Unfortunately, most hams do not really understand antenna principles either.
And I include myself in that category, although I have an Extra Class ham license (AF4WM), have been a ham for some 47 years, and have a few QRP awards stuck away in a desk drawer somewhere.
But I was thinking about the OP's question some, mostly because it is a rainy Saturday afternoon and nothing better to do. An RF signal is a sine wave, and when it leaves the transmitter, hustles through the coax, and through the wiring in the antenna, then in a perfect situation it will exit the antenna as an acceptable part of a sine wave. That is, not cut off too soon or too late. Channel 16 of the marine band is 156.8 Megahertz. At that frequency, one wave-length (how long the sine wave is before it starts to repeat) is about 75.3 inches, or 6.275 feet.
VHF Marine antennas typically use 1/4 wavelength, 1/2 wavelength, and 5/8th wavelength ratios. A 1/4 wave antenna then would be an antenna with an electrical length one-fourth of the actual signal's wavelength, or about 18.8 inches. Lots of sailboats (mine included, back when I had a sailboat) just stick a 19-inch whip at the top of the mast and that works just fine.
Trawler people typically use 8-foot antennas, which one might think would be a full wave-length, but actually are not necessarily so. Usually the coax just continues on up into the fiberglass, and the actual antenna is -- surprise! -- only 19 inches.
All of which is by way of thinking that I am not absolutely certain that putting a piece of PVC pipe over the antenna, with some of the antenna sticking out above, would not work perfectly well. Not saying it would, mind you, and if others more knowledgeable about the subject want to correct me I shall not take it amiss.