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Old 02-12-2010, 09:35   #1
R_C
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Determining Prop Nut Thread Size

I am away from my boat right now but I want to order a prop nut anode kit so it is waiting for me when I arrive at the boat in a few weeks. Can anyone verify from the pictures below the thread size? With about a 1 1/16" opening in the nut I think this is 1 1/4" NC7 thread size. Does that look right? Thanks.
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Old 02-12-2010, 13:29   #2
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Your prop nut is determined by SAE J755 specs. See:
http://www.gpcprop.com/catalog%20pdf...20Shafting.pdf
or
www.abycinc.org/committees/P-06.pdf

and page down to the dimensions of shafts page near the end of each document. There you will find the shaft size and all the specs for that shaft including nut thread sizes.
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Old 02-12-2010, 15:22   #3
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I appreciate the shaft specs. But unfortunately, that does not help me right now because I'm away from the boat and I do not know my shaft diameter. I just have the pictures to go by. I was hoping someone could confirm the nut size from the picture.
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Old 02-12-2010, 15:43   #4
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To measure thread dimensions the diameter is to the outside of the threads on the male end, in this case on the propeller shaft. If you have a micrometer that will fit inside the nut try to measure to the deepest parts of the cut inside the nut.

The thread pitch is easy, count how many threads in one inch, again more visible on the shaft.

From your photos it looks like 1 1/8" diameter and 9 threads per inch. However the SAE standard for a 1 1/2" shaft would be 1 1/8 X 7 threads which is not consistent with the estimates from the photo. Other options:

1 1/4" shaft = 7/8" X 9
1 3/8" shaft = 1" X 7

Neither of these match what I estimate from the photos. Are you sure this is not a metric shaft?
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Old 02-12-2010, 15:46   #5
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If you want to see a chart of all SAE prop shaft specs

http://www.techboat.com/Bibliotheque...0dimension.pdf
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Old 02-12-2010, 16:24   #6
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No, I am not sure it is not a metric shaft. The boat is a 1988 38' Marine Trader trawler with a single 135HP Ford Lehman. With about a 1 1/6" opening in the nut visible in my pictures, it doesn't seem likely that the thread size is 1 1/8", which would make the threads only about 1/32" proud on the shaft. If the threads were 3/32" tall, which seems much more likely, that would give me an outside shaft thread diameter of 1 1/4".

I think the angle in my first shaft picture makes the thread pitch deceptive. In the picture below, I do count 7 tpi. This picture also seems to verify the SAE prop shaft specs for a 1 3/4" shaft. The specs list 5" of taper, 2 1/4" from end of taper to end of thread, and 2 3/4" from end of taper to end of shaft. From this information I think I have a 1 1/4" x 7 tpi nut. Does that look right?
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Old 02-12-2010, 17:24   #7
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On a standard propeller and shaft there are usually two nuts and a cotter key. ABYC shows the installation as in the diagram below - The thin nut is the "Jam Nut" and is installed first up against the propeller. Then the thicker nut is installed and tightened up against the Jam Nut with enough torque to remove the thread load on the Jam Nut. Then the cotter key is installed.
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Old 02-12-2010, 17:39   #8
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We now start the thick nut thin nut controversy
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Old 02-12-2010, 17:46   #9
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My prop was installed with the two nuts and cotter pin but the thin nut was installed last, which I know is incorrect. I plan to install one of these:

Propeller Nut With Zinc Anode - Specifications

I'm not sure if there will be room for the jam nut. Once I arrive at the boat I'll only have a couple of days before it is put back in the water. I want to order the right size so it is there when I arrive. That's why I am trying to determine the shaft or nut size from my pictures.
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Old 02-12-2010, 17:56   #10
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We now start the thick nut thin nut controversy
Yeah, that is a major thing and I have seen every variation and heard every rationalization for both ways.
- - My take, derived from a propeller shaft manufacturer, is that the thin nut (Jam Nut) is treated as the "lock washer" and the thick nut is the actual retention nut with the cotter key being the "ace in the hole" if the whole installation is not done correctly.
- - I would suspect from the 2 or 3 boats I see each year with propellers lost that the primary cause is improper "torque" on the nuts. In yards, too many times I see workers installing the nuts - both ways - but never using a torque wrench. They use the 3 grunt method. Mainly because they have no idea what the actual torque setting should be.
- - But, who's to argue with ABYC?
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Old 02-12-2010, 19:40   #11
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Quote:
Originally Posted by R_C View Post
I appreciate the shaft specs. But unfortunately, that does not help me right now because I'm away from the boat and I do not know my shaft diameter. I just have the pictures to go by. I was hoping someone could confirm the nut size from the picture.
Who is the builder and what is the model?
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Old 03-12-2010, 07:29   #12
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Quote:
Originally Posted by fstbttms View Post
Who is the builder and what is the model?
The boat is a 38' Marine Trader sundeck model trawler with a single 135hp Ford Lehman.
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Old 03-12-2010, 07:36   #13
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Originally Posted by R_C View Post
The boat is a 38' Marine Trader sundeck model trawler with a single 135hp Ford Lehman.
OK, well that doesn't help me, I'm not very famiar with Marine Trader. But here are the prop nut/zinc size appellations and their corresponding shaft diameters:

A = 3/4"
B = 7/8"
C = 1" & 1 1/8"
D = 1 1/4"
E = 1 3/8"
F = 1 1/2"
G = 1 3/4"
H = 2"
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