Re: Trying to make sense of RPMs
Also what is meant by the RPMs dropping by changing the pitch? Does that mean the WOT RPMs and max safe for the engine change?
You're still kinda missing this deal . . . the WOT range and power output are determined by the manufacturer's design. The combination of components in your engine makes it's best power from 4200 - 4600. So let's say you put a 17" pitch prop on it, you weren't worried about blowing it up and you went for a full blast, WOT run. Since the engine and boat combo can easily overcome the 17 inch pitch (think of it like a lower gear), the engine will rev up past that range easily. Maybe it would reach 5500 RPM before it either blew up or ran out of power.
Since the power band is between 4200 and 4600, something needs to change. There is only two real ways to remedy this, go up in prop pitch which will make the boat go farther for each revolution which requires more power or change the gear ratio to make the prop turn faster than it is now which will make the boat go farther for each revolution of the engine. Since it's cheaper to change pitch than gear ratios (and it's the right way to do it), most people change pitch. Your engine appears to be reving past the range with a 19 inch pitch. Assuming the tach is right, you need to go to say a 21 inch pitch so that the boat goes a little farther which each revolution which requires more power which will lug the engine a little more and hopefully bring the RPM down to where it makes the most power: the WOT range.
Imagine if you left every thing the same and put in an 800 horsepower Nascar engine. Assuming everything would stay together it would take your 19 inch pitch prop and try to get to it's 9000 RPM designed peak power point. If it couldn't it would stop revving when it ran out of power (let's say 7723 RPM) or it would rev past that 9000 RPM peak point if it could. It would either blow up at some point higher than 9000, or it would run out of power, because above that designed in peak of 9000 RPM power begins to drop off.
The ONLY way to get the engine to achieve it's peak power point or range is to play with propellers until you find those that only let it get into the window. Not under the window, or past the window, but
in the window at WOT.
Think about riding a bicycle with multiple gears up a hill. If you have to pedal too hard and too slow of pedaling to be comfortable, you shift into a lower gear (lower pitch prop), or if your feet are winging along so fast but you are barely moving then you shift into a higher gear (higher pitch prop). You do that to match your body's comfortable RPM for the steepness of the hill and the strength in your legs. Sort of your very own WOT RPM range . . . Since water is flat and you only get one gear (gear ratio) you have to find a prop that gets your legs (the engine) going up and down at the right speed so you don't have a heart attack (deep lug) or spin until your feet fly off the pedals (over speed) . . .
