have a 89 yamaha 150 counter rotation with a 17 pitch ss prop–prop does fine as long as the boat is loaded with bait tank - 4 batteries and all the fishing gear–wot rpm runs around 5300 loaded with 5500 being recommended max–but when i strip everything off to “play” with the family i can easily exceed max rpm (which i dont do)—question is how much can the pitch of the prop i have be changed without really messing up the prop–talked to guy at local prop shop and he really just wanted to sell me a new $500 21 pitch prop he had in stock but said he could change pitch for $50 – thinking about going to around 19 --what do yall think i should do?
Not a huge fan of manually changing the pitch on a prop…IMO its better to bite the bullet and get a second prop. That way you can run whichever will suit you better for that particular trip/season as well as always provide you with a back up in the instance that one becomes damaged…Better safe than sorry, props are so precisely balanced that the smallest misalignment can cause major motor damage and too me it is just not worth the risk.
I will assume that you have a SS prop. If so pitch can be changed. Personally, I would not want any more that 1" pitch changed on a outboard prop.
If you only want one prop for your boat, then what you have now is correct. The prop should be matched to the heaviest load. If you want more speed when the boat is light, then I suggest getting a second prop and changing based on the usage.
Rule of thumb with an outboard prop is 1" pitch change equals 200 rpm change.
tigerfin—not doubting what you say but where did you find the wot rpm suggestion for this motor ? another thing that worries me about changing anything is that ever since this motor has been put in the water it has had a 17 prop on it --was the port motor on a larger boat for many years until i got it–kinda makes me think i should just let it be so i dont cause this motor to have any extra strain on it–might be the death of it! runs good and stong now with good compression and no problems besides the rpms running above what i was told was max --i want to get every bit of life out of it i can before i have to repower
5500- 6000 is a general rule for most all 2 strokes. Like hairball said, 5500-5600 is ideal, but turning up a little more won’t hurt. The biggest problem most people don’t understand is If you don’t turn enough rpm at wot then you are putting a whole lot of stress due to lugging the engine and do way more damage than over revving and really decreasing the life of a powerhead.
Maximum is stated as 5500-5600. Ideal is typically within 2-300 of max rpm. Under propping an engine doesn’t hurt, but overrevving can. The biggest concern with exceeding the max rpm is the forces exerted on the rotating assemblies. Those who race 2 stroke outboards run full synthetics and a richer oil to gas ratio prevent failure. I would also have concerns with the fuel system keeping up at overrevving speeds. Normally they can, but if your lift pump is weak, filter a little clogged, or jets have some varnish you can get a lean condition real quick and say goodbye to your powerhead.
what kind of changes to rpm does “cupping” a prop do? have noticed this prop is straight without the little roll “cup” (i think thats what its called) on the edge of the blades – isnt that there to prevent slippage ? like is stated before i dont exceed 5400 rpm --always keep a close eye and ear on the rpms --normally dont run wot anyway but sometimes it is nice to stretch her out
talked to guy at local prop shop and he really just wanted to sell me a new $500 21 pitch prop he had in stock but said he could change pitch for $50 – thinking about going to around 19
I am confused. You want to get your rpm up when loaded. Wouldn’t putting a 21 or even a 19 make the rpm go down? I believe you need to go to a 15 to get your revs up. Not sure but it sounds to me that you already have the most efficient prop with your 17.
no i want to get my rpm down when light–im good loaded --thats why i was considering a higher pitch on my prop for “playtime” – think ive decided to get a alum 4blade 19p to swap between and it would be a good thing to have a extra in the boat just in case since i do most of my fishing in the nastiest part of most lakes
2 stroke rpm max should be 5500
4 stroke rpm max is 6000 on most, but 6300 on some
a 21 pitch is WAY to much prop for your rig
adding cup will act like you have added some pitch, it will also gice you more bow lift, which will make the boat act lighter, and run faster.
cant really mix a sports car and a dumptruck
if you like the prop at load, keep what you have, and get an aluminum prop for playing
Give Skip’s Propeller a called, speak with Mike. Very nice and non-pushy. I just bought a Stiletto 22P for my 172 Sea hunt. He will let you try the prop out first and return it if it is not what you want. Given the fact that the prop is not damaged in any way.
Typically 1 pitch of adjustment will approximately affect the RPMS by 200. For example, a 17p to a 19p you should lose close to 400 rpms.
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4 blade prop is the answer, you’ll have the torque you want loaded, and +2" pitch will reduce WOT rpms when unloaded
the 4 blade will also allow you to hold plane @ a lower speed, which is quite helpful for wake sports. it will also improve fuel economy at cruising speed