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paul323 11-20-2013 11:50 AM

Prop Shaft Offset
 
We're having a debate on the Peaaron 323 owners site. Two boats of a similar vintage (late 70s), one has a prop shaft offset to port, the other to starboard. The latter has just had an engine repower.

My question is, how is the shaft offset typically managed? Is this something which is permanently molded into the hull, or can the offset me made by shifting the engine on its mounts, and adjusting the position of the shaft strut? Is it possible that during the repower the yard offset the prop shaft incorrectly?

TIA.

Faster 11-20-2013 12:04 PM

Re: Prop Shaft Offset
 
The shaft log, engine beds, and propeller strut are positioned in a line from the get-go.. if I'm understanding your question correctly, an engine repower, nor a realignment, couldn't change the designer/builders' original intentions.

Plenty of boats have propshafts off centerline.. C&C 30, Mirage 33, Ontario 32, as examples.. not sure if this is done to try to offset propwalk, address torque issues or if it's simply a way to make everything fit around the interior...

Are you saying that two similar vintage Pearsons are 'opposite'?? If so there must have been a factory mod during the production run.. are the interiors otherwise identical?

paul323 11-20-2013 12:33 PM

Re: Prop Shaft Offset
 
Thanks (as always) Faster, that answers my question.

Yes, that appears to be the case - that two boats have "opposite" offsets - thus we are all puzzled. We currently have 3 datapoints which appear to contradict each other hull#42 reports port, #68 starboard, #212 port. Gotta be a mistake somewhere. Other folk are intending to check their offset as access allows.

Hull #68 is having serious propwalk problems since their repower, we're trying to work out why.

One possibility that Gasoline was standard, diesel optional; I wonder is they had different offsets for different engines? The Atomic 4 boat uses RH props, and diesels tend to be LH...

(The P323 (as I am sure you know) has a skeg mounted rudder, thus the offset)

Thanks again!

Faster 11-20-2013 12:52 PM

Re: Prop Shaft Offset
 
Seems odd that they'd change it twice during a production run... Any chance Hull 68 was facing aft and got confused ;) (no offense)

Was a repropping part of the repower?.. Also I suppose that if the new shaft rotation is opposite, and it works 'with' the offset rather than 'against' that might increase the effect?

Love for Bob Perry to weigh in on the reasons/rationale for offset shafts.. as he did himself with the Mirage...

paul323 11-20-2013 01:05 PM

Re: Prop Shaft Offset
 
No offense taken :-) Mistake could have been me (#212), but I need to dive on the old girl to double check.

I think this guy is in a bad spot. The rotation would indeed appear to have reversed - he had a LH prop, now has a RH prop. I have asked him what engine he has because most diesels in a 323 have LH props (I think). So I have the same thought - offset + reversed rotation = a problem.

svHyLyte 11-20-2013 01:09 PM

Re: Prop Shaft Offset
 
If the engine spec's changed during the manufacturing run, the direction of engine rotation might have been changed. While prop-shaft off-sets were rarely used to counter the effect of prop-walk, if an off-set was necessary or advisable for other reasons, selecting the side for the off-set to also account for prop-walk affect would have been logical. Hopefully the direction of rotation of the re-power matches the engine it replaced, eh?

jimgo 11-20-2013 01:32 PM

Re: Prop Shaft Offset
 
That's what I was wondering - whether the new engine spins the prop shaft in the same direction as the old engine. As I understand it, the offset is an attempt to address prop walk by changing the angle at which thrust occurs. If the guy just had his engine replaced with one that spins the prop shaft in the opposite direction (as evidenced by the need for the new prop), that would amplify the prop walk, rather than reducing it.

If the assertion above (gas spins one way, diesel the other) is true, that could explain how it happened that the owner wound up with an engine that "wasn't set up properly" for his boat. Assuming that's the case, is there something that could be put in the drive train to cause the shaft to spin "properly"? Could the owner go back to the original prop but "just" run in reverse all the time? I'm not sure how the transmission would like being run in reverse, but...

paul323 11-20-2013 01:43 PM

Re: Prop Shaft Offset
 
"As I understand it, the offset is an attempt to address prop walk by changing the angle at which thrust occurs."

Jimgo, you make good points,..but as for the offset, FYI the principal reason for it on the P323 is not for the propwalk, but the fact that that the rudder is mounted on a skeg. Most boats with skeg-mounted designs have the propshaft offset, otherwise you could not remove the shaft! Reducing propwalk is a side-benefit, not the objective.

I am guessing he will have a talk with the yard which did the repower. I don't know what can be done - and how running an engine mainly in reverse effects the warranty!

This is a weird one.


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