Building the Bouchie Dory – Part 29 – Thwart Risers

 

Thwart risers, sometimes referred to as “clamps”, despite seeming simple are arguably one of the more complicated parts to fit in the boat. What makes them tricky are a couple of factors. First, by the time you get to this stage you tend to have lost common references that would allow you to judge a level plane in the boat. Second, they tend to sit just out of reach of most common clamp capacities. You’ll notice how I have a few really deep throated F-clamps. Even these sometimes don’t make it, but put them on your Christmas list just the same.

Custom made clamps would be a great option for this task. One of these days I might just get around to that project.

Thwart risers don’t have to be beveled on the top surface. They can be left with a small chamfer or round over instead. The bevel does offer the advantage of a broader target for fastenings to hit. Because the riser is often set at an angle to the thwart, the fasteners holding the twart in are quite restricted in their possible placement. Too far inboard and they show below the thwart and only the tip of the screw can grab the riser. Too far ouboard and the screw may protrude through the backside if the riser.

By flattening the top of the riser, you give the thwart fastening more contact surface to strike inboard, gaining the best holding in the riser while not showing below the thwart.

This bevel is probably not constant but you can start by finding the shallowest angle and cutting that in along the whole length of the riser, then finding the steeper bevels either by measuring at different points or by scribing with the thwarts installed. The latter is what I tend to do. Of course this either requires removing the riser for shaping or trying to do so in place which can be tricky. Leaving about a quarter inch unbeveled on the outboard edge can make that a little easier.

In some boats, especially those built without frames like mant plywood boats or small profile framed boats like canoes simply get short sections of blocking fitted instead of long sringer type risers.