It's not worth the effort to switch over to Biodiesel. That F.L. only stops at fuel anyway. Like about a gallon an hour. It takes time, space and effort to get and convert fryer oil to biodiesel. If you are living on a boat, you don't have the time. You certainly don't have the space and energy is better spent on working on the old boat.
Fuel, as it relates to the expense of owning and operating a boat is the smallest expense. No need to complicate it.