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  1. #1
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    Default Prop talk.

    So as Ive posted recently i launched the Rambler. Very happy day indeed. I did so with a salvage prop on her. 12.5 diameter. A bit unsure of the pitch, might be 11. It is cupped. Old Chris Craft prop.
    She made a hair over 6 knots. This was confirmed at slack water both into and with the wind. Shed only pull 2500rpm. Westerbeke W13a. Governed at 3000rpm.
    I spoke to a prop shop and they are suggesting a 13x9 or 13x10. I can run a 13 diameter prop without clearance issues.
    The salvage prop is quite pitted and the edges are eroded as well, so not the best test. I have read that the correct wheel can make quite a difference.
    Fight Entropy, build a wooden boat!

  2. #2
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    Default Re: Prop talk.

    Get "The Propeller Handbook" by Dave Gerr.

  3. #3
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    Default Re: Prop talk.

    Quote Originally Posted by Ian McColgin View Post
    Get "The Propeller Handbook" by Dave Gerr.
    Got it in front of me. Picked it up at the MBBS.

    Prop shop is suggesting a 13x9 or 13x10.
    Fight Entropy, build a wooden boat!

  4. #4
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    Default Re: Prop talk.

    "I have read that the correct wheel can make quite a difference." - Reynard38

    Truer words have yet to be spoken - or typed, as the case may be.

    If your engine is not spooling up to the target max rpm, then you prop has too much pitch or too much blade area. Go with what your prop shop is suggesting, and be prepared to pull the new prop after trials to have it 'tweaked' to optimum performance. Finally, if the prop shop doesn't do this for you, take a file and square off the trailing edge of each blade - they should have a slight square edge (maybe 1/16" for this size prop), not tapered to a sharp edge like an axe or knife. The prop will run quieter this way.
    Hope for the best, but plan for the worst.

  5. #5
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    Default Re: Prop talk.

    I’ve run the calculations and it always comes up that the boat should make over 8 knots. That’s 2 knots above hull speed. I’d be thrilled if she did that, but.....
    13x10 is what the book, and an online prop calculator say I should be running.

    Funny I make my living at 440 knots, and I’m now scrambling for 2.
    Fight Entropy, build a wooden boat!

  6. #6
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    Default Re: Prop talk.

    Quote Originally Posted by Reynard38 View Post
    Ive run the calculations and it always comes up that the boat should make over 8 knots. Thats 2 knots above hull speed. Id be thrilled if she did that, but.....
    13x10 is what the book, and an online prop calculator say I should be running.

    Funny I make my living at 440 knots, and Im now scrambling for 2.
    BTW


    2 knots more than 6 knots is a 33.33% increase in speed.
    I live and drink rum where other people vacation.

  7. #7
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    Default Re: Prop talk.

    Rule of thumb: every inch of pitch = +/-200 rpm

    Kevin
    There are two kinds of boaters: those who have run aground, and those who lie about it.

  8. #8
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    Default Re: Prop talk.

    Or fit a Bruntons 330mm self pitching auto prop.

  9. #9
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    Default Re: Prop talk.

    Quote Originally Posted by skaraborgcraft View Post
    Or fit a Bruntons 330mm self pitching auto prop.
    Pretty cool device, but $4000!
    Fight Entropy, build a wooden boat!

  10. #10
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    Default Re: Prop talk.

    Quote Originally Posted by Reynard38 View Post
    Pretty cool device, but $4000!
    Keep your eyes peeled on Ebay, they do come up, got mine for less than 10% of that price. For a boat which might not have such a difference in load, a fixed prop is a cheap option. I figured i could have used 3 different pitches depending on load, for the same performance, but also get the benefit of a prop that has minimal drag under sail only, and more effective when motor sailing.

  11. #11
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    Default Re: Prop talk.

    This online prop calculator is kind of interesting:

    https://www.vicprop.com/displacement_size.php

  12. #12
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    Default Re: Prop talk.

