Simple 19ft trimaran

First attempt sketches at top, newer sketches found below

These are early preliminary sketches for a simple 19ft trimaran. Infulences on these sketches are the original Farrier 18ft trailertri, Dudlex Dix 20ft trimaran, the Faith trimaran by Gary Lepak, and Kurt Hughes trimarans. The concept favours space, low cost and simplicity over performance.

Some particulars

  • Length overall = 19ft
  • Beam with outriggers extended = 16ft (should this be reduced to 14ft as per trailertri 18?)
  • Beam with outriggers retracted = 8ft
  • Design weight = 400kg (possibly 450kg max)
  • Optimised for small size and simplicity over high performance
  • Dual external boards inspired by tari-tari Bangladeshi boat
  • Internal sleeping area is only one double berth
  • Cockpit is just long enough to have two people sitting side by side
  • Outrigger crossbeams are 100mm diameter aluminium
  • Outrigger volume is approx 700L each
  • Rig (not shown) is conventional bermudan
  • Hullshape is inspired by Dudley Dix 20ft tri
  • Draught is very low allowing access to shallow areas and assists in getting onto a trailer
  • This craft is only for protected waters, not blue water sailing

These above are some sketches for a simple trimaran outline. The idea in some ways is to replicate the initial Farrier trailertri 18. Weight and complexity has been reduced by eliminating wing berths. The centerboard has been eliminated, and instead dual pivoting boards are used, a design I saw recently used on a French crewed Bangladeshi built boat called a tari-tari. Using this setup, the interior is opened up, at the expense of a degree of performance. The design weight of 400kg to 450kg may seem low, but the 19ft Farrier Tramp was 550kg, and that used heavier fiberglass and also had wing berths that add flexibility but also add weight. From the literature the original plywood 18.5ft trailertri is noted at 450kg. Now if this craft has no wing berths, and utilises smaller outriggers so as to promote lightness over performance, then weight could forseeably be brought down to 400kg

Compared to many other trimarans available this one will not be as fast, nor as refined, and has less berhts. However it should be simpler, lighter, easier and take less time to build.

These sketches were done 5 January 2011, and more information including sections are to follow later. Initially I had the boards were angled parallel to the hullsides, after some feedback I was told that they would not be able to rotate aft when required, thus they are now shown inclined vertically. A photo of the boat where I got the idea for the board layout is shown below.

Next step, relocate the pivot point from high up to much lower. If we raise the sleeping floor about 3 inches, then we can insert a fixed 100mm diameter aluminium tube acrosswise, just underneath this raised floor. Then thread through a slightly smaller diameter rotating aluminium tube through this. To this slightly smaller tube, rigidly attach two boards.

As the cabin sides at this lear are above the waterline, and near vertical a relatively modest amount of additional drag is created. Because the boards are much shorter, the sideways force has less distance to create bending stresses on the boards, thus making them less likely to break.

Lastly we need a way of pivoting the boards. If we weld on a strut pointing upwards, we can attach some lines to the top of this strut. By adjusting these lines the boards can be raised and lowered. Note that watertight integrity of the main hull is maintained. I am still unsure if the boards were smaller at the top, then they could be an inch or so closer to the hull, with a little less drag. If an aluminium tube is welded directly onto an aluminium strut, does it need much webbing, webbing would decrease the stress on the board-tube strut connection.

This boat will not be as good as that from most professional designers, but is should be quicker to build. I have seen articles where people take several years to make a very nice boat.

A brief paragraph to express my opinion.
There are many excellent multihull designs to be found in the marketplace. However the very bottom of the market where those of us with little money, limited skills and those of us with limited patience and who are prepared to accept a less refined boat, good plans are harder to find. With little financial incentive at this end of the market, professional designers are going to be reluctant to spend their time knowing they are unlkiely to recieve an economic return. Designs which produce a lower performing boat and have limited berths are not common. Because this market is so small, it plausible that only amateurs can fill this void

Changes since first attempt

  • Beam reduced from 16ft to 14ft in order to reduce stresses and allow for weight savings in crossbeams (not shown as of yet)
  • Sleeping level rasied a few inches, allowing for slightly more double berth width
  • Pivot tube for boards lowered to just under sleeping area
  • Boards no longer require long, strongly built struts, thus saving weight
  • Meshing on hull sides to be added as a safety measure (not shown just yet)