prop shaft straightness

[stringy]

Hi all.
Just purchased a new propeller shaft for my late 50's ski boat and have several issues, one I am not sure about!

The shaft is 7/8" diameter and 64" long. I just ran it between 2 "V" blocks (59" apart) with a dial indicator and it shows a .012" bend, is this ammount of bend acceptable for a brand new shaft?

Have straightened my old shaft 3 times now to .006" and it seems to bend quit easily hence the new one and keep the old as a spare!

[CDK]

A shaft that long and thin bends easily, .012" or 0.3 mm can be considered straight.

Much more important is the condition of the prop, as even a slight unbalance will cause the end of the shaft to rotate off-center and vibrate.

[stringy]

Ta CDK,

Had my prop refurbished and re-pitched not long ago, just ran my dial indicator over it's od and found one blade 3mm shorter than the other two, a visit to the prop basher is in order, as he obviously had a bad day when he did this one.

The prop is a 12"dia 14"pitch and cupped, 3mm would cause problems would'nt it?

It is unacceptable. That blade is also likely thicker or larger in some other aspect. I wouldn't give him a second chance. I wouldn't accept the shaft like that either but bear in mind that the heat of your hand touching it can move it more than that - it's not very much, but is "perfect" too much to ask for?

[stringy]

Just spoke to the guys I purchased the shaft from and they informed me that the shaft should have a max runout over it's 59" length of .003 to .004, mine is running at .016". I'm sending it back!

Also spoke with the supplier of SS bar in OZ and they informed me that 7/8" was only available in 304 or 316l grades not 431 like I thaught I was being supplied!

So, now I am considering boring out my skeg, replace log gland and bore out drive flange to accomodate a 1" shaft, prop is already 1" with a sleve! There are 2 grades of SS, 431 and 630, both recommended for prop shafts. Which is the best grade material considering corrosion is not an issue?

[Carteret]

Don't forget to lap the propeller taper to the shaft. I have seen several shafts break at the taper because fitting (lapping was never done).
http://www.propellerpages.com/?c=tech_info&f=fitting

[stringy]

Just read the link supplied and have never heard of lapping a propeller to a shaft! Will do this when I get a new shaft sorted. Thank's. About to go home and re-fit my old shaft and prop for the weekend's skiing.

[gonzo]

Lapping is the correct way to fit a propeller. It only takes the high spots out. You use valve lapping compound. A badly fitted propeller will vibrate and eventually cause metal fatigue in the shaft.

[stringy]

Thanks for the advice on lapping the prop onto the shaft, did this and it came up good without any work, this prop had been on shaft for the last 2 seasons.

I will machine up a new shaft myself but have one question; there are 2 grades of SS, 431 and 630, both recommended for prop shafts. Which is the best grade material considering corrosion is not an issue?

[wardd]

there are 2 ways to look at this

the out of straight condition is .008" which will give you a reading of .016" on your indicator as you rotate the shaft

if you have any out of round condition of the shaft where it rests in the v blocks this will also affect the indicator reading making it appear more or less than it actually is

an out of round condition may not be measurable with a micrometer

the best way to check shaft run out is between centers if the shaft has centers drilled in the ends

if you have a bench long enough you can clamp the centers in the v blocks and clamp the v blocks to the beanch

when using a dial indicator it is best to have it chocked up as close as possible to the vertical rod and to the base and have the probe of the indicator as level as possible and sticking out as straight as possible out of the indicator to limit what is known as cosine error as the probe travels through an arc

[stringy]

Put 3hours on the boat skiing Saturday whilst running my old shaft that had been straightend to .006", also the prop has been sorted and it rattled something terrible! Pulled engine cover and floor and ran at 400rpm, the shaft was visibly bending and moving the shaft gland at least 1/16".

Inspected under the boat this afternoon and can see where the shaft has been working away at the hull below the log 1/8" from where the shaft sits when stationary, hung off the prop nut with a "T" bar, carefull not to bend the shaft but twist and it moved in the direction of the damage!

After some investigation I found a post on this site regarding skeg to prop dimensions, I have a 7/8" shaft with a 2 1/8" gap between!

So;
Purchased a length of 1" 431 SS on Thursday, enough for 2 59" shafts.
Have another old skeg that is 12deg and the base plate is the same size, so will extend toward the prop by 1 1/2" to reduce the unsupported shaft. Also had a look at whip skegs, but will see how it goes with the bigger shaft and extended skeg.

Probably should add, that this boat was originally running about 80hp and is now making about 220hp with a huge increase in torque. The prop has gone from a 12 x 11 uncupped to a 12 x 14 cupped which it will rev to 4500rpm, my self imposed rev limit! I do understand the boats old running gear will have some issues!!!

James

[SamSam]

This might help...

http://www.aqualoy.com/shafting.asp
click on
"Shafting data brochure"
About half way down is a formula for calculating shaft sizes.

Maybe going from 80 to 220 hp is the problem.

[Dean Smith]

you do not really have a BEND as such , you have sag, because of the diametre, do you understaand the difference? It matters not whether it is mild steel, high ten or whatever, it is the size of the thing
there are many threads of that subject here, with bearing spacing data for shaft sizes
happy researching me boy

[stringy]

Thanks SamSam for the link, will check with the formulas provided tomorrow! and yes the hp increase has exaggerated the problem with the prop shaft, but this has been the only downside!

Dean, researching away, and no I don't really know what you mean by "sag" in regards to a shaft, just to make sure that the weight of the shaft between centres was not causing the bend I set up the shaft between two centres vertically, with no discernable change!

[SamSam]

Notice the safety factors in the calculations. Also in the machining of shafts section, the part about keyways and radiused bottom corners of the keyway slot.