I'm re-building my small trailer used for the Boston Whaler. It is an older trailer (Yacht Club brand). The frame is galvanized, but many of the component parts are regular steel. Leaf springs and associated hardware have a certain life span in salt water. The roller mounts have some nastiness going on, and those will get replaced once the boat is launched.
I bought new trailers in 2004 (for my Formula 24) and 2021 for my CY 338. The latest one is pretty much all aluminum with torsion springs and stainless steel hardware. It seems like it will stand up to the salt.
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This is my triple axle 15,600 lb capacity trailer used for the CY 338. It has been in the salt for about 4-5 launch/retrievals
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Our launch ramp is best described as a road that gradually goes into the water. I sure wish the town would re-build it with a steeper grade, but that tends not to be a thing around here.
I'm more of a fan of bunk trailers, since rollers can/do leave indentations in the hull, particularly when the boat is stored on the trailer for long periods of time. Of course roller trailers generally have an advantage for launch & retrieval, being you don't have to go as deep.