hwsiii
Commander
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- Jan 25, 2009
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Propellers
A boat propeller is an airfoil, just like an airplane propeller or the wing of an airplane. It has a low pressure side where the water comes in and a high pressure side where the water is expelled. In theory the most efficient propeller has one blade, has a very large diameter and has a very slow shaft speed, but that will never be a reality. Propellers have only one purpose on a boat, and that is to convert horsepower and torque for turning the prop and creating thrust to move the boat. How well the prop does its job is considered the efficiency of that prop. It performs this task by speeding up a mass of water to a higher speed than the surrounding water. No commercial recreational propeller company that I know of making propellers gives the boater their efficiency numbers for their propellers. The speed of the water over different parts of the prop varies directly with the distance it is from the center of the propeller. The highest speeds occur very near the tip of the blade. A 16? propeller that is turning 3,000 Gear Ratio RPM has a prop tip speed of 143 MPH, this will give you an idea of why a propeller can create cavitation on the low pressure side of the blade, which is the side closest to the boat. It also can be influenced by very small bubbles created from any irregularities in the bottom of the boat, or any added apurtenances to the back of the boat like speed pitots or depth finder transducers. Most people don?t really realize, that because of its airfoil qualities that a propeller does react to the flow of the water just like a propeller on an airplane reacts to the air as it changes density from low altitudes to higher altitudes and high temperatures to low temperatures. To keep the angle of attack at the ideal location on the blade, the blade has to be designed with a twist in it to vary the pitch angle from the tip to the root of the prop. And as the speed increases the angle of attack that the prop encounters decreases, which limits the efficiency of all fixed pitch props to a very narrow speed range at a given RPM. But by using progressive pitch props some of this narrow speed range is rectified to a degree because of the camber used in progressive props and the way that it affects the perceived angle of attack of the prop at higher RPM.
In order to pick the correct propeller for you boat and motor combination you first have to know exactly what your intended uses and needs are for the boat and understand some of the hull form designs used in your boat. Some design differences that really matter when you decide to pick the correct propeller are such things as lifting pads on a lot of bass boats, deep vee hulls designed for running in very formed seas and flat bottom boats designed to run in calm waters. There are many other variations in hulls that have not been mentioned here, including everything from displacement hulls to speedboats and hydroplanes and catamarans, and every one of them needs the correct blade area and pitch and other specific elements in the geometry of its propeller to run at its most efficient speed, top speed and fuel usage. Many larger boats and some small boats work very well with 4 blade propellers with their larger blade surface area, which normally helps with vibration, better holding power in formed seas and higher cruising speeds with just a very little bit of loss in top end speed and sometimes they can gain in top speed because of a decrease in prop slip.
There are many factors involved in picking the right propeller that is the most efficient and practical for your particular boat, hull design, weight, motor and intended uses of your boat. The facts may be that you actually want two different propellers to accomplish your goals if you intend on using the boat for two very different applications, especially if your motor is in the lower end of the horsepower range recommended for your boat. If you typically use your boat to go fishing, with just you and one other person with a light load and make long runs up the river to your fishing grounds you would typically use a prop that would probably give you the best top end speeds for the distances you are traveling, like in bass tournaments. Then if you occasionally have four or more people on the boat and at those times you are water skiing, you would definitely be looking more for a prop that has a very strong hole shot as well as dynamic thrust for all the extra weight and drag created by towing skiers and the extra passengers. While using your boat for both of these diverse applications you still want to make certain that your motor is always operating within the RPM range recommended by the motor manufacturer at WOT. The same propeller can't deliver both high thrust and maximum speed. A propeller sized for high speed usually has a small blade surface area and maximum pitch. A propeller sized for power or high thrust has a large diameter or 4 blades and/or lower pitch. For many boats you can compromise on an "in-between" propeller, but for either high speed or high thrust there is little in common between them. Within some guidelines it is normally better, on high-speed craft, to use the smallest diameter and the greatest pitch possible, taking into consideration the weight and hull form of the vessel, because if you do not have enough blade surface area you will experience a high ratio of what is called propeller Slip, which can be compared theoretically to having a very large horsepower engine in a race car and not having wide enough tires to transfer the energy from the motor to the pavement without having them spin uncontrollably. Disk Area Ratio is just the percentage of the Propeller blade area that fills the diameter of the circle created by them and 4 blades normally have a larger DAR.
