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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. 
Link Posted: 8/20/2017 11:55:58 PM EDT
[#1]
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.
Link Posted: 8/20/2017 11:59:35 PM EDT
[#2]
But what are the doors on the side for ?
Link Posted: 8/21/2017 12:12:33 AM EDT
[#3]
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But what are the doors on the side for ?
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it's how you launch an icbm horizontally.. while on a treadmill..
Link Posted: 8/21/2017 12:14:57 AM EDT
[#4]
Link Posted: 8/21/2017 12:18:56 AM EDT
[#5]
...
Link Posted: 8/21/2017 1:09:29 AM EDT
[#6]
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. 
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There is a lot more to think about than the size, pitch and power, like the size of the boat and the reasonable speed that a motor of a given hp will push it.  If you put 300 hp on a barge that will go 20 mph, you need a big , low pitch prop that will push you 20 mph at peak output rpms, say 5k rpm.   If you put that same motor on a lite go fast hull that can reach 80 mph then you need a prop pitched to hit 80 mph at the same 5k rpm.  The faster you go the drag in the water increases dramatically, so this prop would probably not want to be as big because of drag and because it is pushing a lighter load.  Also, once you are going 80 mph that large pitch doesn't feel so large because the water you are pushing on is going by at 80 mph.

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.
Link Posted: 8/21/2017 2:51:44 AM EDT
[#7]
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.
Link Posted: 8/21/2017 3:19:57 AM EDT
[#8]
Oh hell I can't sleep soooo.... let's go
Link Posted: 8/21/2017 6:13:24 AM EDT
[#9]
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.
Link Posted: 8/21/2017 6:49:38 AM EDT
[#10]
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. 
Link Posted: 8/21/2017 7:51:20 AM EDT
[#11]
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.

Link Posted: 8/21/2017 8:01:53 AM EDT
[#12]
Link Posted: 8/21/2017 8:26:41 AM EDT
[#13]
Then you've got things like counter-rotating dual props, variable pitch props, jet drives, etc....
Link Posted: 8/21/2017 8:31:27 AM EDT
[#14]
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Quoted:
it's how you launch an icbm horizontally.. while on a treadmill..
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Quoted:
But what are the doors on the side for ?
it's how you launch an icbm horizontally.. while on a treadmill..
why would u want to?
Link Posted: 8/21/2017 8:39:14 AM EDT
[#15]
Should we tell him about Arneson?
Link Posted: 8/21/2017 8:51:27 AM EDT
[#16]
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Should we tell him about Arneson?
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Cool as shit that they work like they do.
Arneson Surface Drives on a 40' Skater V-Bottom
Link Posted: 8/21/2017 9:08:41 AM EDT
[#17]
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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.
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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.
Dont even own a boat. Just curious about the dynamics of propelling them. 
Link Posted: 8/21/2017 9:40:51 AM EDT
[#18]
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Quoted:
Cool as shit that they work like they do.
https://www.youtube.com/watch?v=Of1RY4M8VqM
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Quoted:
Quoted:
Should we tell him about Arneson?
Cool as shit that they work like they do.
https://www.youtube.com/watch?v=Of1RY4M8VqM
Holay Chit Mang!  
Link Posted: 8/21/2017 9:54:43 AM EDT
[#19]
Variable pitch.

Fixed pitch props will have a cavitation point that is lower speed than a variable pitch prop.
Link Posted: 8/21/2017 8:04:26 PM EDT
[#20]
Link Posted: 8/21/2017 8:07:30 PM EDT
[#21]
IDK the science but my lobsterboat had the prop reprofiled a few years back and it added 4knots
Link Posted: 8/21/2017 8:10:01 PM EDT
[#22]
Discussion ForumsJump to Quoted PostQuote 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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Avatar is appropriate ......
Link Posted: 8/21/2017 8:16:26 PM EDT
[#23]
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.
Link Posted: 8/21/2017 8:43:50 PM EDT
[#24]
Pleasure boating


Spirit of Texas Top Fuel Hydro 274.92 MPH @ Parker, AZ



Problem Child - 257 mph TOP FUEL HYDRO (HD)
Link Posted: 8/21/2017 9:41:23 PM EDT
[#25]
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Quoted:
Dont even own a boat. Just curious about the dynamics of propelling them. 
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Herring makes the fastest propellers for performance boats.  Pitch, rake and diameter can all be varied and also depending on the hull design turning the props in or out gives more bow lift or more stern lift.  Most props these days are 5-7 blades with 6 being the most common.  That's what you will see hanging off Merc 6's and 8's as well as Arnesons.

On a bravo style drive hub design such as length, flair, defuser ring, blade shape all make a big difference
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