Prop Knocker?

V

Viking76

Member
Joined
Aug 18, 2006
RO Number
22881
Messages
92
Can using a prop knocker to remove my propeller damage the thrust bearings in my transmission? To remove a propeller with a this device, you screw it (prop knocker) onto the end of the shaft (after removing the prop nuts) and whack at it with a hammer until the prop pops loose. (have brand new trannys, don't want to damage them)
Any thoughts / opinions of this?
 
What you are decribing sounds more like a shaft puller, the porp pullers I've used are more like big gear pullers. See

http://www.henrysmith.com/toolinginboard.htm

The shaft pull is near the end or bottom. If you use a prop puller leave the nut on loose but on, I've seen props fly across the yard if you don't.
 
quote:
Originally posted by JimPend

What you are decribing sounds more like a shaft puller, the porp pullers I've used are more like big gear pullers. See

http://www.henrysmith.com/toolinginboard.htm

Click to expand...


That's what I thought. I googled prop knocker and it is something I have never seen or heard of before.

http://i161.photobucket.com/albums/t213/Herculesrhapsody/DSCF0835.jpg

http://i161.photobucket.com/albums/t213/Herculesrhapsody/DSCF0834.jpg

Here is a post from the folks at the Hatt owners site.

i got a little curious about this problem so i ran a simple analysis to see what force would be transmitted to the transmission end of the shaft when you hit the prop end with a hammer. the analysis was done as a non-linear problem with a time dependent solution. the force from the hammer was delivered to the shaft in the time shown on the time curve graph. the force goes from zero to 800lbf and then returns to zero in 0.05 seconds, the value of 800 lbf is just a guess. the second graph shows the reaction force on the transmission end. it is significantly lower than the input force, and shows more than one peak. the third picture shows the problem setup and resultant stress. it may not be that easy to read at this reduced resolution, but the stress on the shaft is insignificant. the shaft was held in place with two cutlass bearings that allowed the shaft to translate and rotate. the bearings were rubber but with only minimal friction. the shaft was fixed at the transmission end. the shaft was 316ss, 2 inch diameter, 15 feet long, and weighs 162 lbs. the prop was just a round bronze disk at 20 lbs. much too light but an ok starting point. if anyone knows the weight of a representative prop let me know and i will try to dial in the solution a little better.
 
The Algonac prop pullers work so well, I'd hesitate to shock load the thrust bearing, just to remove the prop. Just the thrust bearing in my lowly 72C cost ~ $500, no labor.
 
quote:
Originally posted by KiDa

quote:
Originally posted by JimPend

What you are decribing sounds more like a shaft puller, the porp pullers I've used are more like big gear pullers. See

http://www.henrysmith.com/toolinginboard.htm

Click to expand...


That's what I thought. I googled prop knocker and it is something I have never seen or heard of before.

http://i161.photobucket.com/albums/t213/Herculesrhapsody/DSCF0835.jpg

http://i161.photobucket.com/albums/t213/Herculesrhapsody/DSCF0834.jpg

Here is a post from the folks at the Hatt owners site.

i got a little curious about this problem so i ran a simple analysis to see what force would be transmitted to the transmission end of the shaft when you hit the prop end with a hammer. the analysis was done as a non-linear problem with a time dependent solution. the force from the hammer was delivered to the shaft in the time shown on the time curve graph. the force goes from zero to 800lbf and then returns to zero in 0.05 seconds, the value of 800 lbf is just a guess. the second graph shows the reaction force on the transmission end. it is significantly lower than the input force, and shows more than one peak. the third picture shows the problem setup and resultant stress. it may not be that easy to read at this reduced resolution, but the stress on the shaft is insignificant. the shaft was held in place with two cutlass bearings that allowed the shaft to translate and rotate. the bearings were rubber but with only minimal friction. the shaft was fixed at the transmission end. the shaft was 316ss, 2 inch diameter, 15 feet long, and weighs 162 lbs. the prop was just a round bronze disk at 20 lbs. much too light but an ok starting point. if anyone knows the weight of a representative prop let me know and i will try to dial in the solution a little better.

Click to expand...


