SD 217 I/O Drain
Comments
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Your boat has a similar deck layout like mine. My rear deck drains (there are two, one in each back corner of the deck) go out hull fittings (the "flapper" you mentioned is called a "scupper") located near the rear of the boat. My drain outlets do sit under water when I'm not under power. However, my deck drains are a bit higher in elevation, so they do drain (if the drains don't have an obstruction, your boat may have them sitting a bit lower, which would explain slow or no draining). However, because the deck drains are wide open without any screen covering them to prevent debris from going down them and drain water is not under much pressure, the scupper can sometimes hold back and accumulate debris. I sometime use a hose and flush the drain system under pressure to help keep them clear. Others use compressed air.
2012 SD237 I/O Mercruiser 5.0L MPI ECT/ Bravo 32012 Load Rite Elite Tandem axle trailer