The honest answer is kinda...
its to do with the proportion of the fuel which is used for propulsion as opposed to that used for heating.
My boat has a diesel heater which i leave running to keep the thing dry and warm therefore 40 % of the fuel is at a lower duty rate as heating fuel attracts a lower duty rate and 60% of the fuel i burn is for propulsion which attracts a higher rate.
The declaration us up to the owner. in theory i could lie on the form and claim 100 % heating at the lower rate but this would more than likely come back to bite me (or anyone else who tries) on the butt!
with the rib (which had no heater) we had to claim 100% propulsion at the higher rate...
does this make sense??