Trying to make sense of RPMs

[DYLBOSS]

Re: Trying to make sense of RPMs

One other question. I currently have a 14.5" x 19" prop. If I upgrade to a 21" pitch prop, what diameter should I get? I can't seem to find a 14.5 x 21. I just see 14 1/4, 14 3/10 and 15 3/4 as my options.

 

[QC]

Re: Trying to make sense of RPMs

Don't worry about diameter, but . . . You really need that tach right first. There is no way to determine or confirm the right prop without it Yes, a four blade may still be the right way to go. Oh, and I didn't recommend a 21", I was using it as an example. That may be where you need to be, but without the right RPM we still don't know.

 

[DYLBOSS]

Re: Trying to make sense of RPMs

I guess i'm not sure how to get an accurate tach calculation. Should I just wire up a new one and test it, or should I try and get a mechanic's tach? I don't even know what that is or how to use it.

I just tested the numbers out again today and got around the same. 4600 with trim up at max before cavitation with 32.9 kt GPS speed.

 

[hwsiii]

Re: Trying to make sense of RPMs

Boss, at 38 MPH and 4,600 RPM you are doing fine.



H

 

[DYLBOSS]

Re: Trying to make sense of RPMs

Boss, at 38 MPH and 4,600 RPM you are doing fine.



H

Sorry to keep throwing inconsistent numbers at you. Today was my 3rd attempt at getting accurate numbers. The last numbers I gave you were from a day that was actually quite rough out on the water. Today was perfect, not rough and mostly smooth as silk. What I got today was similar to my first run. Today at near perfect conditions with medium load in the boat, I got 4600RPMs at 41.4 MPH GPS speed. Again, the RPMs want to climb higher, but I stayed at 4600.

Now according to your formula, and assuming that my tach is correct, I'm looking at an 18/19% prop slip. Does that sound right to you? How much is a new tach typically? Maybe I could just swap it out to check for consistency?

 

[hwsiii]

Re: Trying to make sense of RPMs

Boss, if you really can get 5,000 RPM out of the prop you have right now then you need to buy a Solas Amita 4 blade aluminum 21" pitch prop here at Iboats, it will bring your RPM down to where they ought to be, lower your prop slip to where it should be as well and you will get a good increase in your top speed.



H

 

[DYLBOSS]

Re: Trying to make sense of RPMs

ok thanks. Maybe I'll look into that. Is there a reason you suggest the 4 blade 21", rather than just a 21" 3 blade? Wouldn't the 21" 3 blade bring the RPMs down a few hundred in itself? Also, will I lose some hole shot with the 4blade?

 

[hwsiii]

Re: Trying to make sense of RPMs

Boss,

Quote:
What I'm confused about is that my WOT is said to be 4200-4600, yet when I open the throttle up it continues to 5k+ if I let it go.
You are now saying that it turns over 5,000 RPM.

From what you have said it appears that your present prop will turn about 5,200 RPM, by changing to the Solas Amita 4 blade with a 21" pitch you will lose about 600 RPM which will bring you down to 4,600 RPM, which is where you ought to be.

The speeds you have given vary greatly, BUT you always say the RPM go above 5,000, and that is what I base my calculations on.

Your top speed will increase and a 4 blade prop increases your hole shot over a 3 blade prop, as well as it will lower your prop slip very much.

As has been said before, you need to check your present tach, to make sure it is reading right, whatever you have to do to accomplish that.

Now according to your formula, and assuming that my tach is correct, I'm looking at an 18/19% prop slip. Does that sound right to you? How much is a new tach typically? Maybe I could just swap it out to check for consistency?

You are right, I used a 1.86 gear ratio and that was the wrong one, and 19% is too high a prop slip. A new tach ought to run about $ 60.



H

 

[DYLBOSS]

Re: Trying to make sense of RPMs

hwsiii, thanks for all the help you have provided! I will plan to buy a new tach and will also go with the 4 blade Solas Amita 21" prop. I will chime in with my results when all is in place.

 

[hwsiii]

Re: Trying to make sense of RPMs

Boss, buy the tach first and let's make sure you are showing the right RPM BEFORE you buy the prop. If the RPM you are giving me are wrong then the pitch I said will be wrong, so get the tach and check the maximum RPM first, with the prop you have now.



H

 

[QC]

Re: Trying to make sense of RPMs

+1 ^^^^^

 

[DYLBOSS]

Re: Trying to make sense of RPMs

Boss, buy the tach first and let's make sure you are showing the right RPM BEFORE you buy the prop. If the RPM you are giving me are wrong then the pitch I said will be wrong, so get the tach and check the maximum RPM first, with the prop you have now.



H

Sounds like a good idea. I just bought a mechanic's tach online. I'm going to take the readings this weekend, and I will get you the numbers, or at least verify that the tach is correct or incorrect.

 

[DYLBOSS]

Re: Trying to make sense of RPMs

Update......

So, after buying a mechanics tach and wrestling with whether THAT was accurate, I gave up. I bought a Solas Amita 14x21 4 blade prop, slapped it on this past weekend, and WHAT a difference!

Idle in the water and in gear was dead on 650RPMs. WOT got up to 4200 exactly at 45MPH GPS speed with a moderate load. RPMs did not keep creeping up like with the other prop. I did notice that my speed was consistenly creeping up slowly, but the RPMs were not. I guess that's a good thing.

I definitely gained hole shot and top speed hands down. If my calculations are correct, and the tach is accurate (pretty sure it is), I think I'm looking at 6.5% prop slip.

Maybe hwsiii or QC can verify?

 

[QC]

Re: Trying to make sense of RPMs

1.62 ratio, 4200 RPM, GPS speed of 45 MPH, 21 inch pitch . . . I'm getting 12.7 % slip and I believe all of the numbers are within reason now.

If speed was increasing then your slip number may even come down some. But I rarely believe slip below 10% with a typical pleasure boat. I think your numbers are good. Maybe you should have a 19 as it would give you more flexibility with higher loads. But you are OK now if you rarely run heavier.

 

[DYLBOSS]

Re: Trying to make sense of RPMs

1.62 ratio, 4200 RPM, GPS speed of 45 MPH, 21 inch pitch . . . I'm getting 12.7 % slip and I believe all of the numbers are within reason now.

If speed was increasing then your slip number may even come down some. But I rarely believe slip below 10% with a typical pleasure boat. I think your numbers are good. Maybe you should have a 19 as it would give you more flexibility with higher loads. But you are OK now if you rarely run heavier.

Thanks QC. I must have missed a number somewhere. Yeah, I'm planning to hang on to the 19 that I have as a spare, and if I run really heavy, which I almost never do.

 

[NoKlu]

Re: Trying to make sense of RPMs

What happened with the new tach? What was the difference between the two?