View Full Version : Propeller Shaft Questions

Bob Perkins
11-30-2004, 10:28 AM
Hi Everyone,

I'm in need of purchasing advice..

It's time for me to buy a propeller shaft for my runabout.

There seems to be 2 major choices -
303 Stainless - supposed to be good for up to about 300 HP

Aquamet 22 - Super Duper shaft 300HP and up

Price: $150 for 303, $550 for the other (roughly)

Motor will be 300HP.

Any thoughs on shaft materials? (boat will be fresh water trailered). And how about a low cost source?

Thanks for your thoughts.
My out of date site is at:

My Current project is at:

[ 11-30-2004, 10:28 AM: Message edited by: Bob Perkins ]

11-30-2004, 11:58 AM
Have you priced other alloys from Aquamet, such as Aquamet 17 or 18?

11-30-2004, 01:35 PM
I'm sure mmd could comment better, but how about a traditionl bronze shaft?

Bob Perkins
11-30-2004, 05:49 PM
I think Bronze would not be strong enough for that many horses.

I'll look into the Aquamet 19 and less and compare.

There is an entire sub-culture for everything!


11-30-2004, 08:17 PM
I would think Aquamet 19 would be fine for a power boat. The main reason for using 22 is the superior corrosion resistance. This is an advantage in a sailboat with a long shaft log, where the engine isn't run much. The oxygen poor water sitting in the log can promote corrosion. In a power boat, this shouldn't be much of a problem.


[ 11-30-2004, 08:18 PM: Message edited by: kc8pql ]

11-30-2004, 08:25 PM
what diameter are you looking at?

11-30-2004, 08:30 PM
Why not Monel? I just re-used a 75 yr. old Monel shaft - perfect condition.

11-30-2004, 08:34 PM
What Ken said, particularly since it won't be in salt water

Bob Perkins
11-30-2004, 08:46 PM
It will be a 1" Shaft.

If I found a good used one - I'd consider it. Aquamet 19 may be a good compromise. I think the Monel (New) is very expensive also..

Thanks for your input everyone smile.gif

11-30-2004, 09:07 PM
If you are not already committed to your shaft size, I'd consider going up one size to 1-1/8" dia. or 1-1/4" if you are planning to use Aquamet 19.

Gary E
11-30-2004, 09:36 PM
300 HP at what RPM??
What size wheel?
Direct drive or reduction gear? what ratio?

You dont need to limit the shaft size to one that you may twist. The quality remains long after the price was forgoten.

[ 11-30-2004, 09:37 PM: Message edited by: Gary E ]

11-30-2004, 10:32 PM
Yes, chooseing a shaft involves matching HP, prop. size, gear reduction, shaft length and alloy with diameter. A good shaft supplier will do this for you.


11-30-2004, 11:10 PM
Many years ago, with a very different shaft & prop problem, I was very well served by
H & H Propeller Shop
(Formerly Haskell&Hall)
Essex Street Ext
Salem, MA 01970-5246
Phone: (978) 744-3806

[ 12-01-2004, 12:19 AM: Message edited by: JimConlin ]

12-01-2004, 12:34 AM
Bob, just looked at the picture gallery - impressive, which stain did you decide on? looks nice.

Bob Perkins
12-01-2004, 09:27 PM
Thanks All,

The Details are the boat will have a 300HP Mecruiser w/1:1 Velvet Drive 72c transmission (not Purchased Yet)

The plans called for a 1" shaft - which is what I have the strut and stuffing box for already. So I'm committed to 1". The completed shaft will be ~72" from coupling to the prop taper.

All that being said, and with additional research - the AQ22 is probably how I will go - The local guys (H&H Prop) sell it exclusively. So not a lot of choice. I'm going to work on them with price a bit too see if it gets better if I buy the prop too..
Thanks for helping think this out.

BRobinson: I used Behlen's Solarlux stain. It is compatable with epoxy (unlike Oil based stains) and is UV resistent. I mixed my own color - but had issues during application so had to go over it a 2nd time (long story) so it came out a bit darker than I would have wished. In the direct sunlight it will be fine smile.gif

Take Care,

12-01-2004, 11:07 PM
Going with the Aquamet 22 is the right call if you are committed to 1" diameter. The Aquamet 18 would have been below reccommended size at 1" dia, due to lesser torque strength of the alloy.

For such a high-rpm shaft, I would reccommend that the keyways be spooned, rather than straight-cut, to avoid stress risers in the keyway.

Edit for spelling - oops!

[ 12-01-2004, 11:09 PM: Message edited by: mmd ]

Gary E
12-02-2004, 07:28 PM
I never heard of a spooned keyway..
Whutz zat?

There's keyways cut with an end mill or those cut with a staggered tooth cutter, but whut cutz a spooned keyway? a spoon?

12-02-2004, 11:56 PM
Spooning is a technique of modifying the ends of the shaft keyway to reduce stress risers in the key and keyway walls.

