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Author Topic: Reversing  (Read 3742 times)

Bob K

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Reversing
« on: June 09, 2016, 11:15:12 AM »

A silly question, but with prop shafts no matter how well/often oiled/greased there seems to be a slightly greater tendency for water coming aboard when full reverse is applied than going full forward.  This effect seems greater when the motor end of the shaft is below waterline.  Only a tiny amount, but seems to be regular.

Why ?

First thought is perhaps with water being forced forwards against the prop shaft end pressure to overcome oil etc is greater.  However, the central boss of a brass prop normally well shields the shaft end from direct pressure.  Next is perhaps the finish on the shaft may have a slight 'direction' as if an Archimedes Helix.

Just strange, that's all.  Obviously, only use full reverse in an emergency, and keep the shafts regularly lubed.
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Martin [Admin]

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Re: Reversing
« Reply #1 on: June 09, 2016, 11:57:51 AM »


HS93 (RIP) always told me this was because;
 model boat shaft mostly don't have proper glands / seals,
 shaft wear more in whatever your 'forward' direction is and
 ..... not always the "best" design / quality!

Peter made me a replacement shaft for my Springer... beautiful!


http://www.modelboatmayhem.co.uk/My_models/Springer/index.htm

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nivapilot

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Re: Reversing
« Reply #2 on: June 09, 2016, 12:01:12 PM »

Only reason as far as I can see is that, when you go forwards, the prop is pushing and therefore the gap between shaft and tube is closed.....when reversing the prop is pulled away from the tube and opens up the gap?????

Just my theory anyway.
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derekwarner

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Re: Reversing
« Reply #3 on: June 09, 2016, 12:12:03 PM »

No Martin. {:-{.......Bob..you are on the correct track....all to be involved with hydrodynamic pressures  %)

1. With the vessel engine shaft running FWD, this thrust creates a lower pressure vortex around the propeller shaft/hull bearing
2. With the vessel engine shaft running ASTERN, this thrust creates a higher pressure vortex around the propeller shaft/hull bearing

Remember ..all things being equal  ok2......the pressure differential required when running ASTERN to allow water to migrate up the shaft is very small.... O0......

Derek
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Martin [Admin]

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Re: Reversing
« Reply #4 on: June 09, 2016, 12:24:00 PM »


You can't prove anything with facts Derek!
   :P
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Netleyned

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Re: Reversing
« Reply #5 on: June 09, 2016, 03:04:58 PM »

No problem if you use an esc for ahead only O0
Or build paddlers %)

Ned
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Bob K

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Re: Reversing
« Reply #6 on: June 09, 2016, 03:32:27 PM »

Thanks for your responses.  It is no big deal, and as said provided you have quality unworn bearings and regular lubing the effect is fairly minimal, but there all the same.  Sometimes on a crowded lake an emergency reverse is necessary to avoid someone crossing your bows.  Prop shaft bearings are designed to be reasonably watertight but are not perfect.
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Sub driver

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Re: Reversing
« Reply #7 on: June 09, 2016, 06:09:35 PM »

Hi.
Just put submarine propshats in...they dont leak either way and the are totally submerged all the time front and rear and are mostly always fitted in a level plane.
You could always adapt the end of the exposed shaft
 by fitting a valve stem oil seal of suitable dimensions they are designed to keep oil out of an engine cylender head at gt pressure they have no probs with water.
I have used some from a 600 cc m/cycle engine when i made my own propshafts some 15 yr ago for a submarine and they still work without any leakage.
Regards Sub.
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CGAux26

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Re: Reversing
« Reply #8 on: June 09, 2016, 09:27:01 PM »

There may be some validity in what Nivapilot said.  If the axial clearance between the prop shaft and stuffing tube is too great, water can more easily get past the prop end thrust washer going astern. 


I set my boats up with a bare minimum axial clearance, aiming for 0.010" (ten thousandths), or just enough to feel the bump when the shaft is moved full forward and then full aft by hand.  But not tight enough to create drag.  And I get very little leakage under all running conditions.


Lubriplate grease fills my stuffing tubes.
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inertia

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Re: Reversing
« Reply #9 on: June 09, 2016, 11:00:51 PM »

Here's what I use........................life's too short anyway.
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Bob K

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Re: Reversing
« Reply #10 on: June 09, 2016, 11:20:21 PM »

Here's what I use........................life's too short anyway.

Ha Ha !   Put nicely in perspective Dave.  I was only pointing out something that most of us have experienced, a very very minor ingress when going full astern.  Bearings have to have a minimum clearance and a few thousand RPM under water has a slight effect over the several hours of a sailing session.

