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Old 27 April 2006, 07:53   #1
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Another prop question

I've been reading some of the prop-related discussions with interest but can't get my head around one thing.

My engine, according to the manual, should be set up to have a WOT range of between 4500-5500rpm, which seems quite a wide range.

With the current prop (which is I don't know what, but I guess it is stamped on it somewhere) it maxes at exactly 5500rpm which suggests that the choice is spot on for maximum speed. But, I am not going to be using over 5000rpm much as the engine sounds harsh and thrashy over that and the fuel consumption is epic.

So my question is, if I changed the prop to bring the WOT revs down to say 4800, should I expect a big drop in the top speed (currently 35.5-36kt @ 5500rpm) or would it be about the same within a couple of knots? I guess it depends on what the power curve of the engine is (Johnson 115 2str) i.e. how many hp you get at 4800 against how many at 5500?

I am doing this mainly to get (I hope) a better cruising speed at lower revs, so say 3000rpm which gives me about 18 knots now might give me 20-22 knots with a prop change, but I am wondering whether it will knock a lot off at the top end. I don't go at full speed that often but it is still nice to be able to burn people off occasionally if you know what I mean

Would it achieve what I want or doesn't it work like this?
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Old 27 April 2006, 08:07   #2
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Think of it like gears in a car - drive around in 1st gear you accelerate like hell but hit the redline way too early. On the other hand 5th gear would give you crap acceleration but a higher top speed and better economey.

All depends how soon you are hitting the 5600rpm mark - you may find you can go up a pitch or 2 and still be hitting 5600rpm in which case you will get higher speed so you can throttle back a bit.

As you said a lot depends on the power and torque curves - to be honest the best way is to stick various props on and find out!!!
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Old 27 April 2006, 08:22   #3
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Stick with what you've got.
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Old 27 April 2006, 08:35   #4
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The RPM range quoted by an engine manufacturer is usually the range within which the engine is producing at or very nearly the rated horse power.

If you are hitting the top end of the RPM range and your boat leaps out of the water and on to the plane very quickly you can certainly go up in pitch at least 2"

It could be that the previous owner was always carrying large heavy loads or towing and has put the appropriate pitch prop for that. If you are running round with a much lighter load you will need to get an appropriately pitched prop.

If you cant find what pitch prop yours is you can calculate it from engine rpm, gearbox ratio, speed (gps measure if possible as its more accurate) and an estimated prop slipage (I usually start with about 10% for full throttle calculations)
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Old 27 April 2006, 08:46   #5
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Just done the calculation for you and I think youve got a 19"

Whilst a 21" will probably be usable and give you a higher speed at a lower revs I would only consider it if you always have light loads and never want to do any towing of water skiers etc...

Or you could just keep the 19" and swap the prop for towing and heavy loads.
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Old 27 April 2006, 09:38   #6
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Thanks, loads of brilliant answers as usual on RIBnet

I got the impression from something I read that if the prop was too fine it would make the engine over-rev at WOT but maybe the Johnson has a rev limiter?

Usually it is just me in it at the moment. Performance from a standing start seems good to me though I don't have any experience to compare it to but if you whack it open you just get whaaaaaaoooooooOOOWWWW and you're pretty much at full speed which is good

I suppose I ought to have a spare prop anyway, given that I won't stand a snowball's chance of getting one here in a hurry if I smack it on a rock. What sort of price would I expect to pay for a prop on that size of engine, nothing too fancy just a bog standard 3 blade one? I suppose I could get something a bit different just to try (maybe a 21 or something, if this is indeed a 19), and then if it doesn't work as well as hoped, just keep it as a backup. There's only a couple of small scars on the one on there at the moment, and none of them are my fault!

Thanks again
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Old 27 April 2006, 10:39   #7
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Roy is about right there. I'd say that you are slightly under propped if she's hitting max revs so readily. I'm going to be running a 19" and a 21" on mine. The 19" for general use inc. wakeboarding and towing toys etc and the 21" for extended cruises.
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Old 27 April 2006, 13:21   #8
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Quote:
Originally Posted by BogMonster
I suppose I ought to have a spare prop anyway, given that I won't stand a snowball's chance of getting one here in a hurry if I smack it on a rock. What sort of price would I expect to pay for a prop on that size of engine, nothing too fancy just a bog standard 3 blade one?
Sort of depends on whether you want stainless or aluminum. Stainless props run anywhere from $200 to $400 (or more) in USD; aluminums run typically about $80 to $170 or so.

If you're in a rocky area (and moreso if you run shallow a lot) stick with aluminum. You can beat them back into shape with a hammer and a file, and nailing something really solid will mash the prop rather than your gearbox. Plus, you can get two (or three) for the price of one good stainless prop.

jky
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Old 27 April 2006, 13:33   #9
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Quote:
Originally Posted by jyasaki
Sort of depends on whether you want stainless or aluminum. Stainless props run anywhere from $200 to $400 (or more) in USD; aluminums run typically about $80 to $170 or so.

If you're in a rocky area (and moreso if you run shallow a lot) stick with aluminum. You can beat them back into shape with a hammer and a file, and nailing something really solid will mash the prop rather than your gearbox. Plus, you can get two (or three) for the price of one good stainless prop.

jky
[cough]

Alli will do fine then thanks, I can live without a bling shiny prop with that price difference

I looked at the existing prop at lunch time but I can't see any markings on it at all to indicate what make/pitch/anything so all I know is it's black and got 3 blades

would such information be stamped into the back (front!) of the prop against the hub where you can't see it or is it a "best guess" thing as to what pitch it is?
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Old 27 April 2006, 14:58   #10
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Have a look on Ebay - I picked up a mint stainless prop for 49 - postage would be a prob though!!!
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