Quote:
Originally Posted by cwjohm
Well, more correctly they are saying that a 15HP EP motor will generate the same thrust as a 45HP diesel.
What the EP supporters (driven by the marketing hype of OV) are saying is that given the different torque curves of EP vs diesel they can upgrade the prop to generate more thrust per energy input. There is certainly some merit in this argument. However, they destroy their credibility by over extending this argument to make unsupportable claims. Of course they do this in order to make the financial equation seem even moderately viable.
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They seem to be intentionally confusing the issue by not providing clear performance figures but...
The point I was making is running a pair of 15kw motors (30kw total) off an 8-10kw
generator means you will be limited to short bursts where you can use the 30kw supported by the
batteries to make up the missing 20kw.
For typical cruising (running for 5-6hrs down the
ICW for example), you are limited to much closer to what the 8-10kw the
generator puts out and that's no where close to 30kw unless you have a truly massive
battery bank which adds to cost and negates the space weight savings suggested as a benefit. (the longer you run or running on successive days makes it even worse).
The idea that a 15kw
motor WILL produce more thrust than a 45hp diesel is a complete and unmitigated
FALSEHOOD fabricated on intentionally misleading comparisons.
The amount of available thrust while at cruising speed is primarily a function of the available torque AT THE PROP (not the output shaft of the motor) along with the prop selected.
To go back to the tug-o-war examples from many pages back. A 40hp tractor will typically win a tug-o-war with a 1200hp Bugatti veyron on dirt but no one is going to claim the 40hp tractor is better for a road
race or has a higher
cruise speed.
What you typically see happening with EP is they put a large low
pitch prop on the
boat and they set up the
gear box to give you lots of torque at the prop. This essentially gives you lots of thrust but lower cruising speeds and then they compare the thrust to a much larger diesel optimized for typical cruising speeds where the thrust is less but it's able to generate much higher speeds. They further mislead by doing a bollard test where the diesel is often unable to spin up to it's rated output as would be possible while the
boat is in motion.
If the goal is to generate maximum thrust (which is not normal for a
displacement cruising boat), I can spec a prop/transmission combo for a 45hp diesel (or any other motor of 45hp), that will blow away the OV when it comes to thrust. Mostly it's just putting a bigger low
pitch prop on the diesel and run the
power thru an appropriate gear to boost the torque beyond what the
electric motor can produce (at the prop).
Secondarily: You also often see EP proponents point to automobiles using significant smaller motors to get similar performance, which is true but not applicable to how a
displacement cruising boat is used. A small car only needs 50-70hp to maintain freeway speeds. IC engines are typically spec'd for much higher HP because at low
RPM, they have lower torque. Since HP=torque*RPM, they have little HP (relative to peak HP) off the line. Since an
electric motor has peak torque from zero
RPM, the low end HP is much higher relative to the rated HP. But this is all about acceleration. Acceleration is not a primary consideration when spec'ing a motor for a displacement cruising boat.
So once again. 15hp electric is not magically equal to 45hp diesel in a displacement cruising boat. This is
marketing and misdirection to confuse buyers.
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