Re: fuel consumption / engine efficiency
I disagree on a couple of points here as they pertain to 4 stroke I/Os. I would first, however, like to agree with most of Mischief's first post. The hull (when on plane) is most efficient where he says, just after going completely onto plane.<br /><br />Now to the disagreement, the fuel efficiency benefits are not because the engine is near peak torque RPM, it is most economical because the hull is most efficient at those speeds. Boat fuel economy and engine fuel efficiency are two very different things. In fact throttled, spark-ignited, engines are most fuel efficient at WOT. I didn't say WOT RPM, I said WOT. This is why all automotive manufacturers continue to gear wheeled vehicles higher and higher: to get the throttle open further at more points and most importantly at cruise speed. Lower RPM and the same load (vehicle speed) result in a wider open throttle, so there is less restriction in the intake stream and hence, higher fuel efficiency. Remember, this is engine, not boat, stuff.<br /><br />A point in fact is also noted by Mischief, almost all I/O combos are ultimately the most economical at engine idle speed. The fact is that this is the WORST point of throttled engine efficiency (throttle more closed than any other operating point other than off). Almost all hulls are most efficient at their "displacement speed", which just happens to be around idle speed in most combos. You see, hull efficiency is waaaaaaay more important than engine efficiency when it comes to boats.<br /><br />With the above said then, the fact is that if she is propped at the LOW end of the WOT RPM range (similar to what you have now), then it stands to reason that the RPM will be lower for the same efficient hull planing speed. With the throttle open a little wider (lower RPM and same load) you will receive slightly better fuel economy at that specific speed. And if propped within the WOT RPM window, theoretically the engine will not be overloaded at any speed (yes, I understand the overheat and overloading discussion, but again if within the WOT range, the boat should not have any issues at lower speeds). Yes, propping toward the higher end will make for a more flexible boat, but fuel efficiency if both props are equally efficient at a specific boat speed, will actually suffer slightly.<br /><br />I believe that you will get the most benefit by staying at the lower speeds (boat speed) you do now, probably slightly lower. If you check all of the test data you can find, you will see that most I/Os are most economical between 2500 and 3500 RPM, this again is not where the engines are most efficient necessarily, but where the various hulls are. If propped to the correct WOT RPM they are simply stuck with that RPM at the most efficient hull speed.<br /><br />I have posted similar stuff a number of times and it is usually beat up, but what I say is true from an engine efficiency standpoint. Most of the rest of the advice above is correct, but not necessarily for the reasons stated. Marine applications and peak torque have very little to do with each other. Prop load curves are relatively fixed AND there are no hills except the one you climb to get onto plane
<br /><br />Fact is, I doubt if you can make more than a 10% difference no matter what you do . . . The best way to save money is not to throw any at it that will never get paid back. This, of course, depends on how much you run the boat, but it takes a lot of fuel savings to pay back any new stuff. My .02<br /><br />Edit: Texasmark, you posted while I was still writing this, we are close to agreement based on his original question and info provided.
