View Full Version : wet exhaust questions...

10-20-2002, 09:33 AM
I have heard that having your exhaust outlet underwater causes undue "backpressure" on the motor/exhaust manifold gaskets.

Also, "when experiencing motor trouble, cranking a motor for long periods can actually draw water into the motor thru the exhaust".

Any truth to these, and do you think that a diesel (130hp) would be any quieter by doing this.

Just thinking out loud.... thanks

10-20-2002, 12:17 PM
Ignorance being no deterent, here goes: How far under water, that is the question. Now, with the Yanmar GM-10 on Prairie Islander over cranking would not draw water in the exhaust. What it would do is fill the exhaust system with water and if done long enough it would flood the cylinder. Sort of the same effect so I guess I'm only quibling with the word draw. Other diesels would work the same, I should think.


10-20-2002, 02:31 PM
Yes, overcranking the engine will cause water to backup into the engine. I leave my seacock closed until the engine starts. Check your oil and make sure it didn't invade the crankcase. Although my exhaust isn't below the waterline, I would assume it would cause unneeded back pressure unless thats the way it was designed. I learned this the hard, (Dumb@$$) way!

Ross Faneuf
10-21-2002, 01:42 PM
I located Ceol Mor's exhaust too close to the waterline, and will move it before next year's season. I have a seacock in the line, and a high loop, so I don't worry much about water backing up into the engine. But I do worry about back pressure. And, if the exhaust is at/near the waterline, the visual check for cooling water in the exhaust I always make isn't conclusive.

Also, if motor sailing, then theyre's too much chance heel will put the exhaust significantly underwater. I'm going to make sure the exhaust is above water at least to 10 degrees of heel - probably by figuring out how to run it out the transom.

Ken Liden
10-22-2002, 08:11 AM
Underwater exhaust was once quite popular and in recent years is again gaining popularity. The trick is that your exhaust must be routed to a point well above the waterline to avoid backfill of the engine when not in use. Your back pressure must not exceed the maximum back pressure determined by the engine manufacturer. Under ordinary conditions the engine will not draw seawater into the engine. Warning!!!!. I once was called upon to place a vessel in service after the winter layup. Upon cranking the engines for initial startup both engines immeadiatly filled with seawater (underwater exhaust). Unknown to me was that the owner during the winter decided to rebuild the starters. When
re-installing them he put the Port starter on the Starboard engine etc. causing the engines to rotate in reverse direction and pull seawater thru the exhaust. Many sailboats do submerge thier exhaust outlets during sailing without problems so long as there is adequate height in the system.

Phil Young
10-22-2002, 10:09 PM
re backpressure, dunnno, but i have a friend who retrofitted a turbo diesel to a boat with an underwater exhaust. He found he had to do an above water outlet to let the turbo work.

re noise, I had a very noisy generator engine on my boat. A short length of rubber hose from the end of the exhaust down into the water made it much much quieter.

10-22-2002, 10:49 PM
A proper 'wet' exhaust is not that noisy and has nothing to do with the exhaust pipe being above or below the waterline. It has to do with whether or not seawater is injected into the exhaust immediatly after the exhaust manifold. Directing the exhaust under the waterline is asking for trouble.

I suggest you look for other ways to quiet things down, such as: If its not a wet exhaust, make it a wet exhaust; If it is a diesel, look into a number of intake noise suppressors, Walker Air-Sep, being one; Add a muffler; Insulate the engine compartment. Diesel or gas, you'll never sound like a Lexus 400 at idle.

John R Smith
10-23-2002, 03:38 AM
I agree with the above. The exhaust is the least noisy part of our little diesel. Most of the racket which one experiences in the cockpit is mechanical.