Another location to consider if you have a radar arch is to mount it horizontally on and above the arch using four legs of equal length. I plan to move mine to that position soon. It will actually be floating above the radar dome and is a 30watt polycrystaline panel running through a Sunguard 4.5amp charge controller to our house bank or dinghy battery. It really make a difference for anchoring or charging while moored to a bouy and basically offsets our use of the fridge in moderate temps. It is worth the effort of you have off-grid charging needs or anchoring habits and a fondness for power. I would consider the battery or battery bank size in choosing the size of your panel and consider something over 20 watts for sure. Much less than 20 will have a hard time with anything more than a maintenance charging task and if you have a group 27, 85 AH battery or larger, that is discharged down to 60% and you put a 5-10 watt panel on it, the panel will not push enough power to effectively re-charge the battery. I would generally consider anything less than 15watts to be a charge maintainer and panels of 30+ to be capable of recharging batteries typical of our boats.
Figure roughly 5 watts per 100AH of battery capacity for floating/maintenance purposes only. And work up from there...
There is enough "equipment" load on our boat that our 30 watt panel can not quite maintain all 370AH on board unless if I leave the house switch on and let those little loads add up coupled with cloudy weather. So switching off the house loads is required for a safe charging/maintenance margin at 12+watts per 100AH to be maintained.