Prop Principles

[Zara Spook]

From the perspective of the maintenance and life of the motor (in this case a 2011 F60), would it be better to be propped right at the max RPM at WOT, or slightly under the max RPM?

I'm trying to decide between a 13 and a 14-pitch four-blade prop. The 13 would likely have me right at the max, and perhaps a bit over, while the 14 would leave me slightly under.

[DennisG01]

Go to the max. Once you have a few more guys in there, or hauling home those BIG fish, your RPM is going to drop anyways.

[boscoe99]

Better to be under propped than over propped. Under propped, no harm no foul. The motor will protect itself if you are not paying attention. If over propped some bad juju can happen.

Try and get as close as you can to 6000 RPM with the boat loaded. In the high temperatures and humidity of summer time. If you are by yourself on a cool and dry day, mind the tachometer so as to not let the motor over speed.

[DennisG01]

Another reason to go underpropped is that a (competent) skipper can always control the RPM's with the stick.

Think of it this way... if you overprop (higher pitch), it can sort of be like trying to run 45mph on the highway, going up a hill, yet have your car in 7th gear - it will "lug" the engine.

[Zara Spook]

Appreciate the feedback folks. I'm settling in here on a PowerTech SCB4.

For the Yamaha Performance Bulletin (same boat/motor combination) they got 6050 RPM with a 12-pitch three-blade aluminum prop.

The PowerTech is a bit more than an inch smaller in diameter, however, and a different material (SS versus aluminum). It also has four blades as opposed to three.

So I'm trying to figure out what pitch to get here. I'm trying of course to replicate the 6050 RPM at WOT, but want to factor in the change in material, change in diameter, and change in the number of blades.

Any thoughts about what pitch I should get, given all that?

[tmann45]

Appreciate the feedback folks. I'm settling in here on a PowerTech SCB4.

For the Yamaha Performance Bulletin (same boat/motor combination) they got 6050 RPM with a 12-pitch three-blade aluminum prop.

The PowerTech is a bit more than an inch smaller in diameter, however, and a different material (SS versus aluminum). It also has four blades as opposed to three.

So I'm trying to figure out what pitch to get here. I'm trying of course to replicate the 6050 RPM at WOT, but want to factor in the change in material, change in diameter, and change in the number of blades.

Any thoughts about what pitch I should get, given all that?

With all of that information, I would guess it might need somewhere between a 10 to 15" pitch SCB4.

Have you tried discussing with the experts at PowerTech or Ken at Prop Gods?

[boscoe99]

When you start changing brands, models, materials, number of blades and such, all bets are as to how any particular propeller will work with any particular motor on any particular boat at any particular engine trim angle.

You will have to be your own test engineer.

[Zara Spook]

With all of that information, I would guess it might need somewhere between a 10 to 15" pitch SCB4.

Have you tried discussing with the experts at PowerTech or Ken at Prop Gods?

Yes I have. Ken seems to think a 12p would be best. I plan to call the PowerTech folks tomorrow. Appreciate the feedback.

[Zara Spook]

When you start changing brands, models, materials, number of blades and such, all bets are as to how any particular propeller will work with any particular motor on any particular boat at any particular engine trim angle.

You will have to be your own test engineer.

No kidding. I plan to start with the 12p and use the $35 restocking fee if need be. First order of business, however, is to get the tach working.

[zenoahphobic]

No kidding. I plan to start with the 12p and use the $35 restocking fee if need be. First order of business, however, is to get the tach working.

Nice to have the luxury of returning the prop, principle doesn't exist over here.

How things have changed over the years. It used to be that engine manufacturers provided the one prop with each engine. Most people were accepting of that, and if you reckon the engine was straining you would go down one pitch of the same prop! No tachometer.

Seems strange in some way that we seem to be sucked into a world of "optimum" performance, whatever that is, given that the engines we put on our boats today would almost be twice the HP of old, and therefore have the capacity to cope with a greater range of conditions than of old.

Would it be possible to make a lot of money out of changing gear ratios in cars for instance. The 8 speed transmission is still not good enough for some.

Not having a go at the OP, just making an observation that this approach is becoming unsustainable. The outboard consumer is loosing control, the prop gurus are taking over.

[Zara Spook]

Nice to have the luxury of returning the prop, principle doesn't exist over here.

How things have changed over the years. It used to be that engine manufacturers provided the one prop with each engine. Most people were accepting of that, and if you reckon the engine was straining you would go down one pitch of the same prop! No tachometer.

Seems strange in some way that we seem to be sucked into a world of "optimum" performance, whatever that is, given that the engines we put on our boats today would almost be twice the HP of old, and therefore have the capacity to cope with a greater range of conditions than of old.

Would it be possible to make a lot of money out of changing gear ratios in cars for instance. The 8 speed transmission is still not good enough for some.

Not having a go at the OP, just making an observation that this approach is becoming unsustainable. The outboard consumer is loosing control, the prop gurus are taking over.

I think part of the deal is that we're trying to counter the lore of boats' being money-suckers and headaches. If I can get my prop to make my motor run optimally, then I'm less likely to be "breaking out another thousand" on the motor down the road.

[tmann45]

No kidding. I plan to start with the 12p and use the $35 restocking fee if need be. First order of business, however, is to get the tach working.

Great plan!

[Zara Spook]

Again appreciate the feedback here.

I'm about to switch out my 15" pitch prop for an 11" pitch one.

With my tach continuing to be inoperable (which will be fixed soon), I'm wondering how I will know whether the rev limiter is kicking in, if and when that should happen.

Any help would be appreciated.

[DennisG01]

Again appreciate the feedback here.

I'm about to switch out my 15" pitch prop for an 11" pitch one.

With my tach continuing to be inoperable (which will be fixed soon), I'm wondering how I will know whether the rev limiter is kicking in, if and when that should happen.

Any help would be appreciated.

Excuse the sarcasticness, but you'll know because the rev's will be limited.

Now, back to a normal answer...
You'll absolutely know. The revs will bounce automatically and you'll think something is wrong. Stand on your car's throttle while in neutral - same thing will happen.

If you can just barely get the limiter to kick in, I'd call it "good to go" with that prop. If you have obvious throttle left when it kicks in, go up in pitch.

EDIT: Just to be clear, when I say "just barely", I mean JUST barely - like you only have 1% throttle left. I know that's not very specific, but hopefully it explains my thought. Or, go up 1" in pitch.

[Zara Spook]

Excuse the sarcasticness, but you'll know because the rev's will be limited.

Now, back to a normal answer...
You'll absolutely know. The revs will bounce automatically and you'll think something is wrong. Stand on your car's throttle while in neutral - same thing will happen.

If you can just barely get the limiter to kick in, I'd call it "good to go" with that prop. If you have obvious throttle left when it kicks in, go up in pitch.

EDIT: Just to be clear, when I say "just barely", I mean JUST barely - like you only have 1% throttle left. I know that's not very specific, but hopefully it explains my thought. Or, go up 1" in pitch.

Good deal -- appreciate it. My prop guy actually told me it would be better to add cup to the 11" pitch if that happens because the variation in the manufacturing could actually make me end up with less pitch than 11" even if I buy a 12". IOW, right now I could actually have a 11.5", and buying a 12" could result in having a true 11".