The diffuser ring keeps exhaust gases from re-entering the blades at low speeds. If you don't have one, your propeller may be slipping. Only on a thru-hub exhaust system do they aid to keep the exhaust gases in the slipstream (the flow of water between the bottom unit and the propeller hub) from causing cavitation and, in the worst-case scenario, complete blow-out (total loss of propeller "grip"). With this top-of-the-line marine product, you'll be able to spend even more time on the water. Whether you're practicing your casts, snorkeling the amazing marine life, or just drifting on the open sea for fun, you'll need to equip your boat with the greatest equipment to ensure that nothing interrupts your fantastic recreational time. Key Features: It has an outward flare that prevents exhaust gases from re-entering the blades and reducing thrust It prevents the prop from breaking as a result of the exhaust. Product Specifications: SKU: 9500-090 UPC: 014984009836 Weight: 0.14 LBS Quantity: 1