Compression testing is the single most commonly asked question during survey discussions. Years ago, before the advent of real diagnostic equipment all we actually had was the compression test to use as a definitive measurement of engine cylinder condition. Also, I must add that engine room layouts years ago were no way as tight as the average engine(s) installations of today's boats. For the most part, a compression test would take probably half the time as it takes to do today's installations, and in many cases, especially with 6 cylinder inline engines, you could do both in about an hour. --So Much for the old days--

Some boats today have V8 engines outboard banks so close to fuel tanks or hull sides that all carpets, furniture and hatches must be taken out , and occasionally the outboard manifolds must be removed, and sometimes you find that the outboard side spark plugs have not been changed for -a long time ---- broken plugs can sometimes result in additional expenses--- Having said that, I never completely rule out compression testing, but I seldom schedule compression testing without first electronically diagnosing the engine(s).

The time spent up front at the onset of a survey, where I do my initial electronic diagnostics before sea trial , --verifying that an engine has all of its cylinders, and will run and return to idle smoothly-- can save a lot of dollars, and time that is better spent on a rigorous sea trial whose focus is on actual performance. If at that time, and or during sea trials, it fails ANY of its performance criteria, I will not hesitate to show, by visible and actual test procedures, that a compression test might pinpoint one or more cylinders that are not delivering full compression, By than its only a question of which cylinder(s) and not -If-. A little up front effort usually pays off in saved time and money.

Normally I will defer on Diesel engines, because it can be much more "Invasive", usually requiring more dismantling of components, sometimes resetting of clearances etc. and may require follow thru procedures and service that may actually, or border on, repairs / adjustments that a surveyor should not get into.

Outboard engines are also on a per case status, --normally not a problem, but may require an outboard service technician for a complete follow thru-- usually I will observe and, or verify results / findings of the service technician.

In most cases, compression testing will require the owner of the vessel to sign a release / waiver

acknowledging and giving his consent for the removal and replacement of the spark plugs.

There are many things a compression test by itself cannot tell you, it cannot tell you what top RPM at cruise under a load will be, or whether an engine will backfire or miss or vibrate, or knock, or what oil pressure will be when warmed up and returned to idle after a sea trial, -- or if --and what color it will smoke, will it run hot, and so on...

Every once in a while, someone will ask me to just run the engine(s) on land, and do a compression test I always tell them "yes I can do that", but I always remind them of the things I will not be able to tell them, and most of the time the reality dawns, that it might be a very false economy.

"I ALWAYS GO THE EXTRA MILE"