There are ultimately going to be a number of factors effecting the pressure in the tank, the height of the tank (bearing in mind this is a RIB, steady object), such as the effectiveness of the breather on the tank, the ambient air temperature and the length of your fuel line.
As the optimax's are so sensitive to the amount of fuel available it seems entirely reasonable, from what we have discussed here, that if you have got a fuel filter on your transom then if you are pottering up a river for a bit then it might just die on you, due to fuel starvation.
Not an uncommon problem in the marine world?
Obvioulsy running out at sea there is not going to be a massive problem, with possibly a slight dip in top end performance. Blatantly the better fuel you put in and the more of it the more complete combustion you will get, but, with such a sensitive system on optimax's...what can happen? will it get a bit upset electronically?
anybody got any stories to share on what can go wrong?