A Propeller Brass Crown Castle Washer for a 15-spline Mercury Propeller Shaft Hub Kit is a specific type of washer used in the assembly of boat propellers. 
 
Prop Castle Washer: This refers to the shape of the washer, which has notches or slots cut into it. These notches are designed to engage with a cotter pin or a similar locking mechanism, preventing the washer and the nut or bolt from loosening during operation.
15-Spline Propeller Shaft: This refers to the number of splines (or ridges) on the propeller shaft. The washer is designed to fit propeller shafts with 15 splines, which is a common configuration for certain Mercury outboard motors.
 
In summary, a Propeller Brass Crown Castle Washer for a 15-spline Mercury Propeller is a specialized washer used to secure the propeller to the shaft on Mercury outboard motors, ensuring that the propeller remains tightly fastened and operates efficiently.
 
Included one (1) Propeller Brass Castle Crown Washer Kit. Replaces Quicksilver Mercury Crown Washer for Rubber Hub Props 12- 31211Q03 12-31211A2