Thread: Welding up a Alloy boat

Graham asked me to put this post up on how I went about building and welding my new alloy boat. As I said to Graham I’m not a tradesman nor do I have years of experience in welding, but I do have a passion for metal work and boats. Many here would probably know how expensive alloy sport fishing boats can be. I had a few quotes some time ago and I almost fell off my chair. I thought there had to be another option so I went about researching alloy boats. I come across a few companies that specialise in kit alloy boats. I rang to see what they were charging and it seemed feasible to build. I also found a bit of a problem with most of the companies because they didn’t want to change any of their designs to suite what I wanted. I thought this would go for most of them but to my amazement I found one guy, a qualified naval architect that had a passion for small sport fishing boats. He also catered for custom designs which made me do a few back flips with joy.

The alloy used is 5063 H116 marine grade right through out the boat. I’m a made keen game fisherman and my needs are quite different from your standard alloy boat. I chase blue marlin which can grow anywhere from 400 to 1000lbs and black marlin in Cairns which can grow to 1300lb. I drew up what I wanted in the boat, shape, style, and internals and so on. 3 months later we had both come to a final design which was great for what I wanted and needed. Now this is not a kit boat, no such thing ion my book. If you want a kit, go and by a model plain or boat that simply clicks together. I wanted the sheets router cut and not plasma cut as plasma cutting can warp the sheets where as router cutting wont. The sheets came router cut and pen marked for alignment. The hull sheets are 5mm, transom 6mm, the floor 4mm, stringers 6mm and frames 6mm. I wanted this rig over engineered so if I hit anyone I could right through them rather than go over them. Weight can be your best friend in terms of alloy hulls, to little and you will be wearing your kidneys on the floor of the boat.

My brother and myself took all day to cut the small noggins which where holding each section in place. Once we had the sections in the factory we could start laying out the bottom of the hull. I laid the 2 bottom sheets out first; I then placed the keel in and then lined up the pen marks. I then started placing each frame in and taking it in place. I started from the back of the boat “the Stern” because it’s the flattest spot. As I progressed forward the shape of the hull starting coming together. I haven’t welded anything in place yet only tacks. I needed sash clamps to bend the bow up and around which was very hard. Alloy under this much tension couldn’t be good. I soon found out as one sheet let go and hit my left knuckle opening it up. I could see all the tendons and bone working. Anyway, after a stich up it was back to work. The bottom of the hull is now tacked up; all the stringers are in place. At this stage I have quite a few people coming around to either criticise that I couldn’t do it, or just say how the hell do you know what you’re doing.


Well, the critics can get stuffed; we all can do anything we like if we put our minds to it. I studied the drawings for 2 months before getting the sheets delivered. When it turned up I didn’t even need the drawing, it was all in my head. Many people said I could never weld alloy it’s to hard. Well, it is hard to weld if you haven’t been shown how to weld it. I had 2 very good mates show me. One is an alloy boat builder for Lloyds shipping while the other does the entire alloy welding for another company. I had them show me how to set the welder up, what the welder should sound like and what the beads should look like. The showed me the travel speed and once I had a go in front of them I was away and blazing. The welder should sound like a high pitch buzz with almost no crackle. The speed at which you move is quite fast. As my mates said, weld hot and go fast. If you slow down you will simply blow holes through the alloy. It took me around half an hour to pick it up and to the point where I was comftable. I had to have the welder set up right from the steel welding I had done, well the whole 2 or 3 meters of it. My mate set me up with a 4 meter liner and a new larger hand piece to handle the extra amps needed. He used a Teflon liner and we used a bottle of pure argon. The wire I used was Alloy Saffra 1.2mm 5000 grade, very good wire. The only problem with the 4 meter liner is that my machine is a single drive which some times gets a birds nest from hell. This can be frustrating at time; it’s normally a 20 minute job re doing every thing.

After we had placed the side sheets on the boat the alloy had started accumulating oxide on its surfaces. While the alloy was new it was great to weld. But after some time it started oxidising and the welding appearance started to change. I asked my mate what’s going on and he told me that while the alloy sits it will start to oxidise. Oxidisation melts from memory at 2200 degrees, while the alloy wire melts at 650 degrees. In effect the weld would be sitting on top the oxide layer and no penetration would have occurred. I have personally seen this occur in alloy fuel tanks, the fuel actually leaks out beneath the welds. From this point on I have to now sand every square inch of alloy I intend to weld so that way I have true clean alloy to weld to. It doesn’t take that much more time anyway. I also found that welding large areas was becoming a problem as well. My alloy sheets where warping because of heat. This distortion seemed to be from the sudden heating and then cooling. One thing I hate most about welding alloy is distortion. It’s a real pain in the butt to fix and as I quickly found out that your better of bracing the work and doing small welds instead of one big long weld bead. If I braced around what I was welding first and then made small welds the problem was non existent. If I had a meter to weld, I would weld all over the place at one inch intervals, this maybe the wrong way but it worked for me.

I also found that having to weld a section in my boat for the underfloor esky out of 3mm alloy could be a nightmare. I made some bracing up and instead of welding in short bursts I chose to spot weld it, let it cool for 1 or 3 seconds and do another spot weld. This worked great and actually looked like tig welding. Thin alloy warps in one second flat if you’re not careful; I found this out the hard way. Spot welding takes out all the headaches and of course turn the amps right down.

The other problem I found was if you leave alloy out in the weather you will have heaps of issues with porosity. As I found out later, alloy is very porus and these small pores will hold moisture and other impurities which when welded will bubble and spit and carry on and you will have a nice weld with porosity in it. It’s also very hard to get ride off as well. This whole gig was a big eye opener for me. I have enjoyed every minute building the boat but these small issues which may I add could have turned into big disasters if I had not asked what was going on. Alloy as I soon found out isn’t as easy as picking up a welder and blasting some hot sticky stuff on, its very much more than that. I think alloy when new is a delight to weld, when left for a while can get very tricky. I would much rather weld steel than alloy, but knowing what I know now; alloy wouldn’t pose that much of a problem anymore. Remember, I’m not a tradesman. This is my experience at building a boat I have always dreamed of, you could do the same. I think if you love something and have a real passion for no one can stop you. I would recommend building a boat to anyone.

Stu