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Thread: prop pitch?

  1. #1

    Default prop pitch?

    Yeah, I have a 20' Livingston w/cuddy cabin and a old 92 suki 140 pushing her. The boat has trouble getting on step. When it does it runs pretty well. When it runs into a good choppy sea it bogs down on the motor and takes a while to get back up to speed. Do you guys think its just a tired motor or could it be a pitch problem.....Thought I'd ask here so I'm not just pissin in the wind
    thanks

  2. #2
    Member spoiled one's Avatar
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    Can you help us out with a bit more info? What size prop are you running now and what is your wide open RPM?
    Spending my kids' inheritance with them, one adventure at a time.

  3. #3
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    Empty it should be running a 21 pitch and a 19 with a load with that horse power range. I run a 19 pitch on my 22' Wooldrige with a 150 with 5-6 people on board plus all our gear for a day of fishing.

  4. #4
    Member Cap'n Ron's Avatar
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    Default boats and motors are so variable...

    and "Spoiled One" asked the correct questions and the best ones. It does not matter what prop someone else uses on the same motor on a different boat. You need to be able to reach between 5800 and 6000 rpm on most modern gas powered boat outboards (online or if you have an owners manual you can find the WOT (wide open throttle) rpm for your motor....I suspect yours is running well and that you have the wrong diameter/pitch combo for your boat. With an older boat and motor, often no effort was made to match them, I would guess that they are not the original set, someone put that motor on later and it came from a boat where it worked well with the prop that is on it. Prop makes a HUGE difference!

    So...you are correct to suspect the pitch, but you gotta know your rpms to suggest what might be correct.

    Another thought...WoW I didn't know Livingston made a cuddy of that size. I've had a 16' Livingston for a long time (just sold it) and it was the most stable, best performing small boat I've ever been in. Keep the boat if it meets your needs and get the prop right first, then worry if it still doesn't perform that the motor is "tired"...but if that is the case, I doubt it would run smooth once it was on step when empty.

  5. #5
    Moderator Paul H's Avatar
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    Engines have different gear ratios, which effects what prop pitch you choose. The suzi df-140 runs a lower ratio, so will swing a bigger pitch than the older two strokes.

    Sounds like the boat is over-propped, but it if the boat performed well previously with the current prop, I'd think the engine might be getting old. Pull the plugs to inspect them, and run a compression check on the engine.

    What pitch prop is on there now, and what condition is it in? A prop that is dinged up will also cause problems. Are you getting any cavitation?

  6. #6

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    I am still pretty green when it comes to outboards and props. The prop itself looks ok. A few dings but nothing major. How do you know what pitch is on it? I inspected the thing but didn't see any stamped marks or anything. Is it a measurment you take?
    So I guess the first thing is to figure that out and then get my tach working..... then I could answer some of these questions.
    Oh, and by cavitation, is that when the prop is not "grabbing the water"? Kind of slipping if you know what I mean?
    Thanks for the input guys

  7. #7
    Member Cap'n Ron's Avatar
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    Default Even little dings...

    can make a little difference in top performance, but don't think that is the major problem for you. Sometimes the entire prop specs are a shallow engraving on the outer cylinder part of the prop, sometimes just the pitch, it might say 17p for instance; sometimes there is a bunch of numbers, like on a Mercury prop, then a "-" (dash...) the a number which will be the pitch. If there is nothing, a prop shop can usually tell you what the pitch is. And, yep, you need that tach going to tell. I didn't think your engine was worn because the motor seemed to be getting great speed when you were running empty, but bogged down when you put even a moderate load on it = pitch problem IMO. You might have to look really close if the prop has been repainted to discern the numbers on it, as they are really shallow. Do it in the dark with a flashlight, sometimes that makes them more visible.

    Cavitation is often heard as a revving up of the motor silmultaneous with a slowing of the boat...I've had it happen in extremely glacial silty water with a prop that otherwise performed well in clear water. A 4-blade prop will get you on step faster and often solves cavitation problems, but then you really lose a lot of speed at the top end. So, choices have a lot to do with how often you have a big load and what your usual load is. It is worth it to get a just-right prop that approaches max wot rpm's, and it is really hard on the motor if the prop is making it lug to get on step, and that can cause all kinds of problems.

  8. #8

    Default

    hey, thanks for taking the time to give that info cap'n Ron. I appreciate the help.

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