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#1
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prop design- newbie
Hi,
I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning about prop design. My engines are 250 HP Cummings with Twin Disc Gears. My goal is higher speed with lower fuel consumption without causing damage to the drives. I know that the current props are 4 blade and generate 10 mph at 12-1400 RPM depending on current and wind. My current fuel consumption is 1.1 gallon per mile. The weight of the boat is 80 tons and is a displacement hull. Thanks for your help, Marshall |
#2
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On Sat, 28 May 2005 20:47:10 GMT, "MarshallE"
wrote: Hi, I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning about prop design. My engines are 250 HP Cummings with Twin Disc Gears. My goal is higher speed with lower fuel consumption without causing damage to the drives. I know that the current props are 4 blade and generate 10 mph at 12-1400 RPM depending on current and wind. My current fuel consumption is 1.1 gallon per mile. The weight of the boat is 80 tons and is a displacement hull. Thanks for your help, Marshall Pitch: a fixed pitch propellor has its pitch set to achieve an optimal angle of attack at the desired shaft speed, and hull forward speed, both of which contrl the value. Diameter: In general, a larger diameter leads to higher propuldsive efficiency, until physical effects (like cavitation, or blade weakness) are controling. Projected area: In general projected area is adjusted to absorb the available shaft power at the desired shaft speed and target hull speed. Can you say, "over-simplified?" Hope this helps, all the same Brian Whatcott |
#3
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MarshallE wrote:
Hi, I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning about prop design. My engines are 250 HP Cummings with Twin Disc Gears. My goal is higher speed with lower fuel consumption without causing damage to the drives. I know that the current props are 4 blade and generate 10 mph at 12-1400 RPM depending on current and wind. My current fuel consumption is 1.1 gallon per mile. The weight of the boat is 80 tons and is a displacement hull. You fuel consumption looks about right for an 80 foot boat at 10 knots You are pushing the displacement hull a bit hard at 10 knots, do some trials and plot speed against fuel burn, and MPG. Knots/GPH = MPG You should find a sweet spot somewhere between 8 and 10 knots. The speed of a displacement hull in knots is determined by the square root of the waterline lenth in feet, multiplied be a constant k k = 1.3 is the max that you can hope for, fuel burn will be BAD For best cruising MPG k will usually be between 1.0 and 1.1 but it depends on the hull design. In some cases best MPG may come at lower values of k, plot your graphs and see how they come out. For example, for a waterline lenth of 64 feet sqrt(len) = 8 8 * 1.3 = 10.4 knots -- Only achievable with a clean slick modern hull. 8 * 1.2 = 9.6 knots -- Maybe the max for older hulls 8 * 1.1 = 8.8 knots -- Only a tad less speed for about half the fuel 8 * 1.0 = 8.0 knots -- Best economy cruise will be somewhere around here. There is probably little to be gained by messing with the props. If you want to go faster get a new boat with a semi-displacement hull but be prepared to spend big $$$$ on fuel. |
#4
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thanks for both responses...my son gave me a book by Dave Gerr about
propellers today...fascinating topic thanks again Marshall "Chris Newport" wrote in message ... MarshallE wrote: Hi, I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning about prop design. My engines are 250 HP Cummings with Twin Disc Gears. My goal is higher speed with lower fuel consumption without causing damage to the drives. I know that the current props are 4 blade and generate 10 mph at 12-1400 RPM depending on current and wind. My current fuel consumption is 1.1 gallon per mile. The weight of the boat is 80 tons and is a displacement hull. You fuel consumption looks about right for an 80 foot boat at 10 knots You are pushing the displacement hull a bit hard at 10 knots, do some trials and plot speed against fuel burn, and MPG. Knots/GPH = MPG You should find a sweet spot somewhere between 8 and 10 knots. The speed of a displacement hull in knots is determined by the square root of the waterline lenth in feet, multiplied be a constant k k = 1.3 is the max that you can hope for, fuel burn will be BAD For best cruising MPG k will usually be between 1.0 and 1.1 but it depends on the hull design. In some cases best MPG may come at lower values of k, plot your graphs and see how they come out. For example, for a waterline lenth of 64 feet sqrt(len) = 8 8 * 1.3 = 10.4 knots -- Only achievable with a clean slick modern hull. 8 * 1.2 = 9.6 knots -- Maybe the max for older hulls 8 * 1.1 = 8.8 knots -- Only a tad less speed for about half the fuel 8 * 1.0 = 8.0 knots -- Best economy cruise will be somewhere around here. There is probably little to be gained by messing with the props. If you want to go faster get a new boat with a semi-displacement hull but be prepared to spend big $$$$ on fuel. |
#5
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Try
www.fernandocarvalho.cjb.net "MarshallE" escreveu na mensagem m... Hi, I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning about prop design. My engines are 250 HP Cummings with Twin Disc Gears. My goal is higher speed with lower fuel consumption without causing damage to the drives. I know that the current props are 4 blade and generate 10 mph at 12-1400 RPM depending on current and wind. My current fuel consumption is 1.1 gallon per mile. The weight of the boat is 80 tons and is a displacement hull. Thanks for your help, Marshall |
#6
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I read that book as well and found it extremely interesting although I have
not put that information to practical use. Dave Gerr also has a book about boats that I find myself reading again and again, All about Boats -- R. Scott Carroll "MarshallE" wrote in message m... thanks for both responses...my son gave me a book by Dave Gerr about propellers today...fascinating topic thanks again Marshall "Chris Newport" wrote in message ... MarshallE wrote: Hi, I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning about prop design. My engines are 250 HP Cummings with Twin Disc Gears. My goal is higher speed with lower fuel consumption without causing damage to the drives. I know that the current props are 4 blade and generate 10 mph at 12-1400 RPM depending on current and wind. My current fuel consumption is 1.1 gallon per mile. The weight of the boat is 80 tons and is a displacement hull. You fuel consumption looks about right for an 80 foot boat at 10 knots You are pushing the displacement hull a bit hard at 10 knots, do some trials and plot speed against fuel burn, and MPG. Knots/GPH = MPG You should find a sweet spot somewhere between 8 and 10 knots. The speed of a displacement hull in knots is determined by the square root of the waterline lenth in feet, multiplied be a constant k k = 1.3 is the max that you can hope for, fuel burn will be BAD For best cruising MPG k will usually be between 1.0 and 1.1 but it depends on the hull design. In some cases best MPG may come at lower values of k, plot your graphs and see how they come out. For example, for a waterline lenth of 64 feet sqrt(len) = 8 8 * 1.3 = 10.4 knots -- Only achievable with a clean slick modern hull. 8 * 1.2 = 9.6 knots -- Maybe the max for older hulls 8 * 1.1 = 8.8 knots -- Only a tad less speed for about half the fuel 8 * 1.0 = 8.0 knots -- Best economy cruise will be somewhere around here. There is probably little to be gained by messing with the props. If you want to go faster get a new boat with a semi-displacement hull but be prepared to spend big $$$$ on fuel. |
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