    Must be tempting for a hands on kinda guy to reach for the grinder/polisher and challenge himself to find a couple of hundred rpm.

  13. #13
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    Default Re: Prop talk.

    ^ Don't forget the 'tuning fork' to twist the tips of the blades to adjust pitch....
    Hope for the best, but plan for the worst.

  14. #14
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    Default Re: Prop talk.

    Repitching, It's a surprisingly brutal process isn't it. Done cold with heavy things and loud noises even.

  15. #15
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    Default Re: Prop talk.

    The manager of the prop shop at the shipyard I last worked at phrased it this way:

    "Space-age technology, iron-age methods."
    Hope for the best, but plan for the worst.

  16. #16
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    Default Re: Prop talk.

    I'm not sure how to gimmick up a home method.two vices, one with lever arm added? Vice and really big stilsons plus pipe for the lever arm? Cut a gauge to match the new pitch say 1/3 in off the tips.
    The thing is that a couple of inches of travel forward per revolution translates to (gets calculator, puts mums old reading glasses on and pens in shirt pocket, click click) not very much in degree terms just measuring the angle. A bit less.

  17. #17
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    Default Re: Prop talk.

    Also pitch is not constant: it varies across the blade.


    Kevin
    There are two kinds of boaters: those who have run aground, and those who lie about it.

  18. #18
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    Default Re: Prop talk.

    Well, guys, you are flitting about the enormity of what makes a good prop a good prop, and beginning to realize the black art of prop tuning.

    Of course, a five-axis milling machine might help.
    Hope for the best, but plan for the worst.

  19. #19
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    Default Re: Prop talk.

    After consulting the book “Propellor Handbook” and running through many equations this morning it’s looking like a 13X10 is the way to go.
    If I could swing a bigger diameter prop it’d be better, but 13” is all that’ll fit.
    Fight Entropy, build a wooden boat!

  20. #20
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    Default Re: Prop talk.

    Reynard, will you stipulate 3, 4, or 5 blade? Cupped blades? Skewed? Semi-cavitating? The choices are mind-boggling...
    Hope for the best, but plan for the worst.

  21. #21
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    Default Re: Prop talk.

    3 blade, non-cupped. And yes they are!
    Fight Entropy, build a wooden boat!

  22. #22
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    Default Re: Prop talk.

    Good choice - less prop-induced vibration in the driveline and hull than with a 4-blade prop.
    Hope for the best, but plan for the worst.

  23. #23
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    Default Re: Prop talk.

    Spoke to a very well regarded prop shop in Brunswick GA today. They had be take some measurements and we determined a 14 diameter prop will fit with sufficient tip clearance.
    Going with 8 pitch.
    The test prop is a 13X11 that is cupped.

    Stay tuned. Im after that elusive knot.
    Fight Entropy, build a wooden boat!

  24. #24
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    Default Re: Prop talk.

    There is a lot of debate over what, precisely, is "sufficient tip clearance". In my experience, what is sufficient on a metal or 'glass boat is not so good on wooden hull. Just sayin'.
    Hope for the best, but plan for the worst.

  25. #25
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    Default Re: Prop talk.

    David Gerr’s book “Propellor handbook” states that for the range of prop rpm my boat operates I should have at least 10% tip clearance, or 1.4”
    I’ll have 2”.
    Fight Entropy, build a wooden boat!

  26. #26
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    Default Re: Prop talk.

    14x8 installed. Result? Motor will now pull to 2850rpm at WOT. Average of several 2 way runs is 6.7 knots at 2700rpm, so she picked up @ 10%. Also the speed at idle is lower which makes docking easier.

    Here’s the old and the new.

    315C59A5-5F66-403A-BC67-597D8CCC7866.jpg
    Fight Entropy, build a wooden boat!

  27. #27
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    Default Re: Prop talk.

    Quote Originally Posted by Reynard38 View Post
    14x8 installed. Result? Motor will now pull to 2850rpm at WOT. Average of several 2 way runs is 6.7 knots at 2700rpm, so she picked up @ 10%. Also the speed at idle is lower which makes docking easier.