That is my knocker and boat. The knocker works great and I have never heard of any problems. Usually pops the wheel in one or two hits.
I have 2 1/2' shafts and 28x26" wheels. The puller to fit is cumbersome and usually it still needs a whack to pop it .

I am sure the force of a prop hitting bottom on a heavy moving boat is much more stress than a couple whacks with a hammer. YMMV
 
I ve seen reputable yards whack the props off... I wouldn't worry about the bearings. That said the safest way is to use an hydraulic pulled since it pulls on the shaft.
 
I second Diver Dave's endorsement of the very effective Algonic Prop Puller that I purchased long ago at George VP's recommendation. No banging!
 
I would suggest disconnecting the shaft from the transmission first or hit the shaft with a brass hammer.

A tapered bearing or a ball bearing only needs a small imperfection from dropping or hitting it to cause wear. Over time it will destroy the whole bearing race.
 
I have only used gear-puller type prop pullers, and, as stated earlier, they also can require a few blows. But NEVER use a hammer directly. Use a short length of 2 X 4 and bang on that. The wood won't hurt anything...
 
Buck Algonquin doesn't make the prop puller any more ( I called them). Tried one similar to the Dymex however, there is to much blade skew in my DQX props. Prop shop loaned me a prop knocker. Gave it 3 moderate whacks with a heavy copper hammer. Prop did not loosen. Gave up due to concerns about those tranny thrust bearing. One fairly medium hit (copper hammer again) in the aft direction on the front edge of the prop hub and the prop came right off. Thin coat of water proof grease on the shaft taper when the props were put on was most likely beneficial. The props are now in the prop shop to increase the pitch and bring the WOT down to the MFG recommended number. Tweeking the system after recent repowering.
 
Opinions vary, but I would never hit the prop itself unless I was to the point of being willing to replace it. I would have used a steel hammer on the knocker, I want the shock value of the hit and don't want to mute it at all.

I'm not worried about those prop thrust bearings anyway. Were not talking about trying to drive the shaft into the bow of the boat. Were talking about "ringing" strikes on an object that itself has a lot of mass. Few prop pullers, even the big ones work with just pull itself. It usually takes a ringing blow on the end of the shaft, with the puller at full load to get the prop to let go (loosen but leave the nut on, have personal experience as to why). I didnt' come up with this myself, this is a pretty tried and true approach used my most yards. If they were having to replace thrust bearings every time they pulled a prop, we would know about it. Technique matters though and I'm not sure just saying "hit the end with a hammer" is enough to carry the meaning.

I disagree with Seabob4. Using wood to "soften" the blow is not a common method. At least, not one I've ever seen a yard use. Maybe it works, I'm just stating that my opinions don't agree. In fact, the wood actually takes the blow and makes if more of a "driving" force instead of a ringing one that will vibrate and cause the metal pores to vibrate and release. I still say you want a steel hammer and a good short, but stout, "ringing" blow. You need to be dead on to make that happen and any sideways force will be counter proctive.

Just my opinion, but one where I'm putting my money where my mouth is. I would have no reservations performing on my own boat, at my own risk, again because I'm just doing what professional yards that I came to trust over many years have learned to do. This would include one of the guys who is also one of the better transmission guys on this coast.
 
Seems plausible. To a non-engineer (that'd be me) the prop knocker seems to be another form of a tuning fork or an opera singer hitting a certain note and breaking a crystal glass. Vibrate an object at a certain frequency and it works with or against itself until the weakest bond breaks.
 
it's all about the ring breaking the bond and the prop and shaft being tapered the breaks the bond and the stress coming undone from the prop being pushed up on the shaft done it many Navy utility -boats
 
quote:
Originally posted by seabob4

I have only used gear-puller type prop pullers, and, as stated earlier, they also can require a few blows. But NEVER use a hammer directly. Use a short length of 2 X 4 and bang on that. The wood won't hurt anything...

Click to expand...


When you use a gear puller type puller, you are striking the head of the gear pullers shaft not the prop shaft. So there is no point in using a wood block to protect anything.

Besides, the wood block would absorb to much vibration anyway. And you want the vibration effect to help pop the prop off the shaft.
 
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