Designing a fast open deck catamaran

My build with the gpet foam was to layout full length panels glass the inside and stitch and glue just like a plywood build. Details are a bit different but overall approach is the same. Strip planking requires a lot of interior glassing and other work I've aged out of. Foam panels glass both sides are not going to be bent to shape. Building a 15m layout table is a bit of a hassle but the boat would go together relatively quickly. My much smaller build Proa File | QB - a 21’ cruising schooner proa https://proafile.com/multihull-boats/article/questing-beast-a-new-21-schooner-proa
Best wishes, I went back and looked at your design, looks nice and well suited to your plans.

 

Yeah, it seems with flat panel foam I could basically make hull parts - bottom, sides, cabin top, deck as whole pieces flat on table and them apply them on the bulkheads. It would also allow to get rid of stringers which would save some time although extra time would be needed to laminate fiberglass on the inner side that would be just epoxy coated in case of plywood. Also more fiberglass and resign for foam version which is extra cost and time. And I would still need to add insulation anyway because just 25 mm of structural foam would be too little for good thermal insulation. Overall quickest build solution for my needs probably would be 50 mm structural foam. It would be both insulation and very stiff structure in one, but heavy and expensive even with cheaper Gpet foam. Below waterline would have to be reinforced with multiple layers of fiberglass because just a single or doble layer of 600 g/m2 fiberglass over foam while fine for overall structure is easy to puncture, even kayak hitting it can bash a hole in outer layer easily. Or landing on some gravel at low tide could crush foam cored bottom causing delamination later. Plywood is more durable maybe next best after aluminium although very vulnerable to rot if fiberglass covering gets damaged and not immediately fixed.

That's nice little proa, I was also pondering about proa for my big boat. Harryproas seem to be most developed however big advantage to a cat is I could just reuse a second hand rig and sails from written off monohull to keep costs down while Harryproa rigs are completely custom with custom freestanding carbon masts and custom sails. That's alot of $$$. Initial version of rig for my 9.5 m cat made from second hand parts cost me around 1000 Euro to put together usable rig while ordering everything brand new could easily be around 15k

 

If all your experience is with XPS foam only I'd suggest you get a sample of gpet foam, they are seriously different products, compressive/impact strength of the "structural" foams san, pvc and gpet are considerably higher as well as shear strength and modulus which is important in a sandwich structure. One issue with gpet that I discovered late was the sheets I got for QB were apparently made from extruded strips about 60mm wide bonded together. The previous sheets for a stripper canoe were monolithic. There was no problem with construction but denser edge of the strips printed through the 270 gsm skin over time.

If 50mm of foam is needed for thermal performance it still seems to me that putting it all in the structure makes sense, total weight shouldn't be much different 50mm vrs 25/25 can't be very much different weight wise and a lot of the difference if the inner thermal layer is less dense will be lost in the weight of attachment and then covering same. And at this stage of life I'd go to some lengths to avoid any unnecessary work inside a boat.

Thanks for the compliment, QB is my last camp cruiser proa build, hope to have enough left in me to play with it for a few years. I admire some of the original thought that has gone into Harryproas but the reality of finished working effectively craft is fairly slim. My interest is in a simple lightweight easily driven trailerable camp cruiser suitable for raid type events in shallow water. And I like fiddling with strange things.

 

When I built my 9.5 m cat I had some pieces of light grade around 50 kg/m3 structural foam I tested against similar density XPS foam intended for use in floor insulation in industrial buildings and while structural foam was more durable both foam sandwich test pieces were possible to fairly easily smash with hammer which may be reasonable approximation to boat hitting something. To resist damages from minor impacts and groundings I glued much more fiberglass than would be strictly necessary on the bottom. So far it has worked well and has survived frequent beach landings and some unintended minor groundings on rocks just fine, but at the same time weight savings from having a foam cored bottom have gone too although that is relatively small area compared to whole hull so overall I probably still have decent weight saving compared to solid fiberglass hull.
If I were to build next boat in structural foam sandwich construction I likely would have to use around 100 kg/m3 foam for hull sides and something 150 - 200 kg/m3 on the bottom to have acceptable impact resistance. Birch plywood has density of around 700 kg/m3 so it would still be better in this regard with thinner fiberglass layer although it would require additional insulation inside. Using thick structural foam for hulls would allow to get rid of additional insulation and stringers. Will have to calculate weight of both versions.

 

I've always been curious about the suitability of birch plywood. It used to be cheap here, but now the cost is the same as hydrotek meranti (about $100 USD for 9mm), so I am now less curious. Apparently Francois Vivier used it for his own boat and did some sort of study on it's longevity, but I can't find any info on how that worked out.

