While shading is an important consideration and everything should be done to eliminate it. I think orientation is just as important, and in some ways more so. We run our boat, except the water maker, on two 35 watt panels. That’s refrigeration, lights, stereo, TV, ham/SSB, computer and other things I can’t think of right now. We only put up the 75 watt panels to run the water maker. This is because the panels articulate both horizontal and vertical.
This is a test I ran was on a mooring in the Channel Islands. The two panels were flat and the sun was at noon but about 15 degs. South, output was 1.6/.8 amps. I tilted the panels south slightly, about 15 degs., the amps increased to @ 4.2/.6 amps. Just a slight change made that much difference. Last fall we spent a week in Tamales bay north of San Francisco in mostly overcast conditions and the two 35 amp panels kept us charged even when we watched movies.
With permanently fixed horizontal panels you’ll be lucky to get 20% of your daily potential output. As Norm stated in an earlier post he only gets full charging from 11:00-1:30 which correlates to what I have found. We get near full charging when the sun rises and it can continue all day, a real advantage over flat installations. Also since most of our charging is done with just two small 35 watt panels, mounted up high and out of the way, I don’t have to clutter up the boat with large arrays which has to do with convenience and safety not cost of additional solar panels.
A boarding wave taking out the panels high up would be so rare I don’t even consider it. In twenty five years of cruising the Pacific coast we've never had a wave hit from the stern that could have damaged our panels, although we have had some that hit from the side that might have damaged a rail mounted panel, but I doubt it. Our 75 watt panels are rarely up while underway, the 35’s are but could be taken down if necessary but we've never needed to. in the last twenty five years we've mounted the panels every way I can think of and have found this method to double or triple our output over any other way we've tried.
Keep in mind that running a generator or motor a few days on vacation for charging is lot different than full time cruising. It gets real annoying fast. Most cruisers learn to live with it but I've never met one whom wished they didn't have to. No matter what the cost panels are it’s still a considerable investment in money and more importantly time, so it makes sense to try squeeze every watt out you can. It will make your anchoring much more pleasurable. Lastly invest in an MPTT controller; they’ll increase your output 15-20%.
Jim