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RPM effects with different pitchs

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rnusr01

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Hi,

Is there a general rule of thumb for this? Example if you go from a 15 to a 17 to a 19 how much does the rpm change? Is it 200 per 2 degrees or something like that.

Thanks
 
It's pretty much all educated guesswork.   :wacko:    Usually each change in pitch is 2-400 rpm depending on the prop..
 
Carl is right.  Each inch of change in pitch can change max rpms by ~200 rpm (on my boat/motor).  Could be 400 rpms for other motors. So, if I were to go from a 15 to a 17 inch pitch on my motor, my max rpms would drop by 2 x 200 = 400 RPM.

Generally start with the lightest boat you will run with.  If that's just you, then do this with just you.  If it's always with your wife and two kids and a dog, then do this with them all on board.  With your "light" weight conditions, you want the max rpms to be at the upper end of the range the motor manufacture recommends.  So if it's 5000-6000, you want to be close to 6000 rpms, so get the pitch that will give you that.  Then, when you have a full (heavy) boat, you won't lug the engine by being below 5000 rpms at WOT.

Save your engine.  Dial in your prop.  Dealers generally just put whatever prop on that they've had good luck with for any given boat, but they should really be asking you how heavy you will run your boat first.  I'm sure most don't.  But you can dial this in yourself.  Props are expensive though!
 
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Thanks for the responses. I am sure the answer is it depends but what would be the difference in speed when changes the pitch from 15 to 17 or do you think it would just lower rpm and not change top speed?

Thanks
 
TomS

I did that and when I change the pitch and recalculate everything but the slip percentage stays the same. Hmm
 
The calculator allows you to keep everything else constant and change which ever variable you want. For example,  you can maintain constant slip, change the pitch, and then lower the rpm a couple hundred to see what the speed effect is. This gives you all theoretical values.
 
OK thank you. It would be nice if you could put in the numbers and have it give you the slip results, change just the pitch and then it automatically adjusted rpm speed etc.
 
rnusr01 said:
OK thank you. It would be nice if you could put in the numbers and have it give you the slip results, change just the pitch and then it automatically adjusted rpm speed etc.
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Not sure I'm understanding. If you know the rpm and speed for any given pitch it will tell you exactly the slip. It just solves for which ever variable you don't know.Also, this is just a theoretical starting point, so use it as a guide and learning tool. I use it to cry foul when someone makes ridiculous claims about speeds from a stock motor that would have required zero slip or even negative. Not possible.
 
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Are you insinuating that I DIDN'T go 52.6 mph on my 2 25" toon 2009 2075 GLi with a 90hp yamaha? Rpm's were 12,500 and I had a 21" prop on it. Had to cut away part of the cavitation plate cuz it was in the way.
 
LOL
 
BulldogsCadillac said:
Are you insinuating that I DIDN'T go 52.6 mph on my 2 25" toon 2009 2075 GLi with a 90hp yamaha? Rpm's were 12,500 and I had a 21" prop on it. Had to cut away part of the cavitation plate cuz it was in the way.
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Behind the Escalade, yes, of course you did.
 
That's all the Escalade can do? 52.6???

Or maybe it's that Canadian math again...
 
kaydano said:
That's all the Escalade can do? 52.6???

Or maybe it's that Canadian math again...
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That's when the Caddy V-8 threw a rod trying to run at 12,500 rpm... Wow, I am so far off topic yet again. Sorry about that musr01.
 
Can we make a forum rule that a little comic relief is always considered "on topic" for EVERY topic, even if it is off topic? Hope that made sense...
 
TomS said:
That's when the Caddy V-8 threw a rod trying to run at 12,500 rpm... Wow, I am so far off topic yet again. Sorry about that musr01.
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I'm sure he was spinning on the ice when that happened...

Still iced over up there, Derrick?
 
Some humor is always a good thing.......no worries :)
 
kaydano said:
I'm sure he was spinning on the ice when that happened...

Still iced over up there, Derrick?
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There was still ice on Saturday night, but I'm told that by Sunday night it was gone. It was 33C plus high humidity on Saturday so that should have helped. Right now we are under a severe storm warning,  crazy rain, chance of quarter to toonie sized hail. Luckily the boat is still in storage. Just realized most of you have no idea how big a toonie is, or probably what it is either, unless you watch Olympic hockey you might remember. 
 
It's two loonies in one.
 
Ok so 6 people on board wot 6000 rpm 40 mph. Should I take the plunge and try the 17 pitch? Wonder what kind of speed increase I might expect any ideas?
 
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