Monday, 6 April 2015

A little more about my trimaran

I received a very nice email from someone asking a little about the skinny trimaran I am developing, so I thought I would give some more detail around my design goals and how the boat is put together.

Design Goals
  • Ultra narrow centre hull with a high beam / length ratio
  • Very light
  • To be able to fit everything except the hull in my car. This meant the max length of the amas and akas needed to be 8 feet and I would need collapsible masts.
  • Modest sail area, Ketch rigged to keep centre of effort fairly low
  • Intended to only be used in a sheltered estuary
  • To be able to sail in skinny water when necessary, so kick up foils needed, or perhaps skegs
  • In order to build a 20 foot trimaran in a 17 foot garage I would make one 16 foot section and a 4 foot section
  • Reasonable upwind performance
  • My goal was not to make really a high speed craft as this would mean going into a whole design spiral of bigger and stronger components. That being said I do hope to achieve reasonable performance for such a modest sail area.

The hull is made from 4mm ply on the sides and closed cell foam underneath sheathed in fibre glass. The idea of the foam was to enable me to develop the underside into a u shape to help reduce wetted surface. In hind sight this is a very time consuming process and probably for very little gain. If I were to do this again I would have a ply underside.
The four foot stern section
The amas are made using 4mm ply and a fairly standard stitch and glue technique. Although this gives a fairly nice shape and was easy to make, without another chine they will not have enough buoyancy when I start using the full 45 square feet of sail.

The akas I made from 8 strips of pine glued and held in a jig. It was not necessary to steam them and was quite a quick method of creating nicer amas than aluminium tube.

Results to date
Well, it certainly is a skinny trimaran! When I first went out I just shifted my weight and with a little stern rudder from my paddle I could steer.....just. I then made a quarter rudder which was not successful, too flimsy and 6 foot from the stern. One particular thing I was really pleased with was the absolute minimum of wake from the hull, as can be seen from the video I have already posted on this blog here

I also found it a bit fiddly bolting on the stern section and akas. This weekend I decided to try adding a shorter permanent stern making it 17 foot instead of 20 feet. This will fit in my garage and there will be no messing with bolting it together. I have also started building a foot controlled rudder which I am hoping will work much better. If the shorter stern section does not work out I may just permanently attach the longer stern but that would mean keeping the boat outside from that point.

Next to do
  • Complete rudder and stern section.
  • Lower amas and at the same time make a simpler connector rather than the current system of using bolts
  • Work out positioning of leeboard, or maybe just use skegs on the amas - I am undecided on this at the moment
  • Make a seat with sides so I don't fall off!
  • I realise that my initial design goal of getting everything except the hull in my car too limiting on the design. I have therefore decided that if this design proves promising I might make longer, more buoyant amas.
Happy sailing!