Magnetic flag
 

How can I attach a magnetic flag to the hull of my hull that's fiberglass.

You can't attach it directly. Affix a ferrous metal plate to the hull first using screws or glue, then the magnetic flag will adhere to the metal plate. Regardless, a large wave at speed will probably knock the flag off and into the water. :head:

I was thinking using something like gorilla sealant cause I'm not going to screw into my hull so I think I'm just going to stick it on my trailer, thanks for the comeback destroyer