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

3D catamaran design refinements

Here is the latest iteration of my catamaran design. I’m still not sure what I will do for cabin, but I guess I’ll figure that one out as I go along.

I’ve added some additional braces under the deck (the deck will be 18 mm). I’ll probably add a 45 degree panel from the underside of the deck to the side of the hull to make the entire structure more rigid.

Another change from previous designs is that I moved the front cross member forward, against the front bulkhead. This will increase cabin space dramatically. The net between the bows should make a nice place to sit and enjoy the water 🙂

…and back to catamaran

Just before I started building, I decided (more or less on a whim) to go back to my initial catamaran idea. I really like catamarans. They are inherently stable, don’t need ballast and look good too. So I tweaked my first designs and came up with this:

It’s going to be a sailing cat and it will have some sort of cabin, although I’m not entirely sure how I’m going to shape that.

Some dimensions:

LOA: 4.80 m (15.8 ft)
Beam: 2 m (6.5 ft)
Draft: 15 cm (6 inches)

Progress in 3D

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Some work-in-progress renders of what I’ve come up with this weekend. I feel I have the design pretty much done. Of course a lot of the fines details need to be sorted, but I’ll get to that once I start building.


I’m not entirely sure about the rigging. I like the simplicity of a sprit or lug sail. These kind of sails have been used for thousands of years, so there must be something good in them. On the other hand, I have zero experience in sailing so I need to learn more first.

I’ve added chine runners for course stability. For sailing lee-boards might be required, but I can easily add them when needed.

Change of plans, from catamaran to box keel

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My initial plan of a catamaran hull has advantages, like being super stable and having lots of storage space down low. Parts of the hulls can also be sealed into flotation chambers to keep the vessel afloat in case of an emergency.

There are definitely also some disadvantages, the biggest being the lack of interior space. The tunnel in the middle means standing-height in the middle isn’t really an option.

So I’ve been thinking about changing the design to a box keel hull. I would sacrifice some stability for a more practical interior. The keel would have to be wide enough to provide some walking space, but small enough to not push the boat up and make it want to tip over. For now I’ve made it about 70 cm wide and 20 cm deep. This gives is about 400 kg upward pressure, which should match the weight of the empty vessel.

Time for another prototype…

1:10 scale box keel
Weighed down with the equivalent of 400 kg, the designed dry weight of the boat
An additional 180 kg to scale, simulating 2 people and some equipment. It all looks good so far…
The same 400 kg dry weight + 180 kg cargo all at one side

As the last picture shows, the box keel is not as stable as a catamaran. By far. I will not be able to carry  the 10 or 12 people the catamaran could. But all in all it does seem to hold up, even in pretty extreme situations. Good results. I think box keel is here to stay.

Until next time…