My 1987 Catalina 22 has stayed dry through many rain storms while tied up at a dock. No setting on a trailer, I am getting water in the bilge, sometimes lots of water. Any ideas on the likely p[laces to check?
The boat likely isn't at quite the same angle on the trailer as in the water. Try using a hose or bucket to pour water on deck to find the low point where the water runs to while on the trailer then look around until you find the leak. Could be hatches, deck hardware, cracks. In my case the hull to deck joint was the worst offender.
Check around the glass in the windows. Check that water is not getting in on the front edge of the slide hatch. This is often an unforeseen result of the boat sitting at a slightly different angle, fore and aft or side to side. Boats sitting on a trailer rarely sit at the same angle as they do in the water.