Considering my current problems with pilings that are too short with
the approach of TS Bonnie, I am re-considering an old idea of mine, a
way to allow a boat to rise and fall with the tide in a slip with
fixed pilings. Of course, I want to also be able to extend the
effective height of the pilings.

First, I have seen where people use pulleys affixed to the pilings
with weights on one end of a rope going thru the pulley and the other
end attached to the boat to keep a boat centered in a slip but this
really doesnt work with a heavy boat with swells or wakes from passing
boats. This seems mostly useful in calm water.

So, I am thinking of a robust piece of C channel running up the piling
through bolted to the piling every 6". A piece of metal is free to
run up and down in the channel and it has a threaded hole into which
the thread of an eye bolt screws. You tie your dock lines to the eye
so the boat then rises and falls with the tide as the metal T in the
channel rises up and down pulled by your boat. Of course, fouling and
corrosion might be a problem so it might be desireable to spray
galvanize it every yr but I assume that fouling would be scraped off
simply by the weight of the boat pulling on the T. I considered
having the metal T in the channel having wheels to make the motion
smooth but this is a nasty environment so simplicity is better. I can
imagine the channel running higher than the piling, perhaps up to 3'
higher thus providing some storm swell protection if the channel is
properly braced.
Any thoughts on this? Would it sell as a product? What would you pay
for such a thing?