 |
14-10-2011, 05:35
|
#1
|
Registered User
Join Date: Oct 2010
Location: Perth west australia, Indian Ocean
Boat: Samson C-Falcon 41'9" composite ketch (designed by Cece Norris)
Posts: 226
|
Must a Prop Shaft Be Perfectly Balanced ?
hello, i'm just putting the boss onto my prop shaft. i had to cut off the old one so i have a slightly ragged end. as it will be spinning up to 1300rpm, does it have to be polished to prismatic perfection, ie a perfectly flat and parallel end to the 'cylinder' of the shaft, or is a few grammes ok?
thnks
Cp'n John
|
|
|
14-10-2011, 05:45
|
#2
|
Senior Cruiser

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 51,768
|
Re: must a prop shaft be pefectly balanced?
I don't fully understand your situation, but perhaps the tolerances listed in ABYC Standard "P-6" (PROPELLER SHAFTING SYSTEMS) will answer your question.
See ➥ http://www.abycinc.org/committees/P-06.pdf
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
|
|
|
14-10-2011, 06:52
|
#3
|
Registered User
Join Date: Aug 2010
Location: Naples, FL
Boat: Leopard Catamaran
Posts: 2,583
|
Re: must a prop shaft be pefectly balanced?
Any imbalance will give you vibrations, and cuase problems down the line. A Machine shop can turn it flat on a lathe.
|
|
|
14-10-2011, 07:47
|
#4
|
Registered User
Join Date: Nov 2008
Location: Miami
Boat: Boatless
Posts: 1,580
|
Re: must a prop shaft be pefectly balanced?
When a shaft vibrates it can destroy the transmission or just wobble itself out of the boat...
__________________
Phil
"Remember, experience only means that you screw-up less often."
|
|
|
14-10-2011, 07:59
|
#5
|
Registered User

Join Date: May 2008
Location: We're technically refugees from our home in Yemen now living in Lebenon
Boat: 1978 CT48
Posts: 5,970
|
Re: must a prop shaft be pefectly balanced?
I'm not sure how balanced the anodes are that we bolt to our shafts...just guessing here but doesn't seem like having the end cut-off not exactly 90deg could make that much difference...also what about the set screw that's in the flange that bolts to the trans...?
__________________
James
S/V Arctic Lady
I love my boat, I can't afford not to!
|
|
|
14-10-2011, 09:49
|
#6
|
Registered User
Join Date: Aug 2010
Location: Naples, FL
Boat: Leopard Catamaran
Posts: 2,583
|
Re: must a prop shaft be pefectly balanced?
Quote:
Originally Posted by James S
I'm not sure how balanced the anodes are that we bolt to our shafts...just guessing here but doesn't seem like having the end cut-off not exactly 90deg could make that much difference...also what about the set screw that's in the flange that bolts to the trans...?
|
All good points. Also coming from a power boat, we use much higher RPM's. But The degree of imbalance of each part adds up. If the degree of non-flatness is less than the weight of the setscrew it probably will not cause any noticable change.
Still on an internet forum I would hate to say, "OK don't worry", and have this poster write back in a few months saying, Hey I followed your advice and ruined my, (transmission, cutlas bearing, etc...).
Especially when the fix is so simple; lathe the shaft to be sure, and the machine shop can test for trueness, and balance of the shaft to insure years of trouble free use after reinstall.
|
|
|
14-10-2011, 10:21
|
#7
|
Senior Cruiser

Join Date: Oct 2007
Location: 29° 49.16’ N 82° 25.82’ W
Boat: Pearson 422
Posts: 16,307
|
Re: Must a Prop Shaft Be Perfectly Balanced ?
If I understand the original post, he cut the end off and the cut is not perfectly perpendicular? If that is the case then (disclaimer) in my opinion the effect would be minimal.
More critical would be problems from a shaft that is not straight or not perfectly in alignment with the engine/transmission.
__________________
The water is always bluer on the other side of the ocean.
Sometimes it's necessary to state the obvious for the benefit of the oblivious.
Rust is the poor man's Loctite.
|
|
|
14-10-2011, 10:34
|
#8
|
Registered User
Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 21,171
|
Re: must a prop shaft be pefectly balanced?
Quote:
Originally Posted by James S
I'm not sure how balanced the anodes are that we bolt to our shafts...just guessing here but doesn't seem like having the end cut-off not exactly 90deg could make that much difference...also what about the set screw that's in the flange that bolts to the trans...?
|
You will notice the anodes are at the far end but often pretty close to the strut and the bearing.
Very much depends on basic physics - shaft thickness, unsupported length, the amt of the off-center mass, etc..
The less off-balance, the less wear, in any case.
There is no such thing as a perfectly balanced rig, but the accy should be as per May's post, or better.
b.
|
|
|
14-10-2011, 11:18
|
#9
|
Registered User
Join Date: Sep 2011
Location: Brooklyn, NY
Boat: C&C Corvette 31
Posts: 13
|
Re: Must a Prop Shaft Be Perfectly Balanced ?
Seems to me this is a better safe than sorry situation. Not to mention that knowing that crooked end is spinning around below me would piss me off.
|
|
|
14-10-2011, 11:22
|
#10
|
Registered User
Join Date: Mar 2007
Location: Santa Cruz
Boat: SAnta Cruz 27
Posts: 7,198
|
Re: Must a Prop Shaft Be Perfectly Balanced ?
The crooked end is hidden in the coupling, and being slightly crooked is not going to make it wobble--the rest of you guys are perfectionists!
|
|
|
 |
Thread Tools |
Search this Thread |
|
|
Display Modes |
Rate This Thread |
Linear Mode
|
|
Posting Rules
|
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
HTML code is Off
|
|
|
|
Advertise Here
Recent Discussions |
|
|
|
|
|
|
|
|
|
|
|
|
Vendor Spotlight |
|
|