"In the interest of conservation, Vivier used birch plywood from Finland for the prototype Pen-Hir’s hull. He also conducted a longevity study on this wood and his method of using it, with promising results"

 

It needs to be epoxy encapsulated for real long term durability like regular marine plywood. When left unprotected in wet environment rot develops in around 7 - 10 years. It is heavier than marine plywood with a density of around 700 kg/m3 , but also stronger. There are some plywood multihulls that were built here 40 years ago using birch plywood and they are still around. Here it is cheapest by far option to build boat structure from. I could just rent a cargo van tomorrow drive to factory outlet and buy all the plywood I need with no middle man involved driving up costs. It is made from locally grown birch trees so my money goes into local forest industry instead of going to China like it most likely would be with structural foam.

 

I've just bought the materials for a C50. The carbon uni for the masts costs $US23 per kg, the infusion resin $US8.56 per kg. The 2 masts weigh about 150 kgs each = 300 kgs. There is about 190 kgs of carbon , 10 kgs of glass ($2.40 per kg) and 100 of resin, so ~$6,500, including the materials for the low cost mould. All plus shipping from China, taxes and duty, about 30-40%. A couple of weeks, maybe a month to build it and the carbon sail track. Second hand sails can easily be recut to suit.

Doesn't compare to your second hand rig, but you don't have to buy a traveller (and controls), any standing rigging, a dolphin striker (maybe a much lighter, or no forebeam), headsail hardware (furlers, winches, sheets, blocks, etc), chainplates or any of the beefing up these require. Also, the unstayed rig is nearly maintenance free, will not require annual checking nor replacing components after 5 or 10 years.

Which is all good, but the cost benefits pale into insignificance compared to the ease of use, safety and low stress aspects. No worrying about the 'angle of death', gybing in big waves, wrestling with flogging headsails or getting caught in irons. You will be able to raise, lower and reef the sails regardless of wind strength or angle, and to dump the sheet on any point of sail and the boat will stop quietly and drift while you reef or have a cuppa and get problems sorted out. You can also easily add a near idiot proof anticapsize fuse.

Glassing both sides of a sheet of foam is far less hassle and time than epoxying ply interiors, especially if there is bloom to remove or any sanding to be done.
Hand laminating boat sized panels uses much more resin, exposes you to more harmful chemicals, takes longer and is not a whole lot of fun. A flat table and infusion is a much better way to go.

Recycled 100 kgs per cu m PET foam is $US30/sq m for 25mm. Plus shipping, etc. It is remarkably tough stuff, but if you are worried about sitting on sharp rocks, maybe make the impact area solid glass, with bulkheads, ring frames or furniture to support it if required. Maybe partial bulkheads above the flooded waterline for each section. Holes in flat panel hulls are way easier to fix while afloat than round or compound curved hulls.

Infused ply is rot (and near bullet) proof.

 

I am very interested in insights to the build process of such a mast - presumably other readers are too.
For me its about personal curiosity, and a latent idea to replace the wooden mast of my 15m²-Jollenkreuzer. (literally translated: 15sqm dinghy cruiser - a locally evolved boats class)

 

Yeah there seem to be a lot of advantages to Harryproa design. The nearly carefree rig is obvious although I wonder how you set downwind sails on it for really light wind sailing. I suppose you could set gennaker from first mast to the bow, but it would apply big loads to top of mast where there is no support. DIY freestanding carbon masts scare me a bit. Carbon is known to fail with no warning if builder screws up something slightly. Probably I'm overthinking this too much.

I wonder how well suited this design is for extended periods in cold climate and more expedition type sailing than typical tropical island hopping. 25 mm foam from my experience is too little so either would need redesign for 50 mm foam (price per m2 would really go up also) or additional layer of insulation foam inside. That big cabin with big windows seems would quickly let out heat.
Mast support hull looks too narrow to house big work table away from living hull. I consider big comfortable work table very important because stuff breaks and it is nice to be able to fix things inside out of rain and cold wind and not bring dirt, chemical smells, oil spills in living area. Hull looks so narrow even storing inside few full size mountain bikes without disassembly would be a struggle. It wouldn't work as walk in storage barn.

What are the total square meters of foam board needed for C50 build? Are there any videos of C50 sailing?

How do you make that? I suppose with infused you mean like it is fully saturated with resign? It sounds like close to ideal material for bottom of a boat.

 

The call of the north speaks through you. It can be merciless there sometimes. I have a lot of thoughts on this enviroment.

