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3 Blade prop to 4 blade prop

Discussion in 'Post Yacht' started by Rich M., Feb 6, 2021.

  1. DOCKMASTER

    DOCKMASTER Senior Member

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    Yep, understood. But 1” of pitch is not going to gain you 4-5 knots no matter what. Do you know of others with same model boat with same power making 24-25 knots? And are you talking about cruise or WOT? Do you have a dinghy on your bow? Big tower? Other sources of added weight and drag? Bottom clean?
  2. Capt J

    Capt J Senior Member

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    1” pitch isn’t going to get you any measurable speed and just going to overload the motors. You’re better off spinning them 25 rpms more with the props you have. You want speed, soda blast off the years and years of bottom paint you probably have. I picked up 2.5-3 knots on a yacht that was less than 10 years by doing that
  3. Stainless45

    Stainless45 Member

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    All good advice. Bottom will be clean and repainted. There's no tower or dinghy. If 26x26 is what she probably came with then whatever I get will be fine by me

    I surveyed a 46 last summer with 4 blade wheels 26x26. She had 6-71's and made 25 knots. So there's that
    Last edited: Mar 13, 2021
  4. OutPost

    OutPost Member

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    How many knots do you think I might pick up blasting the bottom paint off my 42 - there is anywhere between at least 10 and potentially up to 40 years worth :)
  5. mapism

    mapism Senior Member

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    Forget the speed, you'll gain what you'll gain.
    With that background, you'd better do it regardless.
  6. DOCKMASTER

    DOCKMASTER Senior Member

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    I just hope you don’t end up over propped. Sounds like you were propped right at 25” if you got to 2,350 fully loaded and probably why someone previously took some of that pitch out. If you were under propped you would have seen 2,400 + range . Any way, hope it all works out for you. Please report your findings when you get back in the water.
  7. Stainless45

    Stainless45 Member

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    From what I've read those 92's are rated for 2300. They spun right up to 2350 where I backed off because my Detroit guy was along for the ride which leads me to believe I'm leaving something on the table here.
  8. DOCKMASTER

    DOCKMASTER Senior Member

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    Ak-OK, my bad, I thought they were 2,350. Makes more sense now.
  9. Rich M.

    Rich M. Member

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    I just ordered a set of 26x26 four bladed propellers with a small cup from Jr at Johnson propeller and machine in Freeport. Stainless 45 check your private messages
  10. Capt J

    Capt J Senior Member

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    I gained 10% in speed. Think of all of the weight of a can of bottom paint, half of that weight stays on the bottom after it dries. Then the un-even bottom.
  11. OutPost

    OutPost Member

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    Thanks J. I've had two different yards vehemently talk me out of it. I usually get "Unless you can't sleep at night by occasionally dropping a bottom paint flake on the bottom of the bay you'd be (*&^ing crazy to go through all that just to gain a knot."

    It's still on my list, however it's priority level has never been high enough to beat out other projects.
  12. Stainless45

    Stainless45 Member

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    Got my original 3-blades tuned up by Johnson Machine in Freeport to 26x26 the original specs for the boat. Huge thanks to Rich M who gave me his old wheels as spares- 26x28. Now I have a "hot rod" set to try if the ones I have don't get the speed I'm after
    IMG_4004.jpg
  13. DOCKMASTER

    DOCKMASTER Senior Member

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    They look great. Let us know how the numbers look once you get back in the water. Are you doing a blue-fit to check your contact area % on the shafts? Did you do aN NDT on the shaft ends while you had the props off?
  14. Stainless45

    Stainless45 Member

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    My Diesel guy did a check of the shafts there is some run-out on the port but they are straight. Going to see how it is in the water. Will report back
  15. Stainless45

    Stainless45 Member

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    Sorry just realized I never updated this thread. So the 26x28 3-blades went on and she flew right up to 26 knots in the spring... With 1/3 fuel and no water onboard. Loaded her up and lost about 300 rpm and made a little smoke. Took those wheels down to 27" pitch and she seems to have found the sweet spot. 2350 rpm and around 24 knots give or take. At 2000 rpm I'm making 20. Good enough for me.