I bought a new trailer LOL, really a frustrating thing especially when loading in a current, I had a builder tell me once to back in water and go have lunch, once the wood soaks up a little water there like lead weights LOL

Wonder if my triple axle Manning will do this in salt water. It did not on the sea trial but that was fresh water, isn't salt water more buoyant?

I have load master triple and trailer never floats. Fresh or salt... always heard that up north, ppl prefer steel for that reason, but my alun never floats. I back in just enough to cover fenders and front bunks with no issues and drive boat up.. latch and crank i f needed.

Your trailer will not.

The only ones that do it are the aluminum bolt-together trailers with long/large wood bunks.
Often the frames are quite short in comparison to your Manning; as they end them right after the axles and use the wood bunks to extend off the back to meet the transom. This is part of their cost cutting technique; again--all pending the manufacturer. Some are built better than others.
Triple axle trailer more likely than a tandem.
16" tires on aluminum rims, more likely than 15" steel rims. etc. etc.

I have a Owens and Sons Slide-On Aluminum trailer that was custom fit to my 20' Cigarette and the only issue is that it wants to float. Lucky for me, I currently boat in fresh water and it does not float as much as you boaters in salt water. The best method other than add 100's of pounds to the trailer is use this method that is recommended. I generally launch and retrieve my boat without any help and this method works great after you practice a few times. If you take your time and get the boat started on the rear of the trailer you can usually back down only one time and you are all set. I do not ever power load my boat and very comfortable handling the boat alone. This might be more challenging with larger, heavier boats but should be a breeze if your wife(not) or girlfriend gives you a little help. Enjoy the weekend and safe boating!

Also 2 extra tires filled with air..

Floating trailers ... ... . what's next, flying boats?? lol

if you have the guide poles cap off the bottom and fill them with sand and put a cap on top..

Sorry Brian... Trailer... NOT Trainer. I was typing from my phone.... AND - 4 tires have a lot less buoyancy than 6. Do the math.

Thanks for the recommendations!

I have a tandem with no guide poles. It is a "bolt together" trailer. I can see how the long bunks in addition to the four tires don't help. The trailer holds a Sunsation 288 so there is plenty of wood to add to the bloating issue.

I've tried backing in slowly, dunking the entire trailer and pulling out, and many of the other recommendations from above. I'm not a novice but would like to think I exhausted every solution before bolting weight to the trailer which seemed to be the best solution outside of buying a steel trailer.

Seeing that I boat in only freshwater I could go either way!