More Stern Tube problems

Bobobolinsky

Bobobolinsky

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I have a packed fore and packed aft seal, on the stern tube. Between the engine and prop I have a very long prop shaft 7/8", with a plummer block, now changed from white metal to caged roller between gearbox and fore seal. The prop is a 15 x 9.

Prop calc suggests 16.25 x 10 as a suitable prop.

I have changed the engine from a 15Hp Lister ST2W to a Ford 1800XLD, with Hurth Gearbox.

The stern seal was damaged BER and I am having a new one cast.
The engineering company suggest that the prop is too large and the prop could spin off, even though the prop never fell off since the boat was built.

They suggest that I replace the stern tube with a 1" one, so I can fit their dripless and water cooled system.

I have not ordered up the aft seal yet, so I have the opportunity to change from white metal aft seal with white metal/brass bearing, to cutless bearing water cooled. Recommendations and comments please.
 
Shaft is too small for your new engine. Minimum of 1"/25mm required. Virtually nothing uses a shaft less than 1" these days.
 
Other boats in the same class, were fitted with 4108's, BMC 1500/1800's and Pug/Ford 2.5's and same stern tube.

Don't really want to replace the stern tube and shaft, as that is a major cost and undertaking
 
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You can replace the white metal directly with a composite Maritex bearing of the same size, these can be lubricated with water, oil or grease. They can be finished to suit whatever size shaft and stern tube system you have.

This is a solid hard composite bearing, not rubber or rubber in a composite carrier.

Boats like these use Maritex and get very long bearing life.
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3372brochure1.jpg
 
I replaced a white metal stern bearing with one of Neil Y's maritex bearings this winter. It cost about the same as a cutless bearing would have cost but I did not have to fork out for a new carrier because the maritex bearing can be supplied with a thin enough wall to fit the old carrier. Just epoxy into the carrier.
I couldn't find anyone to cast a new white metal bearing so the availability of maritex was a godsend.
 
Checking the Maritex clearance calculator it appears to be OK, but as the size is quite critical and on the thin side, I'll check with the manufacturer.

My figures for a 25.00 shaft and a 26.70carrier are
(make sure it's 25 and not 25.40 as this is not possible in a 26.70 carrier)

bearing size for clearance epoxy fit
ID 25.04 - 25.14
OD 26.40 - 26.50

Min radial displacement 0.04mm made up of
basic clearance 0.03mm
bore closure from water swell 0.0058
bore closure from unscheduled thermal expansion 0.01

I'll get confirmation tomorrow.
 
Tranona said:
Shaft is too small for your new engine. Minimum of 1"/25mm required. Virtually nothing uses a shaft less than 1" these days.
Click to expand...

How do you justify that when his shaft is more than adequate for 15bhp.
I know the new engine is several times more powerful, but with the same prop the power absorption will be the same for the same rpm.

(The OP never said he was changing the engine to go faster)
 
Well, something is not quite right. The Ford engine is between 35 and 55hp rather than 15 (on a good day). If the boat only needs 15hp, why fit something that is 2 or 3 times more powerful than needed?. And that small change in prop size does not sound right but difficult to say without having full details of boat.

Don't think you will find many 30hp+ engines running a 7/8" shaft.
 
Tranona said:
Well, something is not quite right. The Ford engine is between 35 and 55hp rather than 15 (on a good day). If the boat only needs 15hp, why fit something that is 2 or 3 times more powerful than needed?. And that small change in prop size does not sound right but difficult to say without having full details of boat.

Don't think you will find many 30hp+ engines running a 7/8" shaft.
Click to expand...

I agree going from 15 to 35bhp+ seems overkill and without a shaft change, it would be risky trying to use the extra power.

I also think that the prop change would be pointless. I believe its well known that you can take an bit off the pitch and add it to the diameter and vice versa without a major impact. (eg when you can't swing a prop of the 'ideal' diameter.)
 
ianj99 said:
I also think that the prop change would be pointless. I believe its well known that you can take an bit off the pitch and add it to the diameter and vice versa without a major impact. (eg when you can't swing a prop of the 'ideal' diameter.)
Click to expand...

