I know I want longer range vs shorter range, in my future boat. I was asking for an easy way to estimate boat ranges, all things being equal: seas, wind, etc. This helps me in my boat search. Someone already provided a link that said each 18 hp roughly burns .5 gal per hour. So if my engine is producing 36 hp at cruising rpm, then it will burn 1 gal per hour, at cruising speed. From there I can determine average range based on tankage and cruising speed. Does this not sound reasonable?
Now I'm interested in the fuel return line or no return line controversy.
I have a Westerbeke 30b, which is really 27hp not 30, go figure.
It's got an electric fuel pump that puts out 20 gph, no governor, always on, so it is always putting 20gph through the system. I have an observed and measured fuel burn rate of .75 gph at 3000 rpm.
That means that 19.25 gph are being filtered and returned to my tank per hour. I've never seen a diesel that doesn't have a return.
Max RPM is 3600, so I'd be taking and educated guess, but still guessing to say at 3000 rpm I'm making 22hp or so, and putting about 18 of that to the prop (alternator, transmission, drive leg losses siphon off the rest).
I can switch my supply and return paths independently between my two 18 gallon tanks, so I can take from tank 1 and return to tank 2 - in effect polishing (filtering) the fuel. Of course in 1 hour or less I run out of fuel in tank 1, and I better hope tank 2 is empty when I start
I do that sometimes just to filter the fuel, the engine doesn't have to be running, just the fuel pump turned on.
The Volvo's return is just called a leak off I guess, and maybe it's just 'leaked off' to the supply side line. I'd be curious how they do that with different pressures being involved.
As to range - 3000 rpm on mostly flat water gets me about 6.8 kts, if I can use all 36 gals (unlikely level of efficiency there) and could actually find enough flat water to run for 48 hours (36/.75) I could in theory make 326 miles as my range. I typically run at 2800 rpm, 6.5 kts in flat water meaning 6 kts or less in the choppy Chesapeake Bay. The bay takes off .5 kts going north, adds it back going south due to current.
Being conservative I figure my range is 250 miles, I can double that with interior, ventilated jerry can storage (my aft lazerettes are huge) but I'd have to put 250+ pounds of fuel right where I want it least.
I can make that range infinite by simply raising the sails.