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Old 09 March 2008, 16:45   #11
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this one is ally, if its as old as the boat its 8 years old and still ok, paint has lifted in some places and a bit corroded but still very strong, having said that, i have just stripped it down and has gone to Bristol Industrial coatings for a birthday,
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Old 10 March 2008, 05:23   #12
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Matt, is that how you wash your hull ? tie it in a lock and chuck a bottle of fairy liquid in

There are a few in Port Solent that could do with the same trick.


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Old 10 March 2008, 06:05   #13
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I had an Osprey Viper with the factory standard double A-Frame made from Alloy tube not more than an inch OD and powder-coated in orange. It was well tricked out with plates for lights&aerials and cleats for warps. Whoever supplied these, mine was a craftsman's example or alloy forming and welding. I used it to tow a wakeboard and while it groaned a bit on tight turns it held up.
The boat was at least 10 years old and showed no signs of the sort of cracking typical of alloy under cyclical (repetitive) stress e.g. chop induced vibration. I know of at least one other Viper in a local dive club with big hours on it at about 14 years old and the same alloy A-frame and it's in great condition.

That lovely searider matt h appears to be simmering gently is sporting the standard Searider Alloy frame. Although you obviously wouldn't risk towing anything off it they do seem to last extremely well.

It's not just the thickness, design and surface finish of an alloy fabrication that decides durability. It's easy build it strong but if the boat can generate the wrong frequency vibration it'll crack it without any load attached. It can be sensitive enough that adding or removing a small amount of weight say a toplight might tip it either way. FYI I've also seen SS tube suffer vibration cracking too.

jyasaki has identified vibration as the source of grief - it might be worth adding some weight up top to see if you can change the frequency the vibration starts. Strap a diving weight on there - it'll only be worth trying though after you get the A-frame solid again. Lowering the Frame a few inches will change the frequency too -should require a higher speed but it could be induced by airflow and/or road vibration so no way to know if shortening it would allow lower or higher towing speeds. However even tying a strap forward to the bottle rack would help damp the vibes. Think of it as a tuning fork!
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Old 10 March 2008, 08:22   #14
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My "new" boat has an ally (no idea what grade) frame and it appears to be original - I know it's at least 10 years old, but I suspect it came with the boat in about 1978...... It's a single arch made of 40mm OD / 4mm thick tube.

I would avoid powder coating like the plauge - it;s wonderful until the surface cracks somewhere, then the water gets in and the first you know of the corrosion is when it falls over! You have to bolt it on, and I'll lay money I know where the broken surface will start!
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Old 10 March 2008, 16:32   #15
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My Delta's got a black power coated A frame. It can flake off after a while though!!

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Old 10 March 2008, 17:14   #16
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Powder coated epoxy, if properly preped and applied will never flake off.
http://www.hydralon.com/
These folks do and outstanding job and have been in the business for 40+ years. They are in Northam, Southampton. I must have purchased at least 1 million coated parts from them and never had a problem, all of them ali. They are really good.
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Old 11 March 2008, 10:41   #17
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i am busy making a prototype seat for my new brand RIBs.
We make it of aluminium and then anodize it.
It looks very good and will be very saltwater resistant.
And it is not expensive.
The only problem is that there are not much companies that can anodize
big A-frames.
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Old 11 March 2008, 10:44   #18
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Quote:
Originally Posted by Limey Linda View Post
Powder coated epoxy, if properly preped and applied will never flake off.
And there lies the problem.......
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Old 11 March 2008, 12:46   #19
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The problem with aluminium (as well as stainless) is when you powdercoat or paint it etc, you are starving it of oxygen which prevents it from forming it's protective oxide layer. In effect the powdercoat is causing the corrosion rather than preventing it.

Its probably better to leave it bare and just gently wash it regularly. Granted it will go dull and grey (the oxide layer) but thats better than it turning into a pile of white dust!

This is only really true of proper marine grade aluminium mind you, generally anything in the 5000 series family ie 5052, 5083, 5086.
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Old 11 March 2008, 12:54   #20
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The problem with aluminium (as well as stainless) is when you powdercoat or paint it etc, you are starving it of oxygen which prevents it from forming it's protective oxide layer. In effect the powdercoat is causing the corrosion rather than preventing it.

Its probably better to leave it bare and just gently wash it regularly. Granted it will go dull and grey (the oxide layer) but thats better than it turning into a pile of white dust!

This is only really true of proper marine grade aluminium mind you, generally anything in the 5000 series family ie 5052, 5083, 5086.
I am not sure I follow the logic of that. The Aluminium will already have an oxide layer before it is coated. If it gets scratched through the powdercoating and the oxide - that layer will then be exposed to air and oxidise (in the same way that an uncoated component would).
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