Boats are full of compromises, and advances in one dimension can bring problems in another. Nailing the full equation of longevity, strength, and serviceability often is truly challenging.
A perfect example of this challenge is the Nordhavn 47 and 52 rudder upper bearings. The early members of the Nordhavn 47/52 line used fixed race ball bearings. These bearings are durable, very strong, and should never require replacement. But these are steel bearings and if they not always 100% greased, they will rust. If the emergency hand rudder deck plate O-ring fails, water will leak down onto the bearing and it will quickly fail. Even draining the air out of the rear thruster, if not done with some care, could spray this bearing. If this bearing rusts and the rust isn’t noticed, it can fail in dangerous ways.
This bearing is example of an excellent engineering component selection that should support the application well. Yet due to the less-than-perfect conditions often found in the real world, the bearing can fail. Because the failure modes for this bearing can be dangerous, Nordhavn went to a Delrin bushing on the later boats in the 47/52 series. See the white bushing in the picture below.
The Delrin bushing approach to the rudder upper bearing again works very well but, again, has drawbacks. A Delrin bushing is stable, inexpensive, takes loads very well, and unlike the ball bearing approach, the Delrin bearing can operate even when wet with sea water and still show excellent longevity. It’s self-lubricating as well so virtually service-free. Where the Delrin bearing is less than ideal is that it wears fairly quickly. When Dirona got to around 1,500 hours, the bearing was starting to show significant side play and you really don’t want side play on the rudder.
A perfectly reasonable approach to this problem is to exploit the fact the Delrin bearing is inexpensive and easy to change, and just install a new one. On this approach, I recommend having a spare.
On Dirona, we wind up the hours fairly quickly and have just crossed 8,000 hours in 6 years. The approach we took was to replace the Delrin upper rudder bearing bushing with an Oilite bushing of the same dimensions.
Oilite is the trade name for oil impregnated sintered bronze. There are generic sources of the product but this is the brand name supplier: https://www.oilite.com/bearings.asp.
Above can see some nice touches from the machine shop where grease passages are cut all the way around the outside of the bushing.
In the picture below you can see the grease fitting installed in the outer housing. This fitting forces grease into the outside of the bushing and from that single fitting grease is brought all the way around the bush as well as through two holes into the inside of the bush where another grove conducts the grease all the way around.
The machine work was done by Wilson Machine Works in Seattle. I had Wilson produce a spare since I fully expected I would wear them out every 4,000 to 5,000 hours. But, the current bearing has been in there for 6,500 hours and thus far shows absolutely no signs of age. I’m starting to suspect it’s going to never need replacement or service at this point. I still don’t mind having a spare on board.
Regardless of whether you chose to stay with the Delrin bearing or move to Oilite as we have on Dirona, there are a few important installation tips that are worth bearing in mind. When re-installing the rudder arm it is imperative that the two 90 degree pockets that clamp onto the rudder post be 100% square with the rudder post. If you can see any gaps between the rudder shaft and the rudder arm, it’s not installed correctly. This is super important as the loads on this interface are very high and it will work free if not seated correctly. Ensure that the 4 large nuts and bolts that clamp on the rudder arm are properly torqued after first coating in Loctite Blue. Then ensure the lock nuts are properly installed.
In addition its super-important that the steering hydraulic ram be operating exactly 90 degrees to the rudder shaft. Ensure that the rudder arm vertical install height places the hydraulic arm exactly horizontal (assuming the boat is properly trimmed).
Finally ensure that the large bolt that goes through the rod end on the end of the hydraulic ram and attaches it to the rudder ARM be both tight and cotter pinned in place. Forces are high and some of these forces are rotational so you really do need a cotter pin. There have been incidents where this has fallen off and it will yield a boat without steering which, needless to say, can be highly dangerous.
The bolt that connects the rod end to the rudder arm was once installed upside-down for a few hundred hours on Dirona. In this configuration, with a cotter pin in, it’s just slightly loose, with the result that some minor motion was allowed between the bolt and the rudder arm. This has two nasty side effects: 1) it will open up the hole in the rudder arm so proper tightening will no longer stop all motion, and 2) any metal from wear will fall down into the spherical bearing in the rod end and cause it to failure earlier than it should. On Dirona, there is a slight movement between the bold and the rudder arm so I will have to get a machine shop to bush the rudder arm or weld it up and redrill the hole. I will probably request a slight press fit to ensure this issue doesn’t repeat.
Getting all these installation conditions correct is easy, but important to ensure thousands of hours of trouble free operation. None of us want rudder issues.
2016/07/11 update: Bruce Harris of N5260 Stella Maris has really dug into the details of this project and contributed greatly to the owners community by working with the Masters Machine to draft out a repeatable design that can be easily fabricated for other owners interested in making this change. This is a really nice refinement of what we did: