The Hobie Mirage is a sit-upon rotomolded plastic kayak that can be paddled or can be propelled using a novel pedaled propulsor that drops through a slot through the footwell of the boat. I prefer to lift the propulsor out of its slot whenever I switch to paddle while underway, but I do not enjoy being splashed by water spouting into the boat. I designed and built a foam plug that can be latched into the slot when the propulsor is removed while underway. See the boat in action at hobiecat.com website, search on "Mirage." This plug project was part of my user review of the boat, "Love is a Mirage," in the 01 August 2000 issue of MESSING ABOUT IN BOATS magazine. To see my related Instructables, including "Kayak Dolly" and "No-Pour Concrete Boat Ramp," click on "unclesam" just below the title above or in the INFO box to the right. On the new page that appears, repeatedly click "NEXT" to see all of them.
Unclesam

Step 1: Tools and Materials

The propulsor slot shown from the bottom of the boat, one-foot ruler. The plug is made of RIGID plastic insulating foam from the smallest can available, the version that has the lowest amount of expansion. Plastic wrap serves as a mold release, corrugated cardboard is cut to establsh the sides and top of the plug that are above the level of the slot itself. A 6" length of 1/2" diameter aluminum rod takes the place of the propulser's axle, and the boat has latches that will hold the plug's rod in place. The ends of the aluminum rod are rounded over, and two 4" lengths of 1/8" (approx) diameter steel rod are put through two holes in the aluminum rod, one drilled 2" from each end of the aluminum rod. The area around each hole is swaged with a center punch to keep the small rods in place, and the ends of the small rods are bent in toward each other to conform to the shape of the slot. The finished plug is given two coats of latex exterior house paint.

Step 2: Cover Slot at Bottom of Boat

A piece of shirtboard covers the slot at the bottom of the boat, held in place by UCM (Universal Clamping Medium, which some people inexplicably refer to as duct tape).

Step 3: Plastic Wrap Is Mold Release

Plastic wrap is used to cover the insides of the propulsor slot and the footwell of the boat. No mold release spray is used, because it would keep paint from sticking to the foam plug. Cardboard pieces are cut to define the sides and top shapes of the plug and covered individually with plastic wrap. A brick holds one top piece in place. Once foam is injected into the mold, the other top piece will be put in place and weighted with another brick. Modeling clay is packed around the ends of the aluminum rod to keep foam out of the hold-down latches.
This view shows the two shaped small rods that help the larger rod "grip" the completed foam plug.

Step 4: Foam Expands, Hardens

Foam expands out of crevices in the mold structure. Brick weights are removed and mold disassembled once foam has had plenty of time to reach ultimate hardness.

Step 5: Plug Unmolded

Plug peeled out of the molding materials, after being marked with "F" to note forward end, since propulsor slot is not symmetrical. Yellow plastic wrap is still to be sanded off.

Step 6: Fit Test

Plug fits in slot and can be fastened in place by hold-down latches. Plug is not intended to seal slot water-tight, just stop waves and boat wakes from sending spouts up the slot and into the boat. Incidental water that gets in the footwell from paddling, etc., can drain out past the plug.

Step 7: Plug Viewed From Below

Plug in place as viewd from the bottom of the boat. Unmolded plug edges were filed and sanded smooth, and the plug was given two coats of white exterior latex house paint.
U.S.