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Old 11 September 2009, 08:48   #1
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Props explained please..

Following on from IBWET's Prop comparison thread http://rib.net/forum/showthread.php?t=32193

Could someone explain to me about the different types/sizes of props and there effects.

After reading the above thread I thought to myself I could do with a finding the right prop.
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Old 11 September 2009, 09:02   #2
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19x21 will go faster but accelerate slower than a 19x19, 19x19 will accelerate quick but will top out sooner.
First number is diameter second the pitch.
A 19x21 will do the same speed as a 20x20. If you reduce the diameter of the prop by an inch and increase the pitch by an inch you will get the same speed but may get different characteristics.

Above relates to no particular boat and is just an example.
Clear as mud eh!
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Old 11 September 2009, 09:16   #3
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JPS - do you mean the high level of pitch compared to the effect ?

i.e. - bigger/ higher pitch = generally higher top end speed ( if the engine has the grunt to turn it) , but slower acceleration & the potential to not get to the correct power band/ revs on the engine.

lower pitch is the opposite .....

Like gears in a car - 5 th will allow you to go fast , but you wont pull away in it ; 1st will accelerate great , but you hit the rev limiter at 15 mph !

As for the blade area, cupping , no of blades etc etc I would just be guessing !
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Old 11 September 2009, 09:33   #4
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Props & engines are essentially a balancing act. Your engine under no load will rev itself to destruction, the prop is what takes that energy and turns it into forward thrust but also looses some of it thru drag. What you need to do is balance that thrust / drag with the engine's output.

There are essentially three things you can vary with a prop - pitch, diameter & # blades.

Pitch - this is the "screw" that pushes you through the water, and is the theoretical distance the prop will travel per revolution - so an 11" pitch should go 11 inches, a 22" will go twic as far per Rev. (Slip means you actually go a bit less distance in reality). Think of it like gears on your car - al ower pitch is like a lower gear - so better accel & better pulling power, but lower top speed & less MPG

Diameter - this is exactly what it says on the tin. A bigger diameter prop will generally have bigger blades, and so more grip, but will also have more drag. Largest dia is fixed by your cav plate getting in the way.

# blades. - again self explanatory, and again in theory the more blades the more grip, but more dag.


So, the net result of this is that you need to get a prop that won't "let go" at the slightest provocation, and will provide enough resistance to the engine to stop it going over it's max RPM. that matches your boat & engine combo, whilst taking into account what you use it for.

Example - if you have a super lightweight hull, and want to go really fast, you can get a smaller diameter (less drag) and higher pitch & go further per rev, becuase a higher pitch will put the load back on the engine & stop it passing the red line. You can do that with a light boat because it doesn't need so much force to push it along. Problem is, with a bigger / heavier boat, there comes a point where the small blades / diameter will let go of the water due to less grip, so you have to get a bigger dia prop. At the opposite end of the scale, if you are towing a skier, a lower pitch will allow you to get on the plane faster.

My old Yam was a classic example of that - with the prop it came with (small dia, high pitch), once on the plane it went like a scalded cat.... in a straight line. Turn the wheel more than 1/8 turn & it broke out. Show it a small wave & it let go..... Upped the diameter by 1/2", downed the pitch to compensate & got a really useable compromise.


Engine trim will also affect the likelyhood of it "letting go" & getting on the plane, but I'll leave someone else to explain that one.
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Old 11 September 2009, 09:37   #5
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i suppose you could think of it like gearing of a car ,,for a given engine r.p.m the finer the pitch the less speed you will get from the boat but more pulling power ,,then as the pitch gets coarser more speed but the pulling power gets less ,,,,thats why with an old 5hp seagull outboard,,,big slow, fine pitch prop, its possible for it to push a 60 foot barge .but only at a couple of knots ,where as if you put say a modern 5 hp engine on it would push it but the prop would be airating /cavitating or not getting a proper bite in the water ,,,,,water skiers wanted a prop to get quick acceleration up and out of the hole and on to the plane quick but the top speed would be down a little so would use a slightly finer prop than say a boat for distance racing where a coarser prop would give slower acceleration but a much better top boat speed when it got there ,
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Old 11 September 2009, 09:52   #6
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Thanks lads. Not able to digest everything you've put but will sit down after the kids have gone to bed and have a good read.


Motor is the Yam 115 with a 13x19 stainless prop.

When turning tight, the prop can slip very easily. Also can loose grip when things get lumpy. Top end I get around 42-45 knots but not sure on the revs (will check when the boats back in the water. Acceleration seems good but with nothing to compare it too I wouldn't know for sure.
Also I feel as if the trim doesn't raise the bow as it should. I know this isn't due to weight as I've had a move around with stuff over the months. But again this could be my in experience with plaining hulls. Again I'm used to pushing through the water.
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Old 11 September 2009, 09:53   #7
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Quote:
Originally Posted by chewy View Post
A 19x21 will do the same speed as a 20x20.
You think so eh?
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Old 11 September 2009, 09:54   #8
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Quote:
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Clear as mud eh!
No
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Old 11 September 2009, 10:12   #9
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i wanted a spare emergency prop for my 8 hp 4stroke ,,only second hand one i could get was the next engine size up ,,tryed it out on the boat and found it was making the engine overheat and it had far too much pitch ,,so borrowed the boat clubs lathe ,and took off about an inch or so of the blade tips ,,now its just right ,
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Old 11 September 2009, 10:24   #10
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Quote:
Originally Posted by m chappelow View Post
i wanted a spare emergency prop for my 8 hp 4stroke ,,only second hand one i could get was the next engine size up ,,tryed it out on the boat and found it was making the engine overheat and it had far too much pitch ,,so borrowed the boat clubs lathe ,and took off about an inch or so of the blade tips ,,now its just right ,
Wont this have buggered the flow of water coming of the tips of the prop ? Maybe with 8hp it not to noticable & as a back up not any problem- but guess if you did this on a prop bolted to a 50 + hp motor you'd soon feel it ?

Props are not the most efficient things anyway, but I thought that when you get into the full on fluid dynamics of drag etc ( its been a while since I did this at uni, or for anything else on Aircraft) this can have major impacts. Best simple example being the winglets on airliners using the analogy of the wing as a prop blade .
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