Trailer repairs

[aquaplane]

One of the bits of box section on my 20'er trailer has rusted and needs some work.

The previous owner did some patching over a 12" section but holes have appeared over about 6' of box section.

Anyway, I'm looking at cutting out the bit that looks rotten + some more and replacing it.

The existing steel is 1½"x3½" 1/8" wall. Galvanised.

The nearest I can find to replace it is 40mmx80mm 3mm wall. Will it do?

I haven't shown it to the bloke who has said he will weld it yet, I suppose he should have an idea, I'm looking for 2nd opinions.

TIA

[mm5aho]

The box section you mention will work OK. There might be a small "step" where the two sizes join, but that shouldn't be a problem.
The metric sizes, which have been around for about 20 years now, are not an exact match to the old imperial units, but they're close.

In order to then match the old parts that are galvanized with the new, you could consider galvanizing the parts you weld in, then use zinc rich paint on the welds. That would be better than just painting the replacement steel, but there is the zinc fume from welding to watch for (a welder will know what to do).

What some people fail to do with boat trailers that are galvanized, is make sure that the seawater can drain out when taking the boat out of the water. This is different to the holes added for galvanizing. If salt water stays inside, then the rot starts internally. This looks like mostly internal rust.

[SteveN]

Bigger in one dimension and smaller in the other isn't ideal, but I'm sure it's do-able. Is this a road or a yard trailer?

[aquaplane]

It's a road trailer.

I have shown these pix to the welder and he suggests getting a piece of 3mm bent to L section and plating over the rough stuff. I'm not keen but it could be a goer.

[Pete Cooper]

I reckon plating over the rough stuff is a bodge - cutting it out and replacing with new is a better fix.

[mm5aho]

Yes, plating over is a bodge. You'll just get rust from inside out from the water lying between the plated bits. Avoid narrow spaces that water sits inside.

[BlowingOldBoots]

The trailer is junk. Unless you inspect the complete trailer and determine that the wall thickness of the remainder of the steal is satisfactory for the loads it will be subjected too, you have to ensure it is unsafe. If it was me, based on the pictures, I would buy a new trailer.

Hit the rest of the trailer steel really hard with a hammer and see if any of the steel gives way that is not obviously rusted. Of course, it may be OK now but my the end of the season it may have rusted thin where it passed the hammer test today. You can get into big trouble with the law over unsafe loads. Spend the money on a new trailer.

Having said that, the new pieces could have smaller box sections at the joint which slip into the old box sections to allow the piece to be centrally positioned before welding. Some Series III Landrover replacement rear sub frames came like that. It allowed one to chop the back off the frame at a known area which was not prone to rusting, slip in the new frame and then have it welded with a fair chance of it being aligned correctly.

[aquaplane]

The rusty stuff was cut out but the corners left for something to weld to.4mm plate welded over the gaps. It looks a bit heavy duty but it'll do.
Only one bit is affected, almost as if no galv got into that piece.

[ash]

Glad to hear that you got something sorted.

You will be hoping that it is short time for you anyway because the boat and trailer are up for sale.

Ash