remove the following line from your script:
my.polygon = on;

Quote from the manual regarding the polygon flag:

Quote:

---

Only one of the objects involved in a collision should have a polygonal hull. Do not set this flag for moving objects or for animated models.

---

edit:
Next thing is:
If you already have a player movement take a look at it how it handles gravity.
Usually this is done via some c_trace, now add the ACTIVATE_SONAR flag to that c_trace instruction and let your platform react to that.
If the EVENT_SONAR of the ship entity is activated, the player is on the platform.
You could additionally compare the z-position to check if the player is standing on the ship or jumping above it.
If he jumps you don't add the moving vector of the ship to the players moving vector.
If he is not jumping and thus standing you add it to it or rather perform a c_move(player ...) to force the player the same way.

But: Be carefull with the relativ and absolute move vector.
Best thing is to translate the relativ movement of the ship into an absolute moving vector and apply that to the player.

hope this helps a bit.