How To Buy A Used Boat - 10 Basic Engine Checks

When you buy a used boat, you want to make sure that the engine is good (unless you are buying a wreck to refurbish). Inboard diesel engines are generally reliable. So how do you discover if the engine is to be trusted (or not)? Ultimately, you should sea-test the boat you are buying, but there are quite a few simple engine checks you can make on first pass.

If the vessel is out of the water, and water cooled via a heat exchanger, then a cooling water supply will have to be arranged (if the boat has keel cooling then other ways will have to be found). This can be done with a hose pipe (you might need a big one), but make sure that the seller's agent agrees to this, and get them to sort it out for you. OK, so we have a cooling system ready.

1. Is the engine clean and well looked after? A skipper who cares will have a clean and well painted engine, so that any leaks are immediately visible. If the engine looks poorly maintained, then it probably is.

2. Look at the lub oil dipstick - is it clear and is the level up to the mark?

3. Are there signs of water in the lub oil?

4. If you can, check the transmission fluid - level and colour - grey or milky means it's contaminated.

5. Does (Do) the engine(s) have an hours meter? If so, check it. Yes, they can be faked or replaced, but most skippers are genuine. A really good truck engine which is really well maintained will do a million miles without trouble. At an average speed of say, forty miles per hour, that's twenty five thousand hours.

6. Does the engine start easily? A battery shouldn't be flat if the seller wants to make a sale. It could mean that the starter motor is faulty, or cables are in poor condition, and that the flat battery is a cover.

7. Is there a cloud of blue smoke when the engine first starts? If there is, then this could indicate, for example, that the cylinders/piston rings are worn. With diesels, some trace blue smoke is inevitable, but it should not be like a thick fog.

8. Check that the battery is charging properly - the charge light should be out, warning siren stopped and the alternator ammeter should be showing charging current.

9. When the engine is fully warmed up (check the temperature gauge - diesels run at about 82 deg C (180 deg F), does it sound good? Without being able to work the engine - driving the boat - it may not warm up properly. After 20 minutes under load, then the engine should be up to operating temperature. If you cannot get the engine up to temperature, then that could be a problem. Engines running cold will have excessive wear. Running over temperature is bad too.

10. What color is the exhaust gas? White? That's not a good sign. Some blue smoke is fine - a slight haze would be right. Black smoke indicates unburnt fuel and could be a sign of injection problems (some black smoke when the throttle is fully open is not a problem).

These are just a sample of over fifteen basic engine checks that I make when checking an engine - and we haven't even covered turbos yet.


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Phil has been refitting boats for over 50 years, power and sail. There's a whole lot more guidance about how to buy a used boat and you can get the full list of his own engine checks at boat engine checks. Save yourself time and maybe even money. A lot could depend on a trusty engine!