Forgive me for a lengthy post....bearing failures on boats are almost totally caused by water intrusion. SO:
Start with a clean and dry hub, install new bearings and real "Bearing Buddies", which are made with stainless steel, not the many cheapies made of plastic or metals that corrode easily. They cheapies are only a couple of dollars cheaper, and they fail in a few years.
Once assembled, pump them up with your grease gun until the diaphrams are fully pushed out, per manufacturers instructions. Then take your trailer for a short drive....maybe even to the boat ramp to launch, but PUMP UP THE DIAPHRAGMS BEFORE launching. They will likely be clear in because there was still a lot of air in the hub that escaped during your short tow.
When you return home, pump them up again. Now, they are likely full, but watch carefully that they remain full.
If your hub seal is good likely you will be good for months, but make it a habit to look, much like you notice a low tire, etc. As long as there is positive pressure in the hub, no water can enter and your dry bearings will last for many many years. I recently took a badly rusted boat trailer to the dump that I purchased new in November,1984. The bearings had never been repacked and were still as quiet as new after 30 years......this on a trailer that was in nearly daily use in a semi-commercial operation. Of course I had installed bearing buddies before its first launch.
Greases: Use any marine bearing grease. Massive advertising claims on the tube will not prevent bearing failure, if water is in there they will fail.
I maintain a fleet of boats and trailers that are in constant use, launching daily for about 4 months a year, and less often for 8 months a year. The proper use of Bearing Buddies has reduced our bearing failures to zero. We never repack bearings ever....unless a seal has failed and caused the buddies to go flat and lose positive pressure. For most of our boats that means 15 to 20 years with no bearing maintenance except for keeping them full.
Bearing Temps: Hub temperatures should be around 15 degrees above ambient when towing, if you have brakes on that axle they may be 40 degrees above ambient if your drivers are hard and late brakers. When I detect temperatures higher than that I usually note that the towing drivers are those that power toward red lights, stopped vehicles, etc and generally do not look far ahead......and wind up using the brakes much more then safe and alert drivers.
Then, finally, flip a jack under the axle at least annually or before very long trips and spin the tire and listen. Hope for near silence, not a rough and growling noise. I just returned from a large tournament event, many boats and trailers were there from 300 to 500 miles away. I was stunned to notice that a couple of trailers went by with obviously failing bearings, a soft but clear growling sound than was indicative of failing bearings. Fancy trucks, expensive boats but total ignorance will still result in an expensive and possibly dangerous failure.
History: in the 1970's and early 80's I used to repack bearings twice a year, still no bearings lasted more than a couple of years, I was changing them constantly. I tried every brand of grease to no avail. I have never had a bearing fail since switching to Bearing Buddies except a trailer that we purchased with "red eye" brand plastic buddies. They soon jammed up with dirt causing the diaphram to stick tight.....which of course makes them only a grease cap and not a bearing buddy. Regular checks caught the bearing failure before hub damage could occur, but the failure mechanism was clear.
End of long post!
