Model Boat Mayhem

Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: Davenotdone on January 19, 2013, 07:54:03 AM

Title: Prop shaft
Post by: Davenotdone on January 19, 2013, 07:54:03 AM
Hello everyone. Hope you could help me with a prop shaft problem. I am fitting a new prop shaft to a model tug. Got one made up by a well known model boat factory and all seemed well, it came with the stuffing tube, shaft and oiler as requested. It spun freely and i duly fitted it ( resined it into the hull ) and when dry i tried to align the motor to it. I then found out that at the oiler end the shaft is a nice fit in the bearing bur at the prop end you can ever so slightly move the shaft in a up and down motion as if the bearing at that end is to big ( or the shaft too small ) Should you have any play at all in the bearings or should the fit be ' snug ' ? I have rang up the factory in question and they say to send the hull back for them to check, i am mentioning no names to give them the benefit of the doubt so what should i do, run it as is and hope for the best or send it back? The model is only 24inch in length ( no deck or wheel house fitted as yet ) so hopefully the postage will not be high. The end of the shaft at the motor end runs below the water line so obviously i want it to be as watertight as possible. I would like you thoughts on this matter as i have learned a great deal from all the the Forum and it's witty members. Regards, Dave.
Title: Re: Prop shaft
Post by: chipchase on January 19, 2013, 09:45:15 AM
 You are right Dave the inner shaft should be a snug fit you should not be able to move the prop up and down. If you have movement on the shaft this will make the prop oscillate.  You do not say if the prop is to run in a kort nozzle, if this is the case you will catch the inside of the nozzle with your prop and cause damage to either the prop or the nozzle.
Hope this helps.  :-))
Title: Re: Prop shaft
Post by: Davenotdone on January 19, 2013, 05:46:41 PM
Thanks for the reply, yes it does run in a Kort nozzle and the play is only just noticeable so it would not hit it as is but obviously it won't get better over time, i think i will play safe and just send the whole thing back, regards, Dave.
Title: Re: Prop shaft
Post by: boatmadman on January 19, 2013, 11:26:31 PM
I think you have to bear in mind that commercially produced shafts and tubes are machined in a mass production environment and to a cost.

Consequently, its not surprising to find a bought shaft with a little play in it as its unlikely the bushes were drilled and reamed to size due to the extra time required to ream properly.

You may be able to get around the problem by putting a little waterproof grease in the tube which will stop water tracking in.

Alternatively, if you can remove the bush from the tube you could get a new and more accurate replacement machined - if you know someone with a lathe.

Title: Re: Prop shaft
Post by: john s 2 on January 19, 2013, 11:28:03 PM
Can i suggest that you take a careful look at your propshaft? Most are made by pressing in the bushes at each end. More expensive shafts use oil seals bearings etc. So i see no reason why the end bush can not be removed by gripping with a mole wrench and twisting out. If this can be done then all thats needed is a better made bush. This will save the cost of returning the hull. Also you ca check the replacement bush. Ive done this myself . John.
Title: Re: Prop shaft
Post by: Big Ada on January 20, 2013, 02:52:31 PM
Could it be that a threaded portion of the shaft is in the bearing thus making it sloppy?.
Just a thought.
Title: Re: Prop shaft
Post by: Netleyned on January 20, 2013, 03:07:10 PM
If you can get the bush out as John suggests
then just send the bush back (lesspostage :-))

Title: Re: Prop shaft
Post by: Neil on January 20, 2013, 11:38:49 PM
pack it with some grease and oil and you'll never know the difference......... and really not worth worrying about unless you are going to run the thing at 20000 rpm...............I've never had any problems with leaks once packed with grease and I don't work to the fine tolerances that you are worrying about.