The power developed by any marine engine is available at the propeller shaft in the form of torque and horsepower and the propeller is the instrument to convert these to thrust. High-speed runabouts and fast cruisers require relatively low thrust at high speeds and therefore operate at low slip, whereas a tugboat propeller requires tremendous thrust and a lot of prop slip. In general, horsepower available and shaft speed determine the propeller diameter, while shaft speed and boat speed determines propeller pitch. The pitch of a propeller divided by its diameter is a term called "pitch ratio?. For best efficiency, the pitch ratio of boat propellers should be somewhere in the range, .8 to 1.3 for heavy and average boats, 1.2 to 1.5 for medium and fast boats and 1.4 to 1.9 for really fast bass boats and high speed offshore boats. The higher the pitch your engine can turn near top horsepower and RPM, the faster your boat can go theoretically.
Propellers, the Black Art Part
The supposed black art part of propeller geometry and sizing is not really a black art. What it really is about is having enough knowledge to know what blade geometry is needed for a particular boat and motor combination, taking into consideration the use, weight, HP and hull form of that vessel. Many people think if they know of another boat exactly like theirs, with the same size motor using let?s say a 13? x 19? stainless prop that they should use the same thing. And that may be a very true statement, as far as it goes. So he orders a 13? x 19? stainless prop from XYZ company, and when he takes his boat out with his new prop, expecting his boat to perform just like his friends does, it is a miserable failure. It raises the bow so high on takeoff that he can?t see over it and then it won?t even reach 75% of the manufacturers recommended RPM, as well as it starts to porpoising at about 65% of the recommended RPM, and he can?t imagine what is wrong. He thought just because he ordered the 13? x 19? stainless prop that it was going to perform like his friends propeller. What his friend was actually using was a zero rake (for stern lift), progressive pitch prop (the prop actually has a 17 Pitch on the leading edge progressing to a 19? Pitch in the middle of the blade to a full 21? pitch at the trailing edge, and it is called a 19? Pitch by the manufacturer because that is the AVERAGE Pitch of this prop) with a moderate cup (adding more stern lift and theoretical Pitch) on the trailing edge and he has a 2:1 lower gear ratio. What he had purchased was a 30 degree progressive rake (adds a LOT of bow lift), fixed pitch (bad hole shot and acceleration with a very narrow speed window) and cupped on the blade tips, NOT the trailing edge (adds MORE rake and bow lift) stainless propeller for his boat. Pitch is a major consideration in choosing the right prop, but in almost every case the blade geometry of stainless props is much more important than the pitch. You can buy a prop that is one inch in Pitch different than what is actually needed for the boat and it will make a small difference in the RPM and speed of the boat, but if you purchase a prop with the wrong design elements (Blade Geometry) it will make a much more pronounced difference in the RPM and speed. Prop manufacturers and boat manufacturers as industries, have not educated the general public as to which props are designed for which general set of hulls and uses. As long as propellers for boats have been made there is NO industry standard for cup on a propeller blade, the Navy is the only organization that has set a standard for exactly what a single cup or double cup has to conform to in order to meet those definitions. It is so bad in many situations, that even in a propeller repair shop a medium cup by one person working there is considered a heavy cup by another. That is why getting a propeller cupped does not always meet the expectations of the person that is paying for it. It would be very easy for them to standardize each of the different elements in propellers to a certain standard that is industry wide, making it much easier for us to decide which prop fits a particular boat, motor and use. If you don?t think this is true, just buy a couple of aluminum props with the same diameter and pitch from different manufacturers and see what the performance differences are, they are usually pretty close to each other. Stainless Steel propellers cost a LOT more money, and part of that price includes the cost of engineering the prop to perform certain functions that are very specific and beneficial to certain hull forms. There are only a limited number of hull forms, and it is not very hard for prop manufacturers or boat manufacturers to give guidelines as to what elements in a prop are suitable for that hull form. Many people will say that is not true, because there so many different engine combinations that can be used on any boat, differences in torque and where the torque is in the power band, differences in gear ratios, differences in horsepower and many other variables. Different types of hull forms require different prop geometries to get the maximum performance for that hull, and these elements in a prop designate what hull forms can benefit the most or the least from these elements, no matter what horsepower, torque range, gear ratios and low or high horsepower. The diameter and pitch will take care of all these variables, if the horsepower is sufficient to get the boat on plane. The lower the horsepower the slower the speed, the higher the horsepower the higher the speed, but having the right Rake, Cup, Blade Geometry and engine height play MUCH more of a Role in performance than having just the right Diameter and Pitch.
