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Protect your engine with an exhaust swan neck

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What is the issue?
Most cruising vessels have an inboard diesel engine with a ‘wet’ exhaust system. Salt water is injected at the riser, which is the outlet from the exhaust manifold and a mixture of water-cooled exhaust gas and water is then passed through a series of bends and components until it exits the boat at the exhaust outlet, typically at the stern.

The problem is the exhaust pipes are subject to taking water in through the outlet. This is a common and dangerous occurrence and is entirely down to poor exhaust design. If this happens in quantity the exhaust system will backfill right up to and into the pistons.

Exhausts should be as high above the waterline as possible but they are always vulnerable. Following waves can push water from behind into outlet pipes and they may go a long way under when heeling. Long distance cruising boats are more subject to this as they are typically loaded more and so they go down a few inches on the waterline making them more vulnerable. You will know your engine is full of water if you cannot turn it over, generally after a long sailing period or having been hove to.

Why address this?
Taking in a volume of water via the exhaust outlet will at best disable the engine. At worst, if it backfills into the pistons, it could cause untold damage.

How to address this?
Fit a stainless steel swan neck, or gooseneck/water lock hose, at the stern exhaust outlet to stop water flowing back in and straight down into the engine. The swan neck provides an expansion chamber so that when sailing, any following sea that comes into the tailpipe hits the expansion area and then falls out.

Swan Neck or Gooseneck on the exhaust outlet
Drawing: Tony Gibson
The purpose of a swan neck, which vary in size and shape depending on the engine size and stern layout, is to muffle noise and provide back pressure for engine operation and prevent water coming back into the engine.

This invidious problem has a tendency to catch you out when you least expect it. It was only when we hove-to that this issue manifested itself upon our vessel. We noticed the boat slow to start after hoving-to but paid little attention to it. It was only when a real blow came and we hove to for a night stop that we found the extent of the problem we had. At this stage, it backfilled right back to the pistons. I believe we came within a hair's breadth of losing our engine and perhaps even the boat itself in the dangerous waters we were sailing at the time.

Being out in the deepest South Pacific, and at a time when my circumnavigation budget was running low, I could not address the problem at the time. To prevent it happening I resorted to hanging over the stern and tapping a bung into the exhaust from that point on. Literally, at the merest possibility of backfilling, I was over the stern.

The first thing I would do with any vessel now is to check if it has a swan neck, if not I would set one in place. A stopper pipe helps for following seas and will improve the situation, but it will not halt an inflow when the outlet regularly dips below the waterline in a big seaway as ours tended to do when hove to.
Please note

It is worth checking that the vessel also has a siphon break fitted to the water injection line.

In the event that you have fallen victim to it First, check that the starter motor is not the reason for no engine cranking. Franklin Marine suggest trying the following steps:

  • • Remove exhaust hose from manifold.

  • • Remove rocker cover.

  • • Cut up an old credit card and place a piece between each exhaust valve and rocker arm

  • • Crank over engine, to pump out water.

  • • Remove card and refit rocker cover.

  • • Refit empty exhaust hose and start engine.

If this does not work:

  • • Remove all injectors.

  • • Crank over engine to blow water out.

  • • Refit injectors, bleed injector lines, and start, then leave motor running till back in port and redesign exhaust.

With thanks to:
Michael Harpur, Yacht Obsession.

Diesel Engine Cooling System

Marine Diesel Engine Cooling System

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