What are the pro/cons of each?  Is there a big advantage of having a dual axle boat trailer over a single axle boat trailer?  Thanks

Tandem trailers are really a necessity on just about any glass rig over 19' long.  I've seen 19 1/2 footers sitting on single axle trailers but I wouldn't want to do it.  That's a lot of boat back there.  

The only cons of a tandem are more difficulty in moving the boat around in a driveway or garage, Weight and more maintenance. (hubs bearings, tires, etc.)

Like he says they are necessary on big boats.  I have a tandem axle now and hate it.  There is no way to move it around by hand.  It also takes 40 acres to turn it around but I do feel save pulling the 3500 pounds down the road.

Ride is much smoother too .....

if you get brakes on both axles, you have more braking power which is a good thing. and a dual just ride so much better,

My uncle has a 21 foot open bow on a single axle and its a horror to tow. beats you and your truck up and hard to stop.

kb here    i have a ranger 195   which is 19'--5"  and have a single axel  i dont travel like i use to.  if i did i would have traded off the single and got a tandum axel  my fishing partner has a 520  and some times i pull his boat with   my truck he has a tandum .  man there sure is a lot of difference  the dual axel pull's a lot easer  and smoother , so long runs they are the best   kb

Having driven through pretty croded freeway type roads getting to the lake, I experienced blowing a tires on a single axle trailer and being in the middle lanes durin gbusy times. New set of depends were required!!!!!! :-[    With dual I could have gotten over pretty easy and not flipped the rig. Just added safety, but important when you have to travel

My boat came with the original painted trailer, but living at the beach I need a galvinized trailer - the original is rusting bad.  My boat is a 2000 ProCraft 17' long.  I was thinking about a single axle trailer.  

Would you buy brand new or lightly used. Where is the best places to find a trailer to match my boat?

Yeah, the bigger boats need a tandem trailer and if you do any amount of traveling the tandem's are sweet to pull. I pull mine with my Tahoe and I really cant tell its behind me.....even on a hard stop.

I'm definitely in favor of tandem axles.  I had a 2nd axle added to  my trailer for just that reason even though the weight of the boat doesn't justify it.  Peace of mind.  Also tandem axles seem to track better, eliminating the need to oversteer going around corners.

If you fish a 19 or larger boat you need the tandum trailer. Especially if you tow any distance. Most of the fishing I do requires a tow of at least three hours one way. The tandum is a lot more stable than a single axle. We usually tow at 70 mph or above and you need the extra axle.

Like a Tandem also. Took a bit of getting use to backing up after having a single but once you have it down it is a piece of cake. Like the added safety involved also.