How to bond an aluminium patch over a small crack in a dinghy

A crack in an aluminium hull is a fatigue failure, and fatigue cracks grow from their tips. If you bond a patch over a crack without dealing with the tips, the crack carries on growing under the patch and reappears past the edge of it. A small hole drilled at each end blunts the tip and takes the stress concentration away, which is why stop-drilling comes before anything else.

Aluminium's oxide layer is the other issue, and it is more of a problem in a marine repair than anywhere else, because salt water attacks an imperfect bond line relentlessly. Abrade to bright metal, degrease, and bond within the hour. Do not touch the prepared surface with bare fingers.

Be realistic about what this repair is. A bonded patch on a small dinghy above the waterline, or on a hull that is not structurally loaded at that point, is sound. A crack through a structural member, a chine, or anywhere that flexes hard in use needs welding. If the boat carries people offshore, get it welded.

1. Clean the area and find both ends of the crack. Use a torch at a low angle, the ends are usually further than they look.
2. Drill a 3mm hole at each end of the crack, just past the tip, and deburr both sides.
3. Cut a patch from aluminium sheet of similar thickness, large enough to overlap the crack by 25mm all round, and round the corners to a generous radius.
4. Abrade the hull around the crack and the whole bonding face of the patch with 80 grit until both are bright.
5. Degrease both with isopropyl alcohol on a lint free cloth and do not touch the faces afterwards.
6. Mix a marine grade two-part epoxy and spread a thin layer on both faces.
7. Press the patch on, work it down from the centre outwards to push air out, and clamp or weight it evenly.
8. Tool the squeeze out into a smooth fillet all round the patch edge, then leave it 24 hours before removing clamps and a week before hard use.