On Wed, 05 Mar 2008 20:56:00 +0100, Heikki wrote:



Well, I am certainly no expert, but as far as I have understood it, the foam
does not matter much at all. The important thing is that the tabbing does
not try to make a sharp 90 degree turn, but a round curve, or at least two
45 degree turns. Doesn't matter so much what is underneath, as long as the
tabbing gets in the right shape.

I'm sure someone will correct me if I am talking total nonsense...

-H

The only purpose of the foam is the prevent what is known as a "hard
spot" on the hull. Otherwise you would just tab the plywood directly
the the FGP of the hull itself. A trick I picked up here on the web
is to cut a few slots in the plywood about 3/4" up from where it joins
the hull and about 3/8 of an inch wide. You then run some fiberglass
from one side to the other through the slots. This strengthens the
anchoring of the bulkhead to the hull. I am very concerned about
doing this right because the bulkheads will provide support for the
tank (100 gal. - about 500 lbs.). In my case I had to have three
tanks made to replace the single original tank which was installed
before the deck was put on. This allowed me to get them into the
engine room where the tanks are fitted vertically against the hull. (I
know this is not the optimal mounting position for fuel tanks but I
did not design the boat. I am restricted by the original design. This
bulkhead placement of the tanks lessens the righting moment of the
hull by placing the weight too high above the center of gravity and,
to my mind, is a design flaw but one I am more or less stuck with
short of redesigning the entire interior of the hull. The original
tank was foamed in but everything I have studied says that this is a
terrible way of mounting tanks because the foam holds water against
the aluminum tanks and accellerates corrosion. By installing these
bulkheads/ribs I can run stringers between them and fit the tanks
against the stringers and have the support this weight will require.
Even if I had decided to foam them in it would have been impossible
because with the deck on there is no space to get back there to pour
the foam. Anyone having to replace a fuel tank in an OI 41 certainly
has my sympathy. I am going to end up spending as much time getting
these tanks in as I will spend on the installation of the new motor
that is going in.