H
A boat propeller is an airfoil, just like an airplane propeller or the wing of an airplane. It has a low pressure side where the water comes in and a high pressure side where the water is expelled. In theory the most efficient propeller has one blade, has a very large diameter and has a very slow shaft speed, but that will never be a reality. Propellers have only one purpose on a boat, and that is to convert horsepower and torque for turning the prop and creating thrust to move the boat. How well the prop does its job is considered the efficiency of that prop. It performs this task by speeding up a mass of water to a higher speed than the surrounding water. No commercial recreational propeller company that I know of making propellers gives the boater their efficiency numbers for their propellers. The speed of the water over different parts of the prop varies directly with the distance it is from the center of the propeller. The highest speeds occur very near the tip of the blade. A 16? propeller that is turning 3,000 Gear Ratio RPM has a prop tip speed of 143 MPH, this will give you an idea of why a propeller can create cavitation on the low pressure side of the blade, which is the side closest to the boat. It also can be influenced by very small bubbles created from any irregularities in the bottom of the boat, or any added apurtenances to the back of the boat like speed pitots or depth finder transducers. Most people don?t really realize, that because of its airfoil qualities that a propeller does react to the flow of the water just like a propeller on an airplane reacts to the air as it changes density from low altitudes to higher altitudes and high temperatures to low temperatures. To keep the angle of attack at the ideal location on the blade, the blade has to be designed with a twist in it to vary the pitch angle from the tip to the root of the prop. And as the speed increases the angle of attack that the prop encounters decreases, which limits the efficiency of all fixed pitch props to a very narrow speed range at a given RPM. But by using progressive pitch props some of this narrow speed range is rectified to a degree because of the camber used in progressive props and the way that it affects the perceived angle of attack of the prop at higher RPM.
In order to pick the correct propeller for you boat and motor combination you first have to know exactly what your intended uses and needs are for the boat and understand some of the hull form designs used in your boat. Some design differences that really matter when you decide to pick the correct propeller are such things as lifting pads on a lot of bass boats, deep vee hulls designed for running in very formed seas and flat bottom boats designed to run in calm waters. There are many other variations in hulls that have not been mentioned here, including everything from displacement hulls to speedboats and hydroplanes and catamarans, and every one of them needs the correct blade area and pitch and other specific elements in the geometry of its propeller to run at its most efficient speed, top speed and fuel usage. Many larger boats and some small boats work very well with 4 blade propellers with their larger blade surface area, which normally helps with vibration, better holding power in formed seas and higher cruising speeds with just a very little bit of loss in top end speed and sometimes they can gain in top speed because of a decrease in prop slip.
There are many factors involved in picking the right propeller that is the most efficient and practical for your particular boat, hull design, weight, motor and intended uses of your boat. The facts may be that you actually want two different propellers to accomplish your goals if you intend on using the boat for two very different applications, especially if your motor is in the lower end of the horsepower range recommended for your boat. If you typically use your boat to go fishing, with just you and one other person with a light load and make long runs up the river to your fishing grounds you would typically use a prop that would probably give you the best top end speeds for the distances you are traveling, like in bass tournaments. Then if you occasionally have four or more people on the boat and at those times you are water skiing, you would definitely be looking more for a prop that has a very strong hole shot as well as dynamic thrust for all the extra weight and drag created by towing skiers and the extra passengers. While using your boat for both of these diverse applications you still want to make certain that your motor is always operating within the RPM range recommended by the motor manufacturer at WOT. The same propeller can't deliver both high thrust and maximum speed. A propeller sized for high speed usually has a small blade surface area and maximum pitch. A propeller sized for power or high thrust has a large diameter or 4 blades and/or lower pitch. For many boats you can compromise on an "in-between" propeller, but for either high speed or high thrust there is little in common between them. Within some guidelines it is normally better, on high-speed craft, to use the smallest diameter and the greatest pitch possible, taking into consideration the weight and hull form of the vessel, because if you do not have enough blade surface area you will experience a high ratio of what is called propeller Slip, which can be compared theoretically to having a very large horsepower engine in a race car and not having wide enough tires to transfer the energy from the motor to the pavement without having them spin uncontrollably. Disk Area Ratio is just the percentage of the Propeller blade area that fills the diameter of the circle created by them and 4 blades normally have a larger DAR.
