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3 Blade prop to 4 blade prop

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

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  1. Rich M.

    Rich M. Member

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    Currently running 26x28 3 blade Nibral cupped props on a 1988 46 Post powered by 550hp 692 Detroits with M1.5 Allison trans pulling 2250 RPMs . I'm thinking of going to 4blade props I hear that 4 blades are smoother and better mid range but a little lose of top end .Has anybody done this?
  2. DOCKMASTER

    DOCKMASTER Senior Member

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    I can’t offer any real world Post info for you. However, not sure you will see much gain for your money. If you have vibration you need to figure out why. Simply changing from 3 to 4 blade will be hardly noticeable if noticeable at all. Also curious why you want more mid range? Not sure if you gain much there either. You may get a little more stern lift from a 4 blade, but again, questionable if you will notice The difference. If you are getting full RPM while fully loaded and have no issues with your 3 blade props, personally I would spend the money elsewhere on the boat vs buying 4 blade props.
    AnotherKen likes this.
  3. Rich M.

    Rich M. Member

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    The current props are pretty banged up and have to be reconditioned not sure if I would be better off putting the money towards new ones
  4. NYCAP123

    NYCAP123 Senior Member

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  5. Capt J

    Capt J Senior Member

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    I would invest in new props then. 4 blades will be a little smoother and give you a little more stern lift, they probably will also give you a better cruise speed. But make sure to get proper numbers from what you have now so you know exactly what pitch you need. Generally you lose/take off 1" of pitch when going from a 3 blade to a 4 blade, but I'd also recommend making sure you're correct WOT rpms.
    sound boating likes this.
  6. Rich M.

    Rich M. Member

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    Lowering the pitch by one inch when going up in number of blades is what I've been reading .I was just hoping to see if anybody had some real world experience. I'll have to call a prop guy by me like S+S or Lawrence Johnson and pick their brains. I think I may go down 2 inches to get max RPM up take some of the load off the engines and let them breath easier
  7. Capt J

    Capt J Senior Member

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    Take accuracte numbers with full fuel and water, and a photo tach at WOT and speak to one of them. S+S has a very good reputation. I haven't heard of Lawrence but I'm not from your area.
  8. mapism

    mapism Senior Member

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    With boat wheels, there is only one thing you can always be sure of: ALL of them are wrong.
    The name of the game is finding those which are the least wrong ones.
    When you know what the builder originally installed (unless we are talking of a Mickey Mouse boat, which is NOT the case of Post, as far as I understand), the best way to go for the least wrong props is to stick to the original builder choice.
    I'm assuming all other things being equal of course, i.e. boat not repowered and not heavily modified in her hull, weight distribution, etc.
    There's nothing inherently worse in 3 blades, as well as there's nothing inherently better with 4.
  9. chesapeake46

    chesapeake46 Senior Member

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    I thought a three blade gave a little bit better bite at idle and thus better maneuverability in close quarters and a four blade gave a little bit smoother ride at cruise and possibly a little more speed at cruise. Not 5 knots or anything like that tho.
  10. Capt J

    Capt J Senior Member

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    Yes to all accounts. 3 blade will give a hair more speed at top end.
  11. cleanslate

    cleanslate Senior Member

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    I'm with some of the others. Put your money elsewhere. Stick with the Post factory specs. props you have now. I assume they are O.E.M. props/size. Post ran that boat, your model, a thousand times over with different prop configurations. Post/any builder wants their boat to perform at it's best. What you have is the best set up for your boat. Don't mess with it.
    On the other hand if you really want four blade props. Get the size from your prop shop that you talked about and shop around for a used set of reconditioned / barely used props in the size the shop recommends and try them out. I would not by new. Then you are stuck with them.
    I bought O.E.M. specs props last year from a prop shop locally last year 24 x 26 3 blade Nibral for $700 each. They have been on my boat since last summer and perform great with no vibration. My old one's had a tinny shake to them, they are now spares.
    How do I know they are factory spec props? I got my boat's original test reports from Johnson and Towers for MY boat from 1981. It's been a wealth of help/info for me. I learned about this trick from the people here on Yacht Forums.

