Water in the cylinders is enough to destroy an engine, but it's easy to do by mistake, says Vyv Cox

Are you guilty of flooding your engine?

Yacht engines have a lot in common with every other kind of engine, indeed in many cases the same engines used in yachts are also used in various forms of equipment that have nothing to do with the sea, including agricultural, automotive and industrial applications.

However, whereas the vast majority of these are cooled by air passing through a radiator, most yacht engines are cooled by the water they float in, seawater for the purposes of this article.

This can be achieved either directly, where the seawater pumped into the boat passes through the engine (figure 1), or indirectly, where the engine is cooled by a coolant that is itself cooled by the seawater via a heat exchanger (figure 2).

Engine diagram

In each case the seawater passes out of the boat via a waterlock or trap, often made of plastic material, and flexible rubber hose.

The water flow prevents these from being overheated by the hot exhaust gases, (figure 3). The upper bend is there to prevent any water driven from astern by waves from entering the exhaust hose and ultimately, the engine.

In some installations a silencer is also added in the downward leg above the skin fitting in figure 3, adding to the height of the top bend.

While the engine is running, the pressure of the exhaust gases is sufficient to blow water that accumulates in the trap out through the skin fitting.

When the engine stops, any water that is in the exhaust hose runs both forward and back into the trap, which needs to be large enough to accommodate this volume of water.

The damage caused by flooding your engine

In the vast majority of cases this arrangement works perfectly but in one or two circumstances things can go wrong.

What must be avoided at all costs is to allow water to reach the level of the dashed line in figure 3, upon which water will enter the engine.

Figure 4 shows a typical water injection elbow, as used in almost every boat.

If water reaches the inner tube of the elbow, through which exhaust gas passes, there is nothing to prevent it from running forward into the exhaust ports, from where it can flow through one of the open exhaust valves and thence into the engine.

In the event that an attempt is made to start the engine the rising piston will meet incompressible water, causing considerable mechanical damage to the pistons, connecting rods, valves and cylinder heads.

The most common cause of this disastrous situation is turning the engine over for extended periods without it firing.

Water is thus being pumped into the exhaust hose without the essential exhaust gas pressure to drive it over the bend above the skin fitting. Its level will continue to rise until it floods the elbow and enters the engine.

How to stop flooding your engine

The best advice in situations where the engine does not fire within a reasonably short time, in cold weather, for example, or if the cranking battery is degraded, is to close the seawater seacock during cranking until it does.

Headshot of Vyv Cox

Vyv Cox is a chartered engineer and has been sailing for more than 50 years

Turning the engine over on the starter motor will not harm an already wet pump impeller but the seacock must be opened smartly as soon as the engine fires up.

In some installations there are good reasons why the exhaust elbow needs to be raised above its normal position, directly bolted to the cylinder head. In these cases an additional vertical length of pipe, called a riser, is introduced between the engine and the elbow.

The effect of the riser is to lift the elbow to a position above the height of the bend, so that water will always drain aft.

They may also be fitted where the elbow height is below sea level and the bend cannot be fitted higher, due to craft design restrictions. Any water driven forward by wave action is thus prevented from entering the engine.

There have been several examples of trap under-sizing, either in the initial design or when owners have reduced the size of the trap to save the purchase price of the correct one. In these cases the volume of water in the hose was greater than the capacity of the trap.

When the engine stopped the water in the hose flowed forward, filling the trap and the leg of the hose up to the elbow. The result in at least one quite famous case was major damage as described above to more than 40 boats of a new design.

In other cases a small amount of water entered the engine, not enough to wreck it but enough to generate inexplicable rapid corrosion of internal engine parts.

Learning the hard way

Flooded engine

The blackened and damaged piston head caused by flooding the engine is visible

Had I read Vyv’s advice before, I might have avoided a very costly mistake, writes editor Theo Stocker.

The old Bukh DV20 on our Sadler 29 was 35-years-old, but had been a reliable starter, though a little slow in cold weather.

At the end of last winter, on a particularly cold but clear spring morning we went to start the engine. It nearly fired a couple of times and then on the final attempt, the engine stopped dead with a loud bang. The engine now refused to turn over.

Continues below…

On closer inspection, oil was dripping into the bilge – the chain belt that drives the water impeller had broken through its casing. An approved Bukh dealer nearby came to have a look.

With the engine head removed, we could see that the affected aft piston head was blackened and cracked and at least three of the pushrods were badly bent. It became apparent that repair was probably not feasible or economical.

Bent push rods

Bent push rods

New engine being craned in

New engine being craned in

The engine was written off. We opted to replace it with a reconditioned Bukh DV24; neither petite or light, but it fits in the same footprint (and mounts) and is a solid workhorse that got us sailing again.

I’ll never over-crank an engine again, certainly not with the water cooling open.