The "Extendo Prop" is specifically engineered for boats like the Catalina 30 originally designed to use the early Atomic 4 gasoline engines.
All Catalina 30's (and many other designs from the early 70's) were built to be powered with the Universal Atomic 4. With the advent of modern lightweight diesels, boat manufacturers quickly switched from gas to diesel. However, the boats were never redesigned to take full advantage of low speed, high torque characteristics of a diesel. The same engine beds and drive system continued to be used on the diesel equipped boats. This limited the diameter of the prop to 13" even though the diesel engine is more efficient with a 15" diameter prop.
The Extendo Prop solves this problem. The extended hub moves the blades farther aft. Due to the shaft angle and rise of the hull, a 15" diameter can now be fitted. The 15" diameter X 11" pitch is the perfect prop for the Catalina 30 equipped with a Universal M-25XP engine with the standard 2:1 transmission. A NYLOCK shaft nut is included for installation.