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Old 15-07-2008, 20:48   #1
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oil filled shaft tube

has anyone ever heard of this style setup. i bought a bruce roberts 44 steel ketch with a yanmar 3jhte(turbo) and it has a 2 blade folding prop.
the guy i bought it off was a mechanical engineer(he claims)and said he has never had a problem with it. although i like the idea of it and the fact being that it is sealed on each end by bearings i want to put a bigger prop on and probably a 3 blade andi am concerned that maybe the torque of a bigger prop will tweek the sealed bearings. the transmission hooks up to a car uv joint to a typical car shaft to a uv joint that goes to the oil filled tube. it is a great idea as far as never worrying about engine alignment. i have a picture of this set up. is anyone familiar with it?
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Old 15-07-2008, 23:01   #2
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Sounds like a "one-off"

We sailors are famous for "overengineering" things.

I guess...no I know this is what happens when we sail for hours and hours......

we have time to ponder, think, and plan
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Old 15-07-2008, 23:13   #3
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Aloha Seacapture,
Would there be an advantage to a larger prop? You might want to reconsider that and just go to a 3 blade fixed a bit less in diameter if you'd rather have a bit more efficiency.
If the current size prop is mated to the transmission and engine it will be the right size and if you increase size you'll be asking for engine and trans overheating as well as unnecessary added torque to the shaft.
That's just a thought. We've had lots of folks complain about engine overheating problems on the forum which were mostly caused by someone increasing the size of the prop.
Kind regards,
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Old 15-07-2008, 23:36   #4
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If it's using automotive sized u-joints and shafts, I'd guess, without seeing it, any 3 blade you'd try to fit would be OK. Any way to ask him? Your prop size will be limited by hull clearance.

If you could post some pics it would be easier to comment.

BTW - Why are you considering the repropping in teh first place? If he put so much effort into the setup I doubt he screwed up on the prop selection...
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Old 16-07-2008, 02:45   #5
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The Oil Bath Shaft / propeller system has been around for years and is standard on all Feadships.

I ran one for over 10 years, more than 3 circumnavigations in all kinds of weather in glacial to tropical waters and never an issue. Lloyds Class surveys are so impressed that they waived the mandatory 10 year shaft inspection as long as you kept a record of shaft oil analysis.

15 years now of the same world cruising and still never an issue. They did pull the shafts on last special survey and reports were that they were like new.

Not sure how well yours is engineered but I would think you would get more torsional vibration from a folding prop than a nicely balanced fixed one.

If worried look at sizing up the seal, but I would try it first and make sure you have a header tank with sight glass for you to see if the oil turns milky after a year of running.

Even then donít worry as long as the % of water in the oil is small. (Canít remember exactly what Classification allowed but I think it was around 10%).

We never had anything more than condensateÖbut thatís Feadship!

Found these guys who make them for small yachts.Ö..
http://www.evolutionmarine.com/detail.html?id=3001

They may offer better advice
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Old 16-07-2008, 07:13   #6
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Quote:
i want to put a bigger prop on and probably a 3 blade and I am concerned that maybe the torque of a bigger prop will tweek the sealed bearings.
Beware of attempts to improve a boat by changing the prop. This is not really something that is personal preference. There is a correct 2 blade and a correct 3 blade prop for any setup. A sail prop is not the same as a power boat prop. The size and pitch should be optimized. The science of propellers is pretty well understood and as noted above it is a function of the engine and transmission not subject to a lot debate. Under sized and you give up power and over sized just adds extra wear and tear and usually no additional speed or power.

Any prop shop can do the computations for you for free. This isn't something you do just because you think one size may might be better. If you think switching from a folding two blade to another prop is something you want then that is OK, but guessing at the size can be an expensive and poor decision. If done properly there will be no impact on the drive shaft assuming the right prop was on the boat in the first place (might not be). You'll just need to measure the total prop clearance to help make the right choice.
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Old 16-07-2008, 07:38   #7
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Excellent summary Paul
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Old 20-08-2008, 04:16   #8
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interesting, we supply this type of set up as a retro fit, it should be better than having the shaft supported by rubber. As the shaft is held more rigidly you will also see more hp at the prop, so you can optimise the prop and may find the prop would be different for this set up when compared to a rubber supported shaft which can have losses of around 12% (mechanical)

What you have to watch for is debris damaging the rear seals, do you have a way to check if these have let water into the tube?
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