Beam design for a demountable catamaran

some doubling but mast loads will need to be carried regardless
thee seem to be main resistance structures in the cuddy

 

Please don't take what follows as criticism or shooting your ideas down. I'm grateful for your thoughts.

I thought about this. It's a good idea but with some issues.

Remember both of you that the structure has to take vertical bending as well as horizontal parallelogram-ing and torsion. So if a wave hits the bottom of one hull you get a vertical moment through that hull.

I wanted standing headroom in the hulls. To get that and retain good access forward is a bit of a trick. I guess you could make the hull cabins flat on top and walk forward over them, but Mr Woods cuddy cabin boats have those nice walkways on the bridgedeck. You could incorporate them in that sort of design but it would undermine the box structure.

Another issue is physical access and fairly rapid fitting of your bolts. If you run a series of bolts between the box bridgedeck and the hulls you have to not only be able to physically reach each of them, but get the holes to align well enough to start each thread.

Then finally you have to think long and hard about dissipating the stress in the hulls and bridgedeck. Internal beams across the hulls are well understood. A bridgedeck attachment along one side of the hull means the hull itself has to take all the turning moments. Someone cleverer than me might be confident in their calculations on that but not this little black duck...The advantage of course is no internal beams to duck in the accommodation.

These last 2 are the reason I went back to conventional beams. You can configure for easy access, use a sharpened stick to align the bolt holes, use a bolt and nut so if it gets stripped or cross threaded you can discard and replace, and the hulls are then any conventional hull.

UpOnStands: Do I therefore understand that the scissor attachments become non structural after the bridgdeck is fitted ? If so see above, if not then they still have to take all the loads, which are not small. You can dissipate the load across the length of the beam for the middle joints but the hull connections still have to be very strong.

 

If you are going to try and bolt beams together then we are really talking about demountable and not folding cats. I doubt that you would trail them often. Finding a bolt hole down a beam will take time. One problem is the reverse cycling of loads - back and forth, back and forth. Your bolt holes will elongate. I see a world of pain with lots of complexity. I prefer Richards approach where the beams get loaded in one orientation and not cycled back and forth.

Find a free beach cat and do the trials on it before you go big scale.

cheers

Phil

 

this shows the "scissors" when assembled. the outboard ends are pinned to the hulls. the inboard ends are pinned to each other.
the aft vertical wall of the cuddy cockpit has been removed to show the cuddy compression member blue
the starboard hull has been cut away to show only the receiving structure in the hull dark red.
the short vertical strop lifts the center ends of the scissors to pull the hulls in onto the cuddy

 

I do not use nuts/bolts. There would some registration pins molded into the mating sufaces but the key elements are the tensioned scissors and the mast assembly which pushes down on the cuddy and up on the hull edges.

this shows the scissors in the packaged state

 

guzzis3;793563]Please don't take what follows as criticism or shooting your ideas down. I'm grateful for your thoughts.

Hi Guzzis3, thanks for your concern, though not really worried around the concept but it does seem to have some merit in the application with a bridgdeck style but the size range seems too small for commercialisation. I looked at and was approached 10-12 years ago when the smaller trailable tris where being discussed at length- seemed there was some players already moving that way & the market kind of starting to look cluttered & unfocussed so left them to it... the direction of a small cat looked much better to me as tooling costs were considerably less though the segment I was talking to wanted two hull flying tris, now the bleeding edge just want to fly... the cats we considered would have have nested with a flat segment to the inboard skin, kind of like a buttock line cut above the waterline so they transported "Flat to Flat". We had also done some geometry work for an existing trailerable/demountable tri so the assembly/demounting could take place just using the trailer and derrick attachments etc, with that once the geometry was sorted "as possible" we ended up just making cradles for an existing trailer & the assembly movements done with a small truck mounted crane that was also the tow vehicle. Tow vehicles are a vital aspect as the choice limits the size/weight of the finished package as legally towable so an important consideration.
I'll add some comment within your text with some colour.


I thought about this. It's a good idea but with some issues.

