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Commander
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- May 31, 2002
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Do you want to know why your boat works the way it does?
I?m going to use a 300 HP rated, 350 cubic inch, I/O for an example. The curves would be similar on any other motor, just different numbers. Doesn't matter if it's an I/O or an outboard, the physics are the same!
The motor has no idea what it?s bolted to, it has specific power characteristics based on its RPM. It will make 292 HP at 4800 RPM no matter what is bolted to its rear if it can reach 4800 RPM.
350 (5.7L) dyno chart from GM Marine Engines. Original at:
http://www.gm.com/experience/technology/gmpowertrain/engines/specialized/marine/marine_engines.jsp
It redlines at 5000 RPM. It keeps making power after that, but it?s not designed to spin any faster than 5000 RPM. Expensive pieces may go BOOM if you insist on revving it higher. It makes peak power (292 HP) at 4800 RPM.
Chart #1
Now we take the same engine and run it through an outdrive with a 1.5 reduction. It gives us the following theoretical speeds with 10% prop slip:
Chart #2
Hopefully nobody here thinks that they can just keep putting bigger (more pitch) props on their boat and go faster! Since this motor redlined at 5000 RPM, could you just put a 27 pitch prop on it and go 77 MPH? If you were willing to overrev it could you really go 92 MPH?
The answer to both the above questions is NO!!! The reason why is that each boat has a certain amount of POWER REQUIRED to go a certain speed. Any excess power the engine can make over that amount is available for acceleration.
Chart #3 shows how the 292 HP motor above would perform in a given boat with a given load. This graph is the table above put onto a graph with the POWER REQUIRED curve for a boat added. Each boat will have a different POWER REQUIRED curve depending on boat type, size, weight, etc. The numbers will be different but the shape of the curve will be about the same. This curve is typical of a 20 - 22 foot, 3000 - 4000 pound runabout. The bump at 20 MPH is where it comes on plane. You can see that it takes a minimum of 75 HP to get this boat to plane.
Chart #3
The big black line is POWER REQUIRED. Look at a speed (say 30 MPH) on the bottom then go straight up to the POWER REQUIRED line. From there look to the left and you will see how much power is required to push this boat at that speed (about 80 HP in this example). You can see that as you go faster the line gets steeper. The faster you go, the quicker the drag goes up. This is why you can?t just hang a big prop on your boat and let it rev. It runs out of power to push the boat.
Now look at the colored lines showing each prop pitch. This shows the amount of power available at WIDE OPEN THROTTLE at the speed on the bottom. Look at the 19 pitch (yellow) line where it crosses 30 MPH. About 230 max HP would be available at 30 MPH. Since you only need 80 HP to push the boat at 30 MPH you just close the throttle and choke the engine down till it?s only making 80 HP. If you want to go faster shove the throttle open and the 150 HP available above the 80 required to go 30 MPH will start pushing you faster.
If you want to determine the maximum speed of this boat/engine combo, look where the lines from each prop pitch cross the POWER REQUIRED line. That point is as fast as the boat can go with that prop. To go faster requires more power than is available.
You can see that max speed in this case would be right at 60 MPH with the 23 pitch prop. The 27 pitch runs out of power at about 58 MPH, and if you want to go over 54 MPH with the 19 pitch you have to over-rev it.
Now let?s look at acceleration. Look at 30 MPH on the Power required curve. See how you would need 80 HP to go 30 MPH? Any of the 3 props can do this. You just close the throttle until the engine is only making 80 HP. So why does a lower pitch prop accelerate faster and pull harder? Look at the 27 pitch (blue) line where it crosses 30 MPH. This shows that with the throttle wide open at 30 MPH this prop has 140 HP available to put into the water. It needs 80 HP just to go 30 MPH. So the difference (140 HP ? 80 HP) of 60 HP is the amount of power available to accelerate the boat. Now look at the 19 pitch prop (yellow line). It can also easily cruise at 30 MPH at part throttle. But when you go to WOT at 30 MPH it has 230 HP available at the prop. Take away the 80 HP required to go 30 MPH and you see that this prop has 150 HP available to accelerate the boat. Which prop do you think will accelerate better, the 27 pitch with 60 HP more than required or the 19 pitch with 150 HP more than required? Hopefully everyone reading this will say the 19! (Most people will probably want to ignore this little blurb: Bottom line - it?s actually the excess torque that accelerates the boat. That?s why big blocks are so much fun! But I?m trying not to get into a torque war here!)
