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Thread: need advice on (re-)shaping old bronce props

  1. #1
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    Question need advice on (re-)shaping old bronce props

    Hello Guys,

    i got an old bronce prop that i want to use on my boat, but the diameter is a little to large for my engine, so i need to cut it down. As i have to adapt the leading edge to the new diameter as well to preserve a good shape, i would like to use the chance and go a little further into propeller modding.
    the boat's specs:
    -9ft deep-v hull with spray rails, no pad
    -weight (including driver) approx. 530lbs
    -35~40hp@5000rpm, 14:19 reduction
    -runs 36mph with an (old!) aluminum 3blade 9.5"x14"
    -prop runs fully submerged; should not be too sensitive to ventilation (plate slightly above keel), but is not intended to become a real surface piercer

    the "raw" propeller's specs:
    -Michigan AJC-466, 2blade 10.5"x15"
    -progressive pitch varying from around ~14" at the hub to ~16" near the tip
    -this results in rake varying from around zero at LE to ~20deg at TE

    i put some sketches onto the prop:
    Name:  2blade-ideas.JPG
Views: 263
Size:  173.2 KB

    -black is the absolute minimum portion that has to be removed in order to provide sufficient clearance.

    version A: remove as little as possible, keeping the outline very similar to the original (just a little smaller)
    version B: create a distinctively round LE, with or without cleaver-style TE (remember: fully submerged! not sure if a cleaver gives any advantages in this condition?)
    version C: more "aggressive" LE with a smaller transition radius, strongly inspired by other "round ear" racing props

    What do you think- which of these styles looks most promising to you? other ideas/hints? Or another prop that has proven to perform well in a similar application that i could model mine after?

    best regards
    simon

  2. #2
    Team Member fs5's Avatar
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    I like the look of c , but don't take to much out of the leading edge till you try it.

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    This is what mine looks in the same direction. Its more like C.
    Attached Images Attached Images  

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    Team Member Roflhat's Avatar
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    I would avoid the cleaver shape if you're running the prop fully submerged. The more blade area you remove the less pitch the prop will have

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    Good to see I'm not the only one unafraid of modifying a prop.

    If you're determined to do this, I'd start small (design A) and see what happens.

    Whatever you do, don't try to heat and beat cup into a cast bronze prop--experts can get away with it, but not we mere mortals (the alloys separate--a mess). I once added trailing edge cupping to a prop with epoxy (and it worked for quite a while). Great for testing.

    You'll also have to rebalance it after any significant cutting.

    Good luck!

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    wow; didn't expect to get so much response- thank you all!

    so the quintessence is, that C might be the best "target shape", but the path to go there runs in smaller steps with intermediate tests to avoid loosing too much blade area at once. Sounds rational...

    @hupiveneilija: nice boat :-) the hull looks very similar to mine, but on the pic, it looks like you are running at much higher enginge/shaft height, i.e. completely surface piercing. is this true or an optical illusion due to perspective? If so, do you have experiences with this (or another) prop on the same boat, but fully submerged?

    @Fastjeff57: thanks for your warning about heating! Although i did not plan to do any bending for now, i thought about cupping for later fine-tuning. Is it possible to cold-work the material instead?

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    I wouldn't recommend it. Cast bronze is not malleable and it likes to crack instead of bend.

    Jeff (working on my 8th homemade prop)
    "We live at the bottom of an ocean of air." - General Marvage Slatington
    Likes daleskill liked this post

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    Team Member fs5's Avatar
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    Jeff , I like your epoxy idea . Sounds like a great way to test a bit of cupping .

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    Team Member hupiveneilija's Avatar
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    Quote Originally Posted by simon-2 View Post

    @hupiveneilija: nice boat :-) the hull looks very similar to mine, but on the pic, it looks like you are running at much higher enginge/shaft height, i.e. completely surface piercing. is this true or an optical illusion due to perspective? If so, do you have experiences with this (or another) prop on the same boat, but fully submerged?
    Thats not illusion, i run this high to make it spin freely because of 26in pitch. It also works much lower but speed i get is also lower.

    Two blade props were speedprops back then, when motors were assemblied much lower than these days, because of lower drag.

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    Team Member smittythewelder's Avatar
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    Maybe before you start trimming the blades, make some shim sticks of various thicknesses and try a range of motor heights. And angles, maybe first testing with the propshaft about parallel to the bottom as a datum. If you are having to either pick the bow up or drive it down with the motor angle much off of parallel from the bottom, that might point to a separate issue. Then when you are cutting down (and re-balancing) your prop in small stages as suggested, maybe try moving the motor height to find the sweet spot again (all of this with reference to the tachometer so you're not over-revving the motor).

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