Prop pitch..how does it work

Discussion in 'Bubba's Outboards' started by squirtspop, Feb 7, 2007.

  1. squirtspop

    squirtspop New Member

    Messages:
    968
    State:
    Glencoe, Arkansas
    I know props differ in pitch but how does this pitch affect the boat?

    Does a higher number of the pitch make the boat go faster or slower at the same rpm as a lower number pitch?

    Reason I'm asking this is I replaced a 17 degree 3 bladed with a 19 degree pitch 3 bladed. The guy I bought it from said that I'd have to be careful not to over rev the engine with the 19. Don't really understand that statement to much. Would putting on a different pitch prop make the engine put out more rpm's?

    I don't usually push the throttle all the way forward anyway but thought I'd ask these questions. Thanx for any inputs.
     
  2. jim

    jim New Member

    Messages:
    2,579
    State:
    Jacksonville NC
    Squirtspop,I could fill up this page explaining pitch but you can get a much better explantion at Mercury Motors website.Pitch does have a dramatic effect on performance and your friend was right to warn you.You CAN seriously over rev an engine by using the wrong pitch.The ideal situation is to use a pitch that allows your engine to run in the max rpm range for the engine at wide open throttle (WOT).Again Mercury can explain it better than I can,but do check before you open that engine up.:smile2:
     

  3. ShilohRed

    ShilohRed New Member

    Messages:
    4,339
    State:
    West Tn

    higher the number the slower your motor will turn it. but give more speed to a point.
    Your motor should turn the 19 200 RPM's less as its harder to turn. If the 19 allows your motor to turn within specs of what the motor states it should turn you have the right one.
    There is times the 19 may be find with a light load. But not allow you to get on plane good with a load.
    Pete
     
  4. Bobpaul

    Bobpaul New Member

    Messages:
    3,039
    State:
    Supply NC
    The pitch is the theoretical distance in inches that a prop will move forward in one revolution.

    The greater the pitch, the more resistance to the engine and forward movement.

    All things related to the boat operation need to be considered when propping an engine for a specific hull.

    If your 17 pitch was working well, the 19 will probably be way too much, with not much chance to over rev.
     
  5. Mr.T

    Mr.T New Member

    Messages:
    2,553
    State:
    MO
    The 19 in prop will load the engine more -- making it work harder, taking a bigger "bite" of water on every turn, and resulting in a *lower* top-end RPM than the 17 inch prop. As Pete alludes to, the rule of thumb is about 100 rpm change per 1" pitch change, assuming the diameter is the same.

    More pitch = lower top-end RPM and higher top-end speed, at the expense of a slower hole shot (won't accelerate as fast).

    Less pitch = higher top-end RPM, lower top-end speed and a faster acceleration.

    The goal is to use a prop that puts your engine in the manufacturer's recommended max RPM range at wide-open throttle, under your "typical" load situation.

    If you always run around with a lightly loaded boat (no passengers, not a lot of other weight), you will probably need a higher pitch prop to keep the RPM in the right range, but you will find that the boat will be more sluggish when you load it with passengers and other weight -- more load on the engine with the extra weight and it won't be able to maintain the same maximum RPM as when lightly loaded.

    Conversely, if you usually have a heavily-loaded boat, you'll need a lower pitch prop to get the engine up to the proper RPM range and then there'll be some risk of over-revving the engine when you're lightly loaded - in that case the engine isn't working as hard and will spin the prop faster.
     
  6. GMC FishHauler

    GMC FishHauler New Member

    Messages:
    1,335
    State:
    Waco, Texas, Un
    how does the pitch effect the cavitation?
     
  7. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    Read it here: http://sites.mercurymarine.com/port...26_49291:126_49299&_dad=portal&_schema=PORTAL
    All you ever wanted to know about props. I just got back from taking a friend's to the prop shop. He bought a new stainless prop over the internet, same pitch, but he lost 600-800 rpm's because he didn't know they came in different blade designs, depending on your boat, etc. Now he's out nearly $400 and getting ready to spend another $100 or so to adjust the new prop to perform like he needs it to. Both props are supposed to be 17" pitch, but the blade angles are totally different. The prop shop has a computer program to read what's there and tell them what to do. Carried the old one and the new one so they can repair the old one, too. Point is to be certain you know what you're buying.. :smile2:
     
  8. jim

    jim New Member

    Messages:
    2,579
    State:
    Jacksonville NC
    Mercury also has a phamplet called "Everything you ever wanted to know about props" It was the best few dollars I ever spent,because it made it clear that I didn't know anything about props!!!!!!!:lol: :smile2:
     
  9. Bobpaul

    Bobpaul New Member

    Messages:
    3,039
    State:
    Supply NC
    Engine set up, hull design, weight distribution, and a few other things might affect cavitation, but not the prop.
     
  10. Mark J

    Mark J New Member

    Messages:
    9,407
    State:
    Four Oaks, NC
    Some boats like bassboats in particular may need a raked prop to get the bow down.
    Beyond pitch and diameter there is all kinds of blade designs for specific purposes.
    If you dont have a tach. Get one.
     
  11. Katmandeux

    Katmandeux New Member

    Messages:
    1,618
    State:
    Checotah, Oklahoma
    Too much pitch could, at least theoretically, result in cavitation, in the same way that too high an angle of attack will cause an airfoil (wing) to stall.

    An engine that is mounted too high on the transom will result in disturbed flow to the prop, and can cause cavitation, as well. Also, check the hull forward of the prop for anything that will disrupt the flow of water over the hull.

    After you fix everything else, if cavitation is still present, you may want to consider cupping your prop.
     
  12. Bobpaul

    Bobpaul New Member

    Messages:
    3,039
    State:
    Supply NC
  13. Katmandeux

    Katmandeux New Member

    Messages:
    1,618
    State:
    Checotah, Oklahoma
    I doubt that it actually happens, simply because no one would be likely to build/sell a prop pitched high enough for it to occur. In theory, though, it's possible.
     
  14. squirtspop

    squirtspop New Member

    Messages:
    968
    State:
    Glencoe, Arkansas
    I bought the boat used and it had a 115 hp Merc on it but it didn't last too long after I got it. I traded it in on a used trade in 75 hp Mariner and the dealer changed the prop to the 17.

    With all this information now, I'll dig into boat weight, length, etc., then check out the rest of the info. Thanks guys for all the inputs.
     
  15. Katmandeux

    Katmandeux New Member

    Messages:
    1,618
    State:
    Checotah, Oklahoma
    Mark is right...get a tach.

    Without one, you're a blind man in centerfield.

    A question just popped into my little brain: I've had motor/tach combos where the tach ran off of the alternator...if the tach quit, you knew to check the alternator. Is this always the case?
     
  16. Bobpaul

    Bobpaul New Member

    Messages:
    3,039
    State:
    Supply NC
    The tachs I've seen are powered through a pulse put out through the rectifier/regulator.

    When you can't keep a charge on your battery, you might also find your tach doesn't work. Testing input to the rectifier from the stator/alternator, will give you an indication if it's the stator/alternator or the rectifier.

    Most often it was the rectifier.