Model Boat Mayhem

Please login or register.

Login with username, password and session length.
Pages: [1]   Go Down

Author Topic: P bracket purpose  (Read 551 times)

KitS

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 529
  • Getting back into the hobby after years adrift...
  • Location: Lydney, Glos. UK
P bracket purpose
« on: August 15, 2021, 02:07:31 pm »

I've always understood that the purpose of the P Bracket, or whatever they're called, was to transmit the thrust of the prop through to the hull structure, but a recent attempted refurbishment of a Nautix RAF Hants & Dorset launch has caused me to question this.


Both shafts of the model were quite stiff, and it wasn't the fastest boat on the lake by any means as well as sounding 'strained', so I stripped it all down. BOTH shafts were bent, and neither lined up with the holes in the P Brackets, one shaft by over half its diameter! No wonder they were bent, pretty poor assembly to my mind.


Having sorted this out by sawing off the tubes of the P Brackets and filing and re-soldering them in position with the straightened shafts locating them, it all ran a lot sweeter, without the props in place......


With the props screwed on, and located up against the P Brackets, as I thought they should be, the noise when running was HORRENDOUS, and neither motor-shaft-prop assembly seemed to run smoothly. Investigation showed that the props had the manufacturer's name moulded into the rear face of the moulding, and when this rubbed against the P Bracket the noise and vibration were generated, obviously not a good thing.


I can only conclude that it was designed so the thrust of the prop was transmitted up the shaft, through the flexible coupling, along the motor's shaft and sent to the hull via the front bearing of the motor!  :((


Hardly a good idea I'd have thought.


I intend to file/cut/sand off the moulded script and fit a thrust washer so the props put their power through to the P Bracket, but I'd be pleased if anyone has any better ideas.


Logged
Regards
Kit

chas

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 509
  • now in los montesinos Spain.
Re: P bracket purpose
« Reply #1 on: August 15, 2021, 03:20:36 pm »

Two knee jerk ideas to consider. Firstly to check the balance of the props, some can be a long way off. Second is to put a collet on each shaft, up against the tube, the type that are held in place with a grub screw. They would ensure that the thrust is imparted to the model, not the motor. I'm sure you'll get some more, probably better ideas soon.

Logged

Perkasaman2

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 933
  • Model Boat Mayhem is Great!
  • Location: North East
Re: P bracket purpose
« Reply #2 on: August 15, 2021, 03:50:19 pm »

Fitting a p bracket avoids an unsupported propshaft and reduces vibration providing they are correctly aligned  with the axis of the shaft. there is usually a brass washer between the prop/locknut and bracket to help smooth running. A certain amount of endfloat is required at each end  of the shaft to avoid binding but not too much as this protects the motor bearings from premature wear. I hope this explanation is useful. The shaft is also lubricated by a grease/oil mix. The exposed shaft passing through the p bracket is lubed by water.
Logged

KitS

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 529
  • Getting back into the hobby after years adrift...
  • Location: Lydney, Glos. UK
Re: P bracket purpose
« Reply #3 on: August 15, 2021, 04:05:20 pm »

They are correctly aligned now.  :-)


There isn't any sort of washer between the prop and the bracket at the moment, so I'll install one., after I've filed off the lettering.......


I'm not sure there's too mu8ch scope for balancing the props, they're all moulded in one piece, and from some sort of nylon material too, and they have grub screws which are meant to to tighten them onto the shafts, but the screw threads seem to be stripped.  :((
Logged
Regards
Kit

Baldrick

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 1,152
  • Model Boat Mayhem is Great!
  • Location: Nether Effingham (Perfideous Albion)
Re: P bracket purpose
« Reply #4 on: August 15, 2021, 04:28:02 pm »




 My concern would be the shape and stability of the P brackets.  From the photograph they do not appear to be so much a V shape which is dynamically stable, but rather a truncated parallelogram which is not. That and they appear to be quite thin metal means they could be vibrating harmonically.
 Could I suggest putting the boat in it's cradle and running the motors at slow speed but at a level which vibrates and then (carefully) pinch the support brackets with your fingers and see what difference that makes to the sound level .
Logged
The early bird catches the worm.
But it is the second mouse that gets the cheese.

