Hi,
Its a 2"(50mm) x50.
The situation is that I started with the cortina motor on a 12v gel cell which was quite good. Saw my clubmates boat fitted with an 850 on 12v and was impressed. Decide to try upping the voltage with the cortina motor..plan was 16 cell, 18 cell and then 24v if necessary. I had previously hit the prop on a rock and had stripped the plastic universal joint but had glued it back as a temporary measure. Tried 16 cells and the uv stripped again. Thought that changing batteries to get duration and performance was a bit counterproductive so elected to buy and fit the torpedo 850 and accept similar performance that I had observed on my clubmates boat. Bought a new UV and there we are. presently a new 850, 12v gel,new UV,50mm x50 prop and a mktronics 25 amp speedcontroller waiting to sail. Bill