The best prop for an Irwin 37 is a 16 x 12 x 3 prop. It is 16 inches in diameter It is 12 degrees of pitch It is 3 bladed This prop turns left hand. This prop uses a 1 1/8 inch shaft. THis is the best combination. I think your transmission will dictate what way your prop turns on the shaft. A velvet drive will turn left, I believe.