Welding Repair to Deck

[Underdog]

I know Salty Monkey has started a thread on general steel boat questions and they have run to welding topic there, but I have a specific question I am hoping someone may have experience with.....

Doing a refit on my steel boat and last couple of days decided to remove all from fwd lazarette/anchor locker. There was some corrosion under the deck where the hawse pipe passes through. The anchor winch and hawse pipe area are on a 12mm pad of steel which looks like it was welded to the 3mm galvanised deck, with obvious stringer/support framing beneath. Trouble is the corrosion had blossomed (as it does) outside the 12mm pad, and when I hit it with the needle gun it went through the 3mm deck.
I want this fix to be as near to permanent as I can get it.
My thoughts are either 1) to cut away the entire 12mm pad, and the 3mm deck sheeting beneath and replace it with a larger (in size) 12mm one that gives more 'meat' for any corrosion issues that may (if I get the lockdown less that 100% correct) occur. Or 2) to slice away the corroded section only and join a section of 12mm on to the existing edge, obviously with supports underneath welded to this as well.
I have a mate that can precision cut the plate for me, camber it as I need etc.

Any thoughts on what might be the best way out of my ideas (or any other better ones) to go, and if there are any issues I need to think about joining galvanised steel 3mm decking to the 12mm pad?
Also what precautions are needed when stick welding a steelie? Do you need to disconnect/isolate 12v and/or 240v systems when using the welder?

[DeepFrz]

Sorry I don't have any experience so can't answer your questions. If you don't get an answer here, try the Metal Boat Society. There are some very experienced guys here though, so you should get an answer.

Welcome to the Metal Boat Society

[billwa]

I have welded my boat (while in the water) in several places. Have never had to take special precautions, and have never had any problems - just make sure I have a good connection on the welding return curent lead and make sure it is clamped to the weld as close as possible.

This started as emergency repairs to things like exhaust and engine mounts, now I do fairly frequently.

I stay away from any part of the boat that keeps the water out though, but to a good welder even these areas shouldnt present a problem.

[boden36]

If the bit that has rusted was 3mm and was doing the job OK, cut it out and replace it with another 3mm bit. Use a thin cutting disk on an angle grinder, and position a wet towel to catch most of the sparks, or you will have a very rust-flecked deck.
Use short runs of weld and a good fitup to minimise distortion.
Regards, Richard.

[Wand]

If it's just a small area and there are no structural issues, I suggest some of that epoxy car filler bulked with stainless steel shavings or fibreglass. It's cheap, makes a strong mend, goes a long way and will never rust.

[Hillbillyfunk]

long time welder here, I'll bet that the corrosion has gone into places that cannot be seen, if it was m boat I wold take off the and replace the entire pad, a thicker one can't hurt. Grind/cut at least 15mm more around what you are going to remove, remove ALL of the corrosion, that sutff is cancer, you can't just pretty it up with bondo and expect it to be all right.

if grind off the galvanization around the the repair are as well, that stuff doesn't weld nicely you'll have to paint over it once it's done. It is not nessisary to unhook your AC/DC things bt it can't hurt, do you want to risk and an accidental spike blowing through your chartplotter/radar/ect?

keep a fire extengisher and water nearby, take a fire watch when you are done

[captjcook]

Take your compass off the boat when welding! Avoid any doubling of plates or creating inaccessible voids.
Cheers!

[capt_douglas]

I work on a steel boat, so I may be a bit excessive but when we find rot like that we tend to cut until we find good steel, then weld in a replacement. Since the 3mm deck is rotted and you can have some heavy loads I'd consider going with 12mm if you can afford it. Another problem may have been the galvanize 3mm deck. Welding can vaporize the zinc and promote rust and rot (BTDT).

As for precautions, we always weld to a dead boat (nothing powered up, not connected to the dock) and have several folks around with fire extinguishers rated for hot metal as well as a hose or two. I've seen more than one boat burn while at the dock due to heat or sparks.

If you're going to be welding in a confined space then proper ventilation and breathing gear are a must. I'll assume you're going to take proper protections steps for your body.

[Underdog]

Gentlemen (and Ladies, if applicable),

Many thanks for the advice. I have decide to remove the entire 12mm pad, cutting through the 3mm steel is much easier than attempting 12mm, going out into good clean steel and enlarging the pad size and maybe thickness as well. I will weld to the stringers under the deck and make a flat pad, obviously with a higher raising on the sides where the cambered deck falls away from the horizontal. A single, angled transverse brace added to the existing under-deck support structure will ensure integrity for the cleats I have positioned in that area, and welding to the edge of the (galvanised cleaned off) deck steel will avoid any inaccessible voids. Your points on how to set up the cutting are helpful. I will use a wheel cutter and hand finish near the under-deck supports, building a screen area around the cutter, with wet towels etc to reduce spray of rust streak forming bits. I will de-power and disconnect the ship as a precaution and remove the compass (I hadn't thought of that possibility!).
Once welded, using a MIG, I will do the usual lockdown & prime and paint etc, and fit a removable sleeve to the through deck hawsepipe hole to allow for periodic inspection and treatment as required. Hopefully this will make the area secure for at least another 17 years!
Again, many thanks to all of you for your input, but if you see any flaws, or better ways to build this particular mousetrap, please keep it coming.

[Wand]

Originally Posted by Hillbillyfunk

Grind/cut at least 15mm more around what you are going to remove, remove ALL of the corrosion, that sutff is cancer, you can't just pretty it up with bondo and expect it to be all right.

Can't agree hilbilly. Rust is not cancer; a piece of rusted steel denied any moisture will not infect neighbouring good steel. I'm looking now at a rusted end of a corrugated sheet that has been protected from the weather (and therefore dry) for years; the rust stopped when the source of moisture was removed. In my experience with rust holes on steel boats, if you remove all flaky rust and then treat with an everyday chemical rust converter, a stainless or glass reinforced epoxy filler will provide a strong and permanent replacement for small holes in 3mm steel plate decking where there are no structural issues. And if you make sure that the cause for the original rust (ie, moisture pooling) is removed, you'll never get rust there again.