The distance between the POWER REQUIRED curve and the POWER AVAILABLE curve at a certain speed is the amount of excess power available for acceleration. Where the curves (POWER REQUIRED and POWER AVAILABLE) meet there is no more power available for acceleration, so the boat can?t go any faster. (Note - I used these kinds of charts very successfully when racing cars in the late 80?s and early 90?s. The prop pitches are analogous to the gears in a car. You shift gears at the speed where the pitch lines cross to maximize the power available at the rear wheels. If these pitches were gears and the POWER REQUIRED line was farther to the right like it is in a car (air friction is much less than water friction), you would shift from 1st to 2nd at 54, then 2nd to 3rd at 63. Shifting by the tach wouldn?t maximize your acceleration because the gear spacing wasn?t even like it is in this example.)
You can see from this chart that the 19 pitch prop has more excess power available than the others until about 53 MPH. This means that it will accelerate quicker (or pull harder if you?re towing skiers or tubes). You would also be over-revving your motor if you keep letting it wind past 54 MPH. It can do that because it won?t run up against the POWER REQUIRED curve until it?s turning about 5500 RPM. This a classic case of an ?Under pitched? prop. But it might be desirable if you?re pulling a heavy load and don?t intend to do top-end runs.
The 27 pitch prop (blue line) shows an ?Over Pitched? prop case. It has the least power available for acceleration and the engine?s peak power isn?t produced until after it crosses the POWER REQUIRED curve. It?ll never get there because it hits the wall about 58 mph. It has no power available to accelerate any farther. Although if you?re looking for an economical cruise this prop might be a good choice.
The 23 pitch would generally be considered optimal for this engine. This is what you end up with if you adjust your prop pitch to achieve the WOT RPM specified by the manufacturer. You will have max speed from the boat, decent acceleration, and you won?t risk blowing up your motor! It puts the engine?s power peak right on the POWER REQUIRED curve.
FUEL ECONOMY
You can see that if you want to cruise at 30 MPH any of the props could do it. It only requires 80 HP. So you close the throttle until the engine is only making 80 HP. But which one would give you the best gas mileage? The biggest pitch prop would be your friend here. At 30 MPH the engine with the 19 is turning 3000 RPM, the 23 is turning 2500, and the 27 is turning 2000. Look up these RPM on the engine Dyno Chart (Chart #1) to find the max power available at that RPM. With the 19, you need 80 HP out of the 250 available at 3000 RPM. So the throttle is open about 80/250 or 32%. Same 30 MPH for the 27 pitch turning 2000 RPM, it needs 80 HP out of the 150 available to it at 2000 RPM. So the throttle is open 80/150 or 53%. Since the 27 pitch is turning less RPM, it needs more throttle to deliver the same power (80 HP) as the 19 pitch at 30 MPH. The same amount of fuel and air is required to be pumped through the engine to deliver 80 HP to run the boat no matter what speed the engine is turning. Since the 27 pitch engine is turning slower, it must take in bigger bites of fuel and air to pump 80 HP worth through it. The engine must make 80 HP to drive the boat, but it must also produce the power to suck in the air it needs.
Put your fingers together over your mouth and suck in a lungful of air through the cracks. Now take your hand away and suck in another lungful. Much easier without the restriction isn?t it? Just put a couple of fingers in front of your mouth and you can easily tell the difference. You tried to suck in about 30 cubic inches of air to fill your lungs if you?re a normal size person. A 350 cubic inch 4-stroke engine running at 2000 RPM is trying to suck in 5833 cubic inches of air PER SECOND through an opening about the size of your mouth! It takes some serious wasted power to do that. (NOTE: This is a big reason diesels are more economical than gas engines ? they have no throttle plate, the air intake is wide open all the time. Speed on a diesel is controlled only by the amount of fuel squirted into the cylinders by the injectors.)
This is why the bigger pitch prop with the bigger throttle opening will give you better fuel economy. The wider throttle opening required for the 27 pitch to make the 80 HP makes it easier for the engine to suck in the air that it needs. So the engine has to develop less power (therefore use less fuel) ABOVE the 80 HP required to drive the boat to suck in its air. This is also the reason higher gears (overdrives) in your car give you better gas mileage.
