All other things being equal, such as boat length, engine size, driving and use, etc. which gets better fuel economy a full inboard or stern drive? Or are they roughly equal? Specifically I'm refering to ski boats and run abouts in the 19' range.
Here are the things that I believe matter to fuel economy
Size of motor
Weight of boat
Prop
Final gear ratio
All things being equal I would think the draft on the inboard might cause alittle more gas usage, just a thought! No scientific evidence to prove.
Last year I had a 4.3 Carbed in an I/O. This year I have a 5.7 Black Scorpion Fuel injected. Last year?s boat was 18; this year?s boat 22'. It appears this year?s boat is getting better mileage. Fuel injected more efficeint than carbed.
The stern drive will get better fuel economy simply due to the ability to trim the boat up out of the water. The OP said all else being equal, so the only real variables are weight placement (engine mid-ship vs stern) and trim. The trim is what gets you the fuel economy. In the long run, the fuel savings is rather miniscule, so I wouldn't make that the determining factor in which boat to choose. The ability to trim also allows you to make rough water conditions more pleasant.
not many inboard diesel ski boats! Actually, i'm fairly sure there are none....
Technically, the inboard is more efficient, but... Ski boat hulls are much 'wetter' than a typical I/O, and the prop/gearing on a ski boat is much lower, they need to do about 38 mph tops for a dedicated ski boat. All things considered, if you take two typical off the shelf bowriders, one I/O, one inboard, same engine, the I/O will get better mpg's.
A question- you said ski and runabouts. What are you really looking for? cause a ski boat, a wake boat and a run about are actually different things. Sure it gets confusing because everybody claims their boat does that.
An inboard ski boat or wake boat are going to be better in every way when used for the purpose they were designed. An in/out runabout is going to more fuel efficient when "running about".
