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Location: s.w. florida | The dings in the prop are most likely from the rudder. The radial pattern of the fracture indicates that the shaft broke not at once (as in impact) but in a progressive fashion. Bear in mind the progression may have been a very short time from the first crack. Also note that discoloration is evident at the edges of the key-way, both sides. The deepest section of crack also begins at the key-way. Removal of the taper and close inspection will determine either crevasse corrosion or point fatigue. I would guess the latter. Three things you can do to lessen the chances of a repeat performance:
1: Buy a high quality shaft.
2: Spoon cut the key ways. Some shops don't do this on shafts under 2" but I have all mine done regardless.
3: Never install props in the water, always do it on land and ALWAYS lap the tapers. Lapping the tapers insure that the key doesn't take the rotational load it wasn't intended for.
FWIW most marine adjusters and unfortunately insurance surveyors wont be able to ID a progressive crack and will take your word you hit something. I'm not saying........ I'm just saying.
Congrats on finding the prop. Not easy.
BTW Frank and Jimmies does good work.
Edited by baitkiller 1/8/2008 1:41 PM
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| From the photo it appears to be a classic bending break, 90 degrees to the axis of the shaft. Improper alignment is most likely the cause. What does the other face of the fracture look like? Take both sections of the shaft to a Metallurgist, he/she can put it under a electron-scan microscope and tell you exactly what happened. He/she should be able to tell you how many cycles (shaft revolutions) it took for the shaft to fail. |
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Location: marysville wa | I have nerver heard of shaft breakage unlees some one hit something and shock loaded it .
In bristol bay we are bouncing on th bottom all the time , geetting webbing in the prop ect ,
I saw one guy hit a cork line at speed and rip his shaft from the back of the engine . but in all the years fishing I have nevr heard of some one lossing a shaft without hitting some thing , we over load the boats to the max . harsh fwd to rev is a everday every 5 min thing up there its parts of the job
they are only 32 ft boats but 15ft wide and can pack 20 klbs of fish , its been a while but i think they are running 2 inch or bigger shafts ,
almost sounds like you need bigger diameter
how big are the ones that are breaking ? I am no expert but in commercial fishing in ak Its amazing the abuse given to a boat , its a tool not a toy . we have broke rudders off , holes in 1/4 plate in bow and stern ect , our fenders are 50 seires tires |
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Location: Cape Coral FL USA | baitkiller - 1/8/2008 4:39 PM
2: Spoon cut the key ways. Some shops don't do this on shafts under 2" but I have all mine done regardless.
Can you elaborate on this for me? I'm an inboard dummy, although I once owned a Shammy.
Thx |
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Location: s.w. florida | fabricator - 1/9/2008 1:53 PM
baitkiller - 1/8/2008 4:39 PM
2: Spoon cut the key ways. Some shops don't do this on shafts under 2" but I have all mine done regardless.
Can you elaborate on this for me? I'm an inboard dummy, although I once owned a Shammy.
Thx
I don't have a picture I'm sorry. What it means is that the end of the key-way (fat side of taper) is dished out. the key way does not abruptly end in sharp corners. Picture the shaft made of plastic. you push a red hot teaspoon against the end of the key-way transferring sheer loads to a radii instead of the cut edges.
Is that clear? if not maybe someone with more word craft can do better. I just dumped all my work pics after the new year and don't want to dig thru discs.  |
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| Don't make the prop end of the shaft too tough or too fatigue resistant by radiusing everything and spooning the keyway. I'd much rather have the shaft fail first at the prop than first at the transmission coupling. The later type of failures usually open up a pretty big hole in your hull. |
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Location: s.w. florida | GMC your point makes sense on the surface and your postings are always well thought out, but considering what I have dealt with in the field it is still better to make all things tight and strong. Ill explain.
Shaft couplings are taper matched on the lathe at the shop. They fit nice and tight and the only common failures at that end that I see are (generally) gross alignment / failed mount type failures. They are not subject to crevice corrosion.
An impact upon well built and installed running gear will rose bud the wheel or break a blade, not break the shaft.
The prop end is a different kettle of fish altogether. Bad installs with excessive protrusion are common, ill fitting hubs, keys and (Lord bless us and save us) shims are often seen. I honestly see a 25%+ ratio of loose props on all inboards I deal with.
Divers should only do prop changes on planning hulls for testing and get-home.
A loose prop will break a shaft at the key way.
The prop is not loose because it wasn't tightened enough.
The prop is loose because it was never properly installed.
Lap your props people. Its easy. |
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| My previous post was a bit of a joke, it shouldn't be taken seriously. |
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Location: Saugus, Ma. USA | DDHAND - 1/1/2008 5:54 PM
... always wondered why the prop spins down instead of up and into the hull? Just luck?
Because the shaft points down - it isn't parallel to the waterline. Also, it's like bread and butter always landing butter side down. 
Edited by jobowker 1/10/2008 4:16 PM
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Location: East Islip | Im guessing electrolysis. |
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| THE most important factor on a shaft like this one, which I assume is a tapered bore, is that the prop is properly seated on the taper. If it is not, the prop under load will ocillate with every revolution, causing fatigue in the saft. If this is the case, it is not a question of if it will break, only when it will break. What usually causes improper seating is improper installation. As a previous poster said, the prop should first be lapped to the shaft. This is basically "dry fitting" the prop to the shaft, but with the use of a valve grinding compound. This smoothes the shaft and the bore on the prop, and gives the installer a frame of reference during final installation, to be sure he has it properly seated. Another cause is movement of the key in a tapered keyway. If the keway is tapered, and the key slides up the taper during installation, the prop will not be properly seated on the shaft, and will ocillate and fail. A third cause is excessive overhang between the prop hub and the rear of the shaft strut. The distance between these with the prop final installed should never be more than one diameter of the shaft. If this is the case, it can allow the shaft to bend, especially in situations of backing down hard on a fish where frequent high cavitation occurs. An unbalanced prop can also cause breakage, but generally would have to be run in such a condition for a long time. This would be serious operator error or unwillingness to do proper care and maintenance. One other poster mentioned mis alignment of the engine, which could cause shaft breakage, but it would not do so at the strut, but rather at the transmission coupler where all of the stress is applied. |
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Location: Long Island N.Y. | dpdash - 1/8/2008 10:31 AM
Definately did not hit anything
Just wondering if you know you didn't hit anything, was there any noticeable damage to the strut or anything else it might have hit like the other shaft or wheel? |
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Location: Cape Coral FL USA | Another dumb question - what IS the proper nut torque, and how is that determined? I would imagine shaft diameter, or thread diameter, but I'd rather ask than guess. |
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What causes a shaft to snap?
