Increased diameter also increases blade area in most cases and this causes additional drag. Four is that shaft length and may or may not be omitted, 13.25 is the diameter and 15 is the pitch. Aluminum is very common, featuring good performance and price, and the ability to be repaired easily. Aluminum is somewhat soft and easily nicked or split, and it also flexes under load, compromising performance. Stainless steel offers excellent power transfer with minimal flexing, its hardness is well documented making it extremely durable but more difficult to repair.
Stainless props can be very expensive, and reign supreme in high performance applications.
Composite technology is claimed to offer performance and durability similar to aluminum props at a lower price. Not too many years ago, composite propellers were carried by boaters as lightweight emergency spares. Today composites are used as primary propulsion by many boaters and a couple of manufacturers. Composite propellers offered by companies such as Comprop are not designed to be repaired, and under hard impacts the blades will depart the hub. This feature is said to enhance the survivability of the prop shaft.