hwsiii
Commander
- Joined
- Jan 25, 2009
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When you are looking to improve the performance of your boat you should have a better understanding of what to look for in a new prop versus your old prop, and the other changes that can increase your maximum speed and cost you nothing except your time and a little bit of labor. ALL boat motors should always be able to attain the minimum recommended RPM for the motor you have installed with your normal weight load, if you do not do this you can expect your motor to lose some or a lot of it's longevity, just depending on how low your maximum RPMs are and how bad this is Lugging your motor. When you cannot reach your manufacturers recommended upper RPM with the normal load you carry, then your motor is Lugging at ALL RPM levels, not just at WOT. Whereas, being able to attain the maximum recommended WOT RPM will increases motor longevity and fuel efficiency as well. With today's ethanol fuels lugging of your motor creates even more carbon deposits on the pistons and rings than in the past, which over a period of time creates coking of the pistons and rings, which eventually leads to massive engine failure.
1. Before you make any decisions about changing props, you need to know if your outboard motor is at the best height for maximum performance with any prop. Anytime you see water coming up the transom of the boat or the leg of the motor when you are planing means your motor is mounted too low on the transom. Your motor normally should be at least 1" above the bottom of the transom of the boat. Your prop WILL gain RPM and speed by raising the motor higher, until it starts ventilating. On inboard outboard boats you cannot change the height of the out drive, unless you have an old OMC stringer drive, so the following does not apply. Normally the anti ventilation plate should be at least 1? and preferably 2" or higher above the bottom of the boat for the best speeds and the least drag created by the motor. And if you change from an aluminum 3 blade prop to a 4 blade or SS prop you need to raise the motor one more notch again to see if you can gain more RPM and speed, because these props tend to hold the water better and thus can be raised higher in the water column.
Motor Transom Height
When the boat is running in the top 20% of your speed you should be able to see the anti ventilation plate very clearly at the top of the water and have water just splashing over the top of it.
Correct Anti Ventilation Height
2. Then you need to know how much prop slip you have in order to determine if you are using the wrong prop pitch and/or blade geometry for your boat and motor combination. Just getting your motor to turn the correct RPM at WOT does NOT mean your boat is running efficiently and you are attaining good fuel efficiency and maximum speed from your motor. If your Prop Slip is over 15%, excluding heavy cruisers, when using a GPS, then you should consider changing to a prop with a different blade geometry than the prop you now have, to improve your performance, speed and fuel efficiency at all RPM.
Prop Slip
Checking the amount of prop slip you have, will give you a very good indication of how well the prop you are running is at converting Theoretical speed to Actual speed. Every time your prop slip is higher than 15% you should definitely consider changing prop design, but you need to make sure your speed numbers are correct, and that means using a GPS for speed and running the speed tests in 180 degree opposite directions to account for any wind or running water that influences the test results, add the two numbers together and divide by two for Actual speed results.
3. An easy and inexpensive change for performance, if you are experiencing high prop slip, is to go to a 4 blade prop with more blade surface area. The things that need to be taken into account are how much difference there is in the blade geometry and how many RPM you will lose having the 4th blade versus the three blades. As a normal course of events if you have an aluminum 3 blade prop on your boat you can expect about a 200 to 300 drop in RPM, changing pitch for pitch, if you buy a Solas Amita aluminum 4 blade prop and they run about $ 120, here at Iboats. Solas aluminum props are also made with the newer squeeze casting pressure process, which makes for enhanced mechanical properties because of the fine grain structure caused by rapid solidification of the prop and the high pressures used in the process, as well as the blades can be made thinner and stronger because of this. This particular prop is known as more of a stern lifter and you would normally expect to lose about 1 MPH at WOT depending on how much prop slip you have, but if your prop slip is about 20% or higher you might even gain 2 or 3 MPH.
But, If you are running a stainless steel prop and still having high Prop Slip problems, then you would want to consult with an experienced prop man or a prop shop that will let you exchange props, as normally stainless props have much better blade geometry and don't have as high a Prop Slip as aluminum props do. But you can't just change Stainless props pitch for pitch the same way you can't just change all aluminum props pitch for pitch, because of blade geometry differences between models of props, even from the same manufacturers, you have to have the knowledge to know the difference between what you have and what you need for your particular use.
