I'll have to disagree with disagree-able ;-)
So, I really think local conditions dictate and your results may vary. Same reason I tossed the idea of a masthead navigation light after scaring ourselves several times overlooking a tacking boat near a finish line at night flying a masthead tricolor that simply wasn't where you were looking. Perfect for offshore, not good in close.
Where I moor in Catalina has both a mooring field and an anchorage. LOTS of times people will go through the mooring fields on a no-moon night and with no urban lighting and more boats around than you would find in a typical anchorage, might well hit you before they see you. Especially when they are distracted, finding their way through the moorings or looking for one, etc. So, an anchor light just isn't in the right place, which is why most will put some LED's near the coach roof or pulpits.i'll disagree.
a light at the masthead is very easily seen under all but the foggiest conditions, as well as anywhere else you my have it mounted. with the exception of a light mounted elsewhere and you happen to be approaching from the opposite direction with the light being in the eclipse of an object such as the mast or sail...
and if you are close enough to the vessel that it is out of your upper peripheral vision, then you are very likely close enough to the vessel to actually see it even in the darkest of nights.... and if it is a night with absolute darkness, the masthead light will catch your eye easily...
in addition, many, many times the fog hangs on the water about 10-15ft thick... with the masthead in very clear weather, and a light shrouded in the fog cannot be seen very far, but one that is above the fog and is visible from above has a much better chance of being seen than a hidden one....
sometimes the fog is layered a bit higher and the hull can be clearly seen, but the rigging cant.... and a light isnt going to be seen unless its less than 5ft from the surface of the water....
and if its a complete white out with fog, a light has little use. and if its a clear night, then there is no problem...
ive NEVER seen the time or condition where the coach roof/boom area of a vessel was visable but the hull and mast were not.... the fog just doesnt layer that way... its either the lower portion of the hull or the upper section of the mast.
so no matter what theory you want to argue, it doesnt matter, because 2 conditions will dictate whether the light can be seen or not..... the weather and how observant the "look out" is....
personally I want my light above the fog in the best place for a fisherman or ferryman to see it, as that is where they are located when looking out their wheelhouse window.
too many times ive seen the layered fog, and i know for a fact in these conditions a masthead light is better.
So, I really think local conditions dictate and your results may vary. Same reason I tossed the idea of a masthead navigation light after scaring ourselves several times overlooking a tacking boat near a finish line at night flying a masthead tricolor that simply wasn't where you were looking. Perfect for offshore, not good in close.