See this post #25 in this thread for a little more on throttle opening vs fuel economy:
http://forums.iboats.com/showthread.php?t=224894
I?m going to use a 300 HP rated, 350 cubic inch, I/O for an example. The curves would be similar on any other motor, just different numbers. Doesn't matter if it's an I/O or an outboard, the physics are the same!
The motor has no idea what it?s bolted to, it has specific power characteristics based on its RPM. It will make 292 HP at 4800 RPM no matter what is bolted to its rear if it can reach 4800 RPM.
350 (5.7L) dyno chart from GM Marine Engines. Original at:
http://www.gm.com/experience/technology/gmpowertrain/engines/specialized/marine/marine_engines.jsp
It redlines at 5000 RPM. It keeps making power after that, but it?s not designed to spin any faster than 5000 RPM. Expensive pieces may go BOOM if you insist on revving it higher. It makes peak power (292 HP) at 4800 RPM.
Chart #1
Now we take the same engine and run it through an outdrive with a 1.5 reduction. It gives us the following theoretical speeds with 10% prop slip:
Chart #2
Hopefully nobody here thinks that they can just keep putting bigger (more pitch) props on their boat and go faster! Since this motor redlined at 5000 RPM, could you just put a 27 pitch prop on it and go 77 MPH? If you were willing to overrev it could you really go 92 MPH?
The answer to both the above questions is NO!!! The reason why is that each boat has a certain amount of POWER REQUIRED to go a certain speed. Any excess power the engine can make over that amount is available for acceleration.
Chart #3 shows how the 292 HP motor above would perform in a given boat with a given load. This graph is the table above put onto a graph with the POWER REQUIRED curve for a boat added. Each boat will have a different POWER REQUIRED curve depending on boat type, size, weight, etc. The numbers will be different but the shape of the curve will be about the same. This curve is typical of a 20 - 22 foot, 3000 - 4000 pound runabout. The bump at 20 MPH is where it comes on plane. You can see that it takes a minimum of 75 HP to get this boat to plane.
Chart #3
The big black line is POWER REQUIRED. Look at a speed (say 30 MPH) on the bottom then go straight up to the POWER REQUIRED line. From there look to the left and you will see how much power is required to push this boat at that speed (about 80 HP in this example). You can see that as you go faster the line gets steeper. The faster you go, the quicker the drag goes up. This is why you can?t just hang a big prop on your boat and let it rev. It runs out of power to push the boat.
Now look at the colored lines showing each prop pitch. This shows the amount of power available at WIDE OPEN THROTTLE at the speed on the bottom. Look at the 19 pitch (yellow) line where it crosses 30 MPH. About 230 max HP would be available at 30 MPH. Since you only need 80 HP to push the boat at 30 MPH you just close the throttle and choke the engine down till it?s only making 80 HP. If you want to go faster shove the throttle open and the 150 HP available above the 80 required to go 30 MPH will start pushing you faster.
If you want to determine the maximum speed of this boat/engine combo, look where the lines from each prop pitch cross the POWER REQUIRED line. That point is as fast as the boat can go with that prop. To go faster requires more power than is available.
You can see that max speed in this case would be right at 60 MPH with the 23 pitch prop. The 27 pitch runs out of power at about 58 MPH, and if you want to go over 54 MPH with the 19 pitch you have to over-rev it.
Now let?s look at acceleration. Look at 30 MPH on the Power required curve. See how you would need 80 HP to go 30 MPH? Any of the 3 props can do this. You just close the throttle until the engine is only making 80 HP. So why does a lower pitch prop accelerate faster and pull harder? Look at the 27 pitch (blue) line where it crosses 30 MPH. This shows that with the throttle wide open at 30 MPH this prop has 140 HP available to put into the water. It needs 80 HP just to go 30 MPH. So the difference (140 HP ? 80 HP) of 60 HP is the amount of power available to accelerate the boat. Now look at the 19 pitch prop (yellow line). It can also easily cruise at 30 MPH at part throttle. But when you go to WOT at 30 MPH it has 230 HP available at the prop. Take away the 80 HP required to go 30 MPH and you see that this prop has 150 HP available to accelerate the boat. Which prop do you think will accelerate better, the 27 pitch with 60 HP more than required or the 19 pitch with 150 HP more than required? Hopefully everyone reading this will say the 19! (Most people will probably want to ignore this little blurb: Bottom line - it?s actually the excess torque that accelerates the boat. That?s why big blocks are so much fun! But I?m trying not to get into a torque war here!)