    Here’s the old and the new.

    315C59A5-5F66-403A-BC67-597D8CCC7866.jpg
    Interesting to see totally different design philosophy between old and new prop blades.
    One with a straight leading edge and curved trailing, the other the direct opposite.

  28. #28
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    Default Re: Prop talk.

    Glad to hear that it has improved your boat's performance. I'm down with the tip clearance. Next thing to check - if you are one of those anal types like I am - is your fuel consumption. I predict that you will see a slight improvement in gallons per mile. Have you noticed any increase/decrease in hull-borne vibration from the new prop?
    Hope for the best, but plan for the worst.

  29. #29
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    Default Re: Prop talk.

    Feels the same. The harmonics have changed a bit. Sweet spot @ 2700.
    I ran her for about 90 minutes yesterday. The fuel guage didn’t budge.
    Fight Entropy, build a wooden boat!

  30. #30
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    Default Re: Prop talk.

    So hook it up then.
    Glad you've found some improvement in her.
    If one advances confidently in the direction of his dreams, and endeavors to live the life which he has imagined, he will meet with a success unexpected in common hours.
    -Henry David Thoreau-

  31. #31
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    Default Re: Prop talk.

    Or one is upside down in relation to the other...
    Hope for the best, but plan for the worst.

  32. #32
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    Default Re: Prop talk.

    Quote Originally Posted by mmd View Post
    Or one is upside down in relation to the other...
    Yep, sorry ‘bout that. I was quite happy to get the old one off!
    Fight Entropy, build a wooden boat!

  33. #33
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    Default Re: Prop talk.

    I shouldn't presume I s'pose.

  34. #34
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    Default Re: Prop talk.

    I just re-read your OP, Raynard38. I note that your new prop allows your engine to turn up to 2850 rpm, but it is governed to 3000 rpm. If you have opportunity to pull the prop again and your prop guy is good, he can re-pitch the prop down a bit to bring the rpms up to the engine top rpm - a gain of 5% in rpm. This will bring your WOT speed up a bit, maybe to seven knots. It is common practice in the fishing fleet here to pitch the prop to allow the engine to run fifty or a hundred rpm or so above manufacturer's stated engine rpm, and allow the governor to limit engine speed, not the prop.

    What's that old saw? Oh, yeah, the ninety-ten rule: Ninety percent of any task takes ninety percent of available resources - the remaining ten percent of the task takes another ninety percent of available resources."
    Hope for the best, but plan for the worst.

  35. #35
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    Default Prop talk.

    Quote Originally Posted by mmd View Post
    I just re-read your OP, Raynard38. I note that your new prop allows your engine to turn up to 2850 rpm, but it is governed to 3000 rpm. If you have opportunity to pull the prop again and your prop guy is good, he can re-pitch the prop down a bit to bring the rpms up to the engine top rpm - a gain of 5% in rpm. This will bring your WOT speed up a bit, maybe to seven knots. It is common practice in the fishing fleet here to pitch the prop to allow the engine to run fifty or a hundred rpm or so above manufacturer's stated engine rpm, and allow the governor to limit engine speed, not the prop.

    What's that old saw? Oh, yeah, the ninety-ten rule: Ninety percent of any task takes ninety percent of available resources - the remaining ten percent of the task takes another ninety percent of available resources."


    Pitching for a bit of " over-rev," is also common when rigging recreational powerboats. That way, when you have a full crew aboard; the boat has gained weight due to accumulated gear; a Bimini top has been installed; the bottom grows some fouling ; its a hot humid day; or any combination of those, all of which will tend to cause a drop in rpm, it gives you some room.

    Kevin


    Edit: typo and added a sentence


    Sent from my iPhone using Tapatalk Pro
    Last edited by Breakaway; 05-15-2019 at 04:46 PM.
    There are two kinds of boaters: those who have run aground, and those who lie about it.

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