Re: Is it just me or is the boat to slow
You're saying the exact same thing I said but mushing it all up.
If the engine turns at 5000 RPM with a given prop, but makes it's peak power at 4750 RPMs, it DOES go faster at 5000 RPM with the same given prop than it does at 4750 w/same prop.
Nobody ever said that
with a given prop the boat would go slower if the engine can turn at a higher RPM. Put a 10" pitch prop on your boat. Higher RPM will equal faster speed
with that given prop. However, it will be NOWHERE near the maximum possible top speed your boat is capable of because your engine will rev WAY past max power and you'll be putting much less than your maximum HP into the water.
Your car works the same way. Do you really think your car is faster at 5000 RPM in first gear (maybe 20 MPH) than it is at 2000 RPM in top gear (probably 70 MPH)? Props on boats are just like gears on a car. If you want max top speed you gear it so the power peak coincides with the drag curve. If you want more acceleration, you gear it so the power peak is to the left of the drag curve (underpropped - low gears on cars). If you want max economy, you gear it so the power peak is to the right of the drag curve (overpropped - overdrive on cars). You'll have less top speed because you can never make it to your power peak, but greater fuel economy because you'll have less parasitic power losses. Your throttle will be open further to maintain a given lower speed and you will have less internal pumping losses in the motor. This is the reason a Corvette has 2 overdrive gears - to maximize fuel economy. It's slower in 5th and 6th than in 4th, but gets better cruising mileage in the overdrive gears.
If an engine makes 320 HP at 4750 RPM but only makes 300 HP at 5000 RPM, and you have a given prop, and the engine can run up to 5000 RPM with that prop, it goes faster @ 5000 RPM and 300 HP than it does at 320 HP @ 4750 RPM.
With a given prop, that's true. But put a prop on it where it turns 4750 at WOT and you'll be 20 HP (about 2 MPH) faster than the boat that's turning 5000 RPM and only putting out 300 HP.
In your "given prop" example, the only way you'll hold it at 4750 is to REDUCE your throttle till it's putting out less than 300 HP. At WOT you're putting 300 HP into the water with the engine turning 5000 RPM. Top speed on your boat will be where the amount of drag it creates (water and air) takes 300 HP to balance it.
At 4750 RPM, it obviously takes less than 300 HP to go that speed since it's going slower than it will at 5000 RPM. To hold 4750 RPM you have to REDUCE your throttle till the engine is only putting out the amount of power required for that speed. It has to be less than 300 HP, because you already know the top speed of the boat at 300 HP.
If you go to WOT from 4750 RPM, any additional power above what was required to maintain that speed goes into accelerating the boat. At WOT and 4750 it is putting 320 HP into the water but it takes less than 320 HP to maintain that speed (we know it only takes 300 HP to go 5000 RPM) so it accelerates. At 4900 RPM it will be putting 310 HP into the water. Again, we know it takes less than that (300 HP at 5000) so it keeps accelerating. When it hits 5000 RPM and 300 HP it can't accelerate any further. The amount of power it is putting into the water balances the drag on the boat. So you're going as fast as 300 HP can push you.
If you're smart enough to
increase your prop pitch (not a given prop anymore) so at WOT the engine is turning 4750 RPM, now it will be putting 320 HP into the water. It can't accelerate any further because that would take MORE power, but it has LESS power above 4750 RPM. Top speed will be the speed where it takes 320 HP to balance the drag created by the water and air on the boat.
Top speed all comes down to power. Doesn't matter if it's a boat, car, plane, or rocket. Gear it so it's putting the maximum possible power to the water, air, or ground and that is as fast as it can go.
320 HP beats 300 HP every time.
If you had a drag curve for your boat you could easily calculate the top speed. You set the drag equal to the HP delivered (that's why you go faster by putting more HP into the water). This one is for a car and includes only air resistance. For a boat you would need to include water resistance also. You can see why most people don't worry about calculating it:
We had a thread on all this a few years ago, with nice pictures and explanations of POWER REQUIRED vs POWER AVAILABLE. Did you ever read it? Your boat will be at it's fastest when you get the peak of the power curve onto the drag curve. If you have too much pitch, your max power is on the far side of the drag curve and you never get to max power. If you have too little pitch, like in your example, peak power is to the left of the drag curve. You're revving your engine past the power peak and not maximizing the amount of power you can put into the water to attain the maximum speed:
http://forums.iboats.com/showthread.php?t=225803&highlight=secrets