The power developed by any marine engine is available at the propeller shaft in the form of torque and horsepower and the propeller is the instrument to convert these to thrust. High-speed runabouts and fast cruisers require relatively low thrust at high speeds and therefore operate at low slip, whereas a tugboat propeller requires tremendous thrust and a lot of prop slip. In general, horsepower available and shaft speed determine the propeller diameter, while shaft speed and boat speed determines propeller pitch. The pitch of a propeller divided by its diameter is a term called "pitch ratio?. For best efficiency, the pitch ratio of boat propellers should be somewhere in the range, .8 to 1.3 for heavy and average boats, 1.2 to 1.5 for medium and fast boats and 1.4 to 1.9 for really fast bass boats and high speed offshore boats. The higher the pitch your engine can turn near top horsepower and RPM, the faster your boat can go theoretically.
Propellers, the Black Art Part
The supposed black art part of propeller geometry and sizing is not really a black art. What it really is about is having enough knowledge to know what blade geometry is needed for a particular boat and motor combination, taking into consideration the use, weight, HP and hull form of that vessel. Many people think if they know of another boat exactly like theirs, with the same size motor using let?s say a 13? x 19? stainless prop that they should use the same thing. And that may be a very true statement, as far as it goes. So he orders a 13? x 19? stainless prop from XYZ company, and when he takes his boat out with his new prop, expecting his boat to perform just like his friends does, it is a miserable failure. It raises the bow so high on takeoff that he can?t see over it and then it won?t even reach 75% of the manufacturers recommended RPM, as well as it starts to porpoising at about 65% of the recommended RPM, and he can?t imagine what is wrong. He thought just because he ordered the 13? x 19? stainless prop that it was going to perform like his friends propeller. What his friend was actually using was a zero rake (for stern lift), progressive pitch prop (the prop actually has a 17 Pitch on the leading edge progressing to a 19? Pitch in the middle of the blade to a full 21? pitch at the trailing edge, and it is called a 19? Pitch by the manufacturer because that is the AVERAGE Pitch of this prop) with a moderate cup (adding more stern lift and theoretical Pitch) on the trailing edge and he has a 2:1 lower gear ratio. What he had purchased was a 30 degree progressive rake (adds a LOT of bow lift), fixed pitch (bad hole shot and acceleration with a very narrow speed window) and cupped on the blade tips, NOT the trailing edge (adds MORE rake and bow lift) stainless propeller for his boat. Pitch is a major consideration in choosing the right prop, but in almost every case the blade geometry of stainless props is much more important than the pitch. You can buy a prop that is one inch in Pitch different than what is actually needed for the boat and it will make a small difference in the RPM and speed of the boat, but if you purchase a prop with the wrong design elements (Blade Geometry) it will make a much more pronounced difference in the RPM and speed. Prop manufacturers and boat manufacturers as industries, have not educated the general public as to which props are designed for which general set of hulls and uses. As long as propellers for boats have been made there is NO industry standard for cup on a propeller blade, the Navy is the only organization that has set a standard for exactly what a single cup or double cup has to conform to in order to meet those definitions. It is so bad in many situations, that even in a propeller repair shop a medium cup by one person working there is considered a heavy cup by another. That is why getting a propeller cupped does not always meet the expectations of the person that is paying for it. It would be very easy for them to standardize each of the different elements in propellers to a certain standard that is industry wide, making it much easier for us to decide which prop fits a particular boat, motor and use. If you don?t think this is true, just buy a couple of aluminum props with the same diameter and pitch from different manufacturers and see what the performance differences are, they are usually pretty close to each other. Stainless Steel propellers cost a LOT more money, and part of that price includes the cost of engineering the prop to perform certain functions that are very specific and beneficial to certain hull forms. There are only a limited number of hull forms, and it is not very hard for prop manufacturers or boat manufacturers to give guidelines as to what elements in a prop are suitable for that hull form. Many people will say that is not true, because there so many different engine combinations that can be used on any boat, differences in torque and where the torque is in the power band, differences in gear ratios, differences in horsepower and many other variables. Different types of hull forms require different prop geometries to get the maximum performance for that hull, and these elements in a prop designate what hull forms can benefit the most or the least from these elements, no matter what horsepower, torque range, gear ratios and low or high horsepower. The diameter and pitch will take care of all these variables, if the horsepower is sufficient to get the boat on plane. The lower the horsepower the slower the speed, the higher the horsepower the higher the speed, but having the right Rake, Cup, Blade Geometry and engine height play MUCH more of a Role in performance than having just the right Diameter and Pitch.
H
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