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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 15915
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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1993 OMC 50HP 2 stroke. Took it out today and on the way

Customer Question

1993 Johnson OMC 50HP 2 stroke. Took it out today and on the way back to the marina, the RPM's all of a sudden started to increase. The boat slowed with little forward motion. I powered down immediately. Tried to power up again slowly. The boat moved forward
up until I got around to 3k RPM then the same thing happened. RPM's started to increase with no increase in movement. Was able to limp the pontoon back to the marina at around 3k rpm. My guess after researching is that I've spun a prop. So the first question
is ...does this seem likely. Second question is if I buy a replacement, what do I get ? I have no idea about angles and such. The pontoon has always been slow, so I'm not looking to drop $400 or $500 if I don't need to or if it wouldn't create a dramatic speed
increase. The current prop appears to be aluminum. Appreciate the help.
Submitted: 2 years ago.
Category: Boat
Expert:  Jason replied 2 years ago.

Hello my name is ***** ***** am I speaking with?

A spun prop hub definitely fits the symptoms. You can send the prop out to have it re-hubbed by a propeller repair shop. On average it runs 75 to 100 dollars. If you do not know where a propeller repair shop is in your area just call your closest dealer. This isn't brand specific, so you could call any dealer or boatyard, as all dealers/boatyards deal with propeller repair shops. And they will be able to tell you were your closest prop repair sop is, or they would just send it out for you. If you want to replace the prop, your existing prop will have its size stamped on the inner hub. Just pull the prop off and look for numbers on it. There will be 2 numbers with an X in between. First number is ***** diameter, 2nd number is ***** So if yours is stamped 10X13, that would be a 10 diameter by 13 pitch.

Does all that make sense?

Expert:  Jason replied 2 years ago.

I think that should do it. But I work for tips so I do want to make sure you are happy with my service before you go. If you had a further question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.

Questions do not close out, so if you have to come back later on with follow up questions to the issue you still can even after doing a rating.

Thank you

Customer: replied 2 years ago.
The pontoon is in the water 2 hours away so I can't really pull the prop off to look. I am seeing aluminum props for around $100 to $150 on ebay. I assume they would have everything needed for replacement. Is there any way to look up the size/type prob through Johnson (the engine manuf) so I can order it?
Customer: replied 2 years ago.
I assume there is a factory recommended prop?
Expert:  Jason replied 2 years ago.

Unfortunately no, it doesn't work that way. Picking a propeller from scratch, which is what we would have to do here, is done based on math. I have prop calculation software that I use. The software factors in a bunch of parameters and spits out out a perfect prop size. It is not foolproof however. There are 3 ways I can do it.

1. I can do it by boat style and length - The calculator is 75% accurate

2. I can do it by boat style and weight - The calculator is 85% accurate

3. If I know the full throttle RPMS, speed, and current prop size - The calculator is 95% accurate.

But other than that, there is no such thing a s a factory reccomended prop. Weight along makes a difference in prop size. For example, when this boat was bought new at the dealer. If you told the dealer the most people the boat would ever have on it would be 4, the dealer would choose 1 size. If you told the dealer you would usually have 8 people on it at a time, it would be a difference size.

Does that all make sense? What size/length is the boat. I can get give you a rough idea of what size it should have on it.

Customer: replied 2 years ago.
Great...Its a 1993 20' sportfish by Grumman. I think the max weight it will support is 1950 but no idea what it actually weights. I know the max horsepower motor it supports is a 75HP but has the 50HP Johnson. Total people it is weighted for is 9. We commonly have 6 to 7 people on it. Its WOT RPM is around 5500 and travels around 10MPH according to GPS. If it helps.....its always been poky but the engine appears to have steady pressure on both cyl. and has had the entire plugs, wires, ignition, and power pack replaced. If I can get a prop that helps with speed...thats awsome. Finally, is there a "kit" I have to get to install it?
Customer: replied 2 years ago.
Oh pontoon
Expert:  Jason replied 2 years ago.

Got it.

I ran the math and you would be looking at either a 10x10 or 11x11 prop, or as close as you can get to those dimensions. So if you found something that was like a 10.5X11, or an 11X10, as long as you are in that ball park you will be good to go. No kit would be needed to install the propeller.

Was there anything else, or does that cover if for you?


Customer: replied 2 years ago.
Would this do it? a 4 blade and I have a three...does that make a difference?
Expert:  Jason replied 2 years ago.

No, when you go 4 blade the dimensions change. You did say you would prefer a higher top end speed, so you would want to stay with a 3 blade prop. (the extra blade creates more drag, which makes it slower).

Click this link, this will work -

Expert:  Jason replied 2 years ago.
Hello, just checking in. I noticed no rating as been put through yet so I wanted to make sure you didn't have any further questions on the issue.
Expert:  Jason replied 2 years ago.
Hello. Just checking in, no rating has been put through yet so I was seeing if you were making progress, or had any further questions on the issue.

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