A friend of mine asked for some help putting thru hull on his '90 18.5' Concord Mach 1. The only problem I see is when the boat is idling the exhaust will be under water. I have seen other boats setup like this and was wondering how you keep the water from going back thru the exhaust when the boat isnt running? I know they sell tips with flappers but will that completely cure the issue? Thanks for all the help.

Pressure from the exhaust.

Even when it isnt running?

The angle, flappers, valve inside the tip. Lots of boats have exhaust below the water line when at rest.

Definitaly flappers...

water cant defy gravity with the motor not running the water level inside the exhaust system wont go any higher than the level it is outside the boat the engine is hopefully high enough in the hull to not be "under water " the only time this doesnt apply would be if the engine died while under way the wave that crashes into the transom would be enough to force water up the exhaust and into the motor thus explaining the " flappers" either internal or external ( and sometimes BOTH )that prevent that from happening

You might have to change the exhaust risers to a taller version. Mercruiser makes spacers to do this for stock manifold systems. While the boat is running there isn't much of a problem, however during idle, and shut down times water can be pushed past the flappers. Also during starting and cranking water can be sucked up into the exhaust system depending on cam spaec and timing. Reversion I believe it's called. I'm no expert but look into it a bit before you leap. The cost of an engine or 2 due to water damage for a little sound effects might be a big mistake if not done properly. Just my 2ct's worth.