Cat bulkhead design

Call it "gasket", wear plate, or a density tie. It is a neoprene gasket( I prefer drive belt sheet). It is used to prevent the steel plate from cutting into the wood or fiberglass beam. It is also a sacrificial layer (shear plate) to absorb the load as the beam and the bulkhead moves.

We are still in the "connection method" discussion. A choice to be made. Fallguy ran a thread about this type of connection and was discussed heavily.

In crossbeam design and load calculations, It is covered by DNV and LR rules. Ad Hoc might steer you into crossbeam sizing as published in Professional BoatBuilders magazine.

There is a long way to go. Attached is my sketches in understanding the load applied. Might enlighten you. This is not design "yet". Just a basic understanding.

 

You may want to review a similar question from several years ago with solutions/comments HERE and HERE.

 

@Rx @ Ad Hoc @fallguy
Thank you and your posts, unfortunately I want to make a rather strange, foldable catamaran and unfortunately I have to stick to "montero engineering". Although unfortunately I am not an engineer and I have major shortcomings in this field.
I'm also lazy and no rich , so I have to use non-standard solutions.
I think the screws would hold, but why put any tension on them? Thx fallguy .

 

Unfortunately, solid beams made out of wood have fibers aligned in the longitudinal direction for stiffness. Even built up members will have plywood on the sides (web, for shear) but still lumber for the top and bottom planks whose fibers run longitudinally. If you pierce it, it may split, just like bamboo.

Since it is trying to spread apart at all times and trying to rotate most of the time, and you want to disassemble it after sometime, you have no choice but to strap it.

I presume it is a car deck and you have some decking (plywood perhaps) on top. The following image might help you in finding the "q" (glue line or bolt line) or the bolt spacing you need.

 

Don't despair it isn't so hard. Make two bulkheads that go trough the deck ending flush with the top of the beam. Tab them into the hull before the deck. Build a flange all around the opening between the bulkheads and close it with a flat plate, laminate over the outside to tie it into the bulkheads. Install the rest of the deck and tab it to the bulkheads. If you have hollow beams fill the end with a solid core making sure to taper it so there isn't a hard transition. Wrap the end of the beam in several layers of biax glass. Put the beam between the two bulkheads and drill two holes for the bolts, from one bulkhead to the other. For added security you can install gusstes between the ends of the bulkheads and the deck, right on top of the hull/deck joint.
The engineering is simple: the bulkheads need to be made with a solid and stiff core, at least at the top, plywood is cheapest. The beams also need a strong core, wood or plywood. Alternatively you can do solid glass inserts or high density foams (if you work with polyester and don't want to complicate yourself with wood).
One M20 A4 bolt will probably lift the entire boat several times over, we will be more concerned to have adequate material around the holes in the beam and bulkheads, at least two bolt diameters in all directions. I recommend you don't use an actual bolt but a smooth rod threaded at both ends for nuts.
The added benefit of this arrangement is that the bolts are entirely above the deck and you don't have to worry about sealing or tool access.

 

You cannot engineer the ‘poor man’s solution’ in all cases.

In your boat and my demountable powercat; the most vital connection is this one and here you err in demanding ‘your’ way. RX has done you a great service and so does Rumars, and Ad Hoc.

If you do not apply tension to the bolts; they will be subject to the rotational loads even more and be more prone to fatigue. You are taking a Black Sea cat design and insisting it be used on a local lake instead.

Anyhow, not wanting to beat you up. I share with you a thread where I asked a question, but got a different answer than I wanted; to my benefit.

Remain open minded, or become it.

Shear calc help https://www.boatdesign.net/threads/shear-calc-help.61381/

 

No.
The task is: transport a 9mx5m powercat using a 3.5T EU van.

I also want to transport my wife and small children.

2xMerc60 = 240kg
2xcrossbeams = 140kg
2x7m floats, you tell me (?). 2m extensions for transport only.

Weight limited to the vehicle's payload.

 

You want to put everything inside the van? Or are we talking a trailer behind it? How should the cat look, just two hulls with a net and inflatable chairs or something more elaborate?
All of this doesn't change the basic structure I described for the bulkhead to beam connection.

 

There is a problem in design when constraints cross, but it is always present.

You have a constraint to be under a certain vessel weight.

A vessel of a certain size and speed requires a certain amount of strength in beam and connection. The first constraint does not drive the second. Weight can be considered, but not reduced below a reasonable safety margin.

Have you considered the Woods Skoota 24? I think it may be quite perfect for you and far cheaper. You are getting into unfriendly design moving 60hp engines mounted to hulls. Honestly, I can’t imagine the hassle versus leaving engine on and in the middle. If you wanted to get fancier; you could perhaps swing two small twin engines on the bdeck as well.

How did you see this working?

Cat hulls are typically not stable alone, so you cannot put them in the water and float them and walk out the pieces. For your constraint, you want a folding cat. If you want a far easier build the Skoota 20 is smaller, but could work, only not as nice as the 24.

I mounted a 50hp engine on my skiff by hand. It weight 247 pounds. It is at the peak of what two humans (we had two guys one one side and me the other). I cannot imagine a yard assembly of your design without a crane or a James Bond styled articulating trailer. The irony is I wanted to build such a trailer for my demountable cat and only realized it was nonsense when I hired a crane to help me assemble the boat and two more to get it loaded on a trailer and a travel lift at the marina.

 

No trailer , doka van . I want to fit everything, maybe I'll send the motorcycle by courier.

 

Yeah .
There are many problems, such as the pontoon bridge.
But they are not unsolvable. Dynemaa ropes, pulleys, winches.
I know how much the engines weigh and I don't want to carry them .

 

Short of an inflatable boat, there is not way. You can't fit a larger volume into a smaller one.

 

I'm a bit allergic to anything inflatable. My first build was an inflatable. Maybe inflatable furniture would be a good idea for this project.

It was originally designed as a single outboard for calm rivers and easy loading and unloading. I wanted to transform it into a Mediterranean. And the problems began.

 

I don't really know if it's possible with those engines. You only have about 1200kg to play with for everything you need to carry. If you put the hulls on the roof you need some support structure inside to take the weight, that eats into the payload and you need a crane to get them off. If we segment the hulls matroshka style to fit them in the van we have problems with the connections. If we split the hulls horizontally into a bottom "surfboard" light enough for the roof plus two engine pod extensions we could solve the beam loads with a spaceframe, but that leaves us with solving the side panels watertight connections. With the weight limit the side panels won't really be stronger then inflatable sides wich you don't want.

 

Is this your own design, top to bottom or a modification of an existing design adapted to fit your need? The numbers don't add up.