Crossmember strength

As my designs show, my plan is to connect the 2 pontoons using 2 box sections and a deck. Below is a detailed design of the box section, showing 3 bulkheads and openings for storage.

Cross member, 200 x 20 x 20 cm

My initial plan was to make the sections using 18 mm plywood. That would be plenty strong I think. But it would also be plenty heavy. So I’m considering using 9 mm plywood. Or maybe use 18 mm for the verticals and 9 mm for the horizontals. But can I get away with that?

How much strength do I need?

The pontoons will weigh around 100 kg each when finished. I plan to use them for storage when underway and I will probable have one or two batteries on board. Let’s say the total weight of each pontoon could be anything up to 200 kg.

Total width of the boat, and therefor the box sections, is 2 meters.

In an extreme scenario one pontoon might be in the water and one might be suspended in mid air, for whatever reason. That means each box section need to be able to carry 200 kg / 2 (box sections) = 100 kg as an absolute minimum. To account for dynamic loads, I would like a safety margin of 100%. So each box section needs to be able to carry 200 kg at 2 meters length.

Anyone care to help me calculate / guesstimate the minimum required wall thickness?

Box section, x-ray view showing 3 bulkheads inside

Holes in the bulkheads

While driving home from work, I was thinking about how to secure stuff stored inside the hulls. What if I have some supplies, batteries and whatnot on board? I don’t want everything sliding back and forth while sailing. I need something to tie stuff to…

So I drilled some 5 cm holes in the bulkheads. This gives me something to attach a line to. Ideally I should also have attachment points along the length of the hull. Not sure how I should go about that. Suggestions are welcome!

She’s upright

This weekend the bottom of the first pontoon was closed and she could be turned upright. Another milestone 🙂

I’ve also started work on the second pontoon. The glued together sides are on the garage floor. Bulkheads are done already, so I should be able to start gluing her together next weekend. It’s going to be a crowded garage.

Next is filleting all the joints. It’s pretty hard to get epoxy thick enough so it doesn’t run. I need to add about 10% filler material, measured by weight (150 g of epoxy needs 15 g of filler). That makes it so thick it’s hard to mix. In the end I kust checked every few hours and smooth out any runners. Not perfect, but good enough for me.

I’ll put injection resin on the inside to make the wood waterproof. I may also paint the inside, haven’t decided on that yet.

Making some sawdust, finally

After lots of thinking and designing, the time has finally come to make some sawdust and see if I can actually make my ideas a reality.

Raw materials, underlayment/Russian softwood

This pile should be enough for both pontoons + parts of the middle deck. I’m not sure what to do with the cabin, so I’ll get wood when I get to that part.

Sides for pontoon 1

Laying out and connecting the planking for the sides of the first pontoon. I made the connection with a simple butt join and glued a piece of plywood on top of the butt. Not the prettiest, but should be plenty strong for what I need.

Making a mess

Sawing the bulkheads sure makes a lot of dust. I cut bulkheads for both pontoons. Hope I got the numbers right  🙂


Just follow the plans…

More bulkheads
I love rounded edges
Reinforced nose

I will round off the edges and add a few layers of glass later. Should do the job. The front section, up to the first bulkhead, will be sealed permanently for make an air tight chamber. I wonder if I should fill it with foam or something else? I will impregnate the wall inside and out with epoxy.

Closing the bottom

That actually went a lot easier than I expected. I dreaded having to adjust the curvature multiple times, but somehow it fit on the first try. Very happy with the results 🙂