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Thread: Prop questions - 75hp Mariner

  1. #1

    Red face Prop questions - 75hp Mariner

    I have an Alaskan Model Wooldridge with a 75 horse Mariner. I currently have a 4 blade aluminum 17 pitch prop. I'm running from Bethel up the Kuskokwim to the swift water and gravel. I'm reluctant to put on a stainless because of the chances of hitting sunken logs etc. and busting up my lower unit.

    I'm looking for a balance of fuel economy and speed. Right now I feel I'm running a bit slow at the top RPM though I rarely run that high. I seem to get much better fuel economy if I run around 4300rpms which I know is not the recommended top end but at $6 a gallon economy is important.

    My questions are:

    Anyone out there running a 19 or greater pitch on a 75hp Mercury / Mariner?

    Which would be better in swift and shallow water? a 17pitch or 19?

  2. #2

    Default In my very limited experience...

    I would think you would want the lower pitch prop as that will get the boat on step quicker, kinda important if you have limited "runway" in shallow water. More importantly though, it sounds like your prop is not letting you get to the upper range of your motor's rpms. Find out the range and work from that, load the boat how you would run it typically and choose a prop that way, but the general consensus is you should be able to get into the motor's recommended rpms at WOT.
    Hope any of that helps,

  3. #3
    Member MARV1's Avatar
    Join Date
    Jul 2006


    SS props run more efficiently than aluminum props. Running the same pitch gets a couple hundred rpm's more also and that equals more speed. How long is the boat? I have only used SS on my outboards, durability is a must. I have hit logs, rocks, shallow water without breaking my lower unit on my Yammi so far, knock on wood.
    The emphasis is on accuracy, not power!

  4. #4


    Jim I had been thinking the same thing about the lower pitch in fast water. Just wanted to confirm with someone else.

    As for the WOT...I'll have to find out what my recommended RPM is but I for sure don't want to run there. I'm telling you over 4300 and you can actually see the fuel hand falling.

    And for the SS prop. I'm on the fence about it. I know people that only run a SS. I'd like to have one for around town and up the main Kusko. It's big and open but maybe not in the swift shallow water. If I did get a Stainless would I want a higher pitch than the aluminum because of the increased rpm?

  5. #5
    Member SusitnaAk's Avatar
    Join Date
    Feb 2007


    All depends on the weight of everything, The higher the No# or more pitch is more speed and more loading the motor, The lower no# more rpm,s more weight, Think of it like dropping from 4th gear to 3rd to bring Rpms up. I think like B Jim said boat would jump up on step quicker, Ck with the book but 5300 to 5500 WOT> with the boat loaded would be about right, If you put to high a pitch on there you throw that third guy in there the boat might not even get on step. It is a balance of the weight rato: to Hp. If going hunting Gas, gear,People Dead anaimal, I would be leaning the other way towards a 15 pitch. SS prop can take a few small hits way better than the Aluminum, Makes no differance to me if you hit what prop you have, If you hit hard enough that lower unit is going to take on the chin.I wouldn,t run in that part of the river without a Jet then. Let us Know what you come up.


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