hi guys, another question about my old Century boat..my thru transom exhaust doesn't have any flaps, nor does it have a riser on the outlet of the exhaust manifold (this all appears to be original), I noticed in latter production years of this same boat that flaps were added to the exhaust tips....I'm wondering if there is a general rule of thumb in regards to height of exhaust manifold compared to waterline on the boat that would dictate need for flaps or a riser...Thanks!