2 metre Catamaran

I’ve mentioned a couple of times that I’m working on a 2metre catamaran and I thought I ought to share a little of it with you.

An obvious starting point was to build the hulls. The lines are derived from the main hull of the Midnight Oil trimaran, designed by Andy McCullough. I used a CAD package to narrow the beam and stretch the height, calculated the displacement and got an experienced builder/designer to check them out. With his seal of approval I started on the plug.

The plug was constructed from cheap plywood on the bread and butter principle. I like ply as the lines enable you to sight along for smoothness and symmetry - attached is an image of the bare plug.

The plug was then finished using epoxy and elbow grease, and the result is shown in a couple of the atachments.

I then used the plug as a male mould; I firstly laid up the top half, removed it and trimmed it and then secured it back onto the plug with waterproof tape. I then laid up the bottom half, overlapping it with the top by about 1cm. When both halves are trimmed they neatly go back together using the overlap to align them and to provide a surface to glue them. By the way, laminating consisted of a layer of peel-ply, a layer of 200g carbon, a layer of 200g glass then another layer of peel-ply. The first layer of peel ply makes it easier to glue in place the internal structure.

In the bare moulded state the hull halves are quite floppy but when glue together they’ll form a tube and be fine and, of course, internal structure will help.

to be continued, but an image of the current build state is attached (sorry about the distracting backdrop).


Wow, That’s a big 'un!!

Looks like the beam is as wide as the hulls are long???

Is there a plan for daggerboards, etc for lateral resistance??

The beam to length ratio is 75% so the crossbeam is 1.5metres long.

I do have plans for fins and rudders and they’re currently under construction.

As the crossbeam has been mentioned perhaps I’ll say a few words about that.

The crossbeam is built around a blue foam core that I had cut with a hot-wire by a chap who specialises on model plane wings. The finished beam bolts to the hulls so it was important to build it properly aligned and true and simply drilling holes through the finished crossbeam wouldn’t be accurate, or strong enough so the key element of the crossbeam build was to incorporate the mounting holes as an intergral part.

The beam was constructed on a straight stiff board that doupled as a jig and a worksurface. The front of the board (which has to be straight) is the datum line (which equates to the leading edge of the crossbeam) and a centreline for the beam is marked at 90deg to the datum line.

The starting point was to accurately drill two pairs of holes into the jig board, each pair was positioned 70cm from a centreline, the front holes at the required distance from the leading edge (datum line) and the rear holes about 12cms or so behind the front ones. It is important that the holes are absolutely square to each other so checking diagonals is a good idea. Remember, measure twice, cut once. Into each hole I glued some offcuts of CF tube to stand proud of the jig board by about 10cm, again accuracy to ensure these tubes are vertivcal relative to the surface of the jig board is important. The inside diameter of the tubes is the same as the diameter of the bolts that will joint the crossbeam to the fhulls. At this stage I had a building board with four accurately positioned tubes standing proud from it. I then screwed a length of board to the front of the building board (along the datum line so that it formed a ‘wall’ at the front of the board.

The crossbeam was to be built with a curve in it so the next stage was to find the middle of the foam core and mount (i used some double-sided tape) a wooden block on the bottom of the core precisely in the middle. By aligning the wooden block with the centrelineand gently pushing down on the ends of the foam core the crossbeam takes on the required curve and it is poosible to mark it for the location of the mounting holes and these can be cut through the foam; absulute accuracy isn’t important and they need to be a bit oversize. When the holes are cut position the beam so that the carbon tubes on the jig protrude through the holes, accurately align the wooden block with the centreline and push the leading edge of the foam core hard up against the ‘wall’; glue the ends of the foam to the jig board with some daps of 5mim epoxy. Remove the ‘wall’ You now have the foam core accurately positioned relative to the datum and centrelines and pierced by the CF tubes.

Make a ‘putty’ using epoxy and chopped carbon fibres and fill the holes in the foam core around the Carbon tubes and when set use a Dremel type mini power tool to, carefully truncate the tube and fair the putty to the line of the foam core. This gives you a much better bonding, load spreading, surface area for when you laminate the foam. Plug the top of the carbon tubes (I used blutak) and laminate the top of the core (at this stage I used 200g carbon cloth and 300g unidirectional carbon tape, topped with peel-ply. Let it cure.

Using a micro saw, cut through the carbon tubes below the crossbeam and through the dabs of 5min epoxy. The crossbeam will now be free from the jig board and will retain its shape. From the bottom, gently push a drill into each carbon tube and drill through upper surface laminations. Fill and fair around the carbon tubes on the lower surface of the crossbeam, plug the tubes, and laminate the bottom as per the top. To consolidate the beam I then enclosed it in a carbon kniited tube, wetted out with epoxy, plugging and marking the carbon tubes first.

Some additional details: I had the trailing edge of the foam core cut with a semi-circular recess into which I glued a plastic tube and I cut way and boxed in a small area of the rear of the crossbeam before I did any of the above work. The reason is that the cat will have two rigs, one per hull, so will need two winches (and two rudder servos) After completion the boxed in area will be opened up with a small hole and will house the receiver with the tubes carrying the wires to/from the batteries (also one pack per hull), winches and servos.

BTW, at this stage the beam is longer than you actually need and I’ll cover what to do with the ends later.

Hope this isn’t too boring or teaching granny to suck eggs?