Hi, James,
Thanks for the response, I have also had some other useful to-and-fros
off group, including gathering prop knowledge.
I like Erics figures, especially the 500 watt one.
They bode well for off-grid charging, using a smaller battery bank, and
limiting the speed to gain running time rather than adding batteries.
I think part of it is a mindset issue. For years, the only considered
negative impact of going faster has been the amount of fossil fuel
required.
I will be doing some fiddling this summer (in relatively protected
waters) using my 15 foot dinghy, and the DC generator off the Atomic 4,
using it as a (puny) motor. (Note to self - must buy some oars!) Gives
me better access to the drivetrain than reaching under the sailboat
cockpit, and I can use a smaller 'motor'. Will give me a better idea of
how it all works. Maybe if I get the bugs out of a small setup, I can
then do things like adding a small generator to run it to gain range, to
allow the backup outboard idea to be put to rest.
Easier to fiddle with props, too, on a small boat that comes out at the
end of the day.
I will keep an eye on the heat!
As far as the system being 'just enough', I am trying to look at the
whole picture, including backup ICE options that are part of the system.
I am not fixated on electric being a sole means of propulsion.
As I get ticked off with the electric proving to be underpowered, I can
upgrade it. And that goes for the dinghy or the sailboat.
But it is looking more and more like there are good reasons why group
members are generally opting for the motor sizes that they have, and
that is because they work!
Having said that, if I go for a large motor from day 1, I never get to
find out what the useable minimum might be. I am trying to avoid the ICE
mindset of fitting a 25 HP engine into a boat that has a hull speed
requiring 6HP, 5HP of which is used for the last 3 knots. (very rough
figures)
Finances drive it, but it is an interesting challenge aside from that.
Appreciated,
John
3b. Re: Quick Question re power outputs vs RPM/time
Posted by: "James Massey" jcmassey@netspace.net.au technikjames
Date: Tue Mar 29, 2011 1:12 am ((PDT))
G'day John, All
At 02:10 AM 29/03/2011, John wrote:
>Hi,
>Just another of my quick questions to try to get a handle on all of
>this.
>
>Taking it that many people use less than full power in order to increase
>range, and assuming the batteries were regular lead acid wet batteries,
>are the two scenarios below going to give ROUGHLY the same running time
>at the same power/rpm?
>1 - A 10 kw motor set to run at a 5 kw battery draw.
>2 - A 5 kw motor running wide open.
>
>What I am trying to find out, is there some kind of hidden advantage to
>have the larger motor running under less stress, or are the performances
>proportional? I am assuming no appreciable heat buildup.
No appreciable heat build-up is the key to the answer. Heat IS the
limiter to the kW rating of a motor, a 5kW motor can only do 5kW
under certain conditions, and you can be certain that the marketing
salesweasel who wrote the spec sheet picked the absolute optimum
point to call the motor by its' maximum rating. Its' real world 100%
duty rating will likely be quite a bit less, depending on RPMs, load,
enclosure cooling, etc.
The electric drag racing guys are driving motors to many 10's of
times more power than their motor was rated to, as the rating is
usually a 1-hour rating (won't over heat at 'x' amps for one
hour). Their motors don't reach that critical temperature in the 10
to 15 seconds that they are over-driving their motors. However, if
you don't meet the designated datapoint, you may melt down a motor
well under an hour at less than the nominal power.
Heat is the tell-tale of efficiency, and in a motor, heat comes from
amps ( x the resistance leads us eventually to the watts) a bigger
motor has less resistance, if it has half the resistance, at a given
output power it will have 1/4 of the waste heat of the other motor of
twice the resistance, and therefore better motor efficiency.
However, the bigger (a.k.a. heavier) motor may (in a
light-displacement hull) be putting the wetted area of the hull into
an inefficient zone, reducing efficiency by more than the efficiency
gain of the larger motor, so it is an 'it depends' thing.
The bigger motor may have a better or worse torque/speed match to the
propellor, bringing in a more significant efficiency issue (and
adding to the attraction of getting a >properly engineered system
designed for your hull<. Which for most of us would not be available,
so "by educated guess and several prop changes" you will select your
best system.
In my opinion, buy as big as you can afford that will reasonably fit.
Allow budget to try some alternative prop pitch/diameters to maximise
efficiency at your typical performance point. One day you may find
that with wind and tide against you, you'll be glad you have reserve
power to get to a safe place, instead of trying to put in a system
that is 'just big enough'.
Just my opinion, and opinions are ... well, you know how that goes!
Hope this helps.
Regards
[Technik] James
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