At 11:12 PM 21/05/2010, John Frances wrote:
>My personal feeling is that everyone wants to "overpower" their boats.
Whilst this is true, what is the ramification of oversizing the motor
and controller?
The answer to that is, very little. If you're "rolling your own",
there is very little risk in over-sizing the electric drive system.
If you don't have hard information (i.e. a comparable boat already
converted) then you'll be making some educated guesses, but not with
the same dramatic differences that you get with prop changes (for example).
Lets' look at one thing at a time:
Weight: Will your boat suffer if the motor is a bit heavier than it
otherwise might be? In by far the majority of cases, probably not
enough to be measurable.
Efficiency: A larger motor has lower DC resistance than a smaller
motor, so the losses due to the electrical conditions are lower. It
has larger bearings, bigger fan, etc., but at lower revs the
electrical efficiency gain should outweigh the mechanical loss increase.
Torque: all else being equal, bigger motor = more torque.
Speed (RPMs): All else being equal, a bigger motor is happier at
lower revs than a smaller one - so you can use the more torque to not
need to gear down so much. This may make all the difference in making
the installation clean and easy.
So to the "roll your own" vs "buy a package" debate.
In my opinion, it is much, much better to "roll your own". You put it
together, you know how it works, you can fix it when it stops. For
cruiser types, this is an important issue. For day sailor types, much
less so, depending on where/what you're sailng.
Buying an "all in the box" package and connecting the ends teaches
very little. Some people don't want to understand their systems, if
this is the type of person you are then that's fine, but you may be
better off getting a hybrid package put in, instead of a straight
electric. If you just "put in a box" then you will get caught out
when you find yourself in a situation that exceeds the "box"
capability - sooner or later, most likely a flat battery.
Since you are here on the electric boats discussion list, you are
unlikely to be in the "don't want to understand" category. You may be
in the category of "want to understand but need it to be easy" group,
or "don't have time to get the skills together", etc.
The "need it to be easy" group of course ends up sub-divided into a
whole bunch of other sub-groups, of differing reasons: Need it to be
cheap, need it to be able to be fixed with a paperclip, etc.
"as cheap as I can get, and have the time to spend, but want
everything to be compatable". Buy a forklift - seriously!. An old,
running, electric forklift (cheap when it needs its' battery
replaced) will give motors, cables, contactors, possibly a hand
throttle, but you probably will not want to use the controller out of
it, as they often have quirks that you will not want to deal with.
This will probably be 24V, 36V or 48V, all pretty safe to work with.
"fix it with a paperclip" - no such thing, but if your boat is damp,
non-marine electronics will not be happy, so may be better to use a
"rectactor" controller, built using contactors and diodes, uses
battery switching to give various steps - typically three or four
steps, which may or may not suit your situation. I don't know of
anyone offering such a thing as a package, but simple to put
together. For someone with machining skills, a drum controller like
out of a 1920s electric car can be built, exceptionally reliable if
built correctly.
"just need it to be easy" - you have a choice of one of the "marine"
solutions (which inside the box are often just typical forklift-type
parts) or buy a package from someone like Ryan at EVsource, which
will supply you all the bits and he'll ensure that they are all
compatable with each other.
"need it to be very simple, but don't have time" - you are close to
the "don't want to understand" category, but it isn't that you don't
want to, you do understand, in principle, but don't have the time to
be messing about with it and not a big deal if it stops, so there is
no point in knowing all about it. For you a 'tight' package -
Torqueedo and similar if smaller, or one of the higher power
integrated systems on offer would be the way to go.
"need it to be used by others" is another group again - and brings up
the "which others?" question. For the "others" and "don't have time"
groups, it isn't the propulsion package that really matters - it's
the information available to the user: remaining range at this
setting, etc. How do you keep the user informed about the system, in
a meaningful way?
Hope this helps
Regards
[Technik] James
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