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Posted: 8/20/2017 11:55:58 PM EDT
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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Posted: 8/20/2017 11:59:35 PM EDT
But what are the doors on the side for ?
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Posted: 8/21/2017 12:12:33 AM EDT
Quote History Quoted:
But what are the doors on the side for ? |
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Posted: 8/21/2017 12:14:57 AM EDT
 Cavitation! explained HD |
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Posted: 8/21/2017 12:18:56 AM EDT
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Posted: 8/21/2017 1:09:29 AM EDT
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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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Posted: 8/21/2017 2:51:44 AM EDT
To take things a step farther then there are surface piercing props that reduce drag at high speed by getting blades out of the water.
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Posted: 8/21/2017 3:19:57 AM EDT
Oh hell I can't sleep soooo.... let's go
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Posted: 8/21/2017 6:13:24 AM EDT
I run a four blade. I'm not interested in top end speed as much as getting on plane quickly. I fish shallow water coastal the most.
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Posted: 8/21/2017 6:49:38 AM EDT
The different variables of a boat prop is almost mind boggling. Then it also devolves into a 9 vs 45 debate with some also. At least it does in the mud boat world. The two vs three blade debates look like a 9 vs 45 debate here.
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Posted: 8/21/2017 7:51:20 AM EDT
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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Posted: 8/21/2017 8:01:53 AM EDT
Quoted:
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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Posted: 8/21/2017 8:26:41 AM EDT
Then you've got things like counter-rotating dual props, variable pitch props, jet drives, etc....
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Posted: 8/21/2017 8:31:27 AM EDT
Quote History Quoted:
it's how you launch an icbm horizontally.. while on a treadmill.. Quote HistoryQuoted:
Quoted:
But what are the doors on the side for ? |
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Posted: 8/21/2017 8:39:14 AM EDT
Should we tell him about Arneson?
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Posted: 8/21/2017 8:51:27 AM EDT
Quote History Quoted:
Should we tell him about Arneson?  Arneson Surface Drives on a 40' Skater V-Bottom |
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Posted: 8/21/2017 9:08:41 AM EDT
Quote History Quoted:
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. Quote HistoryQuoted:
Quoted:
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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Posted: 8/21/2017 9:40:51 AM EDT
Quote HistoryQuote History Quoted:
Quoted:
Should we tell him about Arneson?  https://www.youtube.com/watch?v=Of1RY4M8VqM
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Posted: 8/21/2017 9:54:43 AM EDT
Variable pitch.
Fixed pitch props will have a cavitation point that is lower speed than a variable pitch prop. |
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Posted: 8/21/2017 8:04:26 PM EDT
Quote History Quoted:
Dont even own a boat. Just curious about the dynamics of propelling them. Quote HistoryQuoted:
Quoted:
Quoted:
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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Posted: 8/21/2017 8:07:30 PM EDT
IDK the science but my lobsterboat had the prop reprofiled a few years back and it added 4knots
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Posted: 8/21/2017 8:10:01 PM EDT
Quote History Quoted:
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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Posted: 8/21/2017 8:16:26 PM EDT
It's a compromise between power and speed. Just like a cars ring and pinion ratio. Only it's compounded by no trans and no tractive surface.
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Posted: 8/21/2017 8:43:50 PM EDT
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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Posted: 8/21/2017 9:41:23 PM EDT
Quote History Quoted:
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 |
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