1980 Johnson 50 hp, plows badly getting on plane

[Quick3201]

Now have 11.75X17 3 blade alum prop. Johnson 50hp 2 cycle on a 14' run about (heavy). Now does 27 mph by gps at 4200 rpm. Really strugles to get on plane. Solas chart has 12.5X13 pitch, too much of a change? Also offers 12.25X15. Hope to only buy one more prop. Any opinions???
Thanks in advance,

 

[steelespike]

I wonder if the motor may have some sort of issue.What brand and year boat?
Based on your prop size 11.75 X 17 I think you have a small gear case.Does the prop appear to fit right?
A 17 is the largest prop readily available for the small gear case .Your 14 footer may have worked with a 17" prop.If your sure the motor is at 100%
Assuming your rpm is accurate you need to make up 1300 rpm I'm guessing the gear ratio is around 2.20. Do you know your gear ratio? I would start with a 12.5 X 13. A 13" prop seems really small for a 14 footer.
Sometimes older boats absorb water in the foam under the floor does the floor seem strong?Sometimes if you raise the bow on the trailer and leave the plug out
water will drip constantly out the drain if water is present.I would suggest a motor evaluation and check for water before a prop change.
My 88 50 hp large gear case on a 18 ft utility 375 lbs dry about 1000 all up. uses a 19" prop and it screams Its hard to imagine your little 14 is any heavier all up weight.

 

[Quick3201]

I'm trying to figure out how much smaller to go. Motor runs smooth, accelerates good, just doesn't get out of the hole well and top RPM is low for the motor. Top is 4200 rpm, should be 5200 to 5500 rpm. It's like the Limbo, how low can i go?? :<)

 

[steelespike]

Rule of thumb is 100 to 200 rpm per inch of pitch. that puts you at about 12" pitch. For an additional 500 to 1000 rpm.

 

[Texasmark]

Your WOT rpms are telling on you! If you propped to get into that 5000+ range you mentioned and where I like to run 2 strokers, you'd be dropping pitch and in doing so would improve your hole shot. I'd say that you would want that 13" since you said your boat was heavy. That's 4" drop x 200 per inch puts you right at 5000. You would surely notice some improvement there. Now the question is.....what happens to your MPH at WOT. Unless something magical happens with the boat to water interface you probably will be where you currently are at 27 or less. Assuming slip remains constant....a wild assumption, you are looking at decreasing pitch 4/17 = 23% while your rpms are expected to go up to 5000/4200 currently = 19% So WOT speed loss would be 19/23 x 27 = 22 mph. So if you went with the 15" pitch you would have a halffast better hole and only loose 2 1/2 MPH but your rpms would still be out of the WOT max range at 4600 which is better than where you are. Gotta decide what's important or get a bigger engine and you can have both!

Edit. Terminology: Plowing means that the bow is running too deep. It's a misleading notation for your current problem. Just to let you know as it can confuse some folks in trying to picture your problem in their head. The hole shot is from the time you firewall the throttle from an idle until the water is no longer touching the transom and the bow comes back down. Then if you are plowing it will come down too far, tend to nose dive into oncoming waves, spray is exiting in the first quarter of the boat length and it is hard to steer with a reduced speed and rpm from what they would be if the bow were up where it belonged.

 

[Quick3201]

Based on your analysis; it's NOT plowing. When given full throttle the bow comes up sharply and stays there a long time. Then it settles down. Moved adjustment pin to closest hole in transom, that helped some. Thanks for the advice.

 

[ttankmoran]

Speaking of WOT Rpms telling on you!!... First of all unless I have missed something even with the small gear case you should still have the 2.42 gear ratio.

Now the facts; You have a heavy boat, probably water logged, you have a 17 pitch prop, and you can make 27 MPH at WOT.

Now assuming 27 MPH, 17 pitch prop, and 4200 RPM would make for a NEGATIVE slip number.

I predict you have a faulty reading from your tach because a 17 pitch prop, 27 MPH and a more realistic (with a heavy boat) slip number of 13-14% puts your RPM closer to the 4700-4800 RPM range

I also predict that if you move down to a 15 Pitch prop that your slip number will drop, your hole shot will improve, you WONT lose any speed, and your RPM will climb to the 5300-5400 RPM range.

But you should definitely figure out whats making you boat so heavy, you should have no problem touching 30 with that boat and engine.

 

[steelespike]

I wonder if the motor may have some sort of issue.What brand and year boat?
Based on your prop size 11.75 X 17 I think you have a small gear case.Does the prop appear to fit right?
A 17 is the largest prop readily available for the small gear case .Your 14 footer may have worked with a 17" prop.If your sure the motor is at 100%
Assuming your rpm is accurate you need to make up 1300 rpm I'm guessing the gear ratio is around 2.20. Do you know your gear ratio? I would start with a 12.5 X 13. A 13" prop seems really small for a 14 footer.
Sometimes older boats absorb water in the foam under the floor does the floor seem strong?Sometimes if you raise the bow on the trailer and leave the plug out
water will drip constantly out the drain if water is present.I would suggest a motor evaluation and check for water before a prop change.
My 88 50 hp not a large gear case on a 18 ft utility 375 lbs dry about 1000 all up. uses a 19" prop and it screams Its hard to imagine your little 14 is any heavier all up weight.

Not a large gear case but performance still shows what your setup can do if all is right.