[MHml] outboard gearing

[TOFS Darwin]

Just been a casual observer - up until now.

I would hate to see the repair bill to small vessel D/E system in the event
of a lightning strike - and no propulsion! 

 
Peter Gordon

-----Original Message-----
From: multihulls-bounces at steamradio.com
[mailto:multihulls-bounces at steamradio.com] On Behalf Of Diane Selkirk
Sent: Wednesday, 14 February 2007 13:42
To: Informed discussion of multihull issues
Subject: Re: [MHml] outboard gearing

Gary Pearce wrote:
> On Wednesday 14 February 2007 04:15, graeme at kizio.com wrote:
>   
>> I think that a lot of the Solomon comments are marketing
>> spiel, rather than engineering fact. But I've still not
>> heard a good explanation, beyond the above, why smaller
>> electrics can replace larger diesels.
>>     
>
> This article explains things a little bit although I still do not 
> entirely "get it"
>
>
http://www.ossapowerlite.com/tech_library/fuel_efficiency/fuel_efficiency.ht
m
>
> Of course these guys are selling a system. Then again Nigel Calder sold
his 
> brand new boat to get the yard to build another exactly the same only with

> the OSSA powerlite system so there might just be something worth checking
out 
> here
>
> Gary
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>
>   
Much of what Solomon Tech. writes is utter BS.  They say that the diesel
engine has all these losses from alternators, water pumps, etc. when the
output quoted for a diesel is at the flywheel after all these parasitic
losses.  

http://www.solomontechnologies.com/faq.htm#10.htm

They also go on about how much torque an electric motor develops at low RPM
compared to a diesel.  That's why diesels have gearboxes :)

Consider a real diesel - the Yanmar 3YM30, versus their hypothetical 30 HP
diesel motor.

It's specs are here:
http://www.marinedieseldirect.com/repower/specs/yanmar/3YM30.pdf

Note the power - 29 HP at the crankshaft, about 28 HP at prop after going
through the reduction gear.


The reason that they can replace a bigger diesel is that the bigger diesel
is seldom putting out full power.  In reality, most people don't motor at
widely varying RPM's; they pick a "sweet spot" where fuel consumption is
reasonable and engine RPM's are some fraction of full power.  On a 3YM30 it
will be around 2800 RPM.

So the diesel is loafing along true, while the smaller electric motor is
doing all it can to push you along at hull speed in flat water.  

HOWEVER, when the wind is in your face, and waves are pushing you back, then
the diesel owner will throttle up and get more power from the engine.  The
guy with the smaller electric motors just has to go slower because he has no
reserve of power.


The OSSA stuff isn't as much BS, but they do a lot of hand waving with
bigger more efficient props with their electric motors.  That would work
with a diesel too.


The reason Nigel Calder gets all this stuff is that he has to find something
new to write about.  A super techy boat full of electric propulsion, and
NMEA 2000 (another barrel of worms) gives him lots to write about!  As he
said in one Professional Boatbuilder magazine <paraphrasing> "I've got kids
to get through college - this is a good deal for me with boats getting more
complex"


There is also a reason why commercial ships seldom use D/E.  It's more
costly, seldom more fuel efficient (unless you have a large hotel load that
varies like a cruise ship), and is inherently more complex.  

Vessels that like D/E are cruise ships because they can raft mount the
gensets and make a more vibration free boat and also get better fuel
efficiency with some of the gensets shut down to match the load.  ICe
breakers like them because they generator lots of torque down low for
ramming / milling ice.  And some ferries with right angle drives because
they offer some installation advantages and fewer motors.  That's about it
in the commercial world for D/E.  If I suggested D/E for a tugboat or a high
speed patrol boat or a fireboat I'd be laughed out of the office.

Evan Gatehouse









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