Re: [Electric Boats] power requirements - predicted vs observed

 

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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