Re: Up a pitch for better GPH/MPH?
That was very well stated
Yes it was, nice to see a little bit of reality on these threads every once in a while! Good job Aloysius!
I don't think that anyone is claiming that overpropping or underpropping isn't harmful to your fuel economy or engine if taken to extremes. I know QC and I agree that both extreme overpropping and extreme underpropping are detrimental.
What we are referring to is that the manufacturer's specify an RPM "range" within which to prop your engine, typically 4600-5000 RPM.
We're probably talking 100 RPM either way at cruising speeds. In most cases the effect will be minimal. Propped anywhere within the recommended WOT RPM range, top speed will vary maybe 1 or 2 MPH, acceleration will vary maybe 1/10 or 2/10 of a second to 30 MPH, and fuel economy will vary maybe 1/10 or 2/10 of a gallon per hour at cruise. How important any one of these particular parameters are to you will dictate how much hassle you put into propping your boat within a "gnats ***" of the optimum RPM within the range.
If your engine makes max power at 4800 and it turns to 5000 at WOT, then you are technically "underpropped", and your boat will probably accelerate a liitle better at lower speeds such as pulling up skiers, similar to a car in a lower gear. If you're turning 4600 RPM at WOT you are technically "overpropped", and your boat will probably get a little better fuel economy at cruising speeds, similar to a car in high gear. If it turns 4800 at WOT you are right at the power peak and you are propped perfectly for top speed. You'll give up a little tiny bit of acceleration at lower speeds to the smaller prop, and give up a little tiny bit fuel economy at lower speeds to the bigger prop. But you'll be 1 or 2 MPH faster at top speed.
At cruise speeds, the motor with the bigger prop will be turning less RPM, therefore requiring a wider throttle opening to deliver the same amount of power as the motor with the smaller prop turning at a higher RPM.
As Aloysius pointed out, this wider throttle opening will result in lower manifold vacuum. If the lower manifold vacuum is indeed sufficient to activate the enrichment circuits, then the smaller, higher RPM prop could get better economy. Again we're probably talking a difference of 100 RPM, in most cases atmospheric conditions and temperature will have a bigger effect.
On the other hand, if the difference in manifold vacuum is NOT enough to activate the enrichment circuits, then the lower RPM engine with the wider throtttle opening will be suffering less pumping losses and will deliver better economy.
It's no different than a car, truck or airplane. If you're geared or propped for acceleration, you won't get max top speed or max fuel economy. If you're geared or propped for top speed, you won't get max acceleration or max fuel economy. If you're geared or propped for fuel economy, you won't get max acceleration or max top speed.
You only get one of the three with a single gear ratio and fixed prop like you have on a typical pleasure boat. Cars and trucks have transmissions and airplanes have variable pitch props to overcome these limitations. You could do the same thing on a boat, but the gains aren't worth it in most cases. The prop slipping in the water acts like a torque converter in an automatic transmission and minimizes the effects of the single gear ratio. At lower speeds it slips more, which effectively raises your gear ratio. For example, from a dead stop my boat has about 90% prop slip, at cruise it has 12% prop slip, and at WOT it has 4% prop slip. Same reason the original Powerglide auto tranny only needed two gears while manual transmissions at the time needed three or four.
