MULTIFLEX Trailer Spool Roller Spool rollers are made of non-marking black rubber with a heavy duty plastic sleeve. Spool rollers are designed to support the weight of your boat on the trailer, and aid in launching and loading. 4Inch & 5Inch Long. Heavy-duty rollers fit the curvature of heavier boats for better support and ease of launching and loading. Gradual inward taper helps center boat. Spool Roller Plastic. Pipe Reinforced.