yes, prevention is better than cure.  

 Get rid of the bearing buddies imo. Can be more of a headache than and asset.

 Easier to check/replace your bearings at home rather then on the side of road in the heat and flies.

Boat trailer bearings should be replaced annually. Definitely wouldn’t repack old bearings and even more so if heading up north. I’ve seen countless trailers left on side of road in the middle of nowhere because bearings have cooked  

All tow vehicle/trailer cominations will sway once a critical speed is reached. Know what that speed is for your setup because you don't want to discover it when you put your foot down to pass a truck like I did on the Minilya Exmouth road, no damage.

Scary video from Qld police - www.facebook.com/QueenslandPolice/videos/caravan-safety-reminder/896458804063754/

Considering the work involved is the same and the relative cost - I'd go new bearings.

We went up in Sept - the fishing is worth the long drive.

More harm than help in my books. Pull your hubs off and inspect your bearings, if good repack em and add some new end caps. If in doubt rip em out and replace with a fresh set. I'd be getting an entire hub and bearing assembly and all of the associated tools and doo-dads ( don't forget a drop sheet and plenty of rags! ) to replace a hub / bearing in the field.

For the sake of $40 I just replace my bearings and races annually and makes for an easier job.

 .. and just pumped up the bearing buddies, I'd just replace them. Likely to be close to toast. As others have said, don't be that bloke by the side of the road--have them fail badly eough, and you'll wreck your stub axle, and be beyond just having to replace the bearings by the side of the road.

 I use them... but I replace my bearings every 18-24 months... and my boat is kept 500m from the ramp!

 like everyone else has said, its a pain doing it at home, imagine what its like in 40 degrees and flies up your nose and ears..

100%....pull then down, clean inspect re-grease or replace if sign of corrosion.

Plus take a few spare sets with tools to replace.

If you don't take note of all the comments above  and decide not to replace them, then drop in at our roadhouse at Wooramel because I have just received a new shipment of grease, bearings, seals, u bolts and plates and you just might be needing some.

 I travel a lot, and see lots of bearing failures. But only with others, learnt my lessons long ago. I am currently on probably  about 8000km on my current set, but that includes the worst of dirt roads. And they are Ford Slimlines on a tandem weighing in at over 2700kg loaded up. Really marginal for that weight, IMHO. I check heat every time I pull up, first thing I do. I used to do it with the finger on the hub test, until a mate gave me a Thermo  gun. Really interesting looking at variations, ie, on a warm day, the side facing the afternoon sun on a long run will be as much as 10 deg C warmer than the cool (shaded) side. I regard checking them every 200k or so as necessary, but don't delude myself in that respect. They might feel fine, but, as they fail, things go downhill really quickly. So, as a minimum, inspect every 12 months. You can do a quick inspection between stripdowns by the following

1. Jack up and spin the wheel. if you have discs, it will be necessary to remove them to be able to hear properly. You are listening for a rumble in the bearings. Good bearings will make no more than the slightest hiss, for want of a better description. Bearings which are heading to failure will rumble, you'll understand what I mean first time you hear them.
2. 2. Check for vertical play. Do this by grabbing top and bottom of the wheel, and try to rock it. They should be no movement. If there is, but the spinning part soiunded good, you just need to tighten up the nut enough to stop the rock. Timken explain this as tightening firmly, then backing off to the next hole that will line up for the split pin. As good a method as any. Firmly does ot mean putting a length of pipe on your 10" shifter and jumping up and down on it. Just apply pressure until it is properly bottomed--it is a tapered bearing, after all. 
3. Grease is really not much good for more than a couple of years anyway, it starts to go hard. So you will be repacking/replacing every couple of years max. When it comes to repacking, well, it is actually less work, IMO,  to completely replace--you don't have to do the messy job of cleaning the old ones out. Just press your old cups out, pack the new ones, job done. 

If the grease appears "milky", means it is water contaminated, replacement is a must.  The best way to completely cover yourself in the event of failure on the road is to have a complete spare hub and bearing assembly.  get a spare mount with the matching size stub, and you will always have a spare assembly ready to go.

There is a lot of good information above, but I'll outline what I do.  My rig is more or less 3.5 ton and I tow annually to Denham and numerous times a year to Jurien

1. I always replace my bearings, races every year.  It simply isn't worth risking it in my book

2. I got rid of bearing buddies years ago.  I lost several of them on long trips and I don't really think they are effective.  I now only use dust caps.

3. I have two full sets of hubs.  When its time to redo the bearings, I take the spare hubs to Martin's Trailer parts.  I buy 4 sets of bearings, 4 dust caps, 4 seals and other bits and pieces.  I ask them to punch out the old race and press in the new ones.  They always do it, which makes it a lot easier and is a great service.

4. Now I can replace all the bearings in all 4 wheels at once.  I always put in a new split pin and new dust cap....nothing reused.  The 4 hubs that come off get stored for next year.

5. I have bought a bearing grease press for the bearing.  Got it from SuperCheap Auto.  This pumps grease into the bearing and is an absolute god send.  Saves manually pressing the grease in

6. I have a plastic box with spares that always travel with me if I'm going any distance.  This has a spare hub, bearings, seals, a few dust caps, couple of castle nuts, 6 or so wheel nuts, split pins, wheel brace, hammer, pliers, 2 x huge adjustable spanners, greese gun, grease cartridges, grease tub, rags, wooden blocks (for jack support).  Probably missed few things. 

Another thing I have learned over the years is not to overtighten the castle nut.  Just tight enough that there is no rocking but wheel should spin freely.  If you have it too tight, the bearing will run hot and the grease will destroy very quickly.  Pays to recheck the play after 100kms or so to see if the bearing has seated in further.

Great advice from you guys, very helpful to newbies like me 

Use timkens or koyo don't use the crap Chinese brand like Ark 

talking to leeman mechanic business is booming replacing these crap Chinese bearings even on new trailers that don't even make it few hours out of perth 

This is what you will likely see if , upon spinning a wheel, you hear the bearings rumble. These witness marks are when water has entered , and sat in the race. If you see this, replace. They won't last long at speed. The whole plot doesn't need to be full of rusty, emulsified grease to be close to failure.

 $40 a set, and if you were going to go through the trouble of taking everything apart to "inspect and repack", you're 4 fifths  there anyway, you may as well just replace. 

Once you've done a bearing change on the side of the north west coastal hwy in 40'c, covered in ants, dirt, grease, sweat, blood and tears, you'll do exactly as I do and change them on my lawn every 12 months!!

While we are at it, any tips on changing them? If not confident, what is a reasonable price to pay to have a trailer mechanic do it?

 cheers for all the comments guys. Sorry for late reply been busy. Ended up replacing everything, hubs, bearing discs and pads.

 4 months on from my last post and I must admit I've been slack and done donuts (life got in the way). Now heading up North in a few weeks and keen to change the bearings. Not confident to do myself but probably should learn. Have a dual axle trailer with bearing buddies (save the comments about them please). Called Towsafe and they are booked out until after I return.

Does anyone have any reccomendations of any trailer mechanics (ideally NOR) I should try?