Light weight big open deck catamaran

It is interesting that people will do more framing at greater expense and a lot more work. Not only to initially buy the framing materials and build it but also to fair it all and then put up with the inconvenience of framing by hiding under shelving or inside lockers. Not to mention the difficulty in cleaning and all the crevices for mould to hide in.
We just built a 7.8 metre by 3.5 metre powercat as a retirement boat after we sell our 44ft bob oram sailing catamaran. Unfortunately age and declining health is inevitable and we would like an easier boat to continue our nautical life. The boat is 25mm pvc foam and no framing stringers etc anywhere. All surfaces are smooth painted surfaces with not one single internal moulding or veneer etc. Not only do I have a much simpler quicker build and cheaper build and finish I also have much better resale value.
If you have to pay rent for your build site you also need to add that as extra expense for a more complex and labour intensive build. Not to mention the depletion of your most precious resource of all, your time on planet earth wasted building instead of sailing. All these factors need to be considered when assessing build cost.
We cnc cut the foam and glassed one side before assembly. The precise curves and lines created by the cnc machine translate to extremely fair hulls when assembled. The thickness and stiffness of the foam throws nice fair curves and creates very little final fairing, saving materials and most importantly torture boarding. Please note it is not called "fun " boarding.
Now I know guys who will build boat after boat and never sail them or use them briefly and then sell them And start again. For some the pleasure is in the building process and they tend to be happiest covered in resin and bog dust, these tend to be contented individuals If somewhat "different". For others it's the eternal pursuit of the unicorn, the boat that is faster better stronger cheaper that actually pilots and maintains itself whilst serving caviar and a nicely chilled wine to all aboard before turning down the bed and leaving a lovely after dinner mint on their pillow. It also attracts fish and calms the seas whilst keeping the wind at the perfect speed and angle to maintain 20 knot boat speeds without so much as a ripple on the ocean. Unsurprisingly these people never quite seem to reach that state of fulfilment. The nirvana that the Buddhist monks seek.


Running at 20 knots 4 on board. Twin 30 Suzuki 4 strokes.
 
We just built a 7.8 metre by 3.5 metre powercat as a retirement boat

Your powercat is very impressive!
Could you post some more details and photos of her please?
Is she your design?
I also like very much the video on your YouTube channel of your 3.5 metre cat dinghy - are plans available for her?
 
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.

Yeah it is quality stuff and even quite durable in marine environment without any epoxy coating, My 9.5 m cat has deck platform made from it and it is 8 years old and no rot despite the fact there is no fiberglass covering. I suspect it will probably last another 4 - 5 years and then may need replacing which is easy because plywood deck is just bolted on to aluminum frame. I suspect during production birch plywood gets some anti rot treatment because natural birch wood rots quite quickly.
Looked at density and it is around 700 kg/m3 while okoume is around 500 and meranti around 600 so it is a bit on heavy side, but it is also stronger.
If you look at it from different aspect then aluminum also is used for performance catamarans and hulls usually have 3 - 4 mm thickness. aluminum has density of 2700 kg/m3 and just comparing weight 3 mm aluminum is roughly comparable to 12 mm birch plywood or 16 mm okoume. Aluminum also need extensive framing so it is kinda comparable to plywood hull construction.
 
It is interesting that people will do more framing at greater expense and a lot more work. Not only to initially buy the framing materials and build it but also to fair it all and then put up with the inconvenience of framing by hiding under shelving or inside lockers. Not to mention the difficulty in cleaning and all the crevices for mould to hide in.
We just built a 7.8 metre by 3.5 metre powercat as a retirement boat after we sell our 44ft bob oram sailing catamaran. Unfortunately age and declining health is inevitable and we would like an easier boat to continue our nautical life. The boat is 25mm pvc foam and no framing stringers etc anywhere. All surfaces are smooth painted surfaces with not one single internal moulding or veneer etc. Not only do I have a much simpler quicker build and cheaper build and finish I also have much better resale value.
If you have to pay rent for your build site you also need to add that as extra expense for a more complex and labour intensive build. Not to mention the depletion of your most precious resource of all, your time on planet earth wasted building instead of sailing. All these factors need to be considered when assessing build cost.
We cnc cut the foam and glassed one side before assembly. The precise curves and lines created by the cnc machine translate to extremely fair hulls when assembled. The thickness and stiffness of the foam throws nice fair curves and creates very little final fairing, saving materials and most importantly torture boarding. Please note it is not called "fun " boarding.
Now I know guys who will build boat after boat and never sail them or use them briefly and then sell them And start again. For some the pleasure is in the building process and they tend to be happiest covered in resin and bog dust, these tend to be contented individuals If somewhat "different". For others it's the eternal pursuit of the unicorn, the boat that is faster better stronger cheaper that actually pilots and maintains itself whilst serving caviar and a nicely chilled wine to all aboard before turning down the bed and leaving a lovely after dinner mint on their pillow. It also attracts fish and calms the seas whilst keeping the wind at the perfect speed and angle to maintain 20 knot boat speeds without so much as a ripple on the ocean. Unsurprisingly these people never quite seem to reach that state of fulfilment. The nirvana that the Buddhist monks seek.


Running at 20 knots 4 on board. Twin 30 Suzuki 4 strokes.

There are pros and cons to every material. I personally like working with wood and plywood better than with foam and if I could use infusion to apply fiberglass covering then I could avoid messy hand laminating at least for large surface jobs. Infusion also should make better more consistent surface finish so less fairing and sanding.
 
And that is the most important thing. To be happy!! We only get one shot at it, more apparently if you are Buddhist ( i must remember to look into that ), but for most of us you only get one roll of the dice. If doing something crazy like building the titanic out of tongue depressors floats your boat, pardon the pun, then all power to you my friend. It's way better than sitting in a chair looking at a square screen and being brainwashed.
 
Was looking for specs for aluminum catamarans and found this NEW BUILD - Mumby 48 Performance Alloy Sailing Catamaran - SeaBoats https://www.seaboats.net/new-build-mumby-48-performance-alloy-sailing-catamaran-1543612
Only 6100 kg empty. So a plywood construction of similar scale should end up similar or lighter even if not using lightest possible plywood. Although aluminum has many advantages it probably not the most DIY friendly material. Would be quite some learning curve to do proper marine grade welds. Easy to mess up compared to just glueing together pieces of wood.
 
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