottom flanges then the final 15 mm ply web. The main mast cross beam has 100 x 50 mm centre flanges instead of the 75 x 50 mm top and bottom flanges. All exterior surfaces are covered with 300 gsm glass cloth and epoxy. The entire build should be epoxy saturated as per West system. The plywood is glued and screwed to stringers etc. How many screws? Try 5,000 countersunk brass screws. Hope you have a good portable power screw driver.