IO/OB vs 4 stroke ob

[jb426]

Buying new boat. Which is a more fuel efficent motor. It will be either a 220 hp io/ob pushing a 20 ft fish and ski or a 150 xl hp mercury four stroke pushing a 20 ft tracker targa (alum). Not looking for advice on which boat to buy just in general which is a more fuel efficent motor

[boatfixer]

When you say 220 hp, which one? The 4.3 V6 or 5.0 V8? Because the V6 is MPI and the 5.0 is carbed.

[jb426]

4.3 merc cruiser 220hp.

[boatfixer]

The short easy answer is the 150 hp will burn less than the 220..

Outboard or I/O, an easy calculation is to take the hp and divide by 10, or just remember 10% of the hp at WOT. That's gallons per hour. Obviously, things like weight, lake conditions and throttle position will all be factors in actual fuel consumption, but the most accurate way to draw a comparison is to figure the theoretical maximum amount of fuel the enige can burn in identical situations at wot.

Another calculation uses fuel weight and a specific fuel consumption rate.

At peak performance, a gasoline engine can burn about a half pound of fuel per unit of hp per hour.

Gasoline weighs 6.1 lbs per gallon

So, GPH=(fuel consumption rate x hp)/ fuel weight

GPH = (.5 x 220)/6.1

This calculation is supposed to be more accurate.. It shows less consumption than the 10% way but with fuel injected engines, it may be a more accurate way to figure.

If fuel consumption is your only concern, this is as good as I can do. Other things like maintainance are other things to consider, not sure where you stand there.

[delcecchi]

And one other thing.... An OB is lighter and easier to work on that an I/O stern drive.

One more thing, the bellows on an I/O will sink the boat if it goes bad. And they seem to be painful to replace.

[jb426]

thank you. exactley what i was looking for. im leaning towards the ob