prop pitch

[hakksaw]

hello fellas, i have a question about prop pitch.  i have an 88 135 merc with a 15x17 prop on an 86 dynatrak 180.  would i benefit at all by switching up to a 19 or 21 pitch?  i understand the concept of pitch, only if i don't look at the diagrams too much!  don't know whether this motor would have the rpms to perform with the steeper pitch, is it pretty much trial and error?.  don't wanna bust anything up.  thanks and merry christmas!

[cart7t]

Well, that depends. You're ultimate goal in picking a correct prop (there's more to this than just the correct pitch) is maximum performance, both in holeshot, mid speed and top end.  A lower pitch prop will improve your holeshot but you'll lose top end performance. A higher pitched prop will increase top end speed but at the sacrifice of that holeshot.  First, you have to answer the first question which is, what RPM's are you turning at WOT with your current prop?  The correct prop will have your engine turning around 5500-5800 RPM's at WOT with a roostertail just at or slightly below the top of the engine cowl height.  

A general rule of thumb is this, for every inch of prop pitch change you gain or lose about 200 or so RPM's in engine performance.  In other words, if you're current 17 pitch prop is turning 5800 RPM's at WOT, trimmed out and you go to a 19 pitch you can expect you're RPM's to drop to around 5400-5500 RPM's.  You'll see a speed increase of a few MPH but of course, your holeshot will probably take you longer to get up onto plane.  

Trimming an engine out for max performance can take some time because it's more than just prop pitch.  The reason there are so many different prop companies out there is that there are different props that are designed to do different things even though the prop pitch between these different brands is the same.  Since boat hull designs are different, there's no one prop or pitch that's right for every boat.  

First though, you need to get us the RPM's you're turning before we can advise you any further.

[Way2slow]

Go larger and you may actually run slower.  With a boat set up perfectly, it still takes a certain amount of horse power for a boat to run any given speed.   If a larger prop drops you below the peak horse power rpm of the motor, it's not going to run as fast and be a whole lot slower out of the hole.  Many times I've seen guys running those big props thinking they were "speed" props, just go to install a smaller pitch prop and gain a couple of mph.   What will usually give you the best performance from a boat/motor setup is get the setup right with a good stainless prop that is right on the redline or even one or two hundred rpm over the red line and then send that prop off and have it blue printed buy one of the better custom prop guys.  That's not cheap, but ooooohhhhh what a difference it usually makes.

[HPBB]

It depends on how you run your boat. I think you might do better with a 19 or 21, a 17 seem small for a 135, but if your boat is heavy it might be the best for it. If your going to run at WOT when your running a bigger pitch might be better. But if you run at crusing speed, say 3500rpm the 17 might be fine.

I know on my boat 17 1/2 footer, I run a 75Hp and run a 17 pitch prop. when i fish tournaments or are fishing I run the 17 pitch .It works the best for me. If I am going out family boating, maybe tubing/skiing or have 4 or 5 people in the boat. I will put my 15 pitch on it. Make pulling the extra wieght better. I have a 19 pitch, but the boat run slower with it than the 17 and has no hole shot.

The best way to find out what prop will work best for you is talk to a shop about how you want to use your boat, and see if they will let you try out a few different props till you find the best on for you.

[hakksaw]

thanks for the feedback guys.  i will get it out and pay a little more attention to the rpms this spring and then look into it.  it sure does seem like the 17 would be undersized. typically, i wuld like to trust the manufacturer on the decision to stick with that pitch, but its not the original motor.  my boat is a traditional tournament style bass boat from ebbtide.  a little old, but shouldn't be much heavier than todays models i would guess.  take care, merry christmas!

[hakksaw]

Took awhile to get back on this subject.  I purchased a used Quicksilver SS 23 pitch and gave it a shot a couple of weeks ago.  The engine was turning about 5600 with the alum 17 pitch.  I gained 12 mph but the rpm dropped to about 4900 which I am happy with.  I see the major differences in plane times too but it sure was fun opening that thing up on some calm water.  I can't imagine what you fellas running the 175's to 225's are feeling at WOT!!  (well, i guess i could, but it'll take a lottery ticket for me to give it a shot ;D