Welding Aluminium Mast

Richard Selman · 15 April 2012, 17:59

Technically this is a rag and stick question however I think I'll get better mechanical advice on this forum. Doing 20 knots+ tends to get us grouped in with the ribbers by the yachties anyway (although not with the PWCers I hope).

The aluminium mast on my 6m cat has a rotation control system that has a wishbone held 300mm from the base of the mast with a bolt right through. To prevent the bolt tearing through the mast section there are two small stainless plates riveted to the outside of the mast. Every rivet on the boat has been replaced and zinc chromated up to control the corrosion however these plates seem to have loosened up, let water in and corroded the rivet holes. Too many tacks, gybes on the water and gusts of wind off the water. There is now too much metal loss to re-rivet with confidence.

I thought about welding on doubler plates however the bolt has always been a route for water into the mast so therefore I thought about drilling out a 36mm hole right through the mast and inserting an aluminium cylinder (possibly machined like a bobbin to minimise weight) and weld around the outside. See sketch attached.

Thinking about getting one of the guys who made up bits for the rib down south to make up the cylinder. However at >9m long I cannot post the mast to someone who would be used to welding on the like.

I have a few welder contacts in the area however they normally work on fishing boats to oil platforms so sometimes not so skillful on fine work.

I therefore have understand how difficult this is before I let them loose.

The mast section is only about 2.3 mm thick - is it possible to weld on this thickness & also not screw up the strength of the main section (the mast is generally only in compression at this point with little bending or flexing)?

The corroded section could be cut out with a 36 mm bore hole. What would be the diameter of the cylinder that should be installed - should it be an interference fit or should I allow a gap?

Do I need to be careful about what grade of aluminium to specify?

Can the holes be cut well in advance of the work or does the metal need to be freshly cut?

Does the hole edge need to be prepped square or bevelled?

Cheers
Richard

mick · 15 April 2012, 18:06

How big are the rivets can't use drill the holes and use bigger rivets
Put some pics up

Trailer Guy · 15 April 2012, 18:48

Hi Richard, I'm with discomick - stick a couple of pics up.

However, if you're concerned about the skills of the contacts you have, why not google/bing/etc 'TIG welder / fabricator' in your local area.

Or how about a motorcycle repairer?

Alternatively, you could always talk to your local steel stock holder (ergo should carry ally too) and they should have some good contacts for TIG welding.

All the best, Ben

martini · 15 April 2012, 19:02

Bolt the plates on instead using nutserts?

mick · 15 April 2012, 19:10

Quote:

Originally Posted by martini

Bolt the plates on instead using nutserts?

Rivenuts can be a pain in the ass ask the Defender boys

m chappelow · 15 April 2012, 19:43

Think i would do what the rest have said before welding ,
Drill or better reamer the old holes out ,re rivet with next size up,any chance you could use nuts and bolts instead of rivets ,

JohnR · 15 April 2012, 21:00

Use 5083 grade aluminium. The hole can be precut, and the insert can be interference fit. Welding the mast shouldn't be a problem for a competent ali welder. The welder will clean the surfaces before welding and will bevel if he thinks necessary to improve the weld.

Good luck!

Cheers
John

Richard Selman · 16 April 2012, 03:55

drilled one side up to accept 6.4mm rivets but stopped because the holes were getting so close they are almost joining. also there is a tube inside the mast to prevent crushing and the rivets were clashing with this.

Richard

billboat10 · 18 April 2012, 04:14

yes you can weld it i know a welder thay can weld a coke can he bangs a pen through the bottom then welds it over its gota be done with tig im a coded welder but only on steel

Alan Priddy · 18 April 2012, 06:40

DONT Weld it!!! Cut the offending part off and replace it with the same profile and rivet it together You might be looking to save a penny know but it will cost you pounds later if you dont do it right. There are hundreds of masts for sale on Ebay Alan P

Nos4r2 · 18 April 2012, 09:31

I'm with Alan P here. Heat+aluminium+grain distortion+stress=dismasting.

It'd be a different story if it was steel, but it's not and there's no way to regain the grain structure in the metal in the welded area. It'll look pretty when done if welded, but the mast is very likely to fail with the grain boundary or heat affected zone of the weld as the start point.

rik_elliott · 18 April 2012, 14:17

Quote:

Originally Posted by Nos4r2

I'm with Alan P here. Heat+aluminium+grain distortion+stress=dismasting.

It'd be a different story if it was steel, but it's not and there's no way to regain the grain structure in the metal in the welded area. It'll look pretty when done if welded, but the mast is very likely to fail with the grain boundary or heat affected zone of the weld as the start point.

I'm with you there. I've taken up aluminium casting as a hobby so have got a real feel for how aluminium and heat mix. I might trust a weld for something lower stress or non critical, but not your mast.

Rokraider · 18 April 2012, 14:54

Quote:

Originally Posted by rik_elliott

I'm with you there. I've taken up aluminium casting as a hobby so have got a real feel for how aluminium and heat mix. I might trust a weld for something lower stress or non critical, but not your mast.

That's an interesting hobby,perhaps you could cast him an insert that could be bolted in place?

Simon B · 18 April 2012, 15:57

I used to wled as part of my job, ALWAYS ask can a mechanical fixing be used, gnerally it is superior.

Richard Selman · 18 April 2012, 17:24

Thanks for the advice. Decided to put bigger stainless plates on and bolt through with 4 fixings per side. Mark at Marine Fabrications is on the job (did my A-frame). I suppose that will be a whole day with a spanner on a stick trying to locate zinc chromate clarted washers and nuts on threads up inside the mast. The bottom of the mast has more fixings than swiss cheese but probably not much worse than an typical aircraft wing.

I think I would have been OK though. These masts are oversized to give buoyancy when capsized so would have been strong enough even with a 36mm hole right through (except the water would run in!). Saw one turtled boat and crew lift right out of the water on its mast in big waves off Abersoch - didn't snap. The hole is so close to the base that there is little stress here. Only a beach cat after all. Not crossing the atlantic anytime soon.

Dismasting's not so bad. It all floats down quite gently even if you are cracking on. Bust a lovely carbon mast last year on the other cat. Luckily it was a free-be. Just a little embarrasing as the whole pile drifts along like in Jaws 2.

Richard

robbo1 · 18 April 2012, 18:42

Try putting your post on this forum. They are a good bunch of tekky lads and lassies on there.
MIG Welding Forum
There is a TIG section as well as MIG and other stuff.

Simon B · 18 April 2012, 19:55

Ally TIG welding is the preserve of Yoda like geniuses, not for the amatuer unless they are coded in Ally on TIG ofcourse. Sounds like he's sorted though.