Pontoons connected

I’ve finished the cross members, figuring 18mm plywood should be plenty strong for the task. I’m not sure how much weight they’d be able to carry, but I feel confident other parts will break before the cross members do.

Connecting the pontoons was pretty straight forward. After some careful measuring, making sure the pontoons where parallel, fitting and gluing the cross members¬†was easy. All the seems between the hull and cross members have been strengthened with 50mm glass fiber. I thing she looks nice ūüôā

The area between the cross members will¬†be decked in. Here I’m adding some¬†blocks to help attach the deck to later on.

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 ūüôā

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!

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 ¬†ūüôā


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 ūüôā

…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)

Added some lee boards

My hope is still to get away with the chine runners only. But people who seem to know more about sailing than me (which is basically everyone) regularly suggest I should add something to keep the vessel tracking in a straight line while sailing. Lee boards seem to be the obvious choice.

In my design I’ve added some reinforcements to the hull and something for the boards to pivot on. I’m thinking this could be done even simpler by just¬†using a piece of rope tied to the cabin sides. We’ll see about that when we get there.

Lee boards

The bottom will also have longitudinal runners, also helping to keep the vessel tracking straight.

Bottom view, showing longitudinal runners

Estimated vessel weight and designed draft

I made some guesses beforehand about weight and resulting draft of my vessel. Before going ahead with the build, I thought I’d make an effort to get a slightly more accurate estimate.

The plan is to use 15 mm marine plywood throughout the design. The transom will get a double layer, as do the bottom runners. I will use the stitch-and-glue method to put everything together. The bottom and sides will be covered in glass/epoxy. Above the waterline I plan to use only epoxy and paint.

3D design of my tiny scow

So I took my design and measured all the parts, put that into a spreadsheet, added some columns to calculate the weight of paint, glass and epoxy and came up with a total weight just shy of 230 kg (507 lbs). With a bottom area of close to 5.8 square meters the vessel is able to lift 58 kg for every cm of draft. This results in a designed draft of 230 / 58 = 4 cm for an empty vessel.

Designed waterline

If we add some payload of let’s say 250 kg (2 adults and the proverbial case of beer) then we get a draft of roughly (230+250) / 58 = 8 cm. Not bad in my eyes.¬†It leaves¬†a healthy safety margin before the sideboards get dangerously close to the water.

Behind the link at the end is my calculation sheet.

Vessel weight calculation (PDF)

Progress in 3D

This post was previously published on tinyhouseboat.wordpress.com

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.