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Old 08 March 2008, 15:34   #1
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Ali A Frame ?

I was thinking of having an A Frame made from Ali tube , and powder coat painting it , has anyone any experience of this. Has anyone any ideas wyh I should or shouldnt have it done in Ali tube? I wont be towing from it , just using for lights and aerials. Any comment or pics appreciated, thanks
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Old 08 March 2008, 15:54   #2
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Hmmm, no idea really, but I wonder would "bad things happen" where metals meet and lekkie gets grounded. I know it plays merry h*ll in Land Rovers - we're just out of the paint shop
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Old 08 March 2008, 15:55   #3
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I would have thought the only thing to be carefull of is you use marine grade ali.Apart from that good idea.
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Old 08 March 2008, 16:11   #4
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Thats how all the a frames are built here. You will have no real problems and the powder coating will last about five years with minimal wear and tear.
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Old 08 March 2008, 16:50   #5
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I had a very old one and it wasn't that substantial. It only broke on one of the four stanchions when I tried to push the boat off some mud. So although it didn't look as massive as a Stainless steel one it certainly proved to be strong.
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Old 08 March 2008, 18:27   #6
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This one is Ali and un painted but made from a type of Ali that resists salt water corrosion.

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Old 09 March 2008, 09:41   #7
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There may be some issues.

Guy at Polaris built my double=tube arch out of 6061, sized to be large enough that a diver could pass through from the transom without resorting to gymnastics. The tube itself is 2", about 1/4" wall.

What happened is that the span is large enough that the top oscillates while trailering (doesn't seem to happen on the water to much of a degree), and the rear legs, which are straight for a short distance before bolting to the transom, weakened and broke. I had a friend sleeve and weld them with aluminum, and they broke again above the sleeves.

Currently, I have the unsleeved portion built up with tape, and the resulting mess clamshelled with ABS pipe and hose clamps. In the past, oh, year or so, there have been no further issues, so no further action has been taken. A friend of mine who routinely follows me says the oscillations have been greatly reduced.

Make sure you use a good anti-corrosion treatment that is rated for aluminum on any stainless fastener touching the aluminum, and the use of nylon washers to isolate the bolt heads would probably be a good idea.

In my case, it's the largish span that causes the breakage issue; the resulting structure is harmonic to the road vibration. With the same tubing, and a normal size frame, you probably would not see anything like this.

jky
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Old 09 March 2008, 10:37   #8
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I think all the quinquari commercial ribs have ali fittings. Just remember you need much larger tube diameters.
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Old 09 March 2008, 10:43   #9
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Thanks all for your advice, I know Ali and Stainless dont mix so what do you bolt or screw the frame to the boat with? Is it ok to use something like someone said a plastic washer between them or what else would be good?
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Old 09 March 2008, 14:29   #10
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Our three Quinquari Humbers all have aluminium A frames and all three have broken at least once. We think it might in part be due to resonance vibrations, but the short, steep Clyde waves no doubt contribute as well.
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