Ali A Frame ?

davis · 08 March 2008, 19:34

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

willk · 08 March 2008, 19:54

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

PeterR · 08 March 2008, 19:55

I would have thought the only thing to be carefull of is you use marine grade ali.Apart from that good idea.

OceanEco · 08 March 2008, 20:11

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.

Biggles · 08 March 2008, 20:50

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.

Pete7 · 08 March 2008, 22:27

This one is Ali and un painted but made from a type of Ali that resists salt water corrosion.

Pete

jyasaki · 09 March 2008, 13:41

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

codprawn · 09 March 2008, 14:37

I think all the quinquari commercial ribs have ali fittings. Just remember you need much larger tube diameters.

davis · 09 March 2008, 14:43

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?

JIY · 09 March 2008, 18:29

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.

matt h · 09 March 2008, 20:45

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,

Pete7 · 10 March 2008, 09:23

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.

Pete

Daibheid · 10 March 2008, 10:05

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!

9D280 · 10 March 2008, 12:22

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!

CJL · 10 March 2008, 20:32

My Delta's got a black power coated A frame. It can flake off after a while though!!

Chris

Limey Linda · 10 March 2008, 21:14

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.

Xcraft · 11 March 2008, 14:41

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.

9D280 · 11 March 2008, 14:44

Quote:

Originally Posted by Limey Linda

Powder coated epoxy, if properly preped and applied will never flake off.

And there lies the problem.......

martini · 11 March 2008, 16:46

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.

Poly · 11 March 2008, 16:54

Quote:

Originally Posted by martini

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).