Ama to beam connection on trimaran - design help

I'm building a small 18' trimaran. I'd like to make the boat more modular. Rather than having the beams be connected permanently inside the amas, I'd prefer that they bolt on to the tops of the amas.

I believe that the Farrier and others use a system like this. My question is what is the preferred internal structure for the attachment point? Single reinforced bulkhead?

And what type of hardware is used? - by this I mean are holes just put through the top and washers used internally or is it common to use a caged nut that's set in the epoxy?

 

The Malcolm Tennant catamarans used a Grp sleeve around the aluminum cross beams. the cross beam was wrapped in thin plastic, then fibre glassed over then built up in thickness. Someone else may may have more details on it.

 

I'll probably be using GRP beams too. I'd really like to be able to break them down as the two beams connected to the amas permanently, although removable from the boat make for very difficult components to manage.

 

On the Ian Farrier cross beams the Bolts or threads appear to be embeded into the cross beam with access through hatches as well around the deck.

I agree it would be easier to be able to have the crossbeams separate from the float until required

 

On some of the larger trimarans, the beam ends are curved down and tapered. They fit tightly into sockets built into the amas that go almost all the way through from the deck to the outboard bilge. Bolts hold the beams from coming out, but the loads are primarily taken through the contact between the beams and sockets. This might be an approach to consider.

The sockets would be molded to the beams. They would then be glassed into the amas with all three hulls jigged into position and accurately aligned. There is typically a bulkhead at the beam, with a slot in the bulkhead to receive the socket. Then the amas can be removed from the beams and the ama decks installed. This all might be overkill for a small trimaran, however.

I would look at how the beams are attached to beach cats. They typically have a molded saddle and a strap over top of the beam. I'm not sure how torsion loads are transmitted through the joint - that may require some sort of bracket or arm welded/bonded to the beam that bolts to the ama.

My Chris White designed trimaran has the beams permanently attached to the amas. There is a pinned junction between the inboard ends of the beams and the hull. The struts below the beam are also pinned at both ends. Stays from near the outboard ends of the beams to the bow and stern take the fore-aft loads. These hulls are demountable, but don't fold like a Farrier tri. However, when demounted and on its cradle, it does look a lot like a folded Farrier.

 

do you want to have the cross beams and float separate while building, up to final assembly?

In Rays blog on the Ray Kendrick site he has left the cross beam area of the decks open so he could store the floats and cross beams separate, when he builds the main hull for his scarab 32. a similar approach may work for you, both boats follow a similar design approach so I would think the cross beam would be similar.

 

Here are some pics from a performance trimaran build blog... Seems they used a lot of carbon and incorporated the sockets into structural bulkheads.
This is from pt-11 tri build. Looks like a strong fast boat.

 

On a Tri of your size the simplest and best way would be how the crossbeams are attatched across the top of the decks of the Piver Nugget.
Two 2 x 1/16th galvanised steel straps pass through the deck and are permanently bolted to the float hull bulkhead. The ends protruding through the deck are then through bolted to the crossbeams, for easy mounting/dismounting.

 

Thanks for all the replies. I've been super busy and haven't had a chance to respond. Lots of great ideas here. If I get a few minutes today I'll try to draw up a quick version of what I'm thinking about to see if the collective minds here think it's an adequate design.

 

I am building a small 20ft tri based on the Ulua proa design. Basically I increased the main hull beam and length and made it a tri. This is how I am attaching the ama's to the beams on mine.

 

IMHO your big alloy plates are overkill.
If you are going to use 1/4" alloy, four straps 2" wide are all you need.

 

Do you have a link to that build log? I would like to see more contruction photos.

 

I always thought the connection between ama and beam of the M 23 pretty clean.
Couldn't find any decent pictures of it but this video shows the idea:

http://www.youtube.com/watch?v=UD_YHeoZ__Q

 

here you go:

http://www.aquidneckcustom.com/blog/?p=148

 

What are your thoughts of this idea:

The bulkhead could either be honeycomb core or marine plywood, or I could even get some other type of foam core. Then I would probably put two layers of 400gsm double bias along with 1 layer of 400gsm biaxial on each side of the bulkhead and the top plate. The backing plate could be something like 1/4" aluminum or 1/8" stainless strap.

Does this seem reasonable in terms of strength?