    If you have J & T detroits , I would call them for your test reports for your boat. Have both your unit and model numbers ready to give to them so they can look it up. This way you can be sure what your boat came with from Russel Post .
  12. Rich M.

    Rich M. Member

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    Thats interesting they are J&T 550HP 692s. I think 26 x 28 is what the boat came with there is a spare set in the bilge with Dominey Propeller stickers on them with 2 sets of serial #s on them like they where reconditioned twice
  13. boatpoor

    boatpoor Member

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    My 93 46' came with 26x28 three blades. A few years ago I took 2" of pitch out of them to get the rpms back up to where they need to be.
  14. chesapeake46

    chesapeake46 Senior Member

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    Curious, how did you loose rpm's in the first place ?
    Props damaged or boat weighs more than when new somehow ?
  15. cleanslate

    cleanslate Senior Member

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    Call J and T Mt. Laurel Nj
    Ask for Jeff Knott in parts
    Ask for the test reports and start up reports for your motors.
  16. mapism

    mapism Senior Member

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    Yeah, I know the theory. Including the bit about less blades for higher top speed, as C J mentioned.
    But there's MUCH more than the number of blades, when trying to find the holy grail of a perfect prop.
    I've seen many boaters thinking/hoping to be able to improve what the builder did, but I've yet to come across a single one who actually achieved such result in any meaningful way.
    Which is what I meant with my previous post, and cleanslate explained even better.
  17. boatpoor

    boatpoor Member

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    The only time I could get to 2450 wot was after a fresh bottom job. After I repitched the wheels I can get there anytime and the speed numbers are almost identical to the sea trial numbers when new. Post did a good job of getting all the available horsepower out of the engines when they were new but after a couple thousand hours they're overpropped especially if you carry lots of stuff with you. Fresh engines would probably have the same result but it's a lot cheaper to take some pitch out of the wheels.
  18. Capt J

    Capt J Senior Member

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    The factory props are useless as a guide. Just this year I had to take 2" of pitch out of 2 different 2020 yachts from 2 different manufacturers that cruise around 25 knots in the 60-70' range, one new and one with 250 hours on her, because both boats were overpropped and neither achieve WOT rpm's for engine warranty purposes. Which I don't get, you do not get more speed overpropping a boat. The faster the prop spins, the more lift you get, more HP the motors make (both at 80% load and WOT) and the more speed you get. I shoot for 50 rpms over WOT rpm......One boat was shy almost 100 rpms with full fuel and water, the other 40 rpms with 1/2 fuel and no owners gear.

    Also as the boat ages, all of those coats of bottom paint add weight and more drag, engines don't make as much HP, hull weighs more, has more gear on it, makes the addage of going by what the factory used 20 years ago, completely useless, not to mention prop technology has changed quite a bit in those 20 years.
  19. mapism

    mapism Senior Member

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    I agree that the addage is useless with manufacturers of 60-70' boats who are unable to prop correctly their boats, because even without knowing who you are talking about, at that size more than likely they are production boats, not one-off requiring tests upon each and every launch, right?
    But in that case, my personal recommendation would be to look at other manufacturers, rather than repitch the props... o_O

    PS: I'm not so sure about technology changes either, unless you rate the shift of propellers production to the People's Republic (often with lower quality metals) as one of them.
    Last edited: Feb 9, 2021
  20. NYCAP123

    NYCAP123 Senior Member

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    All this brings me back to "For me choosing a prop is a black art. So many variables." And recommended he get in touch with a prop shop. I know little about choosing a prop, but I remember like 20 years ago a prop shop in Florida developed a computer algorithm for figuring the right prop for boats. I assume that by now most reputable shops have the technology. Also seems to me that most better boat manufacturers would outsource the choice of props in their designs to someone with that technology because it is so complex.
    AnotherKen likes this.