Robert Wilson of Dunbar - ship's propeller

I was thinking about the ship's propellers on the ferry to Arran and I remembered that I'd seen this in Dunbar harbour. I've been looking at the issue and have begun to wonder how you choose how many blades to have and what angle to set them at. It turns out that some planes and wind turbines can vary the angle (or pitch) of their blades. Wind turbines always seem to have 3 blades but the one in the harbour has 4. This will need looking into. I like the idea that the young Robert Wilson actually did experiments to find out what was going on.