The Jack is a directional antenna, so mounting it on a rail is a little difficult.
Since its on a boat, try a set-top antenna that you can rotate by hand as long as the stations are local. For distance you will need to get something up in the air. Omni's are nowhere near as problematic on boats as they are on an RV, so that's another option, but it needs to clear the deck rigging to get away from rails, etc.
Jeff - WA6EQU
'06 Itasca Meridian 34H, CAT C7/350