Here in the US, you cannot place the fuel tank in the same compartment as
the motor(s). If it is plastic, it has to have at least passive ventilation
with scoops facing fore and aft. Regardless, if it has a fuel sender or
other electrical wiring in the same compartment, it has to be force
ventilated and there are rules for the sizing of the vents and on flow
rates. Since it sounds like you are using gasoline, it might be worth your
while to have a professional look at it since regulations vary
country-by-country.

Something else to keep in mind is that the original designer put the fuel
way up in the bow and may have taken this into account when he did his final
design (longitudinal center of gravity). How's the boat float and run when
it is empty and the bow tank is low on fuel? Any tendency to porpoise? If
it's sits reasonably level at rest and is stable and satisfactory in this
condition when using the boat, then you should be fine putting the fuel
amidships as you are. As a rule of thumb, try to keep the fuel's CG at
around 1-1/4 waterline beams forward of the transom. If your new fuel tank
location moves the CG too far aft, then the most likely affect will be a
tendency to porpoise ...and the porpoising may come and go depending on
speed, will likely be more likely in smooth water and may disappear in a
slight chop (since it lengthens the effective wetted length of the hull).
If your boat sits too low in the stern (a swamping hazard when in rougher
seas with people standing in the stern ...easier for waves to come in), or
it shows a tendency to porpoise (especially if it gets violent), then you'll
want to move your fuel back forward -or- always carry weight in the bow
(ballast) to compensate.

Have fun,
Brian



"Hans-Joachim Sellner" wrote in message
...
Hallo!
I would like to get some advice concerning gasoline-tank-installation in a
motor boat:
After completing the repowering of my boat with a dual-caburated motor I
looked for a reliable position for the fule tank, previously installed in
the bow by the manufacturer. Because there was enough volume near the
center of buoyancy, I decided to place the tank there, tightly screwed and
taped on the extensions of the motor-stringers. Although this position
might be physically the best one, fuel-lines could be held quite short and
servicing is easy, I've some strange feeling: The tank is in direct
neighbourhood in line to the engine, there are at least 5 inches between
them, no separating wall or plate made of epoxied plywood or alu, steel,
... is installed. Now my question:
Are there any rules or regulations (coast guard) prohibiting the
installation of a fuel-tank in nearly the same compartment with the
engine?
By the way: Of course the tank is connected to motor-ground as well as the
copper-lines and the water-separator are grounded to the tank. A correctly
dimensioned blower to suck gasoline-fumes is installed.

What do you think, anything left I've to work on? Any advice will be
appreciated!

Bye, Hajo