Technical Question on Prop Pitch

Discussion in 'Boats and Motors' started by FISHERBALL, Apr 24, 2008.

  1. I just launched my maiden voyage on my '75, 16' bowrider. The motor is a '81 Johnson 70 hp from my '61 15' deep vee. Boat & motor ran great but, as I figured, the prop is not the right pitch. When floored the motor got to maybe 2/3 of how fast it ran on other boat. I know I need a tach to do this correctly. Until then, does anybody have a guess which way the pitch should change? It's, I think, a 17 pitch/13 1/2" diameter. To move rpm's up do I make the pitch higher or lower. Anybody know of a prop shop near Columbus that will let me test one to see what I need? I bought my prop new from the shop north of Home Rd.

    Thanks,

    Fisherball
     
  2. Are you sure the engine is firing on all three. Prop sounds right, might have a bad plug ,an ign or a carb prob.
     

  3. Motor has always run perfectly. Never has to turn over more than 3-6 times to start, as long as it has gas, battery power, etc & is chocked correctly. Even when I'm starting it at the beginning of the season & it's full of fogger & hasn't run in 4 months. It fires right up. It just wouldn't run rpms as high as with other boat. Maybe I missed something but the motor just sounds too strong for it to be that kind of problem. I figure it's pushing a heavier boat that has more resistance on the water. Maybe I'm expecting the pitch to be the problem.
     
  4. Reduce pitch to increase rpm's.
     
  5. Without a tach, how do you know the RPM's are too low? You mentioned it's a heavier boat.... If you are going by the mph, is it possible that you just aren't getting what you expected? Be careful changing props without knowing what your tach reading is. Sounds like a strong motor that has been good to you... not sure I would take the chance before getting a tach on there... just my opinion
     
  6. fishingguy

    fishingguy The Inferior Fisherman

    You don't say what the boats are made from. I assume the bow-rider is glass and if the deep v was aluminum, than that would be the cause of lost speed. Heavier boat, less speed. Be careful about changing pitch, could damage motor.
     
  7. UFM82

    UFM82 The one others want to be

    2,089
    313
    1,268
    But as cautioned, get a tach and check it. You'd hate to go buy a prop and still have it be wrong for the engine. If you truly have lost that much RPM you'll definitely want to change the prop. Lugging an engine can cause damage just like over-revving can. Pre-ignition caused by lugging can destroy your engine.

    UFM82
     
  8. Both boats fiberglass. The volume & pitch of the motor was much lower. You know the sound of a motor when it's cranked, it just wasn't there yet. I will definitely buy a tach b/4 I start spending the big bucks that a good prop costs. This is an early info seeking post. Pay2Play told me what I need to know. I wasn't sure which way to go. I have a Vortec prop with a interchangable hub so I will stay with that brand & probably go to the shop I bought it to start. I've heard they will work with you to find the right pitch. This motor pushed my old boat 33 mph according to the 20-30 year old speedo, the new boats speedo does not appear to work. The pitot tube is missing. This boat is designed for a 115 hp so I don't expect it to go as fast as the other. Personally I've never caught a fish at 30 mph so I'm not going to worry about speed, I just want it to run at the max it can. I'll buy a matching speedo & tach. It'll be a while for me to get the $$$ to buy those, than a little longer to have the cash to buy a new prop. By mid-season it'll be where I want it to be.
     
  9. NUM1FIRE

    NUM1FIRE FIREFIGHTER/1ST RESPONDER

    i agree with everyone elses posts but i have something to add did any on the cables or linkages get messed up any when u swapped the engine from one boat to another? the only reason i asked is because i have a 89 johnson 70 hp and i am trying to get the carbs and linkages in the right spot right now on my engine.
     
  10. NUM1FIRE-I didn't even think of that!!! This is why I love this site. It' very possible that the new cable connections won't let it open the throttle wide enough!!! That's more likely the problem. I know I'll need a different pitch to get max speed but it's running at about 60% of what it did b/4. It wasn't straining or making any unusual sounds, it just sounded like it wasn't floored. If it was a car I'd say it has first, second & third but won't give me fourth or fifth gear. I invested $30.00 in a Clymers manual when I bought the motor & it's paid for itself 10 times over. It'll tell me approximately where the cables should be set. Thank you!!!
     
  11. NUM1FIRE

    NUM1FIRE FIREFIGHTER/1ST RESPONDER

    not a problem i messed with my linkages and cables because i thought something wasnt right and i lost 4 mph it went from 34 mph to 30 mph and my 70 hp was only hitting 4950 rpm's. so i did some readjusting and i am heading back out on sunday and going to try it again but i never put the engine from one boat to another