Joost,
Your observations are correct, but that is the key. Actually the forces we
are talking about are low, but when applying force, it must be relative to a
reference point. I envision you drilling bolt holes through the deck just
outside the perimeter of the new plate. Then tack welding the heads of the
bolts below deck. These are not permanent. Then drilling large holes for
bolt clearance through 6" long steel bars and with mating nuts and washers ,
pull the new plate, relative to the deck itself together. At the same time,
using a hydraulic porta-power or screw jack acro, push upward from the frame
floor at the center of the new plate. This should be sufficient. In the case
that you find the deck rising under this upward force, this can be
compensated by attaching chains on either side of the new plate under the
bolts heads you now have at the new plate perimeter. The other end of the
chains can be bolted throught the floor structure you are jacking against,
but I really do not think this extra bend resistance will be necessary. In
this manner, you are concentrating the forces exactly where they are
required. This job is really not difficult. You will spend more setting up
to do the job than the time it takes to actually make the repair, but this
is absolutely normal.
Steve

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

Thank you for your very detailed answer to my question. This sounds
like a good method. The only concern that I have is determining the
support points on the original structure. As this is the same material
as the new piece to be welded in I´m afraid that it will bend as well.
Ofcourse the original structure is much larger, the whole deck, but
still I´m not sure that it will not be bend also. Can you explain this
a little bit more for me?

greets, Joost

On 27 dec, 10:50, "Steve Lusardi" wrote:
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