Posted: 8/20/2017 11:36:23 PM EDT
| Was talking to a buddy about boat propellors and was watching vids of some damn fast boats and was thinking about what specifically about the propellers that makes them move the boat so quickly. I assume there is some finite number of RPMs a prop can spin in the water and still "bite", so then what does more horsepower do if a lesser powerful motor is still capable of achieving the fastest usable propeller speed? Does a boat with a 7 marine 627hp outboard have a propeller that is 2x the size of a 300hp need outboard? I am interested in the relationship of size and pitch to power assuming there is a point of diminishing returns with RPMs. |
| Most of the outboard props are 14 to 16 inch diameter, pitch ranges vary greatly. A go fast boat may run pitch in the 32 to 34 range while a fishing boat may run a 17 to 19 pitch. Pitch is how much a boat will move forward with each revolution. A 17 pitch would move forward 17 inches per revolution while a 34 pitch would move forward 34 inches per revolution. All props slip, rates vary from as low as 4% to as much as 20% depending on prop design and application. 4 and 5 blade props are becoming common. |
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Was talking to a buddy about boat propellors and was watching vids of some damn fast boats and was thinking about what specifically about the propellers that makes them move the boat so quickly. I assume there is some finite number of RPMs a prop can spin in the water and still "bite", so then what does more horsepower do if a lesser powerful motor is still capable of achieving the fastest usable propeller speed? Does a boat with a 7 marine 627hp outboard have a propeller that is 2x the size of a 300hp need outboard? I am interested in the relationship of size and pitch to power assuming there is a point of diminishing returns with RPMs. To you bite comment, to be efficient a prop should only slip a certain percentage. If you have a high pitch prop targeted to go 80 mph, when the boat is barely moving, the slip is incredibly high, it will just cavitate with to much power applied. Once you get moving closer to target speed the prop will bite better as the slip comes down and more power is applied. |
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It all goes back to the purpose of the hull-engine-prop combination.
And then go one step further. We're running a chevy 5.7l engine with 343hp, high torque as well, in a relatively short 20' true waterski boat. Meanwhile the top speed is only 42mph or so. On the other hand, tie the rope to the dock and we could probably rip the dock down clean off shore. Our props are very thin, 4 blade with very LITTLE pitch. 
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Was talking to a buddy about boat propellors and was watching vids of some damn fast boats and was thinking about what specifically about the propellers that makes them move the boat so quickly. I assume there is some finite number of RPMs a prop can spin in the water and still "bite", so then what does more horsepower do if a lesser powerful motor is still capable of achieving the fastest usable propeller speed? Does a boat with a 7 marine 627hp outboard have a propeller that is 2x the size of a 300hp need outboard? I am interested in the relationship of size and pitch to power assuming there is a point of diminishing returns with RPMs. Your best bet is to find a good book from a naval architecture selection that has a chapter on prop design and selection. It's more complicated than you'd like, and then you are stuck with using an off the shelf propeller. There are a couple around in print that have design nomographs to help choose the prop. |
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Should we tell him about Arneson?  Failed To Load Title |
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Your best bet is to find a good book from a naval architecture selection that has a chapter on prop design and selection. It's more complicated than you'd like, and then you are stuck with using an off the shelf propeller. There are a couple around in print that have design nomographs to help choose the prop. Quoted:
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Was talking to a buddy about boat propellors and was watching vids of some damn fast boats and was thinking about what specifically about the propellers that makes them move the boat so quickly. I assume there is some finite number of RPMs a prop can spin in the water and still "bite", so then what does more horsepower do if a lesser powerful motor is still capable of achieving the fastest usable propeller speed? Does a boat with a 7 marine 627hp outboard have a propeller that is 2x the size of a 300hp need outboard? I am interested in the relationship of size and pitch to power assuming there is a point of diminishing returns with RPMs. Your best bet is to find a good book from a naval architecture selection that has a chapter on prop design and selection. It's more complicated than you'd like, and then you are stuck with using an off the shelf propeller. There are a couple around in print that have design nomographs to help choose the prop. 
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Should we tell him about Arneson?  https://www.youtube.com/watch?v=Of1RY4M8VqM
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Dont even own a boat. Just curious about the dynamics of propelling them. Quoted:
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Was talking to a buddy about boat propellors and was watching vids of some damn fast boats and was thinking about what specifically about the propellers that makes them move the boat so quickly. I assume there is some finite number of RPMs a prop can spin in the water and still "bite", so then what does more horsepower do if a lesser powerful motor is still capable of achieving the fastest usable propeller speed? Does a boat with a 7 marine 627hp outboard have a propeller that is 2x the size of a 300hp need outboard? I am interested in the relationship of size and pitch to power assuming there is a point of diminishing returns with RPMs. Your best bet is to find a good book from a naval architecture selection that has a chapter on prop design and selection. It's more complicated than you'd like, and then you are stuck with using an off the shelf propeller. There are a couple around in print that have design nomographs to help choose the prop. I can go retrieve a couple of my books with useful information if you want. There might be something online, I haven't looked in ages. The problem with design charts is that they generally cover usual operating conditions and radical conditions are "off the chart". |
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Most of the outboard props are 14 to 16 inch diameter, pitch ranges vary greatly. A go fast boat may run pitch in the 32 to 34 range while a fishing boat may run a 17 to 19 pitch. Pitch is how much a boat will move forward with each revolution. A 17 pitch would move forward 17 inches per revolution while a 34 pitch would move forward 34 inches per revolution. All props slip, rates vary from as low as 4% to as much as 20% depending on prop design and application. 4 and 5 blade props are becoming common. |
Pleasure boating
 Spirit of Texas Top Fuel Hydro 274.92 MPH @ Parker, AZ  Problem Child - 257 mph TOP FUEL HYDRO (HD) |
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Dont even own a boat. Just curious about the dynamics of propelling them. On a bravo style drive hub design such as length, flair, defuser ring, blade shape all make a big difference |