Nick
What boat are we talking about?
Have a look at air berths too.
The seapen concept works too as the growth is inhibited by the lack of water change / turnover. A neighbour has one and the crap on his hull seems to fall off as he uses the boat.
The airberth keeps everything dry and out of the water and muffs can be fitted to motors and they can then be flushed. Not so easy in the seapen.
IMHO second hand airberths etc is the way to go as they do hold their value after the initial depreciation
Cheers
Chimo