Hollow with some vent slits. Has an O ring. Part # BH11 UPC 9321977688703 Nearly all stand up paddle boards have an air vent installed on the deck surface. Air vents are used as a way to help prevent delamination by changes in air pressure typically by heat causing air expansion or plane flight (altitude) making air bubbles underneath the shell. Delamination by Heat Nearly all stand up paddle boards use EPS foam cores because the material is lightweight and reasonably easy to shape. But when EPS heats up, it can become unstable and expand, causing delamination. Your SUP can warm up quickly when not in the water. Leaving a SUP exposed on the beach or shore, without a bag covering it, can cause a board to delaminate. An air vent helps prevent delamination by allowing some of the expanded gases to equalize, but it is not a cure. If you are not using your stand up paddle board, you want to keep it in the shade or secure it in a reflective board bag. Delamination by Altitude Changes in altitude are the other main cause of delamination. This can happen when traveling in the mountains, long road trips or plane flights. As the atmospheric pressure is decreased (or increased), it creates a pressure differential between your SUP and the atmosphere. A vent will allow the pressure to equalize, saving your board from delamination. Some vents will work closed by having a breathable membrane and some require loosening and tightening. This is a replacement for a missing or damaged plug for the ocean and earth range of cruiser boards. Follow your user instructions for further details
