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

  1. #11
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    Hi smittythewelder,

    the prop as it is now is definitely too large (would hit the cavitation plate), so i can not test it yet. But i will do the "minimum cutdown" first and try before cutting deeper.

    i'm very limited with engine height as my gearcase does not have a low water pickup (yet? :-D). The water entrance is behind the prop, just below the cavitation plate.
    With the current setting, the cavitation plate is about 1/2 to 1 inch above keel and cooling is still fine, but if i go any higher, i think the cooling might collapse. That's why i prefer to stay fully submerged...

    With standard aluminum props, it runs best at about 3 to 5 degrees trim (positive/up). Trim is continously adjustable, but NOT while driving, so i need to cope with the same trim position for both getting on plane and running WOT. With more trim (~10deg), the hull has less wetted surface and feels to run "higher" above the water level (guess a little faster as well), but it also starts getting seriously wobbly. Additionally, it is difficult to get on plane with such trim angle.

  2. #12
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    Trimming the motor out with the cav plate still running in the water is a major drag inducer--it also tries to pull the stern down. If you need to trim out, try gradually raising the motor until the cav plate is ABOVE the wake. And keep your eye on that temp gage (if you have one).

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

  3. #13
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    hi Jeff,

    my cav plate is already above the wake. Maybe my explanations were a bit confusing, but a sketch is worth a thousand words:

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    as you see, it is kind of borderline and there is little to no space for height variations: a little bit lower and the cav plate gets submerged, or a little bit higher and the water entrance gets out of the water.
    But the prop is still (almost) completely submerged in this condition.
    Simon

  4. #14
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    Hi Guys,

    here's a first update:
    used a compass to construct the new LE-outline:

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    this resulted in a surprisingly nice shape: as the (compass-)centre is located aft of the prop, the axial distance to the LE is higher than to the TE, making the radius smaller (with respect to the prop axis now).

    almost cut through:
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    finished (this step).
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    btw: this is what i did with the pieces i had cut off. Seems to be a different kind of bronze alloy from what you had, Jeff..? This one seems to bend very well: i did this without any heat, only bench vise and hammer.
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    regards

  5. #15
    Team Member zul8tr's Avatar
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    Good start and it looks like cold workable metal. I would blend the cut pointy end points into the remaining blade and I sharpen the cut edge by grinding only on the blade forward direction side away from the cut edge and into the existing blade and checking static balance as I grind.

    Before grinding I would check the static balance to get a start reference to weight removal. When grinding the cut edge to sharpen into the existing blade area I keep the same relative radial grind areas on each blade as close as practical. One can get static balance that is not correct with less metal removed at a larger radius area on one blade compared to more metal removed at a smaller radius area on the other blade. That will not be good when underway.
    “Extraordinary claims require extraordinary evidence” Carl Sagan

    Location: SW Orlando, Fl
    Thanks simon-2 thanked for this post

  6. #16
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    ...."it looks like cold workable metal."

    Right! I tried heat and beat cupping a cast Merc prop and....Don't want to talk about it!

    Isn't prop building fun!

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

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    The Michigan props we raced with fifty years ago would occasionally break. Tacoma prop-man George Lockhart warned me of this when I gave him a 7X14 steel Michigan to rework. Sure enough. George said he only hit it about three times before it cracked (at the blade root, leading edge).

    When you are figuring out an alternative cooling water pickup, you don't necessarily have to have it mounted on the motor, and maybe not feeding the pump. Something like a pitot tube for a speedometer would do what you want, though you'd want something that would pick up enough volume of water to do the job, since with this set-up you'd be ramming rather than pumping water. Until you're on-plane the motor's pick-up and pump would have to suffice. Interesting experiment.

  8. #18
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    hi Smitty,

    this cooling water pickup is annoying; yes. One more disadvantage: as the "new" prop has rake and higher pitch, it is "longer" than my other props. In other words: the prop in its current size is still too large; although the diameter is fine now, it would still strike the water pickup.
    As both pitch and rake are progressive, cutting the prop back in this region would take away the top end of both pitch and rake distributions. I think this would ruin the prop... so i'd rather like to cut the exhaust (w/ water pickup) and get an alternative pickup (transom-mounted seems to be the easiest way..?). But no way to omit the pump and rely on ram pressure alone! too much negative experiences *GG*

  9. #19
    Team Member zul8tr's Avatar
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    Could you mount the prop on the engine and take a pic to see the present clearance.
    “Extraordinary claims require extraordinary evidence” Carl Sagan

    Location: SW Orlando, Fl

  10. #20
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    How about cutting off the entire projecting part of the water pickup and cap it off, by welding or just drilling and tapping for a thin plate (with gasket or sealant). Then rig a water pickup like I was talking about, and run a tube around the side of the towerhousing, above the cav-plate, and entering a drilled hole outside of the now-capped-off factory water passage. Obviously this would send water to the pump.

    You haven't said what the motor is. I'm just guessing that it does have an actual pump somewhere up in the towerhousing. At least one of the race motors, the Yamato 80, has a pickup roughly similar to what you drew, but with no pump, relying on water being thrown into it by the prop.

    Does the gearcase on your motor split along the centerline of the propshaft, or is it one-piece with the gears and shaft loading into it from the back?

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