This is not a how to, mainly because it's dangerous and I didn't do it by the book and there's a chance you could get killed doing it. This is a "what I did".
So here's a story for you, I've owned three boats on trailers in my life and never once had to do any major trailer maintenance except the usual stuff, changing tires, hubs etc. I guess I took things for granted because I never really understood how lame they are made. I can tell you this, I never ever want to be a trailer mechanic as a career...
We're at the ramp and I drove the boat up on the trailer and my buddy is pulling Gigas out of the water with my truck. I hear this scraping sound and think, "huh, that's odd, it sounds like the tire is scraping". I jump out of the boat and look and the left side is a bit lower than the right side so I back the trailer up a bit into the water again and pull the boat closer to the right side. It no longer rubs so we wash the beast down and take off.
About two days later I get this nagging feeling so I start to inspect the trailer closely. The springs are pretty rusty but that's common. And then I look really close. ON THE LEFT SIDE PART OF THE LEAF SPRING BUSTED OFF AND THE AXLE IS SITTING ON THE FRAME in effect I have a three leaf spring that is only sitting on two rusted springs. Holy shit, that's dangerous. Mainly because I didn't realize the only thing holding the axle on boat trailers is the trailer springs, ie if the springs busted completely your axle falls off on the freeway. You have to see it to believe it, look under your boat and follow the axle and be amazed that the only thing holding it onto the trailer is the wimpy springs.
I immediately do measurements to find out the spring size and to my dismay it looks like I have to order a custom spring because mine is a bit longer than the stock ones. I'm doing research and thinking how can that be and then I realize you have to measure the spring when it's off the boat. Duh, I'm glad I got my masters in bio and can't figure that one out.
I get the measurement and then call up a couple of trailer places and learn that 1. they don't keep every spring in stock and I will have to drive the trailer down there, have them measure it and then drive it back home while they order the part and 2. they are completely booked up for weeks. So I decide to order my parts from etrailer.com, great place and I'd recommend it. Make sure if you buy there that you call up the people directly instead of ordering online as one of my parts weren't in stock and they didn't notify me and I waited five days before noticing they never shipped it. I had to call them up, order a different part and then they shipped it immediately.
You shouldn't do this with the boat on the trailer, but unloading the boat meant I'd be doing it at the launch ramp and I know from experience I will always be missing a tool and there's no way I'd want to leave my boat floating while I was scrounging for parts for a week. I did mine with the boat on the trailer, I used jackstands all over the place as well as two jacks. My boat is small, 16'. If I had a bigger boat I would not do this with the boat on the trailer, I had to make sure my jacks were strong enough to lift that boat up safely.
Roughly a week and a half later and I have all the parts and start the install. First I sprayed all the rusted bolts with liquid wrench for a few days to loosen them. That helped slightly, but I still had to use my 3' breaker bar to get them loose. I probably should have just cut them off with a metal blade on a grinder but I live in a really quiet neighborhood and didn't want to make a lot of noise. The problem with using a bar is you can only move it slightly each time because of space and it was a bitch positioning the bar. I had to fight the main support bolt the worst. Do a youtube search and you'll see how to raise the axle up to relieve pressure on that spring but be very careful when you remove it, even at rest it still has enough kick in it to do damage to you if you aren't careful. My tires came off easily because I antiseize the bolts every year. There are two schools of thought on this, some mechanics will tell you to never antiseize it because you can over torque things and stretch the bolts and if you don't do it tight enough they can spin off. I work in a saltwater environment with giant pumps that aren't really made to be used for salt water and we antiseize EVERYTHING because otherwise they will rust to shit. Therefore I always antiseized all the bolts on my trailer when I put them back on.
There were a few bolts that weren't the right size that I had to pick up at home depot, I choose either stainless or the heavy duty galvanized versions. Stainless will rust as well over time so I antiseized those too. I wore gloves and also used a floor mat to save my knees and back. And again, everything was braced all over the place with stands to keep my wife from collecting accidental death insurance.
If I would have taken the boat off it would have been easier. If I had a buddy to help me it would have been even easier. The disassembly was a bitch because of the rust, I'd say this was a 3 beer job and I had to work on it a few hours at a time because of family commitments. Assembly was a piece of cake, but I had to do a lot of readjusting and thinking to make sure the bolts seated correctly on the axle plates. The job probably took about four hours start to finish, had the bolts not been seized it would have taken probably two hours if I had all the parts. I forgot to wear the gloves at first and promptly busted my knuckles on a bolt when the breaker bar slipped. I wore rubber gloves under the mechanic gloves to keep the antiseize off my hands.
I probably saved a couple of hundred on doing the install myself, the trailer springs are shockingly cheap and I ordered extra stuff for my boat because at etrailer your shipping is free if you hit a hundred. It would have probably been worth it to have a professional do it, but that would mean I would have to have the foresight to do it during the winter when business is slower and again, I'm dumb. Truth be told, I like doing things like this to learn how it works, it helps in troubleshooting and also next time I'll know exactly what I need and what I should do.
I also did the bunks, I'll do a separate write up on that.
Trailer tire, axle is sitting on the frame, notice the tight clearance of the tire
Trailer tire with new springs, notice height difference
Old and new springs, notice the busted leaf spring on the old one, this is not good.
Some of the tools I used, note that there are a lot of rachets, that is because I needed one to hold the bolt and one to loosen it. See the gray container, that's antiseize. Antiseize is your best friend, except it gets everywhere and ruins clothes and when you get it on the couch or rug it ruins relationships too. Also note the deep sockets, this is because the bolts are very long.