Flush Skin Fittings: Recycling Our Existing Through-Hulls for Less Drag
The toilet inlet and the toilet outlet on Paikea were about 300mm apart. Anyone who has lived on a boat knows exactly what that means. The inlet was moving — three or four metres forward in the hull — and while we had the boat out of the water, the skin fittings were getting recessed flush at the same time. Two problems, one haul-out.
The standard dome-head skin fitting is a small bump on the hull that creates turbulence and drag. On a passage boat that isn’t trying to go fast that’s a minor annoyance. On a performance cruiser it adds up, and for the watermaker intake specifically it’s a real problem — aeration and turbulence at the inlet means the watermaker struggles to draw a clean, steady flow at any meaningful speed. Flush fittings fix that.
The job looks simple from the outside. It isn’t. You can’t just grind a recess and pot the fitting in — you’ve removed hull laminate and you need to put the strength back. Shayne ground a tapered recess on the outside of the hull around each fitting, then fabricated a fibreglass washer of identical thickness to what was removed and bonded that to the inside. The total skin thickness is maintained, the taper gives a large graduated bonding area, and there are no hard points at the edges.
The fitting itself gets potted in with woven fibreglass cloth — not stitched biaxial. The distinction matters. A stitched fabric relies entirely on the resin to hold the fibres together under localised stress. Woven cloth is mechanically interlocked, which makes it significantly more resistant to the kind of abuse a skin fitting sees — someone knocks it from inside, it tries to rotate, the torque loads one side in compression and the other in tension and wants to split the plies apart. The woven cloth handles that better. Two layers of 300 gram cloth, fibres at 0/90 and ±45 to cover loads in all directions, glassed over the whole patched area to cap everything and lock it together.
The fitting gets taped at the flange so it doesn’t bond permanently in place while the glass cures around it. Once that’s done, flowcoat goes on to give a clean surface, then Sikaflex around the flange and the shank — the shank seal being the one that actually does most of the work long term, because the flanges can move slightly as the hull works and the resin bond at the face can eventually lift. The shank seal is less affected by that movement.
The result from outside is a hull with no protrusions below the waterline. Clean, fast, and the watermaker now has a proper intake. Ideally Shayne would have used TruDesign nylon glass-filled fittings for this job — better material for the application — but we were on a budget and the existing bronze ones were perfectly serviceable, so they got recycled.
In the Members Library
Flush fittings solve the drag problem. But what happens when a seacock itself fails — not the fitting, the actual valve controlling flow through it? Replacing a Seacock While Still in the Water covers exactly that: diagnosing a seized ball valve, and the emergency wooden plug technique that makes it possible to safely replace one without hauling the boat.
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