Yanmar marine diesels: what to check on a used sailboat or powerboat engine

Yanmar, founded in 1912, launched what it calls the world's first practical small diesel in 1933, and small Yanmars like the 2GM20 are common in sailboats.¹ Its bigger engines power many cruisers, sportfish and trawlers. Yanmars are compact and widely supported, with long production runs that keep spare parts available; one reviewer described Yanmar as a conservative engine maker.²

The Yanmars that give trouble are almost always the ones that were run hard on a high rating or ignored at the raw-water end. Check the rating, the records and the exhaust elbow.

Yanmar marine diesels at a glance

What to check

1. The rating

Marine diesels are rated for how hard they can work. A reviewer noted that the earlier 6LY3 carried a pleasure rating of 250 hours a year, while the later 6LY 440 was rated for light commercial use at 1,000 hours a year, and said the lower figure "ain't enough for boaters who actually use their boats."² A high-output pleasure-rated engine with lots of hours has used more of its life than the number suggests. See boat engine hours.

2. Service records

The same review recommends oil and twin filters every 100 hours or six months on the 6LY.² Compare the records with the maker's schedule for the exact model.

3. Small sailboat engines: the raw-water end

On small sailboat Yanmars, look at:

• The exhaust elbow for crusty corrosion; a failing elbow can let water back into the engine
• The raw-water pump and impeller
• The heat exchanger on freshwater-cooled models
• Engine mounts for cracked rubber and rust

Small engines squeezed into tight compartments often have these jobs skipped.

4. Saildrives

On boats with a Yanmar saildrive, the rubber diaphragm seal around the leg has a service life set by the maker. Ask when it was last replaced and compare with Yanmar's interval for that model. Look at the aluminum leg for corrosion.

5. Light loading

Diesels run gently for years, such as a sailboat engine used mostly to motor in and out of the marina at low rpm, can suffer from carbon buildup, wet-stacking and glazed cylinders. One industry journal suggests regularly loading diesels to 50 percent or more.³ Ask how the engine was usually run, and watch it under load on the sea trial.

6. Smoke and oil analysis

A clean cold start, no persistent smoke, and an oil sample sent to a lab. One popular lab has charged around $35 a sample.⁴

Questions to ask the seller

• Which engine model is it, and what's its rating?
• When were the oil, filters, impeller and exhaust elbow last done?
• When was the saildrive diaphragm last replaced? (if fitted)
• How is the engine usually run: rpm and hours per year?
• Has it ever overheated?

Common questions

Are Yanmar marine diesels reliable?

They're widely regarded as solid, well-supported engines. Reliability depends mostly on the rating, the maintenance and how the engine was used.

How many hours does a small Yanmar sailboat diesel last?

With good care, many thousands. On a sailboat engine, the condition of the exhaust elbow, cooling system and mounts, plus how it was loaded, tells you more than the hour meter.

How often should a saildrive diaphragm be replaced?

The maker sets the interval for each model. Ask when it was last replaced and check against Yanmar's current guidance for that saildrive.