MARINE EXHAUST SYSTEMS IN SAILBOATS

Marine Exhaust systems has confounded Marine Engineers, Marine Engine Distributors and Boat Builders since they first installed an engine in a boat   Although Power Driven yachts and commercial vessels continue to have challenges, it is the modern sailing yacht that seems to suffer the most.

Every time we get a call about a starter on engine being seized, the first thought we have is that there may water in the cylinder which as created a “hydro lock”.  All too often, we are correct.  If the situation is caught in the very early stages, the engine can be salvaged by removing the injectors, cranking the water out of the cylinders, and then running the engine under load at 50% or more for a minimum of 3-4 hours.  If the victim is not lucky, the salt water in the engine has corroded the liner, rings and pistons to the point that the piston is almost welded to the liner wall.  In other cases, in the act of attempting or actually starting the engine, the connecting rod has bent.  Before long that connecting rod will break, usually in a catastrophic circumstance.

How does raw (or sea) water get into a cylinder? In our experience, 99% of the time, it is by going up the exhaust system.  There are a variety of ways we’ve seen this happen, but it is always the result of a poorly designed exhaust system.  What works sitting in a marina tied up to a dock will not always work when a sailboat is at sea.  In addition to the Pitch and Roll all vessels experience, sailboats heel.  Monohulls heel the worst, but even multihulls heel to a certain degree.

Marine exhaust systems have gone through a variety of design changes, or evolution, over the years.  The early dry, insulated, straight out the transom was the simplest and, possibly the best.  But the safety, and ambient heat, factor of having a pipe with a temperature of up to 600-700 degrees F running through the boat was a problem.  If the engine was located anywhere other than in the stern of the boat with an unobstructed run out the transom, that also created problems.  So, the water jacketed exhaust was born.  This was not only expensive, it also had the inevitable problem that the eventually the metal would corrode.  If on the dry side, that resulted in sea water in the dry exhaust side.  In many cases this resulted in s/w going into a cylinder.  For a while in the 70’s the dry pipe in a wet can was very popular.  But it, like the water jacketed dry exhaust risked the eventuality of corrosion and water entering the dry exhaust.

In the 70’s the “wet lift muffler” system was introduced.  The premise was simple and, if properly implemented, very effective.  The design required a “wet lift” muffler, sometime called a “pot”, “water trap” or “water lock”.  The outlet pipe, or exit point, in the muffler was inches above the bottom of the muffler or “pot”.  Exhaust gas entering the pot would push the water level down to just above the outlet pipe, both keeping the water level low in the pot while running, but also doing a good job of greatly reducing the noise level of the exhaust.  Neat!  The tricky part was making sure that the “pot” was the lowest part of the exhaust system.  It needed to be well below the level of the water injected exhaust elbow with a minimum of ½” per foot down angle (no matter the angle of pitch, roll or heel) in that hose.  So, for a 3 ft run of hose, the top of the muffler (not the entry pipe on a side in exhaust) needed to be, at lease 1.5” below the exhaust elbow.  Over the years, we have adopted a policy that this measurement be a minimum of ½” per foot or 12”, whichever is greater.

The original design required the exhaust hose to run straight up a maximum of 4 feet above the base of the pot, then down all the way (minimum ½” per foot down angle regardless of the orientation of the vessel) to the exhaust outlet on the hull.  There have been a number of changes to this system by other manufacturers, but not all were created equally.

The theory of this system was that the pot, not only silenced the exhaust, but it “trapped” the water that was in the hose.  So, theoretically, all the water was kept safely in its muffler and a safe distance from the exhaust elbow.  The problem is that, with pressure on boat builders to limit the size of their engine spaces, they allowed less and less room for hose and trap.  This resulted in often too long hose runs and marginally sized mufflers.  The designers assumed that the hose was half, or less than half, full of water because it was mixed with gas.  In addition, they allowed for water exceeding the capacity of the muffler to rise up into the hoses going in and out.  The problem with this theory is that the volume of water to air mixture in the hose leaving the muffler is not consistent.  The gas/water mixture leaving the pot is not always the same.  It takes more pressure to push the water up the exhaust hose than it takes to go in.  So the water level in the muffler increases to the point that it can overcome the back pressure of the entire exhaust system.  At that time, in the words of Dan Durbin, it “burps”.  Everyone has seen a wet exhaust as it gurgles, gurgles, gurgles then “whoosh”, a spray of water erupts out.  That is the “burp”.  During that period, the muffler gets fuller and fuller until it is able to evacuate itself.  The hose leaving the muffler up to the high point of the exhaust is full, or nearly full, of water.  Shutting down the engine in that burping process exceeds the design parameters of some marginal systems.  If the water in the hose between the exhaust elbow up to the top of the “lift” exceeds the capacity of the muffler, water is sitting in the hoses leading to and exiting the muffler.  So there is water “overflowing” the muffler or trap and is in the hoses.  Depending on how much hose and what the angles are, there is the possibility that the pitching, rolling or heeling of the vessel will cause this water to slosh back and forth, sometime gaining in momentum.  A splash of water can pitch up the dry section the exhaust elbow and go into the exhaust manifold.  The water will settle in the lowest part of the manifold it can find, which would be an exhaust valve.  If that valve is open, or partially open, it runs into the cylinder and settles on top of the piston. If, on the remote chance, the valve was completely closed, the salt water will settle on the exhaust valve.  Valves are designed for their sealing properties and to withstand the high temperatures of combustion, not for their resistance to corrosion.  No manufacturer designs an engine to have salt water in it.

From you highschool physics you will remember that water is “incompressible”.  If even a thimble full of water enters the cylinder, the piston will stop before it hits what is known at “top dead center”. The engine will cease to turn.  Depending on the forces placed on the piston, this may result in a bent connecting rod.  If the engine sits long enough for the water to drain past the piston rings, the engine will restart. But, in the event of a bent rod, the damage is done.  That bent rod will break.  Sometimes it is assumed that the engine won’t start because of a failed starter and the operator will wait until they can get to port and a marine engineer.  If that period takes more than 24 hours or so, the salt water will start to corrode the iron liner at the point where rings meet it.

Other causes of mufflers overfilling include siphoning and over cranking of the engine. We will discuss this more in future articles.