Watertight Bulkhead

I expect the vast majority of small-boat sailors have experienced a capsize—and certainly some large-boat sailors as well. Unless you want to take the risk of your boat sinking after a capsize, you either need a boat that is inherently buoyant or the boat design needs to provide for some means of supplemental floatation. Herreshoff’s Fish Class and the derivative Flatfish design incorporate a forward watertight compartment for floatation. The compartment extends from the stem back to the cuddy’s forward bulkhead. The compartment is completely sealed except for a 2″ fitting in the deck that can be opened as needed to pump out any water that might seep into the compartment.

Justine‘s watertight compartment is isolated from the rest of the interior by a bulkhead located at station 8 made from 12 mm marine plywood. Obtaining a good fit when you make this bulkhead is important. And with a sheet of this marine plywood costing about $80, there’s an incentive to get it right on the first try.

I started making all three of Justine‘s bulkheads by making patterns from various pieces of scrap I had on hand: some plywood, and some solid wood. My technique involved making the pattern from several pieces, one for each side of the opening. The watertight bulkhead is approximately triangular, so I made the template from three pieces, and added a fourth vertical piece to help keep the pattern from distorting.

I made bulkhead patterns from narrow pieces of thin plywood. Each piece was first scribed and fitted to one side of the bulkhead opening, then the separate pieces were screwed together, in place, in the hull. I used the assembled pattern to mark a piece of 1/4″ luan plywood to make a full-size template to check the fit before cutting into an expensive piece of 12 mm marine plywood.
After ensuring that the thin plywood mock-up (white piece in photo) fit well, I traced its profile on the 12 mm sheet, then cut it out. I left a bit extra for beveling the bulkhead’s outboard edges.

The bulkhead lies just aft of and against the frames at station 8. It’s fastened to the frames with #8 x 1 1/4 screws. A rabbet had been cut on the forward side of the top of the floor at station 8  and the bulkhead is also fastened there. I drilled for the fasteners, cleaned out the debris, then applied 3M 5200 adhesive bedding compound to all the joints before finally driving the fasteners.

Watertight bulkhead in place and fastened to the frames at station 8. The bulkhead’s top edge fastens into a rabbet in station 8’s deck beam (seen here resting atop the forward deck beams).

The deck beam at station 8 is 1 1/4″ thick to accommodate a rabbet for the bulkhead on its forward side, as well as a housing on its after side into which the forward end of the mast partner will fit.

Front of watertight bulkhead after installing the deck beam and driving all the fasteners. Stiffeners will be added where the red lead stripes have been painted.

If Justine were to capsize, there would be considerable pressure on the watertight bulkhead. So Joel White’s design adds five oak stiffeners on the bulkhead’s forward side to prevent it from collapsing. I also used 5200 on these joints, in addition to screws driven from the aft side of the bulkhead, to secure the stiffeners.

White oak bulkhead stiffeners have been added to the forward side of the bulkhead, and the entire watertight compartment has been given several coats of paint.

To further seal the bulkhead joint, and improve its appearance from inside the cuddy, a curved piece of trim molding is fastened on the aft side where the bulkhead meets the hull. More 5200 was used in these joints.

A molding has been added along the perimeter of the bulkhead, on the cuddy side. This dresses the joint up and allows an additional application of 3M 5200 adhesive bedding compound to further seal the bulkhead’s joint with the hull. In this photo the deck beam at station 9 is in place and the mast step has been fastened spanning the floors between stations 8 and 9.

I finished installing the watertight bulkhead in March 2015.