Open source 12-15m high performance/semi-cruising catamaran design

Always a personal calculation, is the extra speed worth the extra risk? How many skippers would fly a hull at night unless in a race or survival situation?
All foils are retractable and the centerboard is raked aft so the cruiser can take his foot off the accelerator at any time with no penalty.

 

Thats right- you have the option to go fast, you dont have to go fast...

Designing the board to break first is easy and many of the race boats out there have struck objects and broken boards without major hull damage.

Also- the weight summary is still a work in progress remember, its by no means finished so please keep that in mind. I have to add beam tension and compression caps but have not calculated them yet and they're not in there. Theres other things too so its heading towards 4.5 tonne as an early very optimistic estimate... theres still no bimini on this one either and that would be very difficult to live without in reality... ill probably have to add one if it was to be built.

Theres also a structural problem here in that there is no support for the aft cockpit sole. The cockpit floor doesnt have the rear beam to connect it to using the lowering tender idea so i cant run a longitudinal stiffener all the way down the center. It dawned on me this is why the sig45 uses such a big trampoline through there as they cant easily create a stiff floor of such expanse...

 

I am standing on the beach waving goodby to 3800 kgs. Sad, but inevitable I suppose.
My design had a single 25hp outboard centerline mounted in a swing sled so I went to intermediate transverse beams that tied directly into ring frames connection points. My under deck was heavily curved so make a male mold for vacuum bagging that yields a U-channel like beam. Length basically equals the deck span. Pairs of beams are joined back to back to create H-beams. All beams have the same length so the ring frame connection points have to vary in stand-off.

 

got any pics?

 

These beams are from an earlier design with less curvature on the bottom side and top deck curvature to clear water. Top deck is now flat.

 

Yep- need some sort of transverse stiffener to run longitudinals into from the mast beam aft. Unless it hangs under the floor - looks a bit ugly- the only other option i can see is to have a very thick floor.

I wonder how it would go having a thick low density XPS core of say 100mm bonded to skins of higher density PVC core and glass over that... they construct some windsurfer boards in this manner i think?

 

Key design problem with that "bulk" structure would seem to be connecting the deck to the demi-hulls. The basic build strategy is coving and taping. If you can find a way to preventing separation of the skins from XPS at the side edges of the deck due to hull loading, might be OK. I am not so confident that its possible to do this efficiently (light weight). Your basic solid deck is not a problem except where you have to transfer loads into the hulls?

 

The floor is just a floor to walk on - its not really part of the global structure which is all designed into the 2 main beams and it could just as easily be a trampoline or a peice of swiss cheese. The floor doesnt even need to tie into the hulls at all strictly speaking.

The floor simply needs to be stiff or it will feel really springy and cheap and you might aswell have a trampoline instead...

Im seriously considering using XPS for the interior floors - make a sandwich from 4.5mm ply skins and 25mm XPS - ive done this with a table and it was stiff as concrete. Adding the resin coat with masked painted lines and it looks like a luxury teak floor built for pennies on the dollar!
This is what it looks like - pic from my previous boat;


Structurally - i dont see a big problem - theres too much paranoia on this subjuct - they are using XPS for campervans, truck bodies, strucutural insulated panels etc The stress in the core reduces dramatically as the thickness increases. So whilst its not real suitable for a relatively thin core in a highly loaded area such as a hull panel, its utility grows as thickness increases. Core to skin adhesion is not a problem when using PVC outer core on the XPS inner core - like i said before - i believe this method has been used for a long time on windsurfer boards now...

 

This shows a 5mm divinycell deck, heat bent before its glued to a PU or XPS inner core

 

Let me run through my thoughts.
Top deck cannot be "soft"
Top deck must accept reasonable levels of impact - dive bottles anyone? but also outboard motor?
Bottom skin must accept wave impact.
So it seems that the deck is becoming seriously strong even if it is not used as a strength member.
Why waste that material/weight? Why not use it as a strength member?

 

Impact - that is why you have the PVC foam over the XPS. XPS alone under a thin layer of glass would dent if you jumped on it - this is why they use a thin layer on the high end windsurfer decks. They dont really need it, it just makes them more durable and dent resistant when used under very lightweight laminates.

Your not wasting anything because strength is not whats required - whats required is stiffness! You cant acheive anything structurally significant in a catamaran global connective structure with a thin flat deck, thus this is best left to the main beams. The floor only needs to support local loads ie people and equipment etc and the trick to that is getting it stiff so it doesnt feel springy. If its stiff enough - it will be an order of magnitude strong enough as per the engineering design formulas.

 

If the deck is taken as a stressed member then the fore and aft beams and their tie-in structures can be lighter?
but, your design

 

Ok so how will you acheive bending and tortion stiffness from such a thin floor structure? Even with a series of I or H beams- have you calculated their properties? I suggest you do to see where it ends up compared to the main beams which have a section depth of some 500-700mm.
Then account for the material costs and weights and complexity to build. I think youll find its more efficient use of material to put it in 2 larger sections rather than many small ones...

 

Early open bridgedeck cats used aluminum pipes as cross members - problem was tying them into the hulls. Lots of hull reinforcements, bolts working on the aluminum.
The deck is not the primary structure. It works together with the fwd and aft beams. The beams provide the vertical structure depth and the deck provides the horizontal structure depth. Since the deck/hull connections are extremely long the stresses are low. Well, that's my image of it.

 

ok i see what your saying UOS - when you say horizontal structure depth you mean horizontal bracing correct? This stops the hulls moving forward and aft with respect one another. But that is not the problem im trying to solve - the problem is vertical deflection when people are walking on the bridgdeck floor. Even at 100mm thickness- there is still too much deflection according to plate theory calculation i did on the floor panel. The floor will feel too springy even at 100mm thick. I need to add in some more beam structure somewhere - probably of around 300mm depth then i can tie that to the front beam and make a stiffening grid on the underside of the floor plate. The arrangement of where it is to go is bugging me - it stuffs up the tender idea and breaks up the otherwise wide open deck space which would feel nice unless i create a beam around the tender/platform - which is where this is likely heading...