Props Shafts and Stern Tubes

[David HK]

Does each boat builder manufacture its own design of shaft tube and internal exit position / dimension of same to the engine bed or are they standard? 

Thinking if I buy a shell then the engine fitting is easy  ( Ex RN Mechanician  Sweepers to Carriers to O Boats) but the stern tube, shaft and prop are problematic if there are no standard dimensions.

Will be on a standard narrow boat 60 or 70ft with a 42 - 50hp motor, maybe one of those hybrid donks. This because I image the prop is dependent on the weight / width of the boat and gearbox type ( looking at PRM 125D2 or 150D2.  Water seal rather than stuffing box.   Oh, and as a submariner I would be cavitation paranoid even though we would be running opened up on the surface all the time. ?

[Stilllearning]

Welcome to the forum.

The position of the stern tube is largely determined by the height from uxter to skeg, which is itself determined by the desired draught of the boat.

Personally, I prefer a stuffing box, but each to their own, and prop sizing is a whole science all unto itself. Others will advise on that!

ETA on gearboxes, always choose a full hydraulic box over a mechanical one, because they are better.

 

Edited May 23, 2020 by Stilllearning

[Tony Brooks]

People like Vetus and other suppliers provide their own standard stern gear and its up to the hull builder to fit it. There are too many providers to say there is a standard. However as most modern narrowboat stern posts are too short to accommodate the optimum prop diameter the shaft exit will be halfway between skeg and uxter plate (plate above the prop) so you can swing the largest prop possible. However I suspect the boat builder's tolerances are not likely to be very tight so say + or - 6mm or so.

 

Best get over your cavitation fears in you want to go on narrow canals. I think the prop is starved of water for most of the time so will tend to cavitate at times. you are also likely to get a lot of hydrophonic noise if you don't have an engine to drown it out.

[David HK]

The y axis is not really what I was looking for but more the X axis to judge lengths of the tube and calculate shaft length.

[Tony Brooks]

1 minute ago, Stilllearning said:

Welcome to the forum.

The position of the stern tube is largely determined by the height from uxter to skeg, which is itself determined by the desired draught of the boat.

Personally, I prefer a stuffing box, but each to their own, and prop sizing is a whole science all unto itself. Others will advise on that!

ETA on gearboxes, always choose a full hydraulic box over a mechanical one, because they are better.

 

Not least because they seem to stand up to the abuse canals cause far better. The hydraulics ensure it won't go instantly from ahead to full astern.

[David HK]

Thanks Tony. That about says what I was thinking and yes Vetus seems a good one stop shop for anything needed in whatever size. 

Just have to get er done in the usual disorganised way.  Did tighten the pilgrim nut on the port shaft of my minesweeper back in the day  so lots of experience -- said no one ever !

 

Good community her so looking forward to posting more novice questions.

[Tony Brooks]

Just now, David HK said:

The y axis is not really what I was looking for but more the X axis to judge lengths of the tube and calculate shaft length.

 

that will to a degree be down to the hull builder because the inboard end of the the plus the stern gear is fitted to a plate welded across the swim. It will also depend upon where the rear bulkhead is and how long the engine, gearbox & flexible coupling is. I think the stern gear makers will adapt their product to fit your boat. In other words you specify the tube length and shaft length required.

[David HK]

Noted on the gearbox . Good call.

[Tony Brooks]

4 minutes ago, David HK said:

Thanks Tony. That about says what I was thinking and yes Vetus seems a good one stop shop for anything needed in whatever size. 

Just have to get er done in the usual disorganised way.  Did tighten the pilgrim nut on the port shaft of my minesweeper back in the day  so lots of experience -- said no one ever !

 

Good community her so looking forward to posting more novice questions.

 

I would avoid Vetus like the plague unless I was a millionaire with money to burn. I think they sell a complete engine and stern gear package to builders at a very good price and the have a very peculiar idea about the value of the yellow paint on their spares.  For shafts etc. talk to Norrins of Isleworth or one of the other suppliers.

 

Not sure if the do stern gear but for canal props talk to Crowthers of Oldham.

Edited May 23, 2020 by Tony Brooks

[David HK]

Again Tony , Thanks. This looks like its going to cost me some rum in reward for the info!

[BEngo]

Most boat builders will  put the stern tube and tailshaft horizontal, half way up the sternpost, aligned centrally  athwartships, leaving the engine fitter to sort out the rest. Expect tolerances of about 15 mm.    Usually.   Or if there is an engine shaft prop to be included in the sale they will set all that up and weld the stern tube in last.  Saves aligning things.

 

  Normal practice would be to build the engine beds so the gearbox output centre should be  about the right  place and then mount the engine. Measure and buy a suitable length intermediate shaft (once often a cut down lorry propshaft,  nowadays a shaft  with made for purpose cv joints eg pythondrive) then shim the engine to acceptable line up.

 

Somewhere along the shaft there needs to be a thrust bearing.  No michell pads, usually just a deep groove ball bearing, if separate, or it may be possible to rely on the one in most gearboxes.

N

[Tony Brooks]

If you have enough length the get an Aquadrive, Pythondrive or long two element Centatlex coupling in then do so. It will ensure 20 year plus life without repacking a conventional stern gland and stop a flexibly mounted engine banging the shaft around in the shaft bearings.  It also totally eliminates the need for accurate engine to shaft alignment.

 

If your boat is to have a centre engine room then BEngo's setup is probably the way to go

Edited May 23, 2020 by Tony Brooks

[David HK]

Thrust bearing. I was wondering about that. Most install videos I see ( Live in Hong Kong, not many narrowboats to see here! ) Haven't got them so I assumed were gearbox intergarted.

[Stilllearning]

The neatest system I ever saw was on a brumtug where the small diesel was sited above the propshaft and turned round back to front. From the gearbox output there were twin belts running down to twin pulleys on the propshaft, which was very securely mounted with a large bearing on a cross member. This eliminated all vibration, and took up next to no space at all. There was an adjustable tensioner pulley on the longest run, to take up any slack in the belts as they wore. It was a lovely boat to steer, and to live on.

[ditchcrawler]

1 hour ago, David HK said:

The y axis is not really what I was looking for but more the X axis to judge lengths of the tube and calculate shaft length.

The builder will put the engine bed where you want it, if you tell them what engine you plan to use they will build it to suit. So the length of shaft will depend on how far forward the gearbox is and the type of coupling you plan to use. For tubes take a look here https://www.acornengineer.co.uk/engine-room-fittings-fuel-hoses-fuel-filters-seacock-greasers-lubricators-gland-packing-sump-pumps-exhausts-lagging-sterngear-speedwheels-d-handles-acoustic-insulation/stern-tube-prop-shafts

 

 

Edited May 23, 2020 by ditchcrawler

[Tracy D'arth]

49 minutes ago, David HK said:

Thrust bearing. I was wondering about that. Most install videos I see ( Live in Hong Kong, not many narrowboats to see here! ) Haven't got them so I assumed were gearbox intergarted.

Most gearboxes will take the thrust but if you have soft engine mounts to lessen noise and vibration the engine gets shunted fore and aft everytime you change gear. Not good for the shaft sealing or the mounts.

Better with a dedicated trust on the propshaft.. Look at Aquadrive, it includes all the thrust bearing and mounts.

Edited May 23, 2020 by Tracy D'arth