carbonmarine
03-22-2005, 12:26 PM
PITCH - The theoretical distance traveled by the propeller in one revolution. I.E. a 20" pitch prop would travel 20" forward as a screw would going into wood. But since water is not a solid matter we have to factor in slippage. So if we have a 20" pitch prop but it was 75% efficient (because of slip) the prop would really only travel 15" in one revolution. Also the higher the pitch the more load is put on the engine and the lower the RPM will go. As in the opposite the lower the pitch the higher the RPM goes. By fine tuning the pitch you can achieve the correct maximum RPM that the engine was designed to develop, which is very important. The engines were designed to run at the RPM level for that engine. If your props are not achieving this RPM you are essentially wasting that extra horsepower and loading your engines which will cause premature engine problems.
DIAMETER - The distance across a circle at the extreme blade tip as it turns. Diameter is one of the most important factors in the propeller as is pitch. And they have to be the right combination to allow the boat to perform at its optimum. It is also determined by the pitch needed, the speed of the boat, RPM and gear ratio. Diameter also controls the amount of slip in the propeller (as in pitch because water is not solid). Too large of diameter props will produce too much unnecessary drag as too small will produce too much slip.
BLADES - The number of blades is also very important. Believe it or not the optimum propeller would have only one blade because it doesn't have any other blades scrambling up the flow of water in front of it. But balancing it would be impossible. Two blades would be optimum, but to run it on a larger boat would take such a large diameter you couldn't get it under the boat. So the next choice would be a 3 blade. Realistically in most cases this is the optimum propeller. There are reasons for more blades, as in 4, 5 and 6 blade props. The main one being the increase in blade area where it is needed to handle the pitch. If you had a 3 blade prop and the correct pitch was too much for the diameter of the propeller it would be overloaded and start to cavitate (see cavitation). If you couldn't fit a larger 3 blade under theboat you would need props with more blade area. You could go to a 4 blade of the same diameter and then even a 5 blade, if needed. Another reason for multiple blades in the past was to reduce vibration. But with the new computerized scanning technology, Prop Scan® , any number of bladed props can be made vibration free.
CAVITATION - Cavitation is a condition where a low pressure is formed on the surface of the propellers. This low pressure will cause water to vaporize and turn to steam. When the steam bubbles travel into an area with a higher pressure, these bubbles will implode and pit the surface of the propeller. Cavitation can cause severe damage to a propeller. It will also generate noise and vibration. Cavitation has many causes, e.g., damage on the leading edge of the propeller, incorrect blade shapes, and even the shape of appendages in front of the propeller.
VENTILATION - Ventilation is often confused with cavitation, but it is where air is induced into the propeller from an outside source. I.E. the surface, exhaust, or through hull fittings. It is a condition not generally created by the propeller.
CUPPED PROPELLERS - Cupping is a curvature added to the trailing edge of the propeller. It is usually very slight, about ½ to ¾ inch from the trailing edge down toward the middle of the blade and shaped like the bottom of a bowl. It is used to reduce cavitation and in some cases maximize thrust. Cupping basically makes an inefficient propeller run more efficient. It also affects pitch by effectively increasing it and will reduce RPM. We have found cup to be like medicine, a little won't do much for you, too much makes you sicker and the right amount will make you well (if cupping is the right medicine for your sickness). It is also very critical to have cupping very accurate as it will cause vibration. We can measure and adjust cupping to .001".
CAMBER - Camber is a long arching curve from leading edge to trailing edge like a spread out form of cup. Another term is progressive pitch because the pitch progressively increases from leading edge up the blade to trailing edge. It generally increases efficiency of the propeller and will also reduce RPM (effectively increase pitch). It kind of has the same effect as swimming with your hands flat or swimming with them cupped. It is also very hard to correct repair or recreate without the visual aid of a computerized Prop Scan® system as you cannot see the camber with your eye as compared to cup, which you can see.
