Re: WOT question...
QC is correct. Your throttle is open wider to get the higher mileage at the lower RPM with the bigger prop. It takes a given amount of power to drive your boat at a certain speed. The amount of POWER required at a given speed is independent of the prop size or engine RPM, it depends on the drag of the boat in the water.
It takes the same amount of power, let's just say 25 horsepower for example, to drive your boat at 30 mph no matter what prop you're using or what RPM your engine is turning. A gasoline engine is about 25% efficient. That means for your engine to produce 25 horsepower the amount of gasoline it burns actually contains the equivalent of 100 horsepower. The more efficient your engine is, the more of that available horsepower you get out of the gas to the prop. Of that 75 horsepower you aren't seeing, 50 is going into heating up your cooling water, 5 is going into making noise in the mufflers and exhaust pipes, and 20 is being used to pull air past your throttle valve.
The throttle valve is what controls the amount of air going through the engine, which in turn controls the amount of fuel through the carburetor venturi (fuel injection uses a sensor to measure the air flow and inject the right amount of gas). Obviously, if the engine is turning LESS RPM, it takes a bigger throttle opening to get the SAME amount of air and fuel through it in a given amount of time as at a higher RPM. Remember, only a specific amount of power available in each gallon of gas so to make the same power we need the same amount of gas.
This brings us to QC's description of pumping losses. Put your fingers together and put them over your mouth. Try to suck in a lungful of air through the cracks in your fingers. Notice how hard it is? Takes power to pull that air through those cracks. Now take your hand away and suck in a lungful. Much easier isn't it? In your engine, that power you were using to suck past the closed fingers has to come out of the gasoline the engine is burning. With the engine producing the same amount of power at 2 different RPM's, it has to use less of the available power sucking air through the bigger opening at the lower RPM. So it's more efficient.
To relate this to the above example only using a bigger prop, less RPM, and wider throttle opening, then our engine now only has to use 10 HP instead of 20 sucking air past the throttle valve. So to put 25 HP into the propeller it only has to use the equivalent gasoline that contains 90 HP. We put 25 into the prop to drive the boat, 50 into heat, 5 into noise, and only 10 into suction losses. What this means is that we only need the amount of gas that contains 90 HP, not 100. The boat still requires the same amount of power to drive it (25 HP), we are just using less gas trying to pull the air past our fingers! The throttle opening is bigger to get the required amount of air – thus fuel – through the engine at the lower RPM. Although the throttle opening is bigger, the engine is actually using less air and fuel since it's turning a lower RPM. The total VOLUME of air per minute is less since the engine only requires the amount of air to produce 90 total HP instead of 100.
If you have a car with a standard transmission and one of those "upshift indicator" lights that help you get better gas mileage, you will notice that they have you upshifting at very low RPM. This is because the fuel economy comes from decreasing pumping losses - bigger throttle opening (less restriction in the air flow). Same reason free-flowing exhaust helps fuel economy. You get the best efficiency in a throttled engine with the throttle wide open, just above the point the engine is "lugging".
If you don't think your boat is using a bigger throttle opening at the lower RPM at the same speed, try this in your car with a standard transmission. Climb a hill in 2nd gear at 30 mph - this is like your boat at 30 with the 17 pitch. Now go climb the same hill in 4th gear at 30 mph - this is like your boat at 30 with the 19 pitch. Don't be surprised if you have to floor it. I will guarantee you it will take more throttle in 4th than it did in 2nd. And even though the throttle is open wider, you are going the same speed and are getting better mileage in 4th. Your total OUTPUT power is the same, you're just using less power internally driving the engine.
Boatist, if you have a car with a cruise control and overdrive, put it in cruise and let it come to a constant speed, on a level road with the OD off. Rest your foot very lightly on the accelerator. Now turn on the overdrive and when the car shifts up into OD you will feel the gas pedal move down slightly to maintain the same speed. The same thing is happening on your boat.
I wish it did work the other way. We could just keep shifting up into higher and higher gears, using less and less throttle, till the throttle is completely closed and the engine isn't running. Infinite gas mileage!
This is somewhat simplified but hopefully will help illustrate the basic principle of engine pumping losses.
