From the pages of “Capt’n Pauley’s Workshop”
Helm Seat Access Hatch

The access hatch under my helm seat was a deteriorated piece of teak plywood.
The hatch provided access to the back stay fasteners and diesel tank vent in the outer hull.
Any water finding its way behind the helm seat would leak through the plywood panel and into the bilge.
I found a water-tight molded plastic access hatch on eBay at a very reasonable price.
It was just enough bigger than the rough opening to cover it but not all fasteners would have anything to screw into, requiring some sort of back up plate.
I cut a ring from ½” StarBoard to act as a backup plate. A single slit in one edge of the ring allowed the ring to slip though the existing opening.
I held the ring in place with a couple of stainless steel flat head screws, place out of the way of the hatch fasteners.
I used plenty of blue masking tape around the edges of the hatch to control any sealant squeeze-out.

The backup ring slides into the smaller opening by means of a slit in one edge.

The backup ring is held in place with 4 stainless steel flat head screws, placed away from any of the hatch fastener locations.

Another view of the backup ring fastened in place. The access hatch opens up to the hull inner liner and the outer hull.

The finished hatch installed with the deteriorated plywood cover in the foreground.