How to Cut a Boat Transom Door

by Will Charpentier

Cutting a door in your boat's transom need not be undertaken with fear and dread; the process can take one weekend or less--with the right tools--and makes sense if you've added a swim platform on the stern (back) of your boat. Watertightness is no more an issue with a transom door than it is with any other hatch or door and affects neither the buoyancy nor stability of your boat. The only question might be related to the position of deck hardware; a stud finder from a hardware store will help solve that problem.

Pick a location on the transom for the transom door. One major consideration is control cables, hoses or other operating equipment that may have to be relocated. Another is cleats located on the gunwales near the transom door. The transom is a major structural element of the hull. When the boat is tied to a pier, the cleats transfer the stress from the motion of the water in the dock to the hull. If a cleat is near the transom door, the door will affect the load-bearing capabilities and effectiveness of the cleat.

Use a china marker to mark out the proposed outline of the transom door on the exterior of the transom. Use a level and a carpenter's square to ensure that the lines are straight and that the bottom line matches the plane of the deck.

Cut the door opening in the transom with a reciprocating saw, following the lines. Remove the section of the transom that will be replaced by the door and set it aside; it will be used as a pattern to install wood/fiberglass "fill panels" to the area of the hull that's suddenly been exposed to the elements. Measure the width and height of the opening in the transom with a measuring tape.

Set the piece of the transom you removed from the boat on its end on the marine plywood. Trace around the end of the plywood with your china marker. Turn the piece of plywood onto its other end and, in a different location on the marine plywood, trace around that end. Set the bottom of the piece onto a different location on the marine plywood and trace around it to form the sill of the transom door. These tracings form the "fill panels" that will cover the parts of the hull interior that were exposed by cutting the door.

Use the screws and screwdriver to attach the plywood fill panels in place around the opening of the transom door. Apply marine epoxy and fiberglass sheeting to the panels, sanding as required to blend the freshly fiberglassed panels into the hull.

Apply marine epoxy to the quarter-round molding. When the epoxy has cured, glue (with marine adhesive) watertight gasket material onto the flat surface of the molding that will make contact with the new door.

Cut a panel the size of the opening for the door from the marine plywood. Apply marine epoxy and fiberglass sheeting to all surfaces of the new door panel, sanding and reapplying as necessary for a consistent appearance. When the fiberglass has cured, attach a watertight hinge on the outboard side of the door according to the hinge maker's instructions. Attach the hinge to the fill panel on the outboard side of the new transom opening.

Attach a sliding barrel bolt to the inboard edge of the door and mark the point where the bolt, when extended from the lock, will strike the side panel of the door frame. Drill a hole at this point and install the striker plate with marine epoxy.

Use a screwdriver and screws to attach the quarter-round molding to the new side and bottom fill panels to act as a jamb for the new transom door so that the watertight gasket is compressed when the transom door is fully closed and locked.

Items you will need

  • China marker
  • Level
  • Carpenter's square
  • Reciprocating saw
  • Measuring tape
  • Marine plywood
  • Screws
  • Screwdriver
  • Marine epoxy
  • Fiberglass sheeting
  • Marine adhesive
  • Watertight hinge
  • Sliding barrel bolt and striker plate
  • Quarter round molding

Tips

  • If you choose, you can stain the door and door frame, then cover them with marine epoxy without the fiberglass. This will provide a wooden accent on the stern of your boat.
  • Only use a brass barrel bolt on the door and only stainless steel screws.
  • If you're concerned about the strength of a cleat adjacent to the new door, move the cleat forward on the deck or rail of the boat until it is above a lateral member (one of the ribs). Use a stud finder to locate a rib, just as you would find a wall stud.

Warning

  • This project involves the use of power tools and marine epoxy. Appropriate precautions must be observed.

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.