Hello Tom,
I have a Garcia Passoa 47. This is an French built aluminum expedition sailboat. Like you I got into TIG welding in expectation of doing work on the boat and building things for it. Wow!!! Did I ever!!! I will give you my recommendations for gear and stuff based on my many years with the boat and my experiences fabricating things for it and maintaining it.
Welder: Get an Inverter TIG machine with AT LEAST 200 amp capability. I have a Miller Dynasty 200 DX. It is a Stellar machine, and welds aluminum well above it's weight class. Because you are a machinist I GUARANTEE you that buying some cheap, imported piece of crap will have you tearing your hair out, and eventually you will end up with something like a Dynasty. They are light, and run off 220 VAC single phase at 30 amps or less. You can usually get this at your marina dock box.
You will need a GOOD torch, preferably water cooled, but air cooled will do for starters. Weld Craft or CK International have nice units. I use 1/8" 1.5% Cerium doped electrodes almost without exception for almost all jobs. You can use them at low current just fine, and for aluminum they work great. With an inverter TIG, with a square wave and variable frequency you will be using the electrodes pointed. THe old school balled electrodes can work, but the newer machines are great with pointed electrodes. You will want a Sharpie Type electrode grinder. I have found Arc Zone to be a good supplier of electrodes and accessories. For Miller welders I recommend Indiana Oxygen Corporation. They have an Ebay store and have great prices. As a starter for sharpening electrodes, you can use a clean belt sander with maybe 80 grit Zirconium paper, and hold the electrode in a drill. Angle it against the belt (pointing down stream, as it were) and turn with the drill to get a nice point. A nice way to get a bunch of crap off the electrode after dipping in the puddle. I recommend AGAINST breaking an electrode!! They tend to shatter and can get spiral fractures up the rod that cause grief during welding.
For weld gas, use 100% argon. I recommend using gas lenses all the time for aluminum. It really helps keep the gas flow laminar. You typically use around 15 CFH for a 1/8" electrode in a gas lens. For a TON of valuable information on aluminum welding, see Jody's website "weldingtipsandtricks.com". He also posts on YouTube under the same name. the website 6061.com ( and youtube videos) are interesting also.
Get a good variable darkness full face electronic welding helmet. This is a very personal decision. I have several made by Miller, but there are other great brands. Be sure it can go to a minimum shade of 8. I would prefer 7 for small current stainless welding. Being an old fart, I use internal magnifiers in the helmet which help a lot. they just slide into rails inside the helmet. Do not get a fixed shade helmet- it will drive you batty in no time.
For filler metal use 5356 only on marine aluminum. Proper boat building alloys are 5086 and 5083, and 5356 works well with them. It also works well with 6061, and allows anodizing with no color change at the weld. Aluminum Distributing Incorporated (
ADI Metal Distributes Metal for Marine and Industrial Use since 1958-Fort Lauderdale, Florida) and Mcmaster can supply 5086 and 5083 in various shapes. For underwater work use only 5086 0r 5083. Above water, 6061 is ok. If you have to repair a casting (spinnaker pole, gooseneck, etc) try to get some 4943 filler rod (made by Hobart). It is an awesome filler metal for casting work. You will probably need to preheat.
For heavy material, you can preheat to get it to work well with 200 amps. Going to lower frequencies (30-50 HZ) is helpful for really heavy joints. Adding some Helium to the argon mix (25% or so) makes a huge difference also).
I could go on all afternoon, but pity has stayed my hand. Please feel free to contact me with any questions you might have. I am sure that there will be howls of outrage from people about my recommendations, but they have worked exceedingly well for me for many years. In time, perhaps you will have better suggestions. But if you have an aluminum boat, and you are a machinist, you ARE going to need a GOOD welder you can take to the boat. The Dynasty 200 or 280 (the newer units) would never let you down.
Best wishes!!
Michael