There are two kinds of patch, over-lay and inlaid. The overlay patch is just cutting up a piece of steel that is at least as large as the repair area needed and smacking it on over the top and welding all around. Cheap and nasty.

If done properly, an inlaid patch can not be discerned from the surrounding original work and if under water will not slow your boat because of an uneven surface. It will also be as strong as the original work because it will be fully welded, that is, inside and out.

This is a description of how WE did it.

 First cut out the rotten steel. Be sure to use a respirator and ear and eye protection!!! Besides the flying bits of steel, the fumes from the burning paints and other nasties will bite you. Ask me how I know….

Try to leave fairly straight edges and insure the steel is clean and wholesome all the way around to make a good weld. Transfer the dimension to your new steel by tracing with a piece of cardboard or bracing ply or just measure it up, whatever works for you. It doesn't have to be perfect. The bottom edge needs to be close enough so that it will rest in place to start but much of the rest can be adjusted as you go with an angle grinder. Remember, this is butt joining so a gap of at least 2mm to 1/8th inch is perfect. You can get away with up to ¼ inch depending on the thickness of plate you are using. The range of hull plating used is usually from 1/8 to ¼ inch or 3 to 6 mm. Ours was 3/16 or about 4,6mm.

Because I bought a second, better welder before the bilge work, I had the convenience of leaving my weaker machine in the boat so I didn't have to lug around the other machine. Outside I started the welding by installing the “dogs”. These are small plates temporarily welded in place to support and position the new steel. As you can see in Step 1, the dogs at the bottom are just rectangular pieces while the ones above are “L” shaped. Working inside at the bottom, the bottom dogs should keep the steel in line, or a little pressure against the new steel with one hand while you tack with the other should do. Use your touch and eye to judge the alignment. Get it right. Make the tacks inside as shown on Step 1. If you place your weld on just one side of the dogs they easily knock loose later with a hammer.

 Next I would yell to Kay who would be outside with a hammer, to drive the wedges in the “L” shaped dogs to force the steel into a slight curve. As soon as the surfaces aligned about 6 inches up the plate from the last tack, I would have her stop and install another set of tacks and repeat that until I tacked right under the “L” shaped dogs. Then I would go outside to smack off the “L” shape dogs and weld them higher up and start again. Inside again and with the plate tacked all around and satisfied the edges met well, I would cut back the tacks to reduce bead height and prepare for welding all around. With my small MIG running on moderate power, I could weld a pretty long line before stopping to let the metal cool to prevent distortion.

With that done I would go outside and take an angle grinder with a cutting blade and use it to “groove” out the line all around the patch. I do this to get a more uniform width and depth of channel. Then fire up the good machine. For overhead welding you need a different arc then what you would use on a horizontal surface. Many will advise to reduce your current and slow the work right down but the machine I got was so grunty and I was so sharp from having to work with a rubbish machine before, that I just wound up the wire feed speed and ripped! I use good magnification and my jeweller's background was helpful. If you get it right the weld lays flat and dense, no bubbles, pits or globs hanging off. The bead width should be consistent. It should be pretty!

To prevent distortion you have to work in sections. The length of weld can vary depending on the current used and nature and thickness of steel. I was going about a foot (bold but a mig is gentler, if using a stick welder you may keep it shorter) and then doing a line on the opposite side of the patch. To make a smooth start on the next section, I would cut back the end of the previous line with the edge of the cutting blade to form a small groove then start the next weld with about an inch overlap.

Kay and I did over 40 square feet of curved bilge that way and no one could ever tell we had done it at all once it was painted. And in spite of it all being done on the rush on a slipway that the local fishermen would get nervous about being tied up for long periods… we never had a leak.