A Modified Trilars – Part One

By Mike John Kelk (ed.) - Toowoomba, Queensland - Australia

The Design

For those who are not familiar with this design by Jim Michalak, Larsboat is the centre hull and Trilars is the trimaran stuff.

I read other articles on Trilars on Duckworks and made some decisions:

  • Rudder longer for low speed turning;
  • Daggerboard leeboard, not pivoting (for use on deep dams);
  • Spray skirt forward and sides (if it proves I need it);
  • Handle on balance point of float;
  • A small retractable jib to help tack if needed;
  • It will be a two piece boat;
  • Reduce the sail area to a 36 sq ft gaff (so I can use smaller floats);
  • Reduce floats to 7′ with a single beam (so they fit in the car);
  • I will deviate from the plans in other big ways and I will be honest about the trouble this got me into. Jim Michalak is in no way responsible.

After reading only a few words of plans, I had a decision to make. Jim suggested 5mm plywood.  I can get 4mm or 6mm ply at a reasonable price in Australia. So I asked the Duckworks Facebook group brains trust – “which ply do you think?” The advice was: “use 6mm on the bottom 3 panels and 4mm from the sides up”. To make it easier on myself, I chose to make the hull from 6mm and the deck from 4mm. I ordered 3 sheets of 4mm plywood for the deck and floats and 3 sheets of 6mm for the Larsboat. I should have some spare for a clock I want to make. (I run a Facebook group called Wooden Geared Clocks. Take a look, their fascinating.)

If I can put a build in Hulls, I always do. It does help with the build by providing information on fresh water waterlines, etc. It may also hurry the build by providing plywood panel sizes and this sometimes negates the need for forms in the plans and makes a hull pure stitch n glue. Also, plywood is not cheap in Australia, so if I can avoid the cost of plywood for forms I will. And, to be honest, I am was too lazy to make forms.

First you find a file in Hulls that is close to this design and Kattu_02 was close.

And then, you put in the offsets and realise this hull will not draw well in hulls. Back to the drawing board.

I turned to Rhino 3d and drew the frames. In Rhino you draw the boat in much the same way you build the boat. Draw the keel, stem and put the frames along the line. You then join the points to draw the lines of the hull. Developing the panels – well I phoned a friend on upwork.com.

This was the limit of my CAD skills. (This is for my own reference purposes to build one boat as easily as possible and I will not share these files as they are copyright.) I got help to finish the panel layouts.

It turned out I could get the larsboat hull panels out of two sheets of 1/4″ plywood including the transom. Jim’s plans do show a similar result. This below is the front half of the boat. I also faired the hull. Sorry, but the “puter” challenges any human eye on that score as in Rhino you just click on the line and type fair and then you a have a mathematically fair line. If it makes you feel better, it did not change it that much.

This maze of numbers is the dimensioned plank diagram.

I turned my attention to the floats. I changed them to this basically. These are designed in Carlson’s Hulls software.

My floats design

Here is a video of a rough version of this float but it will give you the idea of how it is done.

You measure from the left of sheet and then up to plot the point. Here is an idea of how to plot the points which you join with curves. I used software but it is the same in the real world. This will give you a little glimpse of Rhino 3D.


I took about four hours to mark all the dimensions and draw fair curves for the centre hull. I discovered spring clamps with a big sinkers on top make good spline weights. Any mathematical perfection I gained is lost in this process as the word is “fair curves” not “perfect curves” to my mind.

The panels took a couple of hours to cut with a jigsaw. You turn big sheets of plywood into small sheets of plywood with a half bucket of sawdust on the side. Use a clean cut blade. I recommend not cutting the planks to length or cutting the transom. If you start stitching the planks together at the bow, you can trim the transom end later. It is easier than cutting them short and trying to put wood back. But, I did not start at the bow. I started at the sheet joins. I applied some temporary butt blocks as this hull with be sectional in two halves.

