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Old 02-18-2013, 11:25 AM   #1
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Prop Alternatives

A Huge thank you to Eric for his words of wisdom re this issue ...... My 30x19 prop has suffered dezincification and needs to be replaced.... bottom line seems to be "Pitch Ratios" so when looking for an alternative sized prop I am basing my search on the PR of my olde prop which is .6333( dia.divided by the pitch)......... these prop sizes have a close PRatio.....28x18(.6428)....26x17(.6538).....24x16( .6666)..... Anyone see anything wrong with this logic ????? My current prop performs well (1700 rpm at comfortable hull speed) but is a little Too large for my 1 1/2 inch shaft(IMO)...................
ALSO my current prop is (was) Bronze ....... I have a steel hulled boat with a Monel shaft ............. I'm thinking that I aught to be looking for a steel prop so galvanic corrosion might be minimized ..... comments other than go talk with a good Prop Shop......... john
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Old 02-18-2013, 11:42 AM   #2
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A My 30x19 prop has suffered dezincification and needs to comments other than go talk with a good Prop Shop......... john
Yes, go talk with a good prop shop and a good yard. Sidney Propellor and Philbrooks are side by side. Did your surveyor pick up the bad prop and offer suggestions? Where are/were the shaft zincs?

BTW, how is the steel hull protected?
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Old 02-18-2013, 11:55 AM   #3
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Yes, ALL WRONG...
By that logic 15 X 10 would also be correct.
The P/D ratio is not key and need only be held between .8 and 1.2 generally. Get some good advice on whatever you are trying to do.
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Old 02-18-2013, 12:20 PM   #4
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Being the boat is steel its probable full diplacement, many full displacement trawlers, tug, ship have a slow turning, large diameter, and high pitched prop to move a lot of water at low rpms. It sounds that is how your boat is set up. So I would not go to much smaller. But might even look at a higher pitch with same diameter.

I agree talk to prop people and make sure you tell the what Your intended purpose use is. You might be better to talk to a commercial yard rather than pleasure.
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Old 02-18-2013, 12:53 PM   #5
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Keep in mind that the Breadbasket's gear is 3.9-1 and her engine has four times the power she should have.

I advised JD get a smaller dia prop w much greater blade area. Also a highly skewed 4 or 5 blade would be great if he could find one. Or a Michigan "Work Horse" wide blade prop but they are really designed for heavy loading .... the opposite of JDs situation.

Don't know much about electrolysis but a steel prop used and cheap will be almost impossible to find.

Would a galvanized prop be good or really bad re electrolysis?
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Old 02-18-2013, 01:18 PM   #6
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?????

Sooooo Many Questions ........... jp
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Old 02-18-2013, 01:49 PM   #7
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Sooooo Many Questions ........... jp



Maybe if you gave us the present spec of the boat and running gear we can at least get you headed in the right direction. What was the origal purpose use of the boat. The fewer the varibles changed the simplier it will be.
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Old 02-18-2013, 07:13 PM   #8
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Breadbasket Specs......

O.K. here goes.. She's 26 ft with a 9.5 ft beam drawing about 3.5 feet of water...she has a two foot keel..... ford lehman 120 with a 3.9:1 reduction gear.... has one bearing(Cutless)...with a Monel 1 1/2 inch shaft...
Was used as a cruiser and fishing/crabbing boat by PO ..... cruises comfortably at 1700 rpm approx. 7/8 knots is keel cooled with a dry exhaust................ current prop 30x19(LH) Bronze three Blade ---- I'd like to go to a 28 inch prop because those MAY be available to accommodate a 1 1/2 inch shaft so that I could get away from having to use a sleeve..................I realize that the boat is overpowered BUT it is what it is!!!!! I'm gathering information BEFORE I go to a prop shop thus the question on this Forum-- Input is appreciated.... standing by,,,, john
ps-- weight is about 9000 lbs
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Old 02-19-2013, 07:15 AM   #9
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Not needing to tow a trawl should allow an inch or two more pitch , and give usual cruise speeds at a bit lower RPM.

But with a 4 1/2 -1 reduction gear it may not amount to many RPM reduction .

The diameter and blade area is set by engine HP , so there will be little change there.

