I don't really know...but what I do know is, that just before I retuned to Italy, moored opposite me on the Lancaster was ( & possible still is !!) a BW workboat, it took three "Lengthsmen" 3 days to load the boat with wooden stakes and planks. They then set off to "repair" the bank side a little further down the canal. On passing, their work boat was adrift and partially blocking the canal, the "repairs were incomplete and they had managed to repair up to a clump of weeds bypasing them and continuing thereafter.. So I guess at least three that not only are superfluous, but also (apparently) useless..