distance between strut and prop?

JohnCranston · Post by **JohnCranston** » May 20th, '08, 08:00

What would the perfect distance from the rear of the strut and the front of the prop hub for 1 3/8 shafts?
Thanks for the help, guys.

mike ohlstein · Post by **mike ohlstein** » May 20th, '08, 09:05

1 3/8 "

JohnCranston · Post by **JohnCranston** » May 20th, '08, 09:16

Mike,
Thanks.

dougl33 · Post by **dougl33** » May 20th, '08, 10:02

Funny that you ask. This topic is currently under discussion on boatdiesel. Apparently a Luhrs owner is having trouble with constantly snapping shafts. Here are Tony Athen's comments:

Joe,

Your prop is no different that something on a lathe hanging out there well beyond the chuck and trying to machine it--Only its worse..

Some Basis:

1)---First----Too much prop overhang-- 1" max ( 3/4" is best , (with the taper right at the bearing), especially in a case like this where you have a short length prop hub as the hub only picks-up the last part of the taper..

2) shorten the keyway to where it DOES NOT go past the hub..

3) Best solution if all of these boats use the same set-up---------have the props over-bored to where they go up just past the taper ( and have a shorter key) and then use a custom spacer between the nut and the aft part of the hub..

4) Is the prop hub and taper be lapped in to assure a proper fit ? Are the keys hand fitted and have a radius on all corners? No riding on the key allowed !! Are your props being installed DRY?

Shaft work cannot always just be done by some "book of rules"... In your case, the basics of understanding props, shafts, and tapers rules are not being followed anyway.. When shaft size may be marginal ( which yours really is not), you have to employ all tricks and engineering principles needed to insure that your shaft can transmit the needed power. Just Basic Stuff..

mike ohlstein · Post by **mike ohlstein** » May 20th, '08, 12:03

Rule of thumb is that hub to prop distance should not exceed the diameter of the shaft. Been that way for a long time.

The enemy of good is better......

Harry Babb · Post by **Harry Babb** » May 20th, '08, 12:48

I agree with Mike that the distance from the strut to the prop should not exceed the diameter of the shaft.........(Maximum distance) BUT.....I am of the opinion that if the prop is to close to the strut you can interfere with the water flow thru the cutlass. NOT GOOD

The bigest thing that I have seen that causes shafts to break is not the over hang past the strut but improper seating of the prop on the taper. Most of the time improper seating is caused by the prop being what I call "Key Bound".

I have seen the spacing between the Strut and the Prop in the excess of 2 times shaft diameter and not really causing problems.......I agree with everyone that posted here but the more important is proper seating of the prop on the taper and the way that the key is machined.

A "Short Keyway" positioned close to the small end of the shaft will help prevent stresses that lead to shafts breaking.

One more thought........I read some where once (I believe it was in a Michigan Prop book) that the maximum prop to shaft diameter ratio is 15:1 for applications that are NOT considered high performance.

Harry

scot · Post by **scot** » May 20th, '08, 13:31

Carl · Post by **Carl** » May 20th, '08, 14:46

1/2 the shaft diameter to 1 shaft diameter is where I recommend. It is also where the company that fabricates the shafting stock and Cutless Bearings recommend. 1-3/8" shaft, 7/8"- 1-3/8" is all good, 1-3/8" is easiest to remember as it is the shaft diameter so I would vote for that.

Too close and you can have issues with water flow thru the cultless as Harry said, too large a distance puts undo stress on the shaft.

Some Basis:

1)---First----Too much prop overhang-- 1" max ( 3/4" is best , (with the taper right at the bearing), especially in a case like this where you have a short length prop hub as the hub only picks-up the last part of the taper..

2) shorten the keyway to where it DOES NOT go past the hub..

3) Best solution if all of these boats use the same set-up---------have the props over-bored to where they go up just past the taper ( and have a shorter key) and then use a custom spacer between the nut and the aft part of the hub..

4) Is the prop hub and taper be lapped in to assure a proper fit ? Are the keys hand fitted and have a radius on all corners? No riding on the key allowed !! Are your props being installed DRY?

Okay then...

Standard shaft, a standard bored wheel and a standard keyway with a sled type radius. Lapping the wheel in place, not a bad idea. Make certain the key is not riding, a must. Toss the rest.

15:1 is correct Harry.

Capt. DQ · Post by **Capt. DQ** » May 20th, '08, 16:42

Gentlemen,
Here is your answer:

http://bertram31.com/proj/tips/shafts.htm

R,
DQ

Bruce · Post by **Bruce** » May 20th, '08, 16:50

Dug,
You let the cat out of the bag. Those tips and articles are supposed to remain quiet since Pat doesn't charge for his memberships.

Spoiler.

Capt. DQ · Post by **Capt. DQ** » May 20th, '08, 17:13

Sorry Bruce,

Rum & Coke on me next time.

R,
Doug

JohnCranston · Post by **JohnCranston** » May 21st, '08, 20:50

Thanks for the info, everyone,

Going to shoot for 1" clearance after seating the props properly.

Mikey · Post by **Mikey** » May 24th, '08, 18:49

Damn, Was ready to actually splash next week and decided today to push the shaft as close to the trannys as possible to see what's what. 2 1/2" from prop to struts with 1 1/2" shafts. So now I have another project to complete this Memorial Day Weekend.
Keep'em cold, I'm coming.

distance between strut and prop?

distance between strut and prop?

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