I am attaching plywood bulkheads to the hull of my boat to provide
support for fuel tanks on a Morgan OI41. Best info I have been able
to assemble says to put foam under the plywood before tabbing them to
hull. Question is: What sort of foam? Thickness of foam? Seems to
md that this application would require something denser than the
standard sheet foam available in home supply stores. Any suggestions?

wrote:

I am attaching plywood bulkheads to the hull of my boat to provide
support for fuel tanks on a Morgan OI41. Best info I have been able
to assemble says to put foam under the plywood before tabbing them to
hull. Question is: What sort of foam? Thickness of foam? Seems to
md that this application would require something denser than the
standard sheet foam available in home supply stores. Any suggestions?

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

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.

On Wed, 05 Mar 2008 15:31:15 -0500, wrote:

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

Gasoline weighs six pounds per gallon. Diesel is closer to seven.
Propane is four pounds., Methanol slightly more.

Casady

On Wed, 05 Mar 2008 13:56:36 -0500, wrote:

I am attaching plywood bulkheads to the hull of my boat to provide
support for fuel tanks on a Morgan OI41. Best info I have been able
to assemble says to put foam under the plywood before tabbing them to
hull. Question is: What sort of foam? Thickness of foam? Seems to
md that this application would require something denser than the
standard sheet foam available in home supply stores. Any suggestions?

The foam simply acts to prevent the bulkhead from joining directly to
the hull and forming a "hard place". The bulkhead is held in place by
fiberglass tape glassed in place. As fiberglass cannot be folded
sharply a fillet is used to create a radius that the glass cloth will
form to. You could either form the foam to a radius or use filler to
form a radius.

The explanation for this is that if you attach the bulkhead directly
to the hull you are concentrating any force on an area the width of
the bulkhead. If you separate the bulkhead from the hull and then tape
it into place using at least 8 inch tape (4 inches on the bulkhead and
4 inches on the hull), on both sides of the bulkhead, you have now
spread the load bearing area out to an area 8 inches wide.

Since the foam does nothing but space the bulkhead away from the hull
any foam what will withstand epoxy will work. In fact if you could
simply leave a gap between the bulkhead and the hull and tape it into
place except that it is difficult to keep things aligned.

The so called "tape" overlaps both the hull and the bulkhead by a
minimum of at least 4 inches, and preferably more, and will be rather
substantial. Depending on the bulkhead I would use, say 3 - 4 layers
of 600 - 800 gm. roving, each layer being somewhat narrower then the
previous - say 12", 10", 8", 6". The taping being applied to both
sides of the bulkhead as far as possible.

If you do this you will have built up an attachment that is stronger
then the material the bulkhead is made from so there is no reason to
drill holes and thread pieces of tape through the bulkhead.

If you are in doubt make a test structure. Use two pieces of 6" X 12"
ply and assemble them as a "T" using the above schedule of bonding.
When it is cured try to break it.

By the way. You MUST use epoxy resin and woven glass cloth. polyester
resin and chopped mat will make a much weaker bond.


Bruce-in-Bangkok
(correct email address for reply)

I'd suggest using a structural foam that has some strength in
compression,like 5 lb. density Core-Cell, maybe 3/4" thick. Rip trapezoidal
strips, 3/4" wide (bulkhead thickness) on the top with 45 degree bevels.
Bond it to the hull with thickened epoxy. If you need to kerf it to get it
to bend to the hull, that's OK. Then, clean it up and glass your bulkhead
in with several tabbings of 1708 glass.



wrote in message
...
I am attaching plywood bulkheads to the hull of my boat to provide
support for fuel tanks on a Morgan OI41. Best info I have been able
to assemble says to put foam under the plywood before tabbing them to
hull. Question is: What sort of foam? Thickness of foam? Seems to
md that this application would require something denser than the
standard sheet foam available in home supply stores. Any suggestions?

I am attaching plywood bulkheads to the hull of my boat to provide
support for fuel tanks on a Morgan OI41. Best info I have been able
to assemble says to put foam under the plywood before tabbing them to
hull. Question is: What sort of foam? Thickness of foam? Seems to
md that this application would require something denser than the
standard sheet foam available in home supply stores. Any suggestions?

When I rebuilt my Columbia Sabre, I bought airconditioning line
insulators.
http://www.acehardware.com/product/i...ductId=2126067
I slit them lengthwise, and got 4 strips per insulator. hot glue or
spray adhesive to hull or bulkhead edge. Doesn't really matter which.
All you are trying to do is to create a pocket between the edge of the
bulkhead and the hull, to prevent.....that's right....a hard spot. The
fillet and tape will supply the strength.
Larry Wilson
Columbia 8.7
Columbia Sabre
Urbanna,Va.