Cutlass bearing conversion - water lubrication?

[big_s]

Hi,

I have just replaced a white metal cutlass bearing with a more contemporary and standard style nitrile cutlass bearing. The old setup just had a grease gun that attached to the stern gland.

My question is do I need to add a water inlet to the stern tub to accommodate the change in cutlass bearing if I retain the same stern gland?

Some folk I have talked to suggest the negative pressure forward of the propeller when in forward gear will mean water would not be able to enter the cutlass bearing from outside the boat and a water inlet to the stern tube would be necessary. Others suggest that a water inlet to the stern tube is not required as the max rpm of the propeller is only 1300rpm and water would enter the cutlass bearing from outside the boat.

Anyone able to give any insight into this? Ideally I would be able to avoid the addition of the water inlet into the stern tube. (I realise that an airlock may be an initial fitting issue here)

Cheers.

 

[Tranona]

Important to leave a gap of a minimum of 15mm between the propeller boss and the stern tube to allow water to flow in and fill the stern tube

 

[Daydream believer]

My bearing has a brass section with 2 scoops that catch the water & turn it to the bearing as the vessel moves forward
It is a separate section bolted on the back of the main bearing in front of the prop.

 

[PCUK]

It's easy to fit a water feed to the stern tube so why not fit one whether it is needed or not, then you have no worries?

 

[PaulRainbow]

Useless trivia....

A cutlass is a sword

The shaft bearing device is a cutless

 

[Pye_End]

Does the cutless holder have a couple of holes at one end to allow full flow of water through it, or is it just open at one end?

 

[LittleSister]

Hi,

I have just replaced a white metal cutlass bearing with a more contemporary and standard style nitrile cutlass bearing. The old setup just had a grease gun that attached to the stern gland.

My question is do I need to add a water inlet to the stern tub to accommodate the change in cutlass bearing if I retain the same stern gland?

Some folk I have talked to suggest the negative pressure forward of the propeller when in forward gear will mean water would not be able to enter the cutlass bearing from outside the boat and a water inlet to the stern tube would be necessary. Others suggest that a water inlet to the stern tube is not required as the max rpm of the propeller is only 1300rpm and water would enter the cutlass bearing from outside the boat.

Anyone able to give any insight into this? Ideally I would be able to avoid the addition of the water inlet into the stern tube. (I realise that an airlock may be an initial fitting issue here)

Cheers.

The greaser attached to the stern gland is to lubricate the traditional stern gland itself (i.e. the packing against the shaft), not the cutless bearing.

My understanding is that all cutless bearings are water lubricated (and cooled), regardless of whether they have scoops etc.

Only an occasional little replenishment pump of grease is required for the stern gland, and I have seen warnings (how well informed, I do not know) of pumping too much grease into the stern gland as this travels down and excludes the water which lubricates and cools the cutless bearing.

Water feed tubes, or air escape tubes, are fitted to some of the modern proprietary lip-seal/rubber stern tube seals which many now have in place of a traditional stern gland. As far as I am aware such tubes are not usually required for cutless bearings, though my experience is on smaller boats, and perhaps they are required or advantageous on larger or high-powered vessels.

 

[vyv_cox]

The greaser attached to the stern gland is to lubricate the traditional stern gland itself (i.e. the packing against the shaft), not the cutless bearing.

My understanding is that all cutless bearings are water lubricated (and cooled), regardless of whether they have scoops etc.

Only an occasional little replenishment pump of grease is required for the stern gland, and I have seen warnings (how well informed, I do not know) of pumping too much grease into the stern gland as this travels down and excludes the water which lubricates and cools the cutless bearing.

Water feed tubes, or air escape tubes, are fitted to some of the modern proprietary lip-seal/rubber stern tube seals which many now have in place of a traditional stern gland. As far as I am aware such tubes are not usually required for cutless bearings, though my experience is on smaller boats, and perhaps they are required or advantageous on larger or high-powered vessels.

His original bearing was white metal, which needs grease.
Many motor boats have positive pressure water driven through the cutless bearing, a forward facing scoop being one method. A cutless bearing in a closed housing may well.overheat but possibly not in a slow yacht application.

 

[LittleSister]

His original bearing was white metal, which needs grease.
Many motor boats have positive pressure water driven through the cutless bearing, a forward facing scoop being one method. A cutless bearing in a closed housing may well.overheat but possibly not in a slow yacht application.

Thanks for the correction, Vyv.

