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Hydrolock - What is it and How to Prevent it. (Written by Captain Frank Taylor)



If you have ever experienced hydrolock on your boat engine, you may already know that it can turn a great day on the water into a really bad day in a hurry.  What is hydrolock?  To put it simply, it is when water gets into your engine cylinders.  While this can also happen to car and truck engines, it is more likely to happen with marine engines because of the environment that boats operate in and it is more likely to happen on boats that have exhaust systems that release exhaust under the water’s surface.

First, let’s talk about how the typical 4 stroke gasoline engine works.  There are four steps (hence the term four stroke) that a cylinder goes through for each combustion cycle:

1.        First the intake valve opens and the piston moves down which sucks air and fuel into the cylinder.

2.       The intake valve closes and the cylinder moves up which compresses the air-fuel mixture.

3.       The spark plug fires, igniting the fuel-air mixture, thus pushing the cylinder back down.  This is called the power stroke.

4.       The exhaust valve opens and the cylinder moves back up which pushes the exhaust out. 

This process repeats continuously for each cylinder with each cylinder being in a different phase for as long as the engine is running.

Hydrolock occurs when water is able to enter one or more of the cylinders which would cause a problem in step 2 of the four stroke process.  As we all know, water does not compress.  In step 2 of the four stroke process, the intake and exhaust valves are closed the piston is moving up in a process that should be compressing a fuel-air (mostly air) mixture.  If the cylinder is full of water, it will not compress but the energy has to go somewhere.  In many cases, that results in something bending or breaking.  Usually it ends up being one of your piston rods. 

Normal Rod and Piston

Piston with Rod Bent Due to Hydrolock 



So how in the world does water get into the cylinders of your engine?  Here are a few ways that it can happen:

1.        Engine timing issues – This is the most common reason.  Incorrect timing set on the boats engine can actually cause the engine to turn backwards almost a full revolution when shut down, especially if the engine shuts down unexpectedly at higher rpms.  This causes the whole system to run backwards.  Instead of exhaust gasses being expelled out into the water, water is sucked up into the exhaust manifold and into the cylinders.  This is a good reason for making sure that your timing is correct and to avoid advancing your timing to try and get more performance out of your engine.   

2.       Large waves – A large wave approaching a boat from the stern when the engine is shut down could possibly create enough pressure to push water up through the exhaust and into one or more of the cylinders.  This happens a lot less frequently but there have been recorded occurrences of this happening

3.       Water in the intake – Any occurrence which causes water to be introduced into the air intake will no doubt result in that water entering the cylinders.  Whether it is the result of the vessel being overtaken by a large wave or water entering the bilge by other means.  Obviously, if one of these events happens, you are probably in a bad situation but if you are able to recover from the initial crisis, you may be faced with the task of getting your engine going again.

How do You Recover from a Hydrolocked Engine?

For this argument, we will assume that you were lucky and your engine was not damaged.  Obviously, if your engine was damaged, you are looking at a rebuild or replacement.  If your engine is hydrolocked you will notice that when you turn the key to start it, you will hear the starter engage but the engine will not turn.  This is because the water in one of more of the cylinders cannot be compressed thus “locking” the engine.  If this is the case, do not continue to try and start the engine because you could do damage to the starter or the flywheel.  Take the following steps to recover:

1.        Remove the kill switch lanyard (if your boat has one) to disable the ignition system.

2.       Remove all spark plugs (make sure you label the wires so you remember the proper order when putting them back together)

3.       Crank the engine with the plugs removed.  You should notice water being expelled from the cylinders through the spark plug holes.  Do this off and on for several seconds until no more water is being expelled. 

4.       Dry off the plugs, put them back in and reconnect your wires.

5.       Start the engine and hope that everything sounds normal. 

6.       Find out why it happened.

What are ways that I can minimize or prevent hydrolock?

Most of the ways to minimize or prevent hydrolock are built into the design of the boat so unless you are building your own boat or replacing the engine, you may not be able to control some of these but here are some things to consider:

1.        Timing – This is the one thing that you can control as an owner/operator.  As discussed earlier, incorrect timing can result in hydrolock.  Advancing the timing to get more out of an engine is one of the prime causes.  This is one of the reasons that performance boats have through hull exhaust systems that expel the gasses above the water surface (That and that fact that they sound better).

2.       Engine placement in the bilge.  The lower the engine is in the bilge, the more susceptible it is to hydrolock.  If the exhaust risers are only a few inches above the water surface, you can imagine that is it much easier for water intrusion to occur from the exhaust side of the system than on a boat where the risers are say 15 inches above the surface.  Setting engines and risers higher up in the bilge make hydrolock less likely to occur.

3.       One way exhaust valves – Some manufacturers install one way valves in the exhaust systems of some boat models.  The purpose of these valves is to only allow flow in one direction thus preventing water from being drawn or forced into the engine from the exhaust system.  There are also some aftermarket valves that can be purchased if one is not already installed in your boat, though I have seen mixed reviews on these.

4.       Thru hull exhaust – If your boat has thru hull exhaust and your exhaust is above the water surface, water is less likely to be drawn into your engine via the exhaust system.  There are even conversion kits to convert your current under-surface exhaust system to a thru hull system.  Thru hull exhaust is not for everyone though.  Some boaters prefer a quieter, smoother sound over the bad-ass sound that thru hull will give you.

To give you an idea of how prevalent hydrolock can be, let’s look at one example.

My last boat had a Volvo Penta 5.7 GS engine.  This particular model did not have a one way valve and it had underwater thru-hub exhaust.  The risers were about 11 inches above the waterline.  Over the 15 years that I owned it, I put about 1300 hours on the boat.  I experienced hydrolock 4 times.  Two of the occurrences resulted in damage to the engine.  One engine was replaced under warranty because it was determined that the mechanic at the dealer where I purchased the boat had not tuned it correctly.  Years later, I rebuilt that engine when hydrolock caused me to snap a piston rod after the kill switch lanyard was accidentally knocked off at 3200 rpm.

Shattered Rod and Piston Due to Hydrolock from Sudden Shutdown at 3200 RPM


I hope you find this information useful.  Please feel free to comment or ask questions.



Captain Frank


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