Having a few tissues handy, or a small syringe, illustrates the scale of the issue.  Nominal operating conditions.
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CGAux26

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Re: Reversing
« Reply #11 on: June 09, 2016, 11:22:59 PM »

Further to my post above:


Pics of a shaft/axial clearance setup on one of my boats.  Note the silver collar and nylon washer on the motor end and the prop almost against the nylon washer on the other end.  No visible clearance, but I can "bump" the shaft 10 thousandths in both directions.  The same clearance set in a Kingsbury thrust bearing in a major compressor.
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CGAux26

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Re: Reversing
« Reply #12 on: June 09, 2016, 11:26:58 PM »

OK OK I am guilty of making a mountain of a molehill.  But I am and engineer, accustomed to the notion that the devil is in the details.


That said I go to the pond with the paper towels and turkey baster/bilge pump, too   O0


And this reminds all boaters that the first job of a boat is to keep the H20 out.  If we don't do that, bye bye boat.
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RAAArtyGunner

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Re: Reversing
« Reply #13 on: June 11, 2016, 01:10:13 AM »


 "keep the H20 out"

What is H20???

 Google wasn't any help   {:-{  {:-{  %)  %)  %)
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CGAux26

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Re: Reversing
« Reply #14 on: June 11, 2016, 01:15:44 AM »

That's the chemical used in the processing of beer.
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BrianB6

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Re: Reversing
« Reply #15 on: June 11, 2016, 02:46:14 AM »

"keep the H20 out"

What is H20???

 Google wasn't any help   {:-{ {:-{ %) %) %)

Take a micro brewery tour of Brisbane.
http://foodi.com.au/craft-beer-walking-tour-brisbane/
Ask a few of the operators what they use.  :P
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derekwarner

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Re: Reversing
« Reply #16 on: June 11, 2016, 06:20:30 AM »

two hydrogen to one oxygen = water H2O           O0.....

Although that's not the composition of the Brisbane River.......similar in color to the Murray River  {-).....or the Thames sewage treatment plant water.... :embarrassed:.........Derek
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BrianB6

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Re: Reversing
« Reply #17 on: June 11, 2016, 06:47:23 AM »

If you think that's Brisbane river is bad you should see the Yarra.  <:(
I could send my son in law to explain it all since he is a water purification chemist and advises on how to treat water for several breweries (he prefers red wine)  O0
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Bob K

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Re: Reversing
« Reply #18 on: June 11, 2016, 08:40:42 AM »

You guys can be very cruel.  Funny, but cruel.  A simple question.
I particularly liked "chemical used in the processing of beer".
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RAAArtyGunner

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Re: Reversing
« Reply #19 on: June 11, 2016, 09:20:14 AM »

Thanks Bob, they are cruel because of the footy. O0 O0 O0

I know about water, you know floods, rain, et all, but a "H" twenty had me stumped.

The Brisbane river is the colour it is because when "XXXX" brewery has a bad batch it is dumped into the drains, sad but true. O0 O0 O0. The brewery is close to the River.   :-)  :-)  :-)

 Eventually it ends up in the Brisbane river. Am told it's the reason, why you don't swim in the Brisbane river, most swimmers drown because they drink it.  :o :o :o %) %) %)

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Bob K

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Re: Reversing
« Reply #20 on: June 11, 2016, 10:17:30 AM »


I know about water, you know floods, rain, et all, but a "H" twenty had me stumped.


Note:  There is subscript tag available on this text form generator, which if used would have avoided any confusion with the new H20  Hi-tec 2.4GHz transmitter.
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Tugtower

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Re: Reversing
« Reply #21 on: June 11, 2016, 07:11:36 PM »

I skipped over the majority of the conversation going on here seeing it had no relevance to the issue, so it might have been mentioned......


Have you checked the bushings which house/hold the shaft?, they have a tendency to wear and poor ( cheap ) shafts have a very sloppy fit.


Bronze or plastic bushings, plastic is worse for it, regardless of the thrust washer in place if the bushings are to put it bluntly  ' >>:-( >>:-( >>:-(   >>:-( ' then your going to get water up the shaft regardless.


Suggestion should this be the case, you may be able to pop out the old bushings and replace, or have new ones machined.


Put a new shaft in, suggest looking at the JP slimline stainless, I've yet to have issues with those.


Or if none of the above work you could pack the shaft with the thick black marine grease ( which i use )  which will stop water ingress regardless of any slop in the bushing(s)

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