is this the right prop

[hillbilly.tim85]

i have a 21 ft pontoon with a merc 60 hp four stroke big ft engine i am only getting about 15 mph does this sound right the prop is a 14x10 red line on engine is 6000 wot will over rev the engine any suggetions on how to get more speed or correct prop size

 

[Chris]

Hey ht,

What you should be aiming for, is that at WOT, your motor is running in the max RPM range.

So if you find that you are able to run above the redline, then you need to go up in pitch on your prop which will drop the RPM's down. If you're below the max range, then you need to drop down in pitch which will slightly increase the RPM's.

There are obviously different material types and blade numbers that you can also choose from which will also effect RPM's as well.

Propping is always a bit of an experimentation process, so you'll want to find a dealer that will work with you to try out some different props.

-Chris

 

[hillbilly.tim85]

pitch question

i read somewhere that 2" of pitch will raise / lower rpms 300 to 400 rpms but how much will the speed change at given rpm or are there to many varibles i know that pitch has to do with the distance of travel per revolution

 

[Chris]

Hi ht,

The speed variable is hard to predict as it changes from hull to hull.

There are some prop calculators around that can help, but the variable that is very difficult to predict is prop slippage. The more efficient a hull is, typically the lower the prop slippage percentage will be.

If you think about a prop being like a corkscrew, the pitch is what determines how far your boat would travel with one revolution if you had zero slippage.

Meaning, a 20" pitch would equate to your boat moving 20" in distance. But the reality is that your prop slips, so your boat doesn't travel that far. And in truth, there are a few other factors that will change the distance slightly as well, like cupping.

But, you can see that by using a few key pieces of data like your engine RPM's, your gear-case ratio and your prop pitch, it isn't too hard to figure out your theoretical top speed number with zero prop slippage.

Then once you start factoring in prop slippage that's where the number changes considerably.

Here's a link to one propeller calculator that I found...

Have fun.

http://www.rbbi.com/folders/prop/propcalc.htm

-Chris

 

[Fun Times]

i have a 21 ft pontoon with a merc 60 hp four stroke big ft engine i am only getting about 15 mph does this sound right the prop is a 14x10 red line on engine is 6000 wot will over rev the engine any suggetions on how to get more speed or correct prop size

Hi I found the merc prop selector online for you if you know your boats specks then it might be a good idea to go into this link and put in your boats detailed info if you have it.http://appcenter.mercurymarine.com/propSelector/home.do Well from my experince with this kind of pontoon with this size engine and boat, 15-17 MPH on the speedo meter is just about where you will be with that set up. And If I remember right your max RPM should be at 6000. The quick way to find this info out on the engine is if you are standing at the varry back of the boat looking at the engine on the right side where the engine is mounted to the boat, With the engine raised up and turned to the left, On the right side just inside of the main bracket just past the few wires you see for the trim gauge sending unit if you have one, There should be a white sticker with the engine serial number on it, That sticker will also say the max rpm for the engine. You will want to run the engine with in that number with a light load in the boat, That way you will not do any damage to the engine when you load the boat up with extra people and gear. Play with the prop selector and see what you come up with. From what I have seen in the past you could run any where from your 10 pitch up to a 12 pitch and maybe a 13 pitch but that might be getting to big [IMHO] But when I played with the selector with out knowing your boat specks, It said 13 pitch for over all performance, But I think that is a little to big But I could be wrong as every boat is not the same till you just get out there and try it first hand. Well I hope this helps you out some and good luck with, That is if you want to keep trying to get a little more out of it, But I say you are prety good at where you are if that sticker said 6000 rpms.

 

[Fun Times]

On performance hulls 1 pitch is worth about 2 MPH on avarage, On your boat It would be close to say Almost 1 MPH and maybe 1 1/2 MPH per pitch on avarage. you will get a little performance out of it, But not much. Also have you tryd to trim the engine up just a little to see if you can get more MPH and RPMs out of it, with out going to hi, triming it up will help just a small amount to get a better speed out of it, But keep in mind if you have any passangers sitting in the bow then you will need to keep the engine trimed all the way down or could make the prop come out of the water a little at times and the engine will free rev, And you will loose performance.