P Bracket bearing

[aitchem]

My P bracket bearing is rubber, it's been in 33 years but never been used.
I am going to change it mainly because rubber has a short lifespan, is there anything better to put in there.?
Technology has moved on a bit in 33 years.!

thx
Howard

 

[rob2]

If its never been used, why do you want to change it? Although rubber technology has moved on a bit, but not that much, you may find yourself restricted in choice just to match the dimensions of your sterngear. Cutless bearings are pretty much an off the shelf item.

Rob.

 

[Malish]

H4 marine will be along shortly...

 

[Spyro]

My P bracket bearing is rubber, it's been in 33 years but never been used.
I am going to change it mainly because rubber has a short lifespan, is there anything better to put in there.?
Technology has moved on a bit in 33 years.!

thx
Howard

Do you mean never been changed?
Usually they are renewed when there is too much play. But it's not an expensive item and not difficult to replace so I would. The bonus with complete rubber ones is they are easy to remove. Most common ones now are rubber with brass casing. (google cutless bearing) for a new one you will need to know shaft diameter and housing internal diameter.

 

[sailorman]

Do you mean never been changed?
Usually they are renewed when there is too much play. But it's not an expensive item and not difficult to replace so I would. The bonus with complete rubber ones is they are easy to remove. Most common ones now are rubber with brass casing. (google cutless bearing) for a new one you will need to know shaft diameter and housing internal diameter.

The boat has never been launched

 

[Tranona]

If it is a cutless bearing with either a brass or phenolic shell then it should still be OK. The shaft does not run on the rubber but on a film of water running through the flutes. You could replace it with a new one of the same type, but it will probably be no different from the one you have. The alternative is a composite which is becoming more popular, particularly in high performance applications. H4 who posts here regularly is the wizzo on those.

 

[Norman_E]

Whilst it is mostly exposure to UV light that does the damage, rubber "perishes" with age. After 33 years, regardless of UV exposure, it may well be a failure waiting to happen, as the rubber might now be liable to crumble in use. For the cost involved it is a good idea to change it now rather than have to lift out out again to do so.

 

[Frankie-H]

You do not mention if your boat is going to be launched. If you intend another 33 years on the hard, I would wait until then. Otherwise I agree with Norman E.

 

[aitchem]

You do not mention if your boat is going to be launched. If you intend another 33 years on the hard, I would wait until then. Otherwise I agree with Norman E.

Hope I haven't blown 2k on a berth. :_(

 

[Neil_Y]

H4 marine will be along shortly...

Funny you should say that, thanks to smart phones I do seem to look at the forums most evenings.

Yes there are new materials that have improved over rubber as a bearing surface, wear rates and possible shaft displacement/flex seem to be the main differences.

There are as well as nitrile rubber three types
Polyester based composite elastomers
Thermoplastics
Phenolic based composites
composites can be fiber wound or moulded

Most have gained Lloyds approval so they all work to a greater or lesser extent.

The structures use a variety if fibers and lubricants (these give strength and reduce dry running wear)

None of this has been scientifically or independently compared apart from dry rudder tests but our view is that ...
Melting plastics and elastomeric materials are soft/ softening with heat so we believe suffer many of the same issues as rubber in that they give under pressure allowing more shaft flex. We think that one of the consequences of a hard bearing is that it keeps the shaft running true, and when the shaft does cause the water film to collapse it runs dry against a surface which self lubricates until the water film re establishes. The shaft movement being caused by the boat bobbing or crashing up and down and the torque, thrust and weight of the system

There are some clever materials and some other materials where the composite is faced with a teflon surface to reduce dry friction and wear.

Maritex has shown that it can compete with all comers in a wide range of applications and has exceeded expectations in recent dry rudder tests for DNV. In recent installations such as the Plymouth pilot boat wear rates seem to be less than a quarter of a good quality nitrile rubber.

However they are more expensive and on a yacht with low power thin and hence light shafts you may find a rubber bearing perfectly OK (until you try something better)

You will see many plastic manufacturers quoting the fact that they don't absorb water (this comes from the early days of nylon when it was thought to be a magic material, used in bearings it had problems with water swell and bore closure) The all absorb water but now most absorb very little and it is a known quantity. However thermal expansion is a more significant factor, and as a bearing starts to have problems it usually heats up and then it just gets worse. Softening materials that are layered delaminate at failure, still probably long after a rubber bearing.

Maritex overcame this with a different fibre structure in a material which hardens with heat. Despite the argument that rubber is great as it prevents shaft wear and somehow works great when there are abrasives what we see in practice on commercial and military vessels with Maritex is low wear and reduced shaft movement.

Now time for a beer, it's a very big subject.....

 

[aitchem]

Thank for the science H4, I will print it off and try to absorb at work.
I think I don't need rubber, as I have fitted an Aquadrive, hence no movement allowance is needed.

Howard

 

[Neil_Y]

Thank for the science H4, I will print it off and try to absorb at work.
I think I don't need rubber, as I have fitted an Aquadrive, hence no movement allowance is needed.

Howard

Rubber bearings aren't required for a soft flexible mounted drive, as the flex is enough in the shaft to allow for engine movement, they just adds to the shaft whip and flex (I believe)

We fit many rigid bearings to soft drives with flexible mounted engines that take thrust, these hold the shaft straight and let the shaft flex cope with the engine movement at the front end.

But with a thrust bearing and CV coupling you are able to have a shorter unsupported shaft length from the forward bearing to the coupling as there is less movement.

It's only because I'm a sailer that I've taken an interest in the smaller bearings as they should reduce maintenance, and they are far easier to fit and change than press fit bronze. Most of our business is in 3" + rudder/shaft/stabiliser bearings for top end yachts.