diesel exhaust below water when underway

[M]

The previous owner of my Lancer 25 added an inboard Yanmar 1GM10, and ran the exhaust pipe out over the cockpit/lazarette drain pipe that exits the transom. I recently replaced the original exhaust "hose" which was PVC pipe and melting. I added a higher bend in the hose to keep water from washing back to the engine, although now water builds some, then exits every few seconds in big spurts. A good mechanic told me that is not a problem.

When the boat is underway, by either sail or power, the exhaust pipe exit in the transom is actually just below the water. Is this bad?

Also I've had a hard time starting the engine coming back in since this was done.If it doesn't start at first I close the raw water intake seacocks (2!) and it (so far) starts then. The mechanic is adding a muffler to check any backwash of water to the engine. Any thoughts on any of this? The engine is in good shape, only 300 hours and I replaced the impeller and added a new alternator and cranking battery.
I was not able to speak with the prior owner, but I know the boat stayed at the dock for the last couple years. ANy thoughts?

 

[BobM]

On my S2

My exhaust is essentially underwater too while underway. The exhaust port is under the stern just above the painted water line on the starboard side, if I recall correctly.

There is a water muffler and the exhaust is routed to prevent back flow.

There are pluses and minuses to the location. A plus is that any particulates go right into the water, which should reduce any transom "mustache," which I often hear others complain of. On the minus side, it makes it difficult to see how much water flow I am obtaining at a given time and what color my exhaust is (for diagnosing problems).