He wants to increase both pitch and diameter, presumably to make use of the extra power.

PS The Boat Data Book specifies 7/8" shaft for 20BHP at 3000RPM so it may be OK.
 
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Actually I would like to keep my present prop, prop calc suggested a larger disc and pitch, but that would add to the costs.

I went for the Ford xld 1800 non turbo, because, it is a small, cheap engine, nearly half the weight of the lister and spares are readily available.

Alternatives were BMC 1500/1800, Kubota, 4.108, VW1500 and Isuzu 1500.

The Hurth box is a 100, which is rated at 32Hp @ 3600 rpm

All but the Isuzu have marinisation parts readily available.

The boat is a 197x Colvic Northerner motorsailer LOA 26'10 Beam 8'6 Weight 4.5 tonnes and displacement hull.

The 1800 XlD is 60ps @ 4800 and max torque is at 2500 rpm, I would not expect to run the engine above 2500 rpm, I have not seen a power curve for the engine, but I would expect 25-35 PS@2500

26.7mm is the ID of the stern tube, without a bearing carrier, I just checked it to see if I could run a 25mm shaft without changing the tube but changing the fore and aft seal and bearing carrier. I think .85mm is a little too tight a tolerance for the run out of a 1.5 metre shaft. Not having to change the stern tube, would be an advantage both in cost and time
 
Is the stern tube all metal? is the bore cast or machined? is there a water feed, oil feed?

We really need a total detailed picture of what you have to suggest options, I had been thinking this was a short stern tube of a few hundred mm.

It could still be possible but you may need intermediate bearings in the tube.
 
A quick rough and ready measurement of the polish marks on the shaft, the stern tube is 66.5cm between packings.

Here is the view of the rudder and prop

005.jpg


Internal view

006-2.jpg


Stuffing box

005-3.jpg


Remote plummer block

004-2.jpg


As you can see, access is not good, so replacing the stern tube for a larger one is not something I would like to do, though could do if forced. If I changed the stern tube, I would probably cut it back considerably and fit a dirigible shaft. I'm presently looking at launching late June and replacing the stern tube, would put me back a month (though the launch date is receding with this present weather)
 
Following discussions with the manufacturer (Maritex) the white metal replacement suggested above is a bit thin for us to be completely happy, so you will have to look at bigger tube or more clearance to get a bearing in.
 
Bobobolinsky said:
A quick rough and ready measurement of the polish marks on the shaft, the stern tube is 66.5cm between packings.

Here is the view of the rudder and prop

005.jpg


Internal view

006-2.jpg


Stuffing box

005-3.jpg


Remote plummer block

004-2.jpg


As you can see, access is not good, so replacing the stern tube for a larger one is not something I would like to do, though could do if forced. If I changed the stern tube, I would probably cut it back considerably and fit a dirigible shaft. I'm presently looking at launching late June and replacing the stern tube, would put me back a month (though the launch date is receding with this present weather)
Click to expand...

If was going to the trouble of replacing the engine, especially with a significantly more powerful one, I'd not want doubts about the sterntube and shaft reliability....
 
Well, I've made the decision to replace the stern tube with a grp tube. The intent is to cut the length back, so it is just forward of the rib and supported by a bulk head glassed in. A VP Dripless seal and Maritex tail bearing. The intent is to retain the plummer block, so that a flexible coupling can be used, to the gearbox.

The plan is to cut the present stern tube as close as possible to the hull and grinding/sanding back to flush and then either punching the remnent out or pulling out, using the tail thread and jacking bolts. Line boring using the existing plummer block plus a second plummer block, bolted and spaced to the skeg, utilising the existing shaft to bore between bearings and finally using the same set up to locate the grp tube prior to glassing in.

Maritex bearing to be epoxied into the tail and a tube fitted to supply cooling water from the raw water pump

Need a souce for the grp tube

What method is suggested for installing the cooling tube, though will probably support the tube by glassing it to the rib

Comments please
 
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