The distance between the POWER REQUIRED curve and the POWER AVAILABLE curve at a certain speed is the amount of excess power available for acceleration. Where the curves (POWER REQUIRED and POWER AVAILABLE) meet there is no more power available for acceleration, so the boat can?t go any faster. (Note - I used these kinds of charts very successfully when racing cars in the late 80?s and early 90?s. The prop pitches are analogous to the gears in a car. You shift gears at the speed where the pitch lines cross to maximize the power available at the rear wheels. If these pitches were gears and the POWER REQUIRED line was farther to the right like it is in a car (air friction is much less than water friction), you would shift from 1st to 2nd at 54, then 2nd to 3rd at 63. Shifting by the tach wouldn?t maximize your acceleration because the gear spacing wasn?t even like it is in this example.)
You can see from this chart that the 19 pitch prop has more excess power available than the others until about 53 MPH. This means that it will accelerate quicker (or pull harder if you?re towing skiers or tubes). You would also be over-revving your motor if you keep letting it wind past 54 MPH. It can do that because it won?t run up against the POWER REQUIRED curve until it?s turning about 5500 RPM. This a classic case of an ?Under pitched? prop. But it might be desirable if you?re pulling a heavy load and don?t intend to do top-end runs.
The 27 pitch prop (blue line) shows an ?Over Pitched? prop case. It has the least power available for acceleration and the engine?s peak power isn?t produced until after it crosses the POWER REQUIRED curve. It?ll never get there because it hits the wall about 58 mph. It has no power available to accelerate any farther. Although if you?re looking for an economical cruise this prop might be a good choice.
The 23 pitch would generally be considered optimal for this engine. This is what you end up with if you adjust your prop pitch to achieve the WOT RPM specified by the manufacturer. You will have max speed from the boat, decent acceleration, and you won?t risk blowing up your motor! It puts the engine?s power peak right on the POWER REQUIRED curve.
FUEL ECONOMY
You can see that if you want to cruise at 30 MPH any of the props could do it. It only requires 80 HP. So you close the throttle until the engine is only making 80 HP. But which one would give you the best gas mileage? The biggest pitch prop would be your friend here. At 30 MPH the engine with the 19 is turning 3000 RPM, the 23 is turning 2500, and the 27 is turning 2000. Look up these RPM on the engine Dyno Chart (Chart #1) to find the max power available at that RPM. With the 19, you need 80 HP out of the 250 available at 3000 RPM. So the throttle is open about 80/250 or 32%. Same 30 MPH for the 27 pitch turning 2000 RPM, it needs 80 HP out of the 150 available to it at 2000 RPM. So the throttle is open 80/150 or 53%. Since the 27 pitch is turning less RPM, it needs more throttle to deliver the same power (80 HP) as the 19 pitch at 30 MPH. The same amount of fuel and air is required to be pumped through the engine to deliver 80 HP to run the boat no matter what speed the engine is turning. Since the 27 pitch engine is turning slower, it must take in bigger bites of fuel and air to pump 80 HP worth through it. The engine must make 80 HP to drive the boat, but it must also produce the power to suck in the air it needs.
Put your fingers together over your mouth and suck in a lungful of air through the cracks. Now take your hand away and suck in another lungful. Much easier without the restriction isn?t it? Just put a couple of fingers in front of your mouth and you can easily tell the difference. You tried to suck in about 30 cubic inches of air to fill your lungs if you?re a normal size person. A 350 cubic inch 4-stroke engine running at 2000 RPM is trying to suck in 5833 cubic inches of air PER SECOND through an opening about the size of your mouth! It takes some serious wasted power to do that. (NOTE: This is a big reason diesels are more economical than gas engines ? they have no throttle plate, the air intake is wide open all the time. Speed on a diesel is controlled only by the amount of fuel squirted into the cylinders by the injectors.)
This is why the bigger pitch prop with the bigger throttle opening will give you better fuel economy. The wider throttle opening required for the 27 pitch to make the 80 HP makes it easier for the engine to suck in the air that it needs. So the engine has to develop less power (therefore use less fuel) ABOVE the 80 HP required to drive the boat to suck in its air. This is also the reason higher gears (overdrives) in your car give you better gas mileage.
See this post #25 in this thread for a little more on throttle opening vs fuel economy:
http://forums.iboats.com/showthread.php?t=224894
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