Solas Squeeze Casting
4 Blade props can also have thinner blades as each one of them only has to carry ? of the HP per blade whereas 3 blade props have to carry 1/3 of the HP on each blade, with a 300 HP motor a 4 blade prop only has 75 HP on each blade and a 3 blade prop has to handle 100 HP on each blade. Theoretically this means the blades on a 4 blade prop can be cast 25% thinner than a comparable 3 blade propeller. As well as with the less blade area on a 3 blade prop there is a higher chance for more prop slip on a 3 blade prop.
Most people with outboard or inboard/outboard motors use aluminum propellers to propel their boat. Most people also believe that aluminum props also flex a lot while running, and thus lose pitch and forward speed when propelling a boat at cruising speeds and above, versus a stainless steel prop, and that is because of the aluminum flexing.
The actual difference between most aluminum props and stainless props has nothing to do with flexing of the aluminum propeller, for speeds up to and above 45 MPH, that is why aluminum props are much thicker than stainless steel props. The differences are in the actual time and money spent in the blade geometry of the two props, as well as the cost in materials and that is why stainless props cost much more than aluminum props. There is a lot more engineering designed into stainless props, partly because aluminum can't be cast with the tolerances required for the blade geometry design and partly because the stainless props can be designed much thinner, which also helps lower prop friction.
Heavy progressive parabolic rake, flat rake, zero rake, progressive pitch, trailing edge cup or blade tip cup, more or less blade surface area and thinner blade sections are the things that can reduce prop slip and raise the boat for less wetted surface area, and thus increase speed when used in the right combinations for a particular hull form design and motor combination.
Picking a very good prop for your particular boat and motor combination requires knowledge, but it is not an exact science. I use mathematical software and physics to get what prop fits best in Pitch and Diameter for a certain boat and motor combination, but then that is where the knowledge of prop geometry starts and experience is required to pick a better prop that will conform better to your uses and needs.
Aluminum props are very economical and can produce very good results for most boats, but for the maximum performance from your boat normally you would buy a stainless prop, although you will probably have to test a few of them to get the BEST one for your boat and uses if you are looking for Maximum speed.
You might also try reading this to learn more about Prop Slip and Blade Surface Area.
http://forums.iboats.com/showthread.php?t=381820
H
?2009
1. Before you make any decisions about changing props, you need to know if your outboard motor is at the best height for maximum performance with any prop. Anytime you see water coming up the transom of the boat or the leg of the motor when you are planing means your motor is mounted too low on the transom. Your motor normally should be at least 1" above the bottom of the transom of the boat. Your prop WILL gain RPM and speed by raising the motor higher, until it starts ventilating. On inboard outboard boats you cannot change the height of the out drive, unless you have an old OMC stringer drive, so the following does not apply. Normally the anti ventilation plate should be at least 1? and preferably 2" or higher above the bottom of the boat for the best speeds and the least drag created by the motor. And if you change from an aluminum 3 blade prop to a 4 blade or SS prop you need to raise the motor one more notch again to see if you can gain more RPM and speed, because these props tend to hold the water better and thus can be raised higher in the water column.
Motor Transom Height
When the boat is running in the top 20% of your speed you should be able to see the anti ventilation plate very clearly at the top of the water and have water just splashing over the top of it.
Correct Anti Ventilation Height
2. Then you need to know how much prop slip you have in order to determine if you are using the wrong prop pitch and/or blade geometry for your boat and motor combination. Just getting your motor to turn the correct RPM at WOT does NOT mean your boat is running efficiently and you are attaining good fuel efficiency and maximum speed from your motor. If your Prop Slip is over 15%, excluding heavy cruisers, when using a GPS, then you should consider changing to a prop with a different blade geometry than the prop you now have, to improve your performance, speed and fuel efficiency at all RPM.
