Prop Diameter and Pitch

scooter79

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Aug 18, 2014
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I have a 2007 Mercury 50 hp ELPTO with a 10.5 13 pitch SS prop. there is a factory sticker on the top of the cavitation plate that reads 12X22. The stock prop is 10 3/8 with and 13 pitch. Just trying to figure out why I have such a small prop on this motor. I have this motor on a 2008 Lowe 1652 VTC Roughneck. about a 1000lb boat. If I wanted to go up in size how would that change with pitch and engine performance. I do understand that these motors are very universal and get used on many types of boats. Is this sticker just simply stating the max dia. and pitch that can be used.
 

Texasmark

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The sales brochure for a different year model of the 3 cyl 50 indicates a gear ratio of 1.64. The pitch variance in the same catalog is 8-13. I can see how they can be confusing. It shows the operating rpm at WOT to be 5000-5500. Whatever prop you select try to hit the high number in the range. 2 strokes like to run, not lug.

A 1.64 vs some other pitches common like 2 or 2.3 means that you are spinning the prop much faster so a shallower pitched prop, to do the same work, would be in order. The range of pitches normally available on new engines from Merc as shown in the sales brochure reflects that; less pitch more work....heavier boats. Diameter moves around with the specific prop as long as you have a diameter that will fit your lower unit.

My gut feel, not knowing anything else about your rig is that you probably have THE prop already. If you are running rpms where I said with that prop and a normal load then leave it be!

Mark
 
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jestor68

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That sticker is the gear ratio( 12X22=1.83).

As far as the ratio of dia/pitch; you'll just have to have faith that the prop designers at Mercury know what they are doing.
 
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steelespike

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Do a lightly loaded test run. Get us the wot rpm and gps speed with your present prop.
remind us of the present prop size. We should be able to tell if your setup works as it should.
 

jestor68

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There is a consideration in propeller design called the "pitch ratio", it's pitch divided by it's diameter.

For the type boats most of us operate, the desired pitch ratio is .90-1.5.

Your 1.5 X 13 prop has a pitch ratio of 1.23; which in the proper range for the application. If it was a larger diameter, the ratio would be less ideal.
 

Texasmark

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There is a consideration in propeller design called the "pitch ratio", it's pitch divided by it's diameter.

For the type boats most of us operate, the desired pitch ratio is .90-1.5.

Your 1.5 X 13 prop has a pitch ratio of 1.23; which in the proper range for the application. If it was a larger diameter, the ratio would be less ideal.

Well that may help to explain, or partially explain, the movement of the diameter when the pitch changes in propeller design. But as you said " For the type boats most of us operate....". I ran variants of 13"D props on my last boat and they were well above that ratio. But that boat was in the "pad hull" arena and wasn't in the class you mentioned. Maybe if he gets some numbers like SS requested, we can move forward in helping him.

Mark
 

scooter79

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Aug 18, 2014
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Texasmark- I took the boat out monday morning after I got out of work, the boat runs well but I have noticed at WOT it sounds like the motor wants to come apart. After about 3\4 throttle it seems like the motor is just making more noise, it does gain a really small amount. I have to get a tach and wire it in to get the exact rpm's but my guess would be I'm over 5500. I beleive the motor is prewired with the tach wire, so I will have to get one and install it. I would say the answer to SS question
I would say the RPM's at WOT are about 5600 to 5800 @ about 25mph, this is with a 10 1\2 13pitch SS prop. The reason for wanting to change is the RPM's sound high and when I am on a plane in the flats and need to take a corner the boat falls off plane and the motor will rev up, I know it's loss of cavitation. A buddy told me to go upto an 11 to 11 1\2 prop and about 14 to 15 pitch. I just don't want to hurt the motor doing this.
 

Texasmark

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You are not going to blow up your engine running 300 rpms over the WOT rating. It's not like a pushrod valved 4 stroker. I have run my engines over the upper limit since 89 that I know of, when I got my first boat with a tach. Prior to that I have no earthly idea where they ran. I post my numbers but people accuse me of lying so I just quit. I know. I was there.

On your turn problems, you are ventilating which can be stopped by trimming down slightly in the turn for the easiest fix. Several things can cause it and usually with a ss prop, the opportunity is reduced due to cupping of the trailing edge of the blade. If that doesn't stop it come back and we can talk about what to look at as possible causes.

Mark
 

scooter79

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Aug 18, 2014
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Thanks Mark, I am going to try to get a tach wired in. I think the motor has the wire in the harness. I have looked at the repair and diagnostic manual I have for that motor but it doesn't state specifically. Know to what SS said about the prop ratio, would I be able to go up to a 11 or 11 1\4 14 or 15 pitch it would keep it close to the same ratio. I was looking at going to a 10 1\2 14 or 15 pitch prop. I was shooting to stay arounf the 1.33 ratio. So to keep close to that ratio the 11 inch 14 pitch would put me at 1.27 and the 11.25 15 pitch would put me at 1.33. And again thanks for you help.
 

steelespike

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I'm sure no expert on the diameter to pitch ratio.I wonder if it becomes a concern for certain specific applications.
The list of al props here have 5 ,10 + diameter and 4 11+ diameter.
​ A 13" pitch seems reasonable for your application.
Mercury tests of boats like yours about 600 lb dry weight.Running 13 and 14" props.About 34 mph.
That lightly loaded test should tell us a lot.
 
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