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Time for a new prop for my 1987 70HP merc

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  • Time for a new prop for my 1987 70HP merc

    After beating my prop against one too many rocks in an unfamiliar lake, I need to replace it. I'd like to confirm what I think I need to do in regards to changing pitch.

    This motor has a WOT rpm range of 5000-5500rpm. With the current 17p prop (before beating it up), could get my boat to 34.8mph (on gps) at 5600rpm with only 1 other person in the boat. I calculate a slippage rate of 10.0%. I have no idea if this is high or not.

    My plan is to go from 17p to 18p to bring the rpm's down some, and possibly reduce slippage. Does this make sense?

    Thanks,
    Rob

  • #2
    I would go with the same prop as those are pretty good #s.

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    • #3
      If going one + pitch to a 18 will lose best hole shot and gain slight more top end speed with you alone, worse with more load and or more passengers on board. Can't have best hole shot and best top speed in one single prop as one variable opposes the other. Stay with current 17, buy a new one.

      Happy Boating
      Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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      • #4
        Alright, I'll stick to the 17p. Thanks guys!

        It may be for the best anyways. 17p is a very common pitch I've found, while with 18p I could only find 1 manufacturer.

        Another thing I will do this winter is get a replacement skeg welded in. The skeg is broken off so that the prop actually extends below it almost 2". No point in replacing the prop, without fixing what caused it in the first place.

        Rob

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        • #5
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          Originally posted by Bobbywolf View Post
          After beating my prop against one too many rocks in an unfamiliar lake, I need to replace it. I'd like to confirm what I think I need to do in regards to changing pitch.

          This motor has a WOT rpm range of 5000-5500rpm. With the current 17p prop (before beating it up), could get my boat to 34.8mph (on gps) at 5600rpm with only 1 other person in the boat. I calculate a slippage rate of 10.0%. I have no idea if this is high or not.

          My plan is to go from 17p to 18p to bring the rpm's down some, and possibly reduce slippage. Does this make sense?

          Thanks,
          Rob
          I like your numbers as they are too. 10% is a great number for slippage. On prop slippage, cupping would help and help if you have ventilation in turns, rough water or at high trim angles...whether done with PTT or manually with the tilt pin. Suggest you look at Turning Point (brand) Hustler props which iboats sells at a great price. They are cupped and grab the water better reducing slippage in alum props. Using your same pitch in your current diameter "range" with the added cupping will pull your rpms down slightly and satisfy that worry of yours.....which really isn't a big deal, really. I ran my Mercs 300-500 rpms over top limit all their lives and they kissed me every time I started them. Grin
          If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

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