Need propeller suggestions

[jcweis1]

I have a 2003 Polarcraft aluminum deep v bottom fishing boat with yamaha 90 four stroke motor. The prop that is on it now is a Solas 3411-130-21 15 spline. The problem is that with four large size men the boat was hard to get on plane and would not get over 4200 rpm. Any suggestions would be appreciated. thanks Jeff

 

[QBhoy]

Less pitch. Seems steep if that’s suggested a 21” on it now. Age dependent, I know of 115 models with the same or similar ratios…that won’t turn a 21” well.

 

[brodmann]

A lower pitch prop will get you up on plane faster, reach higher RPM's and make your motor last longer. A 19 pitch 3 blade is what I would recommend. I have a Yamaha 115 on a 19' Sailfish dual console. When I bought it 5 years ago, it had a 19 pitch SS prop. Top end speed was pretty fun, but it took forever to plane and forever to reach top speed of 42 MPH at 4700 RPM's. I now have a 17 pitch Aluminum prop. My WOT RPM's are now 5700 and top speed is 40 MPH.
The pitch of a prop is the "theoretical" distance (in inches) it will travel forward in one revolution.
The recommended "happy range" for my 2 stroke is 4500-5500 RPM's at wide open throttle. I think it's the same for you. You're overworking your motor, burning more fuel and it's not good for the lifespan of the motor. If you have a buddy with a similar motor, I'd see what props he has and try one out. If you have a friend at a local dealership or boat repair shop, he might let you try a few different props. I'd probably start out with a 17 pitch 3 blade. Also, if you move to an aluminum prop, you'll get higher revolutions since the prop will be a lot lighter. Also, aluminum props have a small amount of flex when under heavy acceleration, so that flex will get the RPM's up and then straighten out gradually to allow you to get a full bite out of the prop.

 

[airshot]

What is your max rpm with one person at wot ?? I adjust my pitch to get max rpm or slightly above with my lightest load, then when I add a budy or two I am good.
You might have to watch your tach when running light so you don' t over rev...

 

[jimmbo]

What is the WOT range for that Engine? If it is something like 5500-6000, and you are only getting 4200, you are going to need to drop far more than 2" in Pitch to get even close to that Rpm Range, somewhere along the lines of 6 or 7 inches.
Was the Motor and Boat a Package Deal from a Dealer? If not, who put the Engine on? What year is it, and what condition is it in?
Compression?
Spark to all Cylinders, that can jump a 3/8" Gap?
Is it carbed or FI?
Is the Tach accurate? Perhaps it is set wrong and giving inaccurate Info
How long and heavy is the Boat?
And how was the Solas 21" chosen?

 

[jimmbo]

it had a 19 pitch SS prop. Top end speed was pretty fun, but it took forever to plane and forever to reach top speed of 42 MPH at 4700 RPM's. I now have a 17 pitch Aluminum prop. My WOT RPM's are now 5700 and top speed is 40 MPH.
Also, aluminum props have a small amount of flex when under heavy acceleration, so that flex will get the RPM's up and then straighten out gradually to allow you to get a full bite out of the prop.

If you gained 1000 by dropping 2" in Pitch, your 17" is slipping badly. A 2" change in Pitch is, almost universally, a 400 Rpm change. Perhaps the engine was rigged with a SS in mind, but is mounted too high for an AL.

I call BS on the Flexing Nonsense. That bit of Misinformation has sold Hundreds of Thousands of SS Props. If there is any Flex, it would be less than 1/100", Water is far more Flexible than Aluminium, and will yield long before the Blade will

 

[brodmann]

My bad. I went from a 21 pitch stainless prop to a 17 pitch aluminum prop. And.......You're kidding me, right?​

​

Do composite and aluminum props flex?​

Ace Propeller
https://acepropeller.com › Prop Talk



Of course! All propellers flex. What is often missed is that flex is a good characteristic. For example, if a prop flexed under the load of acceleration, ...

 

[jcweis1]

I bought this boat used and that prop came on it. The manual says 5500-6000 WOT.
Prop dealer says I should use a 3 blade AL, 13.25 dia with 17" pitch.
Just wondering if I should go with that suggestion or if someone else has a better justified suggestion.
The Polarcraft is a 178 SC aluminum deep V fishing boat, so roughly 18' long, The Yamaha 90 four stroke is FI not carb. Spark plugs are new and gapped correctly. I know the RPM's are getting to 5000 just by the sound.

 

[jimmbo]

First make sure the Engine is in good shape and is able to make its power, especially since that engine appears to be severely Overpropped.
Oh, Great, Tachometer Ears

 

[jcweis1]

The engine is in good shape.
Any good racer knows the sound difference between 4000 and 6000 RPM's

 

[airshot]

The engine is in good shape.
Any good racer knows the sound difference between 4000 and 6000 RPM's

Race cars and outboards are very different !! You will never get the correct prop by listening !! Hope you can affored to replace it when she blows.

 

[Scott Danforth]

Any good racer has a tach, oil pressure gauge and a temp gauge

 

[brodmann]

His prop guy is pretty sharp!! Went with the same sugestion I made.

 

[airshot]

Without accurate numbers a suggestion is only a guess!

 

[QBhoy]

Without accurate numbers a suggestion is only a guess!

Totally agree…would also add…that most might even suggest that even with the numbers…it can still remain a best guess. Props just sometimes don’t play by the rules…if there even are any ! Haha.

 

[brodmann]

He initially stated that his max RPM's were 4200. It wasn't until later that he told us about his "magic ears" being able to tell the RPM's by sound alone. I will have to admit, I've wasted a lot of money on props that didn't do what I was looking for. When I give prop advice, I'm trying to help people avoid all the mistakes I've made. I don't want anyone to have to learn the hard way like I did. I have created a very extensive spreadsheet with specs on what I tried and what the resulting RPM's and MPH's were. With enough pieces of information, you can fill in a lot of gaps with the calculations you get with actual results. I'm not trying to impress anyone, just letting you know I'm not just pulling numbers out of thin air.