Designing a fast open deck catamaran

I would think that foam core construction would make sense in that case, a little thicker core than absolutely necessary structurally would add little extra cost or construction time and you would have a well insulated stiff craft with plenty of floatation.

Mulkari said:

waterbear said:

As for Dazcat, they fit triangular keels and boards. As you can see, there is a fair amount of lateral area between the boards and the keels

Click to expand...

That's interesting design although it adds non removable wet surface area which would add drag in light wind downwind sailing when boards are normally retracted. Would be nice to know reasoning behind that. It is definitely unusual design.

Click to expand...

The boards retraced, the lateral pressure point of the hull(s) is somwhere near the bows - producing weather helm, wich is increasing if the bow(s) deeper dives in. On a deep reach, the wide open mainsail creates a further luffing moment, which makes the whole thing statically and dynamically unstable - especially when a wave pushes from aft.

So its a good idea having some dedicated lateral area well aft the CoG, making the ride mutch more stable and controlable. @CarlosK2 repeatedly point this out in his "How to desingn a modern Yacht" thread, and i think he is right on that topic. Power is nothing without control.

Skip Johnson said:

I would think that foam core construction would make sense in that case, a little thicker core than absolutely necessary structurally would add little extra cost or construction time and you would have a well insulated stiff craft with plenty of floatation.

Click to expand...

It all depends. This will definitely be the fastest boat. But if we consider impact durability again it depends on the type of obstacle. Plywood or plywood covered by GFC is quite impact durable . Most of the boats I build were made of plywood. I even tested one in shallow w.w. river. Few dents and chips. HDPE , alu work better in that conditions.
Most of high lattitude sailors choose aluminium . Going further, there are different ways to build aluminum boats.
How far north/south you (Mulkari) want to sail on plywood cat with an outboard engine ?

He wants to build in plywood.

Mulkari said:

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.

Click to expand...

ropf said:

The boards retraced, the lateral pressure point of the hull(s) is somwhere near the bows - producing weather helm, wich is increasing if the bow(s) deeper dives in. On a deep reach, the wide open mainsail creates a further luffing moment, which makes the whole thing statically and dynamically unstable - especially when a wave pushes from aft.

So its a good idea having some dedicated lateral area well aft the CoG, making the ride mutch more stable and controlable. @CarlosK2 repeatedly point this out in his "How to desingn a modern Yacht" thread, and i think he is right on that topic. Power is nothing without control.

Click to expand...

Interesting, that's certainly non standard solution. Most cats usually have either daggerboards or minikeels, but not both. When sailing and surfing fast downwind even above 15 knots my little 9.5m cat with centerboard retracted have no problem maintaining direction. Far better downwind directional stability than similar size monohull sailboats going downwind at half that speed.

Skip Johnson said:

I would think that foam core construction would make sense in that case, a little thicker core than absolutely necessary structurally would add little extra cost or construction time and you would have a well insulated stiff craft with plenty of floatation.

Click to expand...

It would have to be really thick core. My 9.5 m cat have 25 mm 50 kg/m3 XPS foam core and temperature drops fairly quickly when heater turned off with outside temperature around freezing. I think at least 50 mm insulation would be desirable. Structural foam would be heavy and very expensive in that thickness. XPS foam too weak for 15 m boat.

montero said:

It all depends. This will definitely be the fastest boat. But if we consider impact durability again it depends on the type of obstacle. Plywood or plywood covered by GFC is quite impact durable . Most of the boats I build were made of plywood. I even tested one in shallow w.w. river. Few dents and chips. HDPE , alu work better in that conditions.
Most of high lattitude sailors choose aluminium . Going further, there are different ways to build aluminum boats.
How far north/south you (Mulkari) want to sail on plywood cat with an outboard engine ?

Click to expand...

I would like to go as far north as ice conditions allow. Norway, Svalbard, Iceland, Southern parts of Greenland, Canada.

Mulkari said:

Far better downwind directional stability than similar size monohull sailboats going downwind at half that speed.

Click to expand...

Monos are a different game - by rolling on downwind course the sails produce a permanently changing turning moment. Sailing more monos then multis, maybe i got a wrong bias.

In theory the described effect still exists - in practice maybe a fixed rudder is enough "lateral area aft" to stabilise the direction. Next time on a multi i'll pay attention to this topic ...

I just got back from the plywood store. I checked Ceiba plywood. It's not that light. I don't want to do anything with plywood anymore. Except for details.

montero said:

I just got back from the plywood store. I checked Ceiba plywood. It's not that light. I don't want to do anything with plywood anymore. Except for details.

Click to expand...

Yeah, plywood is heavier, but far more cost effective. Here I can buy quality plywood intended for marine use directly from factory.
https://www.finieris.com/products/riga-shipply/
It is available in sheets up to 2.2 x 4 m. Price per m2 for 9 mm is around 20 Euro. Plywood boat also needs less epoxy and fiberglass. Fiberglass covering is needed only on outside and can be fairly thin I think 300 g/m2 are generally used above waterline, inside can be epoxy coated only so there is saving on epoxy and fiberglass too. Building main boat structure from plywood and wood would save me probably around 15 - 20 k. Enough to buy new mainsail and jib which likely would more than offset maybe 0.5 - 0.7 ton higher weight compared to foam core boat. While not as good as aluminium plywood have better impact durability than foam core unless it has a lot of extra fiberglass layers which would negate weight advantage.