Remember both of you that the structure has to take vertical bending as well as horizontal parallelogram-ing and torsion. So if a wave hits the bottom of one hull you get a vertical moment through that hull.
I think the vertical component easily delt with, if the bolts/wires parralell bulkheads & fitted adjacent deck head and underwing at say three locations such as mast beam, companionway and aft/cockpit beam, the horizontal very well catered for by the sheerplane of the underwing/sole, the torsion- well thats the engineering end but the deeper the center module the better

I wanted standing headroom in the hulls. To get that and retain good access forward is a bit of a trick. I guess you could make the hull cabins flat on top and walk forward over them, but Mr Woods cuddy cabin boats have those nice walkways on the bridgedeck. You could incorporate them in that sort of design but it would undermine the box structure.
Agree though a reasonable example in that size range with forward access over the hull portion would have been the Beach marine Macro style so not impossible but a little exposed without the complication of guardrails

Another issue is physical access and fairly rapid fitting of your bolts. If you run a series of bolts between the box bridgedeck and the hulls you have to not only be able to physically reach each of them, but get the holes to align well enough to start each thread.
Here I was more thinking of using some tapered alignment nodes similar to used in matching tooling flanges with the bolting really going fully port to starboard by wire/rigging screw with strong points closely associated with bulkheads within the hulls using T balls or some other hook/attachment- demounting is a complication!!

Then finally you have to think long and hard about dissipating the stress in the hulls and bridgedeck. Internal beams across the hulls are well understood. A bridgedeck attachment along one side of the hull means the hull itself has to take all the turning moments. Someone cleverer than me might be confident in their calculations on that but not this little black duck...The advantage of course is no internal beams to duck in the accommodation.
Seems somewhat similar to bolts in a typical beam saddle style arrangement, whether it's the hull or deck taking the beam loads these are usually again associated with a bulkhead for that purpose & yep the clever engineers need to look hard at this

These last 2 are the reason I went back to conventional beams. You can configure for easy access, use a sharpened stick to align the bolt holes, use a bolt and nut so if it gets stripped or cross threaded you can discard and replace, and the hulls are then any conventional hull.
Agree, your typical alu extrusion is a pretty reliable bet & anodised the direction I would look at on a production /cost basis

UpOnStands: Do I therefore understand that the scissor attachments become non structural after the bridgdeck is fitted ? If so see above, if not then they still have to take all the loads, which are not small. You can dissipate the load across the length of the beam for the middle joints but the hull connections still have to be very strong.[/QUOTE]

It's an interesting area, I've got a Seawind 24 in that size range & have dissasembled it which took some time. I'd love to see something quick & easy in that market but there doesn't really seem to be the interest required, maybe where I'm at everyone seems too stretched for dollars and time... maybe one with a queen/king bed, lounge room & comfy cockpit might be the thing.

All the best from Jeff

 

Ok we seem to have met some cross purposes here so lets sort some stuff.

Yes I'm talking about a demountabe cat and while I don't intend to trailer it often the whole point of my idea was to make the assembly and disassembly quick and easy.

The holes will elongate with shear forces but beams mostly deal with bending and torsion. It should be easy to engineer the flanges to cope with that. The pivots and securing bolts on sango have to cope with the torque through the beam.

UpOnStands: I think catsketcher's comment about nuts and bolts was aimed at my idea.

I have looked at your post about the scissor beams. I think I understand what you've said there but it does not explain to me how you are resolving the forces in the structure. For example imagine a wave comes across and hits one hull. The area below the waterline tries to move sideways while the other hull is still stationary. This tries to bend the bridgedeck. What part of your structure takes this force ? Does your cabin attach rigidly to the hulls and stiffen the structure that way or does the hinges between the scissor beams and hulls take the force ? It looks like your relying on the rig to provide some of the mounting forces, in lieu of bolts. What happens to your boat in a storm if you break your mast or a shroud and your 2 miles off shore ? Because that's when you'll lose a rig, not a nice sunny day with flat water and gentle breezes...

waikikin: I understand why you've chosen to answer in a different colour but the green is very hard to read. Maybe another choice

I'm not looking at commercial ideas. This boat is for me.

Yeah I thought about that. A normal hull from the bridgedeck down and from the connection up a rectangle. It'd give big lockers forward, nice flat deck area to walk on and be easy to build in flat panel as a one off. The concern I had was you'd have to use threaded inserts and strip one or cross thread and suddenly your in a world of pain. So I started thinking about flanges or similar out in space with simple cheap chuckable bolts and nuts. I'm not a huge fan of reinventing the wheel. Aluminium beams are well understood, it's just getting the connections right.

I had thought of a vertical dovetail as connections pinned in place by a bolt. The trouble with tapers of any sort is they can and will bind in service and be absolute pigs to break to get it back on the trailer.

Yes it is doable and I don't hate full bridgedeck cats, but then I'd be thinking about stauntions...or at least clipping on every time I went forward. Doable of course.

