Hi,
I think that this:
"A lower gear ratio will increase the acceleration of the boat. A
higher gear ratio will increase the acceleration of the propeller."
....might have a limitation, and that is that for the prop to
accelerate, the torque it requires must be low enough for the torque
available to drive.
So, if you have a small motor, and a big prop, then, in water, it might
not accelerate the prop faster.
Or am I misunderstanding?
I am looking at a system whereby the goal is to go, say, half hull
speed.
I would probably need the (standard?) prop to rotate slower than a
commercially available setup, assuming the same prop is used.
Or, I could use a smaller prop, or one with way less pitch.
This whole subject is not as simple as it seems, unless you just go
overpowered with everything, and then you will possibly have it covered,
like ICE engines do, or seem to.
It has also occurred to me, ironically, that if ICE installation
designers took as much care as the members here, in working stuff out,
it might be possible to power a 30 foot sailboat with a 100cc engine or
less!
John
1.1. Re: power requirements - predicted vs observed
Posted by: "James Lambden" james@toolboat.com jameslambden
Date: Sun Aug 21, 2011 1:20 pm ((PDT))
Eric,
The advantage of an increased output torque will only be noticed if you
have a bigger propeller. The same amount of power can only be
developed if you inversely change the size of the propeller with respect
to the gear ratio.
I guess you mean the acceleration of the propeller, not the boat. The
4:1 will only accelerate faster to half the speed of the propeller on a
2:1 gear.
A lower gear ratio will increase the acceleration of the boat. A
higher gear ratio will increase the acceleration of the propeller.
With electric motors, their intermittent ratings allow for greater power
to be developed with lower gear ratios.
A larger propeller will be more efficient than a smaller propeller up to
a point, after which it will become less efficient. There is a sweet
spot for the size of the propeller with respect to the boat and boat
speed required.
A propeller must have enough pitch to be able to produce a given speed.
Finding the proper propeller and gear ratio combination is absolutely
key to mastering an efficient system.
James
On Aug 19, 2011, at 6:57 PM, Eric wrote:
> Hi Arby,
>
> I'm with you, "Empirical data definitely rules!"
>
> Like some have said, torque provides acceleration, and power provides constant speed against a given load. We can see this because changing the reduction ratio from 2:1 to 4:1 doubles the output torque at half the rotational speed, resulting in the same power. So gears can increase your torque (acceleration) but you can't mechanically or magically increase the power (constant speed).
>
> I'm impressed with everyone's contributions to this discussion. I know that it is easy to get sidetracked. But we're doing pretty well to stay on topic.
>
> Collectively, we have the chance to change the perception of what electric power can do in the mainstream marine world. Everybody here has a good idea of what electric can do, gained mostly through observations, trial and error. If we're lucky, we might end up with a better way to objectively predict electric traction requirements for a wider variety of vessels.
>
> Keep up the good work everyone.
> Eric
>
> --- In electricboats@yahoogroups.com, Arby bernt <arbybernt@...> wrote:
> >
> > Nice discussion.I'll be glad to work on a shaft measurement package. With just a couple of inches of shaft and clearance, it should be possible to place two strain gages (not gauges), and telemetry to resolve a set of forces. Record the rpm, power, torque and thrust, and the system will be characterized. There is an existing device designed for measuring power output on an automotive drive shaft, sort of a real time dyno for gearheads to tweak drag racing home-builts.�
> > Empirical� data� definitely rules. My great-grandfather, Milton Humason,ran the telescope on Mt. Wilson, and took the spectra-scope data on the accelerating expansion of the universe.�
> > With enough members, a measurement system can be used in many different boats, and the data now assumed can be verified. I'll cover the cost of the device(s).
> > Be Well,
> > Arby
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