Need Prop Pitch Help

Discussion in 'Boats and Motors' started by NUM1FIRE, Jan 30, 2008.



    i bought a 1983 sylvan backtroller 16 ft deep v aluminum boat with a 1989 johnson 70 hp vro outboard motor a few months ago. it would run around 34 mph on top end and then i did some looking at the motor and found the plugs to be gapped at twice the gap they should be, the carbs were opening only a little over half way open. so now i have everything set where they should be and even installed some boysen power reeds. so now i dont know what the top end speed will be but i do know the motor will run alot better so i need some help with the prop pitch.
    the prop i took off i cant find the prop size anywhere on it at all, all i can come up with is the diameter which is 13". does anyone have a simular setup as i do and could u tell me what your prop size is so i could determine roughly what size i need. thanks for any info

  2. I have a 90 Johnson on my 18' center console, and would run about the same with a 13X17 prop. I went lower,13X15, for better mid range and only lost a few mph on top end. WOT was somewhere in the 5k range with the smaller pitch. I would think yours would be either a 17 or possibly a 19 pitch.


    thanks alot u 2 i will wait till spring and try this prop again and get me a tach thanks again
  4. KaGee

    KaGee Monkeywrench Staff Member

  5. Whaler

    Whaler Whaler

    I have a 17 foot Whaler with a 70 horse Johnson . The motor came with a 13 inch diameter by a 19 inch pitch prop. It runs a little over 30 MPH and for the weight of the boat, motor and all the other equipment in the boat I'm happy with that.
  6. UFM82

    UFM82 The one others want to be

    The #1 requirement for determining the proper pitch prop for your boat is engine RPM. All other values will vary dependant upon your RPM. Find the proper range for your engine and then run the boat as your normally would. If the RPM is high, increase pitch. If the RPM is low, decrease pitch. If you are going to replace the prop, look into a stainless model while you're at it. They boost the efficiency and can get you a few more mph. Plus they're more durable than aluminum and they look a whole lot better too. I've run stainless exclusively for 10 years now and have hit a few things in that time but have never damaged an engine so don't let that fear influence you.

    Keep in mind that there are a myriad of props out there and not all are the same. Just because diameter and pitch are the same does not mean the prop will work on your boat. A Tempest won't behave like a Mirage will or a Trophy or a Vengeance despite having the same "dimensions". Different blade geometry, rake, thickness, etc can all determine how a prop will run on your boat.

    All that being said, a 70 hp on a 16' aluminum V-hull should put you right around 38-40 mph if I had to guess and you may not even get that. It takes a LOT of horsepower to go fast.

    Get a tach and check your RPM. Then you can begin the hunt for the prop. Not until then.



    thanks again everyone for all the info this is one of the reasons i really like this website