Prop re-installation tips.

[KWKloeber]

Its a 1" shaft turned by a small toy like engine. Maybe we're over thinking this.  I mean really, the whole "a keyway is not enough", "you have to lap the prop", "what's the exact torque value on each nut".  Please stop.

YBYC!   but I will pass the wisdom onto the C30 owner who, last season, lost his prop because he didn't lap it and torque it properly.  He hit some floatsom, the prop spun, sheared the key (which is what a KEY will do), spun off the nut -- and plunk, now it's a hundred feet below the surface.

I'm positive he'll get a chuckle or two! 

k

[lazybone]

Its a 1" shaft turned by a small toy like engine. Maybe we're over thinking this.  I mean really, the whole "a keyway is not enough", "you have to lap the prop", "what's the exact torque value on each nut".  Please stop.

YBYC!   but I will pass the wisdom onto the C30 owner who, last season, lost his prop because he didn't lap it and torque it properly.  He hit some floatsom, the prop spun, sheared the key (which is what a KEY will do), spun off the nut -- and plunk, now it's a hundred feet below the surface.

I'm positive he'll get a chuckle or two! 

k

Please tell me how to deal with the engine coupling that relies on a keyway? The same exact rotational load as the prop and it cares not about laping. 
Your Cat 30 relies  on a keyway at the engine coupling. Please tell us the solution to its obvious deficiency.  The same exact rotational loads as the prop. 

Larger boats with larger engines address this issue with splines and indexing profiles.

Its foolish to read articles about large boats and large serious engines and assume we should share there concerns.

[Noah]

Some folks are fans of the "lap dance", others not so much...

[lazybone]

Its a 1" shaft turned by a small toy like engine. Maybe we're over thinking this.  I mean really, the whole "a keyway is not enough", "you have to lap the prop", "what's the exact torque value on each nut".  Please stop.

YBYC!   but I will pass the wisdom onto the C30 owner who, last season, lost his prop because he didn't lap it and torque it properly.  He hit some floatsom, the prop spun, sheared the key (which is what a KEY will do), spun off the nut -- and plunk, now it's a hundred feet below the surface.

I'm positive he'll get a chuckle or two! 

k

Lap your prop or your boat will sink. 
Davey Jones's locker.  Arrrrggggg.
You heard it here. 😊

[KWKloeber]

Its a 1" shaft turned by a small toy like engine. Maybe we're over thinking this.  I mean really, the whole "a keyway is not enough", "you have to lap the prop", "what's the exact torque value on each nut".  Please stop.

I'll point out the obvious that one is on a tapered shaft, the other not so much. One retained by set screws, one not so much. You figure out which is which. If you don't want to do it, YBYC, but why make a false argument against what's accepted practice?  I don't get it.

[lazybone]

Its a 1" shaft turned by a small toy like engine. Maybe we're over thinking this.  I mean really, the whole "a keyway is not enough", "you have to lap the prop", "what's the exact torque value on each nut".  Please stop.

I'll point out the obvious that one is on a tapered shaft, the other not so much. One retained by set screws, one not so much. You figure out which is which. If you don't want to do it, YBYC, but why make a false argument against what's accepted practice?  I don't get it.

So your saying because the other end of the shaft has set screws, that makes it more robust to rotational inertia?...  and you claim to be an engineer? Please.  The set screws are for axial loads.

[Jon W]

I had a lap dance once.

[KWKloeber]

Jon. Did that come with or without a torque! 🤓

[Ron Hill]

Guys : The key way makes sure the prop turn with the shaft.

It's the castellated nut with the cotter in it & thru the shaft that keeps the prop on the shaft!!

Don't be $$ foolish and try to reuse the same cotter pin each year!!

A few thoughts

[KWKloeber]

So your saying because the other end of the shaft has set screws, that makes it more robust to rotational inertia?...

NO, that's not what I am saying (as far as how it pertains to a tapered shaft, as our props.)  BUT YES, of COURSE set screws also resist rotational force.  They don't -- can you twist a fitting held on a bimini frame by a set screw.  No.  It's not necessarily its primary function, but anything creating friction or sitting in an indent will take *some* rotational force -- How much force it resists *all depends* (proportional to the friction it creates.)

Nevertheless, what I was getting at (my point poorly made) is that because the shaft is tapered, the rotational force is meant to be taken by **all** the taper surface making contact.  That's HOW and why tapered shafts were developed -- for good or for bad, we're stuck with them on our boats.  In the real world 100% is unrealistic -- only a perfect, machine-shop fit might attain 99%-- so shoot for the best fit on the faces as we can attain.  At some point if there's insufficient contact and transferred nut torque holding poorly-fit faces into contact, slippage can occur and it's a vicious cycle due to the taper.  It's not as vicious a cycle if it was a straight rather than taper fit and, say, were splined like my outboard prop.

Your point about the toy engines is well made -- but at what point does it become critical?  I dunno -- It's like the bearing removal *contraption* -- if the bearing was not corroded in and the strut heated beforehand it may have worked.  But that doesn't mean it's the *way to do it*.  Also consider that, as forces go higher than our toys -- the shafts, taper surfaces also increase to handle the stresses.  Probably at our *size* engine/shaft/prop there's belt and suspenders involved, but what about any boating isn't (or shouldn't be) belt/suspenders driven? In fact, a properly-fit shaft/prop DOESN'T REQUIRE a key.  The taper is what's designed to resist the forces -- In fact, a machinist will tell you it's sufficient to have a proper taper fit and no key -- not so, with a poor taper fit with a key.  JUST LIKE a drill press, no key, no set screws, no spline -- "only" a taper fit on the chuck.  And unlike our shafts/props slicing through water, a drill chuck taper is a tiny shaft resisting high rotational forces (think drilling steel or using a sanding drum.)  Lapping the prop/shaft, and torquing to get the proper tight taper fit are required to maintain that fit.  But again, YBYC.

Many variations and angles of tapered shafts were developed over a hundred+ years toiling by machinists smarter and with more experience than me so I'll go with what they recommend.  There's TONS of resources out there from machinists and shaft companies about properly fitting tapered shafts by lapping and checking them.  There's also resources that explain what happens if the taper slips as far as loosening and slippage and also potential shaft shear failure.  One good one is western branch, a supplier of yacht shafts and couplings.  If you want to, just look and read.  If you don't want to lap, then don't lap.  Totally YBYC. "DWTL, DL" as it were.   But to claim that hundreds of years and untold number of person-years experience is bogus, is just ignoring the evidence.  Your choice, of course.

Bottom line is, if you think that only a key can hold a prop, then please don't torque the nut -- just tighten it enough to prevent prop slop, as it were.  Depend on the belt, rather than the suspenders.   I'll close out my contribution to this thread with the shaft mfgr (not my) recommendation -- wise owners will follow it.  YBYC.

Propeller Fit
In order to properly mate the propeller to the shaft taper, the following is one of the most commonly used methods:
1. The taper should be coated with Prussian Blue.
2. The propeller should then be put on the shaft without the key and gently tapped snug onto the taper.
3. If the Prussian Blue is uniformly removed – the fit is good.
4. If the Prussian Blue is not uniformly removed, the fit is not good. An appropriate lapping compound should then be used and the test repeated.
Since a 100% fit is not always possible, an effective seal such as waterproof grease should be used to exclude water. This seal will also protect against dissimilar metal corrosion.


kk