150HP slant six, fuel consumption

I recently acquired a 24' 1966 Lone Star with a 225(?) engine. The manual has great HP/RPM graphs but no fuel economy (consumption/HP) graph. Has anyone found one?

My other boat is a 21' bow rider with a 3.0L Mercruiser that is quiet, vibration free and VERY fuel efficient (1.2 GPH and my preferred cruising speed of 5 to 7 Knots) and I am very tempted to swap engines 'cause the Merc is such a nice engine to live with. I know the slant six reputation for being indestructible but noise, vibration, and economy are very attractive benefits of the Merc.

Thoughts, numbers, ideas?

Sorry, but that's horrible gas mileage for that combination! My 32 foot cruiser with twin 360 V-8s gets 1.2 mpg at 20 mph.

Jeff

PS: Having owned a combo like yours--mine was a 19 footer--I'm stunned that anyone would run off plane like that. It wastes gas running "up hill" all the time, goes abominally slow, and handles poorly. If you have a Flo Scan, surely you can see that the boat would go twice as fast with the same throttle setting.

These are both sterndrive boats, yes? If you do have a slant six, wouldn't that have the old Dana sterndrive behind it? This doesn't sound like an attractive swap to me, I'd leave the slant six in there till it dies or you can't keep the old sterndrive going. The Mercruiser engine won't bolt up to the Dana sterndrive. As fastjeff says, a boat that size will get a lot better fuel economy at 15-25 knots than it will at 5-7. For that kind of speed you want an auxilliary outboard.

Yes but I don't WANT to go fast - I want to look at the scenery!

P.S. It is a Volvo Penta 270 drive, no doubt about it.

Have you considered a sailboat? Seriously. They go that speed in a stiff breeze and make no noise doing it! If I was smarter, I might try it.

Jeff (clumsy power boat guy)

Well I am not very smart either! I would end up dangling upside down from the rigging!

Me too! Those rag boater guys are smart.

Jeff