Shaft calculator

backman

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Not to hijack the other prop thread with this but having broken 2 shafts in my previous boat, a 17,000# Carolina Classic 28 I'm a practical expert. That boat had 1.5" shafts and mathematically had a safety factor of 5.


While it was as solid and well built a sport fish out there, the entire Carolina classic community was constantly breaking shafts, more often in the northeast than in the Carolina's. The majority of the breakages were in the 1000 hour range and were from continual pounding at planing speed for long periods of time, over and over and over. I broke one at 600 hrs and another at 1100 hrs and had to replace another with a crack at 800 hrs. I had to dye test the shafts every year to be safe. Breaking a shaft offshore in a twin screw boat sucks, but you will get home at 6 knots eventually. 13 hrs from west Atlantis as an example!


this is a good summary to start from:
http://www.wbmetals.com/downloader.asp?filename=Aqualoy_Brochure.pdf

I learned the hard way about a factory safety factor of 5 really being a safety factor of 3 when you factored in things like short 3.5" hubs, where the hub actually sits on the taper and the effects of high torque and continual steep chop on a shaft over time. Lapping the hub onto the shaft , properly torquing the 2 nuts tight as well as minimizing the hub to bearing distance was critical in that boat setup.

So - now that I'm in a downeast and pretty much eliminated those pounding 21 knot rides forever - is shaft cracking and breakage something I need to be aware of and monitor by dye testing annually or is it a non issue in a properly designed set of running gear.
 

BillD

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"Not to hijack the other prop thread with this but having broken 2 shafts in my previous boat, a 17,000# Carolina Classic 28 I'm a practical expert. That boat had 1.5" shafts and mathematically had a safety factor of 5.

Did you weigh the boat on a scale??? Seems quite heavy for a 28 footer ?
 

backman

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Mine was weighed about 2/3 full of fuel at 16,000#. The boat is advertised as 15K empty and holds 210 of fuel. Other Carolina classic owners have seen equivalent numbers, its a tank for its size.
 

Blitzen

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Just curious where was the shaft breaking or cracking?
 

backman

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95% of them cracked the same way, a crack just inside the hub, starting at the key way and eventually circling the shaft and snapping. The ones I caught in time had perhaps a 1/4" deep crack starting at the key way and working its way around.
 

Blitzen

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OK, what is the hub to strut distance, or other words the space between the back of the strut to the front of the hub?
 

traditions

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I have seen a 1 3/4 aquamet 22 crack in the taper.It was on a 35 H and H with a 375 Deere turning a 30 inch wheel,10 year old shaft.That is a big wheel for that size shaft in my opinion,and I am surprised it took that long to snap.Alot probably depends on how hard the guy running the throttle treats the boat.Reversing that big of wheel has to be hard on the shaft.
 

MDI45

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Snapped a 2 1/2 inch aquamet 22 at the coupler on my 45 MDI....My friend snapped one the same way on his 35 Duffy
 

backman

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Carolina Classic sets then up w. about a 1" distance between hub and strut, which is within design tolerance.
 

eyschulman

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If they are breaking at prop look at the article by Tony Athens of Seaboard marine Ca. Under tony's tips.
 

MDI45

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Snapped at the coupling is from not being in line,any size can have that happen.If you have a long run between trans and inside box it is a little more forgiving.
Traditions,i have a little over 7 feet from inside stuffing box to tranny
 

captainlarry84

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I concur on shaft breaking at the coupler. That is from aliment. The process is call kneeing the metal. The shaft it tight in the coupler and the rest of the shaft slight bends on ever turn, so slight that you would never know. Once it fatigues or knees she snaps at the coupler.

Backman, what are you currently running DE wise? Lets get some opinions if you should have concerns:

The Following please:
HP & RPMs
Reduction gear
Propeller size
Shaft size.

Let us winter nerds give you some opinions.

PS On my JC 31 with 1.50 and a 23 X 23 I snapped 3 shafts before bumping it up to 1.75. First time I lost the wheel, second two times I caught it. Three strikes and I was out. I should have done it after the second one cracked.
 

backman

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Eric Langs NB36. assume 20,000 pounds till it gets hauled and I get a real weight.


QSM 670, Max RPM of 2300, typically run at 1850 RPM. 1.5:1 IRM 310 gear, 2.25" shaft and a 26 X 26 prop with #2 cup. We think I am hair over propped and some of that cup will come out this winter.

The calculator says I have a safety factor of 5.

I intend to pull the prop this winter to tune it and address the cup, I will also dye test the shaft at that point as its cheap insurance compared to the cost of a new prop.
 

captainlarry84

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Perfect!

SAFE!!!!!!!! By a lot a 1:5 reduction gear and a 26 blade you are well with excellent specs. A 26 has a min. bore size of 1.75 & a max of 2.25. If you really want to play in safe. Remove the wheel and inspect the keyway slot. Make sure that all sharp edges at the end of the taper are spooned out as shown in the picture. Also make sure that the keyway edges are a little rounded. Fit the wheel with valve compound and mount it without the keyway. Next mark the shaft and the remount the wheel with the keyway to make sure that you come back up to the mark.

454685.jpg
 
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