Should you be able to locate a 2-1 gear box and bear the expense of a new prop, the cruise RPM and noise could be lowered.

Probably hardly worth it.
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Old 02-19-2013, 09:55 AM   #10
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FF,

With all that power I'd opt for direct drive. How'd you change 3.9-1 to 4.5-1?

You say "Probably hardly worth it." Yea I think he just needs a fairly suitable prop for now. Later on a used 27hp Yanmar sailboat engine would be appropriate for his boat. I think the're relatively easy to find.

But now john just needs a workable prop for a fun summer afloat.

Anybody seen a prop in the ballpark?
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Old 02-19-2013, 11:15 AM   #11
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Thumbs up Right On !!!! Eric.........

I will not be re-powering or changing the reduction gearing -- I'm just needing a workable Prop ...... After much discussion I'm leaning toward a 28x18(LH) three blade accommodating a 1 1/2 inch Monel shaft.. regular tapper.............. material???? Bronze would work however seemingly there are all sorts of different "Bronze Mixes".. some better than others .... the boat has now been re-zinced(steel hull) .. don't want the dezincification issue to raise it's ugly head again ............. comments on an Aluminium prop .......... thanks for the input... john
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Old 02-19-2013, 11:41 AM   #12
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I will not be re-powering or changing the reduction gearing -- I'm just needing a workable Prop
Sure listen to the TF members, but be very wary. Your best advice will not come from here, it will come from a, sigh, good prop shop. You may want to consider posting on boatdiesel for another check of your assumptions and prop metal.
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Old 02-19-2013, 12:16 PM   #13
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For you size boat it seems to be over powered with a Lehman 120 and a 3.9 to 1 reduction. You could go with the same diameter prop but larger pitch prop. As far has aluminum prop, aluminum is used as a sacrificial metal next to zinc. Zinc galvanic rating is 3 and aluminum is 7, mild steal is 15, SS IS 21, bronze is 29 so for a steel boat and SS shaft an aluminum prop is not advised.

Anyway talk to a prop shop.
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Old 02-19-2013, 01:51 PM   #14
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Roger......

I'll speak with Sidney Prop. on Thursday ............. I'll have some good info.. on which to base my discussion........ thnx... jp
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Old 02-19-2013, 03:41 PM   #15
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John

Once you have decided on your prop, you may want to check prices in the Seattle area.
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Old 02-19-2013, 04:54 PM   #16
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I ran your situation through my two prop calculators after guessing at your LWL and max rpm (23' & 2400)

Both seemed to "think" the diamater should be 32" and 28" pitch for 3 blades and 30" for 4 blades with your high reduction ratio.

Also both "thought" 20hp would bring you to hull speed 6.5K, 23hp for 7k, and 30hp for 8k.

You may be better to have your FI pump limited to 50hp then use that HP and rpm in the prop calculator as a starting point.
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Old 02-19-2013, 04:57 PM   #17
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Please let us know what you end up with.
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Old 02-19-2013, 06:50 PM   #18
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Sure listen to the TF members, but be very wary. Your best advice will not come from here, it will come from a, sigh, good prop shop. You may want to consider posting on boatdiesel for another check of your assumptions and prop metal.
Best advice yet. Do you have a shaft brush attached to your bonding system? As someone else mentioned, shaft zincs? (yes, I understand you have a monel shaft). A NiBral prop will be your best bet, and will cost you the most, but let the yard and the prop shop guide you on all that once they have seen your entire system.
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Old 02-19-2013, 07:52 PM   #19
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Brooksie,

You say ... "diamater should be 32" and 28" pitch".

That would be like your boat w a 500hp engine and a 60" prop.

You can't prop Breadbasket for the engine because she can only use a small fraction of her power. Breadbasket needs to be propped to her hull. No need for more than 20" in diameter as long as enough pitch can be had to propel the boat easily at hull speed and a bit more (7 knots max). Probably will need over 20" dia to have a prop that will advance through the water at about 8 or 9 knots ... with the proper amount of slip and enough thrust to push the boat (15 to 20hp).
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Old 02-19-2013, 09:15 PM   #20
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Wouldn't the 3.9/1 reduction gear fare better with the largest diameter wheel as opposed to a smaller because of the slow turning of the shaft. Just a thought. Larryw
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