I use a mains power drill for the cable tie holes as it has more RPM and less tear outs. A little stool saves the knees, but it does little to help the back. When using cable ties, keep the keeper of the cable tie close to one hole and it goes easier. Cable ties can be used loosely and then adjusted, but many will need to be replaced with wire as you can only get so much tension on a cable tie.

I have the forward section of the outermost planks upside down. I found out the hard way but easy fixed.

Bilge planks stitched together.

Loosely stitched together.

Forward section.

Glued and glassed taped with outboard bracket in place. The transom has yet to be trimmed.

I tried a piping bag which is used by cooks to ice their cakes but there was a lot of was epoxy left wasted in the bag. I used a tongue depressor to apply the epoxy and do the fillet. You can twist/flip the depressor and eventually get the goo off it and into the join. It is a fast process but messy. I can’t say I enjoy this step in a stitch n glue boat.

I really did not care how messy I was in the bow and stern tank area as nobody sees it with the deck on. You can see the middle frame in place for the two piece boat.

I don’t know where I saw this scarfing technique, but the little stops prevent the join from slipping too far. They turned out quite nicely.

The gunnels went on easily enough, despite them being 42 x 19mm. It was cheaper than the smaller sizes, so I used it. The mast step is lower fore than aft, so the angle of the hole for the mast in the step was pure guess work but I got it close. I decided to use lag screws instead of pintles and gudgeons this time around. They are cheaper. I put some hooks, with rope on, under the rear and fore decks so I could tie little plastic containers for keys and so on. Old plastic seats from the local tip shop are screwed to simple cleats glued to the bottom panel.

Another reason for opting for a dagger leeboard, rather than a pivoting leeboard, was to reduce weight when carrying the boat around. Leeboards are heavy and a pivoting leeboard is pretty much permanently attached.

Because of the angle, the rear deck looks longer than what it is.

I put slash guards on the front deck so if water comes over the bow it should not land in the crews lap. You can see the two middle frames ready for separation later. Leeboard guards on the port side so I can operate the tiller on the starboard side.

The fore deck needs trimming.

I chiselled the glass edges off the tape, using a very sharp chisel. I sharpened the chisel with this multipurpose sharpener. It worked well, but it did put a slight curve in the bevel of the chisel. Not bad though for $39.

Ozito 65W Multifunction Sharpener

You need to blow it out to clean it and, because I don’t have a air compressor, I used an air duster can. It is also good for getting dust out of your mast step.

For USA readers, Amazon has one but it is a bit more expensive.

This wasn’t my brand but it did the job.

It was the day for the great separation. The temporary butt block idea was mine, sorry if it was yours first, and I was not sure what would happen. The butt blocks fell away easily. I thought for sure they would be attached and need to be hacked off in my usual “Dr Hackenoff” style with a rusty saw. You see, I forgot to insulate them with plastic. I did put duct tape around the edges of the frames though. I had to cut through the gunnels, but I put the boat on the floor first (“it can’t fall any further”, I hear my departed Dad saying). I used a combination of saws (including my Dad’s, Dad’s (Grandfather) saw which must be well over 150 – I apologize to all of the Dad’s) to cut a bit through the 1/8″ gap I left between the frames. I heard a creak, groan and plop and it was all over. Quite a success.

This I also call, Boat Henge. I truncated the leeboard guard so I could store the hull on the ends upright.

I sat on that little stool Dad made for me and pondered it for 5 min. More so, because it was over rather quickly and I had to plan what to do next. I decided to start cutting stock for the foils. The plan was to glass everything else at the same time thus saving on gloves, brushes etc. I now use dish-washing gloves – they are stronger and can be reused many times. I used to soak things in vinegar, but no longer be bothered can I.


  1. That type of scarf joint is often seen in timber frame buildings. They also do one where the perpendicular part is angled back the other way. I’m sure it’s very strong, but difficult to do accurately in small wood.

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