Cruising Helmsman – June 2017

(Sean Pound) #1

28


Legend


enlarged


Cruising Helmsman June 2017

SAME FOOTPRINT,


MORE TORQUE. YANMAR’S OLD FAVOURITE


SCORES IN SO MANY WAYS.


PRACTICAL


ENGINES


THE most amount of time I have
spent with Yanmars in yachts was
when I helped a mate deliver his Lagoon 38
catamaran.
Theo had taken delivery of the 38 from
the French factory and sailed it across the
Atlantic and Pacific Oceans via the Panama
Canal. Already the twin 3YM30 sail drives
had clocked up around 1000 hours each
with the only fault being a leaking exhaust
manifold on one of the engines.
A couple of things really impressed me,
one was how quiet and smooth running
the engines were and the other the ability
of the Gori folding overdrive props to keep
the engines running in the maximum
efficiency zone and preventing cylinder
bore glazing from insufficient load.
For example, driving through 2.64:1
reduction ratios and swinging 16.5 inch by
12 inch three-bladed props which, with the
overdrive setting on increased pitch to 14
inches, pushed the 8.5 tonne displacement
38 into 15 to 25 knot head winds with a 1.5
metre swell. Our GPS showed speed over
the ground of six knots at 2500rpm using
2.1 litres per hour each.
Increasing the revs to 2700 returned
6.5kts SOG using 2.9lph, which seemed to
be the best offshore cruising revs.

In calm water with the overdrive on at
3000 revs., the SOG was 7.2kts using 4.0lph.
But to reach wide open throttle (WOT)
we set the overdrive off and at 3700 revs.
averaged 8.3kts using 7.2lph.
As with any cruising yacht it is always
better to slightly under prop diesels so,
under normal loading, it can slightly
exceed the designed WOT revs, in this case


  1. My belly f lab was in no fear of being
    reduced with these engines.
    Cleverly, Yanmar has taken the
    3YM30 concept and morphed it into the
    3YM30AE. Still an industrial tractor base
    engine that has the same footprint, so
    existing engine bearers can be used.
    Even though the new engine has a 14 per
    cent larger piston and develops the same
    amount of power at 3200 revs., it weighs
    only three per cent more. Torque is up 14%
    on the old engine too and at lower revs.


JUST THE FACTS
Yanmar has mated the new engine with a
new sail drive the SD25, though it still has
a dog clutch so rapid shifting is out.
However the new leg does have cooling
water intakes at the aft end where they are
less likely to be clogged by weed.

If you are repowering a cat like the
Lagoon 38, coarser pitch props may be
needed to absorb the increased torque and,
of course power, because the maximum
output is being developed at 400 fewer
revs. Fuel consumption at WOT is slightly
higher but that is to be expected because it
is a more powerful engine.
This engine would also be a good shaft
drive repower choice for the Sparkman
and Stephens 34 and similarly sized fin
keel yachts. Or more than ample power,
say, for a Nicholson 32 in high tidal areas.
The KM2P-1 mechanical gearbox for
shaft drive remains and that is good. The
KM2P-1 has a choice of 2.21:1 or 2.62:1
ratios ahead and a 3.06:1 ratio astern.
What this means is, when going astern
where engine loads are greater than going
ahead as the prop bites into cleaner, less
aerated water at an angle that tries to drag
the stern down, the deeper reduction ratio
allows the engine to reach its torque band
faster for quicker stopping. This deeper astern
reduction eases engine stress when stopping
heavy displacement cruising yachts too.
With the standard 2.21:1 ahead ratio
translating to an efficient 1450 prop revs,
engaging astern brings the revs down
to only 1050 for real stopping power.

ANDREW NORTON

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