John W E

  • I see no ships !!
  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 8,614
  • Location: South shields
Re: P bracket purpose
« Reply #5 on: August 15, 2021, 05:45:17 pm »

Hi there


one way to cure this is to fit a collet with a grub screw in it.  This is so it can be secured to the shaft - and - this will run against the prop tube and take the thrust from the prop.   Therefore you can leave a little space between the propeller and the 'P' bracket.  As also stated it is advisable to fit a little washer between the propeller and the bracket.


john



Logged
Knowledge begins with respect
But fools hate wisdom and discipline

Perkasaman2

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 933
  • Model Boat Mayhem is Great!
  • Location: North East
Re: P bracket purpose
« Reply #6 on: August 15, 2021, 05:46:47 pm »

The props are plastic so I would'nt worry about balance since it is'nt an issue.
Logged

KitS

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 529
  • Getting back into the hobby after years adrift...
  • Location: Lydney, Glos. UK
Re: P bracket purpose
« Reply #7 on: August 15, 2021, 05:50:49 pm »


one way to cure this is to fit a collet with a grub screw in it.  This is so it can be secured to the shaft - and - this will run against the prop tube and take the thrust from the prop.   Therefore you can leave a little space between the propeller and the 'P' bracket.  As also stated it is advisable to fit a little washer between the propeller and the bracket.



Now that idea I like, oh yes!  :-)


It puts the thrust directly into the hull and leaves the prop clear of the bracket.


I'll give that am go, thanks so much.  :-))
Logged
Regards
Kit

Colin Bishop

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 11,360
  • Location: SW Surrey, UK
Re: P bracket purpose
« Reply #8 on: August 15, 2021, 06:17:46 pm »

Just to illustrate what John is saying. Another advantage of the collet against the tube is it helps seal it against water ingress. Much better to take the thrust on the tube whch is securely embedded in the hull rather than on a delicate P bracket.

Colin


Logged

Circlip

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 3,887
  • Location: North of Watford, South of Hadrians wall
Re: P bracket purpose
« Reply #9 on: August 16, 2021, 10:37:36 am »

In the dim and distant, unshrouded prop shafts - on toy boats, were the exception rather than the rule. "P" brackets at that time stopped the prop tube from wafting about purely and simply. The tube served two purposes, on the water end, it served as a thrust face for the prop and filled with grease, helped to stop the ingress of water into the engine bay. BUT, as a rule rather than the exception, coupling between power source and shaft allowed a certain amount of "Float". Sadly, laziness and Huco have screwed this up, literally. Prop shaft thrust isolation was discussed at length on this site a few years ago by a member over the pond.


  Regards  Ian.
Logged
You might not like what I say, but that doesn't mean I'm wrong.
 
What I said is not what you  think you heard.

Geoff

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 938
Re: P bracket purpose
« Reply #10 on: January 14, 2022, 09:33:59 am »

The P or A bracket as its also known was never intended to take any thrust at all as its just too flexible. Its purpose is both scale and to stop the shafts whipping at the prop end. If you use the P/A bracket as a thrust bearing it will distort which is probably why the shaft was bent. The easiest way to transfer the thrust to the ship has always been to use a collet against the actual tube (as has already been suggested). In my experience its important to make sure the collet does not exceed the diameter of the shaft as otherwise it creates drag and can create water pressure up the prop tube.


On my large models I use a tapered collet and two similar sized washers to act as a thrust bearing against the prop shaft. I use double Huco couplings and find that if you leave a little play in the connector then it will slide up and down the splines if put under any great pressure. Ideally you want about 1mm clearance at the P/A bracket if thrusting forwards so no pressure on the P/A bracket and the same internally if the thrust is pulling aft so there is room. The prop shaft, both ends, then transmit the power to the hull depending whether going forwards or astern.


Real ships have internal thrust bearings to transmit the loads to the hull and until they were developed properly it constrained the amount of power that could be transmitted. They actually look like a rod with multiple washers and spacers running inside a female structure of the same shape in an oil bath. Essentially in a very small volume you create a vast surface area which can accept the loadings. They are a precision piece of technology.


Regards


Geoff





Logged
Pages: [1]   Go Up