Joost,
This is a lot easier than you envision. In fact, it is primary reason that
steel is the premier material for hull construction. Of all the materials it
provides the easiest most economical repairs. Steel is actually very plastic
in nature. If this repair had to done in wood, it would be impossible unless
laminated and the joint would be very complex for the necessary strength.

I have built a 58' round hulled sailing sloop in steel using 5' x 10' sheets
of 4 mm steel with severe curves in multiple directions. There are several
strategies and without seeing the job, I cannot recommend the one that is
best in your case. I will suggest a few rules to follow, but you must do
this cold and you must leave a minimum of a 2 mm gap between the replacement
piece and the existing structure all around before welding. This is
important because it prevents the cupping that you mentioned. Secondly, the
heat of welding will relax any stresses in the material at welding time and
will create high spots at the weld after completion. The last thing you do
before the final welding is cut the plate to size. Initially cut the plate
several inches larger in all dimensions than the hole left in the deck. The
material of choice should be low carbon mild steel, as this will have the
lowest memory and be very ductile.

The tools of choice are the wedge and the jack. The action you want to use
is stretch, not shrinkage, as stretch is much easier to accomplish than
shrinkage. While forming the replacement piece, do not tack weld it. You
must allow the plate to move as you force it to assume its new shape.

The next thing you must determine is the support points for the applied
forces. You want to stretch the new plate not the stucture. Feel free to
drill holes, weld threaded rod, loops for prying and clamping and other
fixtures to the structure as they are easily removed after the repair. Find
a stiff, strong, well supported point beneath the plate inside the boat for
the jack or hydraulic ram. You may have to construct that as well. Try to
use the frame floors for this support.

Place the new plate over the hole, position the support jack so it bears on
the center of the plate and with your attached fixtures, pull the edges down
to the structure in an even manner. This force should provide 20-30% more
distortion than you want, this will allow for some residual memory in the
plate. Now this part is feel. You just have to know how much is enough. When
you have applied all the force either required or as much as the structure
will stand without damage, cover every thing up and go home. That's the
trick. Come back in 24 hrs or more and jack it some more and go home again
and wait some more. The plate will, over time self anneal. When the plate
has your desired shape, then mark it out from underneath and cut it 2 mm
undersize all around. Weld from inside first, grind out the weld from deck
side and finish weld from the outside. Piece of cake.

We can have another discussion on the problems and solutions of teak over
steel, because the teak application has caused your problem. Also if you
have created a severe low spot making this repair, which if done correctly,
should not happen, but if you did, take a grinder with a cut-off disk and
split the plate in the offending direction(s), jack from underneath and weld
up the slots as before.
Steve



"joost" wrote in message
...
Hi everyone,

I also posted this question in rec.crafts.metalworking, but perhaps
there are here also people who can advise me.

I recently removed the teak dek from my steel boat. Although most of
the steel of my 4 mm mild steel deck is fortunately still there, but
there is one place where a section of about 20x40 cm is completely
eaten away by rust. I´m planning to cut out this area as
far until the surrounding steel is again 3-4 mm and weld a new piece
of metal in there. Only complication is that the deck is slightly
curved. Not much, but enough to make a flat piece of steel plate look
ugly there.

What I would like to ask is what the normal procedure is to fit a new
4 mm mild steel piece in the hole following the curve of the
surrounding area. The curve is mainly in one direction and in the
middle approx 1 cm away from the "straight line". Perhaps there is
also some (much smaller) slight curvature in the other direction.

I thought about preforming the metal on a roller, but measurement is
not easy for that. I´d rather take some in-situ approach. Only the
material is too thick to deform with simple tools. Perhaps welding
some sort of long lever on the plate and use that to curve it while
tacking every spot that is on the right position? And cutting the
lever away afterwards? Or is it possible to use a propane burner to
heat the material and have it bend itself?

Related to this I´m also not sure if any problems might occur after
the piece is tack welded in the perfect position. Can it deform while
make the final welding all around? I´ve heard that in bad repair jobs
like this, the curving "flips" inward.

I´d be very happy with any comments on this.

Joost