Originally Posted by RussH
Trying to get an idea of fuel usage between a 260HP 350CI engine compared to a 340HP 454CI engine. Would the larger engine use 20% more fuel, 30%, other?
I am thinking about buying a 27' Sportcraft w/ inboard. It has the larger 454CI engine in it but I can buy a smaller 350CI engine for $1,800 that has recently been rebuilt. The larger engine is going to use more fuel than the smaller engine but would it be worth swapping them out? There is nothing wrong with the 454CI engine, it has about 700 hours on it and has never been rebuilt.
It all depends on whether the 260 hp engine has enough power to get the boat on plane and run it without the secondaries opening up. The general rule of thumb is a larger engine running at lower rpm will use less fuel than a lower hp engine running at higher rpm.
As Tony at Seaboard says props move boats, engines only turn them. The larger engine will be able to turn a larger prop which should give the same speed at a lower rpm than the smaller engine.
This boat will require the same hp from each engine to move it at a given speed. Since it is a single inboard prop slip is always something to be concerned with. The boat with the smaller prop will slip more and be less efficient.
It looks like I'm steering you to the boat with the larger engine doesn't it?
Take both boats for a seatrial and run them at say 20 kts. Take a look at the rpms. You should hear the secondaries open up on these engines. If they are open at cruise speed, it will use more fuel than if they only run on the primaries. I'd want to cruise at 3000 or less.
Try to determine what the gear ratio and prop that is installed on both boats.