 

[alahol2]

I replaced a white metal bearing with a conventional cutless bearing 20+ years ago. There is no water entry to the stern tube other than the grooves in the bearing.. I've had no problems but I seem to need to replace the bearing every 7 or 8 years. This is on a 28ft sailing yacht motoring at 5 - 6 knots. 1" shaft, max prop revs 1700.

 

[Ian_Edwards]

The cutlass bearing on my Southerly is now 17 years old and is still tight, ie no signs of any play.
Water is feed into the stern tube with a small feed from the cooling water, take just before the water injection elbow (i think).
That ensures that the cutlass bearing is always lubricated and cooled.

 

[big_s]

His original bearing was white metal, which needs grease.
Many motor boats have positive pressure water driven through the cutless bearing, a forward facing scoop being one method. A cutless bearing in a closed housing may well.overheat but possibly not in a slow yacht application.

Hi. Yes, you understand the original setup correctly. The new brass/nitrile cutless bearing can only allow water from outside the boat (along the grooves of the bearing) unless a water feed is introduced somewhere along the stern tube setup. There are no scoops in the cutless bearing housing. Given that this is inside a yacht that has an electric motor, there is currently no cooling water feed to an engine to tap into.

The experience of Alalahol2 reflects advice I have been given and the longevity of the cutless bearing would seem to indicate not having a water feed in the stern tube is a valid approach. Boat and propeller shaft dimensions are also reassuringly similar.

Given the difficulty of bleeding air in the stern tube, I may introduce a valve section to allow this following the stern tube self aligning fexible tube coupling. As PCUK says, this would hedge bets too, although hopefully wouldnt be needed as an inlet.

Thanks for everyones thoughts!

 

[Snowgoose-1]

In the days of white metal bearings we would fill the stern tubes with grease before launching. The internal manual greaser was nipped up after each day's motoring.
It did not take long though for water to work through to the packing gland.


I switched one boat from white metal cutless to nitrile mainly because of the expense. It worked fine without any modifications. Perhaps each boat is a different case




It

 

[vyv_cox]

In the days of white metal bearings we would fill the stern tubes with grease before launching. The internal manual greaser was nipped up after each day's motoring.
It did not take long though for water to work through to the packing gland.


I switched one boat from white metal cutless to nitrile mainly because of the expense. It worked fine without any modifications. Perhaps each boat is a different case




It

I replaced the white metal bearing on my little motor sailer with Vesconite. Like yours, the stern tube was full of grease. I took this out and left the Vesconite water lubricated, no water flow through it. It worked well. It is shown at the bottom of this page Stern gear along with a similar one.

 

[Beneteau381]

Hi,

I have just replaced a white metal cutlass bearing with a more contemporary and standard style nitrile cutlass bearing. The old setup just had a grease gun that attached to the stern gland.

My question is do I need to add a water inlet to the stern tub to accommodate the change in cutlass bearing if I retain the same stern gland?

Some folk I have talked to suggest the negative pressure forward of the propeller when in forward gear will mean water would not be able to enter the cutlass bearing from outside the boat and a water inlet to the stern tube would be necessary. Others suggest that a water inlet to the stern tube is not required as the max rpm of the propeller is only 1300rpm and water would enter the cutlass bearing from outside the boat.

Anyone able to give any insight into this? Ideally I would be able to avoid the addition of the water inlet into the stern tube. (I realise that an airlock may be an initial fitting issue here)

Cheers.

Stave bearing - Wikipedia.
Or why they are called CutLESS.

 

[RunAgroundHard]

The cutlass bearing on my Southerly is now 17 years old and is still tight, ie no signs of any play.
Water is feed into the stern tube with a small feed from the cooling water, take just before the water injection elbow (i think).
That ensures that the cutlass bearing is always lubricated and cooled.

Same set up on my Rival, but at 175 years.

 

[alahol2]

Same set up on my Rival, but at 175 years.

Wow...

 

[RunAgroundHard]

15 years of course. I had the prop shaft and other work done on the stern gear when I bought the boat. I keep thinking it will need changed, but so far all is good. The other work involved fitting a Halyard Aquadrive, so I imagine there is very little fore and aft play in the propellor shaft at the stern gland.

 

[Ian_Edwards]

In a similar way, I had the cutlass bearing replaced and a python drive (poor man's Halyatd Aquadrice) fitted when I bought the boat. The thrust bearing removes most of the axial movement in the shaft, which probably reduces the wear in the cutlass bearing.