Is the tank built into the boat, or is it a self contained removable one ?
If it's removable buy a second fuel tank and strap it down on the deck. When required just swap the the fuel line to the second tank on the deck (with an extension to the fuel line if needed). Fuel hose and connectors are readily available in chandlers.
I would guess you would be looking at roughly 0.75 litres per nautical mile with your boat and engine. It will be a bit more thirsty at WOT.
Rib Tigga an Avon SR4 Rescue with Honda BF50
MMSI 235910073 Callsign T123046