I have a 17ft that the scuppers are right at te water line, they are new, the type with a rubber flapper. Prblem is, every time I wakk to the back of the boat the water comes flooding in. I know I can just block them off but sometimes I do take some water over the stern or sometimes it rains. Are there any scuppers that acutally let water out and not let it back in. Any weight (too much water from a good storm or a person puts them under the waterline and floods the boat. Thanks for any advise