Prop Slip
Checking the amount of prop slip you have, will give you a very good indication of how well the prop you are running is at converting Theoretical speed to Actual speed. Every time your prop slip is higher than 15% you should definitely consider changing prop design, but you need to make sure your speed numbers are correct, and that means using a GPS for speed and running the speed tests in 180 degree opposite directions to account for any wind or running water that influences the test results, add the two numbers together and divide by two for Actual speed results.
3. An easy and inexpensive change for performance, if you are experiencing high prop slip, is to go to a 4 blade prop with more blade surface area. The things that need to be taken into account are how much difference there is in the blade geometry and how many RPM you will lose having the 4th blade versus the three blades. As a normal course of events if you have an aluminum 3 blade prop on your boat you can expect about a 200 to 300 drop in RPM, changing pitch for pitch, if you buy a Solas Amita aluminum 4 blade prop and they run about $ 120, here at Iboats. Solas aluminum props are also made with the newer squeeze casting pressure process, which makes for enhanced mechanical properties because of the fine grain structure caused by rapid solidification of the prop and the high pressures used in the process, as well as the blades can be made thinner and stronger because of this. This particular prop is known as more of a stern lifter and you would normally expect to lose about 1 MPH at WOT depending on how much prop slip you have, but if your prop slip is about 20% or higher you might even gain 2 or 3 MPH.
But, If you are running a stainless steel prop and still having high Prop Slip problems, then you would want to consult with an experienced prop man or a prop shop that will let you exchange props, as normally stainless props have much better blade geometry and don't have as high a Prop Slip as aluminum props do. But you can't just change Stainless props pitch for pitch the same way you can't just change all aluminum props pitch for pitch, because of blade geometry differences between models of props, even from the same manufacturers, you have to have the knowledge to know the difference between what you have and what you need for your particular use.
Solas Squeeze Casting
4 Blade props can also have thinner blades as each one of them only has to carry ? of the HP per blade whereas 3 blade props have to carry 1/3 of the HP on each blade, with a 300 HP motor a 4 blade prop only has 75 HP on each blade and a 3 blade prop has to handle 100 HP on each blade. Theoretically this means the blades on a 4 blade prop can be cast 25% thinner than a comparable 3 blade propeller. As well as with the less blade area on a 3 blade prop there is a higher chance for more prop slip on a 3 blade prop.
Most people with outboard or inboard/outboard motors use aluminum propellers to propel their boat. Most people also believe that aluminum props also flex a lot while running, and thus lose pitch and forward speed when propelling a boat at cruising speeds and above, versus a stainless steel prop, and that is because of the aluminum flexing.
The actual difference between most aluminum props and stainless props has nothing to do with flexing of the aluminum propeller, for speeds up to and above 45 MPH, that is why aluminum props are much thicker than stainless steel props. The differences are in the actual time and money spent in the blade geometry of the two props, as well as the cost in materials and that is why stainless props cost much more than aluminum props. There is a lot more engineering designed into stainless props, partly because aluminum can't be cast with the tolerances required for the blade geometry design and partly because the stainless props can be designed much thinner, which also helps lower prop friction.
Heavy progressive parabolic rake, flat rake, zero rake, progressive pitch, trailing edge cup or blade tip cup, more or less blade surface area and thinner blade sections are the things that can reduce prop slip and raise the boat for less wetted surface area, and thus increase speed when used in the right combinations for a particular hull form design and motor combination.
Picking a very good prop for your particular boat and motor combination requires knowledge, but it is not an exact science. I use mathematical software and physics to get what prop fits best in Pitch and Diameter for a certain boat and motor combination, but then that is where the knowledge of prop geometry starts and experience is required to pick a better prop that will conform better to your uses and needs.
Aluminum props are very economical and can produce very good results for most boats, but for the maximum performance from your boat normally you would buy a stainless prop, although you will probably have to test a few of them to get the BEST one for your boat and uses if you are looking for Maximum speed.
You might also try reading this to learn more about Prop Slip and Blade Surface Area.
http://forums.iboats.com/showthread.php?t=381820
H
?2009
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