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:
Attachment 63055

-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

I like the look of c , but don't take to much out of the leading edge till you try it.

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

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

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

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?

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)

Jeff , I like your epoxy idea . Sounds like a great way to test a bit of cupping .

Quote:

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Originally Posted by simon-2

@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?

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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.

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).

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.

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

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:

Attachment 63066

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

Hi Guys,

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

Attachment 63167

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:
Attachment 63168

finished (this step).
Attachment 63169

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.
Attachment 63170

regards

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.

...."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

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.

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*

Could you mount the prop on the engine and take a pic to see the present clearance.

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?

Relying on water being thrown into it by the prop has not worked for me (at least). Motor overheated very rapidly with the cav plate just under 4 inches up. Was fine at 3 inches up.

Going to put the inlet on the boat and pipe the water to the LU.

Jeff

Quite an experiment!! That it didn't work out takes away nothing from the attempt.

well, need to take some photos...

Quote:

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Originally Posted by smittythewelder

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.

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this sounds similar to what i planned. There is a sump below the pump, where the original intake tube ends. i can bore a hole from the side into it for new water feed, and also plug the original tube.

Quote:

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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.

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sorry... it's a König 2cyl, 30-40HP. It is the same brand that also made the racing motors, but my motor is from their standard (fishing/sports/touring) line. Compared to the racing engines, it spins rather low at just ~5000. And yes, it does have a pump (and reverse gear) :)

Quote:

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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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unfortunately, it does split. So no way to mount a "traditional" nose cone with LWP.

Jeff- very nice piece! But when you mount it on the transom, i think you should mount it rather behind than below the transom edge, so that the intake's blund leading edge is above the water..?

regards

That's my thinking as well. Gonna mount it on the starboard trim tab, with an extension long enough to always get water.

Jeff

Jeff, what IS that boat?!! Looks like a super wake-jumper. Puget Sound has a fair amount of big ship traffic, sending big rollers across the water, wake-jumping heaven, get all the air you dare.

Don't jump them anymore--too old for the impacts--but I sure love to wake surf! Boat is very maneuverable and does that very well.

Jeff

PS: Been doing this stuff in homemade 8 to 12 footers for 60 years starting next year!

Woulda thought that with such a deep vee, the boat would hit pretty easy . . .

It also has a 10.5 inch wide pad down there (not visible in the photo).

Jeff