How To Prevent Hull Integrity In Aluminium Boats
It goes without saying that the hull is a particularly important part of a boat, and there are things to consider when maintaining it.
Aluminium is an ideal material to build trailer boats from because it’s light, strong, and needs very little maintenance — and in my opinion Australia builds the best aluminium trailer boats in the world. But there are issues with the material than can seriously affect its longevity.
Over time, and with continued exposure to sunlight and salt air, aluminium can become brittle and be subject to fatigue around joins, such as where the transom sheeting is welded to the box or channel section transom crown that supports an outboard.
An example is the early seventies 3.35m Mark One Savage Gull dinghy I acquired in 2004. The hull was designed for outboards up to 15hp (flywheel rated and around 13.5bhp at the prop) but of course these were smooth running twin cylinder cross flow two strokes. But when I started testing single cylinder four stroke outboards up to 6hp on the hull I noticed cracking at the transom/crown join due to the increased vibration. In time the crack would have spread and allowed the transom sheeting to flex more, reducing its ability to handle the thrust of an outboard which tries to pull the transom backwards as the under-hull thrust of the prop pushes forward, creating a twisting moment.
In comparison the transom sheeting and transom crown of my 2003 Sea Jay 3.4 Punt are unaffected by the vibration of the singles because the hull is newer and more strongly reinforced in this critical area. And because the hull is all welded with substantial transverse framing, it can absorb vibration of the singles better the welded/riveted Gull with has large areas of unsupported topsides sheeting which actually flexes with some singles. Repeated flexing in old aluminium sheeting encourages cracking in all sorts of hidden areas.
My suggestion is that if you have an older hull such as the Gull, only buy a used twin cylinder two-stroke such as the Johnson 6 and 8 (a 9.9 is my recommended limit), although the hull appears to be able to handle my 2005 two-stroke loop charged Tohatsu 8 okay because as rpm increases the engine smooths out to almost that of a cross flow engine. I’ve also found the old single cylinder two-stroke Tohatsu 5 won’t damage the hull because as rpm increases the engine smooths out, unlike single cylinder four-strokes.
Another issue to consider if you have an older aluminium hull is to check that all fixtures such as rod holders are bolted to the hull seating or gunwales with stainless steel bolts and washers, never brass bolts and washers as electrolysis can occur between the brass and aluminium, literally corroding the aluminium. If a hull has a flat carpeted plywood floor this should be removed and the bilges checked for stray lead sinkers, which again react with the hull sheeting and can cause pinhole leaks that at the least need ‘bogging’ with marine sealant which over time will dry out and not adhere to the sheeting.
If the boat is kept on a trailer away from damp ground and adequately supported the hull should last for decades. But if it’s been kept on a concrete ramp by the water’s edge, the lime in the concrete can corrode the aluminium sheeting. Also as aluminium is a relatively soft material, it quickly wears through if the boat is dragged repeatedly over concrete, such as when the hull is hauled up a launching ramp while retrieving the car and trailer. My old Gull was fibreglassed around the transom and bottom sheeting (not by me) and while it looks terrible it has prevented leaks in this area. In fact my old Gull doesn’t leak a drop even though it’s approaching fifty years. Not bad for a lightly constructed car top dinghy!
Your tinny is an investment in enjoyment and needs just a little TLC to have a long life.
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