Boatbuilding: The Red Rhombus

One of my first few summers home from college, I got the idea to build a boat. I naively thought it would be cheaper to build one than to buy one, I quickly learned that is not the case. But I also learned a lot about boatbuilding.

I did some online research and located some different plans. I came across the concept of a "one sheet skiff" (OSS). The challenge is to build a boat almost entirely from a single 4' x 8' sheet of plywood.

Several designs look like miniature rowboats, but the displacement was not sufficient to hold two adults. I really wanted to be able to get out on the water with my girlfriend (now wife) in a boat that I built myself, so I kept searching for a plan that would provide sufficient displacement.

The real calculations are much more complicated, but the basic idea is that if you calculate the interior volume of a vessel (how much water it would hold) and multiply that volume by the weight of water, you have a rough idea of how much water it would take to sink. So, by toggling the height in your volume calculation, you can determine how much freeboard you will have at a given weight.

I came across what looked like a good design that maximized volume and still looked something like a boat and would provide enough room and displacement for 2 adults, so I jumped right in and started building.

The first step involved cutting the sheet of plywood into "pickets:"

Then I added 2 extra ribs to the original design for extra strength and a little easier construction:

 

Then finish by joining the tips and sides of the pickets together using more or less standard hard chine building techniques with lots of marine epoxy and sawdust/baking flower fillets.

Waterproofing comes from a couple coats of thinned epoxy.

 

Then the finishing touches: A lot of paint and some oars and oarlocks.

 

So it floats and holds two adults. But unfortunately, I learned the hard way an important lesson about hull design: symmetrical shapes are very prone to tipping. The Red Rhombus is so tippy that I had to remove the seat cushions to keep the center of gravity as low as possible. If I ever build another boat, I'll be sure to build one that is shaped so that the more it tilts to one side, the more water it displaces.

All in all, a good learning experience and not a bad way to spend a summer.