What's the big water flush screw for on the 1953 Evinrude Big Twin 25hp? It is higher than the top drain/fill oil screw on the other side, and facing the same way as the propeller. Is this to flush fresh water through if I was in salt water? Only fresh water around here. Or to drain excess water before winter? I know to run it on land the motor has to be in a bucket and the water level would cover this screw. My 1955 Johnson 25hp doesn't have it.