Talking Torque: Time Warp

One of the last true marine diesels

Recently, as part of a deceased estate, I was offered a Mark 1 Top Hat 25 yacht. The vessel was built by Geoff Baker in Sydney sometime between 1965 and 1972 (Baker built a total of 59 Mark 1s) and was laid up by hand with a far stronger construction than Mark 11 Top Hats.

The Top Hat 25 was designed by John Illingworth and Angus Primrose in the early ‘60s and was 7.6m overall with a waterline of 6.2m (short overhangs for less pounding in a seaway) and a beam of 2.4m. A full keep design with a draft of 1.3m, she displaced 2540kg, with a ballast ratio of 45 per cent. She was way heavier than my first yacht, a 1969 7.6m cold-moulded timber Van de Stadt fin keel and balanced spade rudder design that displaced 1750kg. 

Trouble was, the Top Hat had sunk on her mooring at one stage and her single pot BMW D7 auxiliary (made for BMW by German industrial diesel specialist Hatz) was beyond restoration. A new diesel would have been needed, and considering the damage done to her internal furniture whilst submerged, I decided the restoration needed would be beyond my budget. A tough decision as I really enjoy coastal cruising and the Top Hat has a reputation for being a good sea boat, whereas my first yacht was too light and lively in a sea way. Nothing beats displacement offshore! 

Even though I declined the offer, it got me thinking about which engine would make a good re-power. A mate has a similar age Top Hat with a 7.5hp Volvo Penta MD5A, a beautifully-balanced (for a single cylinder) direct injection diesel that unfortunately was discontinued decades ago due to non-compliance with ever-toughening exhaust emission requirements. The 443cc MD5A developed maximum output at only 2500rpm and weighed 111kg with gearbox, but I've always liked the concept of having a lighter engine when re-powering. 

I'm not a fan of single cylinder diesels in fibreglass yachts because the material is excellent at transmitting vibration. But flexibly mounted, the single cylinder Yanmar 1GM10 has tolerable vibration levels (significantly less than the Yanmar YSM8 of the '70s), and because the engine has been around since 1983, spare parts are very easy to secure. When idling at 800rpm the engine has that wonderful four-stroke single cylinder exhaust beat. The twin cylinder 2GM20 never sounded quite as nice!

THE NITTY GRITTY

The indirect injection 1GM10 is the last of Yanmar's direct raw (sea) water cooled yacht-orientated diesels and overall is a very reliable engine. Richard Minard, my local Yanmar agent, always told me that the main issues with small marine diesels are heat exchanger cooling and that raw water cooling is more reliable because there’s little to go wrong.

The 1GM10 has provision for backup manual starting (though unfortunately not the raised hand starting of the YSM8) and simple push rod valve actuation via a gear-driven camshaft. A thermostat maintains reasonably constant engine temperatures but being raw water cooled and subsequently running colder than heat exchanger cooled engines, the 1GM10 must be ‘worked’ to prevent condensation build up in the sump oil. 

The 35A voltage regulated alternator is mounted high up with the starter motor not far below, so a partial flooding of the engine compartment should not prevent the engine from operating. As there's no heat exchanger expansion tank access to the fuel injector pump, injector and rocker cover is excellent. 

The easily-accessed raw water pump with its rubber impeller is at the forward end of the engine — as is the only belt drive powering the alternator. The canister oil filter is also at the forward end and on its side so that oil won't spill everywhere when being replaced. In temperate climates the oil should be diesel-specific SAE15W40, but thinner grades may be needed in colder climates. I recommend changing the oil and filter every 100 running hours or annually. A sump oil evacuation pump is optional. However the gearbox oil level dipstick may be hard to reach as it's just forward of the prop shaft connection flange.

Like Yanmar's more powerful yacht auxiliaries the 1GM10 is available with the KM2P-1 mechanical gearbox that has ahead ratios different to astern. For example, the standard ratios are 2.21:1 ahead and 3.06:1 astern. When going astern this enables the engine to rev more freely and quickly reach its peak torque band for faster stopping. Only Yanmar has this feature after 40-odd years!

Complete with gearbox the 1GM10 measures 554mm long, 410mm wide and 485mm high, so it's compact for its output. 

According to owner comments the main issues to watch for are faulty water pump impellers, exhaust elbow corrosion after about 1000 running hours (a mate's three cylinder Yanmars in his Lagoon 38 cat had the same issue), back-flooding of the engine in yachts from having an inadequate water-lock exhaust system, and cylinder head corrosion from not frequently checking the anode located behind the alternator. Also engine power loss is sometimes due to incorrect valve clearances, easily fixed by removing the rocker cover and using a spanner and feeler gauge. 

As with all diesels a water-separating fuel filter must be fitted and checked regularly for any rise in water level.

Yanmar offers a full range of instruments, and if the re-power budget allows I'd at least install a tachometer and oil pressure and temperature gauges. A voltmeter and fuel tank level gauge would also be nice — but in my opinion this isn't essential. In some older yachts having Yanmars the instrument panels have been installed at the engine but in my opinion instrumentation should be located in a waterproof perspex-lidded box directly ahead of the helm position.

THE WRAP

The simplicity of the 1GM10 is in keeping with older yachts such as the Top Hats. Manual starting may not be as easy as direct injection diesels that had compression ratios of 16:1 compared to 23:1 but in my opinion being able to start a diesel by hand is invaluable. And being able to secure spares easily makes owning an engine way more pleasurable. 

Hopefully the 1GM10 will reach 40 years!

For more on this engine Google Marine Engines – Yanmar/Power Equipment and follow the links. 

QUICK SPECS

Engine Yanmar 1GM10

Engine type Single cylinder indirect injection diesel.

Rated BHP/MHP* 9.0/9.1 at 3600rpm

Maxcontinuous BHP/MHP7.9/8.0 at 3400rpm

Max torque 18.3 NM at 3000rpm

Piston displacement 318cc

Bore x stroke 75 x 72 mm

Dry weight with gearbox 81kg

*Brake horsepower/metric horsepower or PS