Couple of my boats were bare plywood epoxy and paint. Last one had 200g/mm multiaxial on the bottom and part of freeboard. Epoxy and polyurethane paint .I also made many foam parts and next will be entirely foam . You are not afraid of weight with such dimensions?

jamez said:

I can't remember if I've posted this before, but the late Malcolm Tennant designed a whole bunch of open/podcats in the 12-15 metre range, some more cruisy than others. Getting the plans would be the difficult part, but as you've already achieved a design/build project you might not need to.

Click to expand...

Do you have more pictures of Malcolm Tennant's designs in this kind ? Caught my attention.

Mulkari said:

Building main boat structure from plywood and wood would save me probably around 15 - 20 k. Enough to buy new mainsail and jib

Click to expand...

Why are these rags so expensive? What kind of sail fabric you consider ?

Mulkari said:

It would have to be really thick core. My 9.5 m cat have 25 mm 50 kg/m3 XPS foam core and temperature drops fairly quickly when heater turned off with outside temperature around freezing. I think at least 50 mm insulation would be desirable. Structural foam would be heavy and very expensive in that thickness.

Click to expand...

There is Parabeam https://parabeam.com/ Insulation is one of the applications mentioned in a marine environment.

Mulkari said:

It would have to be really thick core. My 9.5 m cat have 25 mm 50 kg/m3 XPS foam core and temperature drops fairly quickly when heater turned off with outside temperature around freezing. I think at least 50 mm insulation would be desirable. Structural foam would be heavy and very expensive in that thickness. XPS foam too weak for 15 m boat.

Click to expand...

Yes the XPS isn't really optimum structurally for a 15m boat. Corecell (san) and divinycell (pvc) are costly, Gpet foam is quite a bit less expensive and almost as good structurally. I made three side planks for my proa with 25mm gpet foam on top of 65mm XPS covered top and bottom with 750 gsm unidirectional wrapped all around with 600 gsm regular cloth. 0.17m x 1.8m meter planks weigh 3.3 kg each. I hope that I am the last generation saddled with imperial units, metric is much better but there's a lot to unlearn for an old man.

montero said:

Do you have more pictures of Malcolm Tennant's designs in this kind ? Caught my attention.

Click to expand...

Have a look at these both designed as pod cats rather than add-ons. Over 13-14 metres Malcolms racing designs tend to be open wingdeck like the Bladerunner and the cruising boats all have bridgedeck cabins, although some are more 'performance' oriented than others. Wayback Machine https://web.archive.org/web/20010501000000*/www.tennantdesign.co.nz is worth a look.

montero said:

Couple of my boats were bare plywood epoxy and paint. Last one had 200g/mm multiaxial on the bottom and part of freeboard. Epoxy and polyurethane paint .I also made many foam parts and next will be entirely foam . You are not afraid of weight with such dimensions?

Click to expand...

It would be heavier than foam boat, but probably not that much. The whole structure should come out at around 5 tons. Add stuff like engines, sails, water and fuel and it would be around 6 tons which is still very good number. Even 7 tons empty boat would still be pretty much on par with most performance catamarans out there. If aluminum is suitable at this size range then plywood is even better suited from weight perspective.

montero said:

Why are these rags so expensive? What kind of sail fabric you consider ?

Click to expand...

I think it is because you need pretty advanced materials to have mainsail at this size that don't stretch and become too baggy in strong wind when you want to keep it more flat. Then there is a lot of labor too to produce sail. I haven't yet settled on sail fabric, it's really not that important at this stage.

Robert Biegler said:

There is Parabeam https://parabeam.com/ Insulation is one of the applications mentioned in a marine environment.

Click to expand...

That's neat product, kinda like SUP board that becomes rigid after resign is added. Will have to look more into it.

Skip Johnson said:

Yes the XPS isn't really optimum structurally for a 15m boat. Corecell (san) and divinycell (pvc) are costly, Gpet foam is quite a bit less expensive and almost as good structurally. I made three side planks for my proa with 25mm gpet foam on top of 65mm XPS covered top and bottom with 750 gsm unidirectional wrapped all around with 600 gsm regular cloth. 0.17m x 1.8m meter planks weigh 3.3 kg each. I hope that I am the last generation saddled with imperial units, metric is much better but there's a lot to unlearn for an old man.

Click to expand...

Will have to look into that type of foam. What I also like in plywood is flat panel plywood boat is likely fastest to build from scratch. If I replaced sheets of plywood for foam sandwich panels I would have to individually make each panel by cutting foam to size and laminating fiberglass both sides while plywood is mostly just some stitching, jigsawing and gluing together and then fiberglass applied to whole hull only on outside and bilge area inside. Lot's of manhours saved. Biggest drawback is possibility of rot if long term water intrusion happens like for example anchored boat grounds a bit at low tide with no one on board to notice it and a little bit of plywood is exposed and gets noticed only year later when lifted out.