I believe that Ferry companies do not allow you to carry spare petrol cans and request that gas cylinders are turned off when the vehicle is being transported, this is a condition of them ferrying your vehicle. I have to say that I have always had a spare petrol can for the generator and have never turned off the gas cylinder, indeed at one port I was asked if I had any secondary fuel supplies, when I told him I had a can for the generator he said "I will say I didn't hear that" and then proceeded to tell me which lane to enter. They must know that not turning off gas bottles and one or two spare petrol cans would be the least of anyones worries if the ship foundered or caught fire with 200 odd lorries and cars with full tanks of petrol and diesel already on board. Phil
As for turning the gas off when travelling, most underslung tanks have a crash valve built in, so that if there is an excessive release of gas due to a pipe failure it auto closes a valve in the tank, however this is pointless if there is a catastrophic breach in the tank wall itself in the event of an serious accident.