Very high speed propellers experience significant impact loads from cavitation, ventilation, and other transient loads. The stress of these loadings concentrate on any sharp transitional points of the assembly, commonly at the ends of the key. These stresses can be high enough to gall the key and/or keyway, weakening it and possibly leading to jamming or of one of the components' failure. One way to reduce the risk of this is to reduce the acuteness of the transition angle from shaft surface to keyway wall, thereby reducing the magnitude of the stress riser.

A successful method of reducing the transition angle of the keyway at the key ends is spooning. The keyway is cut in the shaft by conventional means, as described by Gary. The end of the key is located in the end of the keyway and a bull-nose end mill twice the diameter of the keyway width is plunged to the depth of the keyway, centred on the point of the end of the key to be fitted. It is then drawn up and away from the key along the axis of the keyway at about a 15-degree angle. This leaves a low-angle cavity leading down into the keyway that looks like the bowl of a spoon, hence the name. This is done at both ends of the keyway.


Gary E
12-05-2004, 12:01 PM
Wow, something new. In over 45 yrs in the field, I can say this is the first time I ever heard of this.

Does not the milling of a keyway using a staggered tooth cutter that provides the gradual slope from the bottom of the keyway out to the shaft's diameter provide the same function?

To make a good joint the key is fit to the shaft and what is mounted on the shaft on all 4 sides, in other words, no slop to allow movement in reversing situations.

12-05-2004, 01:27 PM
There's always something new to learn, Gary. I hope that I never stop finding them.

"Does not the milling of a keyway using a staggered tooth cutter that provides the gradual slope from the bottom of the keyway out to the shaft's diameter provide the same function?"

Apparently not. Without the relief of the spooning at the end of the key, the key is held firmly between the keyway walls and the plane between prop hub and shaft surfaces acts as a shear plane, causing stress risers at the intersection of key end plane and prop/shaft surface planes. With the keyway on the shaft spooned, such stress risers are lessened by dispersal over more riser points.

"To make a good joint the key is fit to the shaft and what is mounted on the shaft on all 4 sides, in other words, no slop to allow movement in reversing situations."

I don't mean to be pedantic, and maybe I am misinterpreting your statement, but the key (and keyway) should bear none of the axial load of the propeller, but act only to resist the torque of the prop. The shaft taper and end-nuts should bear the longitudinal loads. I agree that the fit of the key's top, bottom, and two sides should be a transitional locational fit (machinist's term) in the shaft and prop hub.

As I understand it, the method of "spooning" a keyway was developed/discovered by race boat (Cigarettes, hydros, drag boats, etc.) mechanics who found that, even with high-precision machining of standard keyways, the ends of the keys were galling and causing the key to distort (leading to a prop jammed on the shaft) and eventually shearing the key (bye, bye, shaft & prop). Spooning apparently reduces or eliminates this problem. I'm sorry that my grasp of esoteric mechanical engineering isn't sufficient to explain this in mathematical terms so we could get past anecdotal descriptions and into theoretical ones, but sadly that is the present case. Maybe one of our MechEng forumites could step in and give us some guidance.

Gary E
12-05-2004, 02:04 PM
I guess I confused the meaning of 4 way fit. I mean only radial loads, no axial load or fit at all. Agree that the taper absorbs all of the axial load.

I have no experience with some of those boats, but was involved with a 265 cu in hydro back in the late 50's early 60's, we used standard keyways with no problem. I spose it's about that time I left the race boat behind as the horsepower race started and my attention turned to cars.

The reality of the situation here with the shaft that Bob Perkins wants to use, in my opinion should not be designed on the ragged edge of material limits or he will find himself disabled if the prop hits so much as a broom handle. Better to have a very high margin of safety. Should that shaft fail, the key shear, the prop fall off.. what ever can go wrong might go wrong and then the engine over rev's and blows up or the shaft being a little to skinny bends a little and severe vibration sets in and well, the fit hits the shan.

12-05-2004, 02:22 PM
Gary, I suspect that the 265 cu.in. motor that you raced with was not much more powerful than the 300 hp motor that Bob is considering (forgive me if I have underestimated your hot-rodding prowess :D ). My concern - albeit minor - is the prop rpms. A prop that turns at 4600 rpm is able to create some pretty intense rotational loads on the shaft.

As for the shaft being a bit skinny, Bob has indicated to me via private messaging that he is looking into moving up to a 1-1/4" diameter Aquamet 22 shaft, which sets my mind at ease quite a bit. With a safety factor of 5 applied, a 1" shaft was under Aquamet reccommended diameter by a sixteenth of an inch; going for a 1-1/4" shaft allows a much more comfortable safety margin. Solves a problem with bearing spacing, too.

Gary E
12-05-2004, 02:27 PM
Good, he will be much better with the 1 1/4 Dia..