In summary, there is really only one correct optimum propeller for your boat and it is the correct combination of all these terms above. It is very possible that you could have 2 or 3 identical boats and because of different owners and the way they configure and distribute weight (gear and etc.) it will have an effect on the optimum propeller. Nowadays with the price of boats, cost of fuel, environmental protections and concerns, it is very important that your engines perform to the peak that they were designed to run. In turn Prop Scan® tuned props will give you the best possible speed and efficiency out of your boat. With the new technology available it is very exciting to be able to accurately adjust the pitch of the props in increments of tenths and hundredths of inches instead of having to settle for single full inch increments. As well as documentation of what changes were made, plus the database of other boats with similar characteristics to research from.
DIAMETER - The distance across a circle at the extreme blade tip as it turns. Diameter is one of the most important factors in the propeller as is pitch. And they have to be the right combination to allow the boat to perform at its optimum. It is also determined by the pitch needed, the speed of the boat, RPM and gear ratio. Diameter also controls the amount of slip in the propeller (as in pitch because water is not solid). Too large of diameter props will produce too much unnecessary drag as too small will produce too much slip.
BLADES - The number of blades is also very important. Believe it or not the optimum propeller would have only one blade because it doesn't have any other blades scrambling up the flow of water in front of it. But balancing it would be impossible. Two blades would be optimum, but to run it on a larger boat would take such a large diameter you couldn't get it under the boat. So the next choice would be a 3 blade. Realistically in most cases this is the optimum propeller. There are reasons for more blades, as in 4, 5 and 6 blade props. The main one being the increase in blade area where it is needed to handle the pitch. If you had a 3 blade prop and the correct pitch was too much for the diameter of the propeller it would be overloaded and start to cavitate (see cavitation). If you couldn't fit a larger 3 blade under theboat you would need props with more blade area. You could go to a 4 blade of the same diameter and then even a 5 blade, if needed. Another reason for multiple blades in the past was to reduce vibration. But with the new computerized scanning technology, Prop Scan® , any number of bladed props can be made vibration free.
CAVITATION - Cavitation is a condition where a low pressure is formed on the surface of the propellers. This low pressure will cause water to vaporize and turn to steam. When the steam bubbles travel into an area with a higher pressure, these bubbles will implode and pit the surface of the propeller. Cavitation can cause severe damage to a propeller. It will also generate noise and vibration. Cavitation has many causes, e.g., damage on the leading edge of the propeller, incorrect blade shapes, and even the shape of appendages in front of the propeller.
VENTILATION - Ventilation is often confused with cavitation, but it is where air is induced into the propeller from an outside source. I.E. the surface, exhaust, or through hull fittings. It is a condition not generally created by the propeller.
CUPPED PROPELLERS - Cupping is a curvature added to the trailing edge of the propeller. It is usually very slight, about ½ to ¾ inch from the trailing edge down toward the middle of the blade and shaped like the bottom of a bowl. It is used to reduce cavitation and in some cases maximize thrust. Cupping basically makes an inefficient propeller run more efficient. It also affects pitch by effectively increasing it and will reduce RPM. We have found cup to be like medicine, a little won't do much for you, too much makes you sicker and the right amount will make you well (if cupping is the right medicine for your sickness). It is also very critical to have cupping very accurate as it will cause vibration. We can measure and adjust cupping to .001".
CAMBER - Camber is a long arching curve from leading edge to trailing edge like a spread out form of cup. Another term is progressive pitch because the pitch progressively increases from leading edge up the blade to trailing edge. It generally increases efficiency of the propeller and will also reduce RPM (effectively increase pitch). It kind of has the same effect as swimming with your hands flat or swimming with them cupped. It is also very hard to correct repair or recreate without the visual aid of a computerized Prop Scan® system as you cannot see the camber with your eye as compared to cup, which you can see.
In summary, there is really only one correct optimum propeller for your boat and it is the correct combination of all these terms above. It is very possible that you could have 2 or 3 identical boats and because of different owners and the way they configure and distribute weight (gear and etc.) it will have an effect on the optimum propeller. Nowadays with the price of boats, cost of fuel, environmental protections and concerns, it is very important that your engines perform to the peak that they were designed to run. In turn Prop Scan® tuned props will give you the best possible speed and efficiency out of your boat. With the new technology available it is very exciting to be able to accurately adjust the pitch of the props in increments of tenths and hundredths of inches instead of having to settle for single full inch increments. As well as documentation of what changes were made, plus the database of other boats with similar characteristics to research from.