Sean requested for setup which would perform like a two Yanmar 3YM30
diesel engines. That assumed there would be a genset and two electric
motors with drives able to be raised above the water line
I like these sorts of challenges just to see what it takes.
Using the Yanmar box 2.221:1 and prop speed of 1579 rpm the brace of
Yanmars could put out 42kw/150Nm. There are reduction sup to 3.22 for
this engine and of course deeper for heavier conventional displacement
hulls. Yanmars specs indicate at those revs the rec'd props (cant find
what Yanmar thinks they are) are starting to work efficiently. The
motors are rated to consume 3.4l of diesel per hour at maximum
continuous output (wot @3489 rpm). A single pushes a 15000 mono hull at
approx 6-7 ie it cant be the pressure wave at hull speed. In a 45' small
high length to beam ratio cat they should exceed hull speed.
The Yanmars cruise at 2500 rpm (1131 rpm at the prop). At full throttle
and load at these rpms the motor would put out 35kw and 300+Nm. At 1 gal
per /hr for the pair (Yanmar and owner reports) this isn't happening .
Using the fuel consumption we can gu/estimate the relative outputs -
fuel consumption in ICE being more analogous to power draw in electrics.
Output is more likely to be in order of 10.5kw and perhaps 91Nm (14hp &
67ft/lb)
The options and if you were going to it would be a matter of balancing
the genset and battery or backing down a little on the throttle.
there are two which i found interesting. Neither is a $1000 option but
both are very strong, operating within their design range at modest amp
levels and excellent power.
The Prius a 205 volt water cooled max 30kw motor, with a specific regen
winding and an electronic controller, HV battery, a CVT gearbox and
host of useful stuff if a wrecked Prius is purchased. SH Prius battery
205v 6.5 Ah can be had for $650 online and parts bought separately. The
NiMH packs are prime facie similar to a 48v 50ah lead acid but weigh
53kg and are designed to deliver 90% of its capacity so more similar to
an 80AH lead acid. Spare modules can be purchased, and independent
testing has shown after 10 years of car use the Prius packs show very
little fall in capacity or outputs. They are a handy shape too flat like
half a single bed sponge mattress, so easy to store can be fan cooled/warmed
The second motor is an EMTRAX 40/30kw (peak/cont) BLDC is simply a water
cooled, 12kg good torque 200 or 300 volt motor. These look similar to a
:Lynch motor but are in a different class of output. Claimed efficiency
is 91-94% - no reason to doubt its close to 90%.
The good thing is both these motors can be power from a high volt genset.
The Prius motor at 1579 rpm on 205 requires a 3.55 reduction ratio to
bring it down to the same prop rpm as the 3YM30. It can be loaded as far
as 26kw and 150Nm outputs.
However at a cruise speed based on 3YM30 cruise figures and 1130 prop
rpm the Prius motor can put out up to 28.6kw and 230 Nm at WOT (well in
excess of the diesels possible output) or it will draw about a 56 amps
to make 10.5kw/125Nm or 33amps for 6.2kw/91Nm - that latter one sounds
familiar. Not sure on the Prius C1's weight but its pretty modest less
than 30kg. Sitting on top of a o/b leg its very light. And for cruising
a relatively modest genset is required.
Alternatively the EMTRAX is a pancake style from Slovenia designed for
power assisted glider flight. Max power is at take of and takeoff and in
helicopter landing) is where most aircraft power systems fail so this is
strong. They are now available water cooled and the rate power increases
are minimal. This motor runs at 2200 rpm on 200 volts and 3000 on 300v.
On 200 volts the gearing is shallower than the Prius or the 300v setup.
BTW these are compact with Bravo leg they would weigh under 90kg each
and with the motor mounted to the transom would take up about as much
space as a straw hat. On an outboard the EMTRAX and Prius C1 can be
fully enclosed and the cooling requirement will be very low at cruising
speeds.
At 200v a pair of EMTRAX at max efficiency (continuous output) require a
1:1.39 reduction to put out 47kw / 362Nm at 120 amp draw. This is well
in excess of the Yanmar outputs, but the Gensets inputs will be well in
excess of the Yanmars too if you ever use this level of power. At
cruise the amp draw should drop to 57 amps to give 10.5kw/81Nm torque
or If torque is prioritized rise to 65 amps for 11.8kw /91Nm
At 300v a pair of EMTRAX drop the ratio 1:1.9. At max efficiency
(continuous) output is 71kw /494Nm at a 130 amp draw. Again the genset
will go harder and thirstier than the 3YM30's at full power. However, at
cruise the amp draw should drop to 26 amps to give 10.5kw/78Nm torque
or If torque is prioritized to 48 amps for 13kw /91Nm.
I haven't proffered any speed figures because there are too many
variables here. However i found it interesting to see what it would take
from whats available commercially to match the 3YM30's. All three
electric systems could be improved in economy and performance by using
props specific to their power characteristics and deeper reduction ratios.
The advantage of high voltage batteries is they can run high voltage
appliances. NiMH H or lithium are significantly lighter per KWh than a
genset of the same KW output (without considering fuel - I realize there
is a big difference in what each has the potential to deliver). However
some recommend a genset may be 2-3 times the expected peak load. A
battery means the genset size can be reduced and the battery used to
supply peaks in demand which are by definition relatively short.
Batteries hey can be charged from excess genset output or solar cells,
wind or prop regen or even an exercise bike. It also has the virtue of
being a super UPS if gen sets etc fail; it enables silent assistance
when sailing or docking or to get out of doldrums.
Not sure on the $'s for the Prius in US I have seen auctions down around
$5K for a wreck but you get a lot and there is always the engine and ECU
and scrap to sell and there is a Toyota Dealer in every port where used
and new parts can be had. Ditto for things like common brands outboard
legs on which motors might be mounted. The EMTRAX are $4500 per. Half
the price of the diesels they match and much lighter at 24kg the pair
again a great option for conversion of outboard legs for propulsion of
larger hulls. Mounted inside a transom driving a leg they would take up
as much room as a large head size hat.
I did this for fun not to convert anyone - its just a way point you may
or may not use.
Andrew Gilchrist
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