Trailer Spring Identification:

double eye boat trailer spring example

Double Eye Spring

Measure from the center of one eye, straight across to the center of the other eye. Then count the total number of leaves (the spring in the picture is a 25-1/4", 4 leaf, Double Eye spring)

boat trailer slipper spring example

Slipper Spring

Measure from the center of the eye, straight across to the end of the spring. Then count the total number of leaves (the spring in the picture is a 29-1/2", 4 leaf, Slipper spring)

open eye boat trailer spring example

Open Eye Spring

Measure from the center of the eye, straight across to the center of where the mounting bolt would be. Then count the total number of leaves (the spring in the picture is a 26", 4 leaf, Open Eye spring)

hooked end boat trailer spring example

Hooked End Slipper Spring

Measure from the center of the eye, straight across to the end of the spring. Then count the total number of leaves (the spring in the picture is a 25", 4 leaf, Hooked End Slipper spring)

radius end boat trailer slipper spring example

Radius End Slipper Spring

Measure from the center of the eye, straight across to the end of the spring. Then count the total number of leaves (the spring in the picture is a 24", 4 leaf, Radius End Slipper spring)

When Do My Trailer Springs Need To Be Replaced?

This one is easy! Your trailer springs need to be replaced when the leaves start to separate. Huh?
Think about it like this: Let's say you have a four leaf spring. Put your hand up, with all four fingers side by side, touching each other (each one of your fingers represents a leaf of your spring)If the leaves of your trailer spring look like this, then they are good.
Now spread your fingers apart, if the springs on your trailer have space between them (at the ends of the lower leaves) then it's time to get some new trailer springs.
My springs are rusty! Rust on a trailer spring does not necessarily mean they need to be replaced.
How can I keep them from getting rusty? Well, you can't. But you can try! Coating them with an automotive rubberized under coating can help for a while, but the rust will always win. You could keep your trailer in your garage, and never put it in the water. (Hee-Hee)
The best option would be to get rid of the leaf springs and go with a galvanized torsion axle. The initial cost will be higher, but you can expect to get 8+ years out of it. Leaf springs generally last about 2-3 years.