Yeah getting into that defeats the point of this excercise for me. That would be far too complicated and time consuming. Tensioning rigging screws... tangled wires...

If you read my first post the seawind 24 mast beam was part of my origional inspiration. As you say trailering a S24 is a pain and it's that trouble I was trying to overcome. Detaching the beams from the hulls isn't the problem. It's the fact that you take the bridgedeck apart and have to manually stack everything on your trailer. Just buckets of work. If you split the beams and make a trailer that reliably moves the hulls in and out plus lifts the bridgedeck up with a winch in one piece you've built a very different animal.

There is a seawind 24 for $5k up the coast. I suppose I could buy that chop up the beams build a tailer and see if it works but I was thinking more of buying or building a 26' cat with headroom doing the experiment and if it fails I could just replace the beams (or sleeve them) and put it on a mooring. The trouble with the s24 is even if it works I don't have a boat at the end of it that I'd want to sail anywhere....

 

The cuddy is built basically as a normal bridge deck structure. It resists all racking loads of hull movement and locks the hulls into place.
The loads are transferred from hull to cuddy to hull by the surfaces and registration pins.
The compression loads are fairly easy to design for and build as the area for transfer loads is quite large, the wracking/torque loads are more difficult as the area of loading is much smaller.
The tensile loads are damn difficult. Building the registration pins and passing their loads into the structure would need care.
The cuddy could actually have a dovetail flare on the outboard edges to "key into the hulls but releasing/demounting the cuddy might become more difficult.
This image shows the structure for raising/lowering the cuddy.
The hull cradles slide out sideways to allow cuddy insertion.
So, at the ramp. raise cuddy slight to free the hull. Slide the hulls out. Lower the cuddy. Lash the fore and aft scissors to the cuddy and tension.
The mast with all stays attached is raised. The butt of the mast slides in from aft and the backstays are retensioned; the boom is used to lower/raise the mast.
The stanchions and safety lines can remain in place. But care would be needed not to damaged during transit.

 

I've got it now. Yes very clever. I'm sorry I took so long to understand.

I think torque will be fine. Vertical bending maybe, depends on the details of your cuddy to hulls connection. It's really a bridgedeck cat. If you added an opening in each side of the cuddy and corresponding openings in the hulls you could move between the 3 spaces.

There are some details to think about and I still think on a 7 meter length it will be a bit tall but it definitely has potential.

 

have them, I call them mailslots 1.8 m long and 0.66 m high
not a dignified entry of course.
this shows port hull with outer skin translucent. The cuddy is ready to be dropped into position.

 

Yes I see them now. Good!

 

was watching the launching of a Janus Optimus Demountable cat, here https://www.youtube.com/watch?v=Ua4U8aer2ro
and your original idea of making the trailer the assembly/launch platform is key to making everything simpler and quicker.
the mast/stays must remain tied to the chain plates (dyneema to keep it simple), only the backstays are released enough to allow the mast to be dropped. The trailer moves the hulls apart and into position and mast remains on the cuddy.
I noticed that the Janus crew had a dedicated A-frame for mast raising. Boom is not long enough? strong enough?

 

There are two problems with mast raising, one is weight the other is tilting to the side. If you can raise your mast by hand then you can catch it but if your raising with a winch and gin pole any breeze or misalignment and it'll fall off to the side damaging the base,possibly the mast itself and hull into the bargain. Mast raising is a non trivial problem with large trailer sailers of all types.

Yes my idea was essentially to put the whole folding mechanism in the trailer and leave it at the ramp. Once launched the only difference in the boat is the joins in he beams, which is not a trivial difference. Putting a joint in the most important structural element on the boat has to be considered carefully.

There is a lot of merit in designing a bridgedeck cat that assembles something like you've shown. If you think it through and sort the structural problem along with making the spaces inside and out usable it wold be a nice result.

The demountable 8 meter idea is scary. I kind of think the 7 x 2.9 might be worth a go. It's a small project, affordable and the worst that can happen is it is a disaster and I scrap the boat. It's more likely I'll end up with a boat that feels cramped and doesn't sail particularly well, but it might be ok...

 

Just did a quick check
for 9m mast - same as mark I Hirondelle - with side stay chain plates in line with mast base, the side stays slacken by just 7 mm at 50 degrees from vertical.
Should be good enough to prevent the mast falling sideways.

 

Yes Ian Farrier uses a similar system you can see details if you search on his trimarans. If the shroud anchors are inline with the mast base they will support it all the way up.