I have an Islander B30 and have replaced the scupper hoses which do freeze and split if water sits in them over the winter. The splits are not visible and the water does flow into the bilge. I've learned to make sure the cockpit is covered during the winter and put antifreeze in the lines.
I would think that knowing the shaft diameter is more important than shaft length. When I replaced the prop on my IB30, shaft diameter (mine is 3/4 in), rotation direction and prop diameter were the 3 factors needed.