I have a 38' single screw full keel trawler with a 120 Lehman. Boat weighs aprox 30,000 #. The three blade prop that is on the boat is 26" diameter. I do not know the pitch. Not able to find any markings. A spare prop on the boat is a four blade marked 26 X 26. At 1800 rpm the boat goes about 7 knots, this seems a bit slow. What are the pros and cons of 3 blades versis 4 blades? Would appreciate any other data on boats of this size and power. Frank 38' PT Cheer Men