 

Yeah some of the most beautiful nature up there. But you are right weather can get quite nasty there, but during summer sailing conditions are mostly acceptable. In Norway there are literally millions of places to shelter if bad weather arrives. A metal monohull is generally preferred type of boat for those places, but I like boats that sail fast. Sailing a barge is no fun. And shallow draft of a catamaran is huge advantage. Some catamarans and fiberglass monohulls have sailed to ice free parts of Antarctic peninsula, southern Greenland, Svalbard so it is doable, just never pretend that you have an icebreaker.

 

Thanks. I tried to eliminate the drawbacks of cats and tris, then figured out how to make the result work. Not perfect yet, but a definite improvement.

You could and it does, although none of the owners so far have felt the need. You 'just' need to be careful about pulling it down if the breeze increases. Visionarry (15m, strip planked, sorry about the video quality and the music) has 70 sq m of sail, does windspeed from 5-15 knots without extras. The C50 has 100 sqm.

Probably. Race boat masts are built to the limit, see an undesigned for load (flogging headsail, poorly tuned stays, deep reefed mains and big headsails, etc) and they break. We use higher safety factors and have far fewer unexpected loads. The masts will bend a huge amount before they break (think fishing rods), giving you plenty of warning and automatically depowering the sail.

It's easier to change details like this on a Harryproa than on some of the cats you are talking about. The windward hull sees no rig or sailing loads, you can modify whatever you wish. Once you have done a couple of Intelligent Infusions, you will be knowledgable enough to alter whatever you want to change. We give all the help you require in this process.
My design partner is building a 20m version north of the Arctic Circle in Norway. Plans to live on it up there for a year or so, then head South. The windward hull is different to the standard to make it easy to heat. No raised cabin, and double glazed (I think) windows in the hull.

It's easy enough to make it wider. The boat will be a little slower, but I doubt you will notice. Will also cost a little more. The designs are not fixed in stone, and I am not arrogant enough to think they are perfect. You want to change stuff, let me know, I'll help all I can.

First page of the plans, with materials is attached.

There are none sailing yet. The C50 is a development of Visionarry (see above). Better hull shapes, finer bows, schooner rig, better rudder and layout arrangements and a bunch of smaller improvements.

You drill 1 mm holes every 20mm, put it under vacuum and suck all the air and moisture out (might take a day or so with birch), then introduce resin. 40 minutes later, the ply is ~50% epoxy. Very tough, but heavy. As is solid glass, which also works well.

Ropf,
We will be videoing the build of the C50 in Fiji later this year. Meanwhile, any questions, feel free to ask.[/QUOTE]

 

Vivier's boat made of birch ply is for sale and seems to be doing well 15 years later.
Pen-Hir, the architect own boat – François Vivier Architecte Naval https://www.vivierboats.com/en/pen-hir-the-architect-own-boat/

 

Interesting, do you know how far the build process is? It is within easy travel distance. I could just cross Baltic sea in car ferry and drive the rest of the way or fly there whichever is more convenient. Mods to make it better hold heat would really interest me. Harryproas are such a weird boats I would really like to see one at least close to finish, preferably sail one too before commiting to build one.

Some more questions.
Windward hull where most of the weight is supposed to be is shorter. What is the reasoning for it? Intuitively it seems it would benefit from being same length. Better length/beam ratio, better load capacity, more internal room.
What is the expected windward performance? It is generally accepted that multi mast rigs are poor performers to windward because of extra drag and bad air caused by mast and sail in front. What kind of WMG numbers could be expected? What would be max wind and sea state where some slow progress to windward could be maintained?
Are there many parts that require custom metal fabrication like rudder fittings, boom fittings?
Rudders/boards seem to be attached only at the very top while having a long lever arm. Could this cause issues under high load situations? I can't wrap my head around how the kick up system is supposed to work both directions.
This design seem to be very weight sensitive so I assume building from plywood would be bad idea?

How wide it is as per plan, and how much more wider would be reasonable?

Thanks, video helped to get better idea.

That's exactly what I would expect. If well cared for it should last decades.

 

Tender boat that fits into a slot in the deck and can propel main boat by dropping rear end down is kinda neat concept, but I wonder how well would it work in waves throwing it up and down, possibly slamming against the deck. I suppose you need fairly calm water to dock it back to main boat. Imagine you arrive back from shore trip and there are 1 m steep waves blown up while you were away. Wouldn't it bash it all together. Nice thing about inflatable boats is they don't get damaged and don't damage main boat when impacting. I suppose you could just build normal deck where tender boat goes and have normal inflatable boat for shore trips. Inflatable boats also are better suited for landing on rocky shores.