Hi hanermo,
Interesting point and I agree with each point, but it doesn't address the question. The Gerr formulas are stated to cacluate the propeller shaft HP required to push a hull. So it's supposed to be the HP applied to the propeller AFTER all of the parasitic drag and driveline efficiency losses. So regardless of the power source, the number is supposed to be how much power needs to be supplied to the prop for a given speed. Everything that you mentioned were losses before the power made it to the propeller shaft.
So all of this is just derailing the conversation. Accurate info, but not what we're discussing right now.
Let me try to simplify the topic to keep this conversation on track. The Gerr power formulas claim to quantify how much power the propeller needs to push a hull at a given speed. These numbers have been verified through years of observations with ICE engines and are accepted to be accurate. However, through observation, one can see that an electric drive can push a hull at the same speed with much less power applied to the propeller.
My boat at 3kts uses about 17% of the power via electric compared to what Gerr predicts. That is so far off, that I think that Gerr is predicting something other than what I am measuring. By 5.5kts, my observations are 42% of what Gerr predicts. Gerr must be using some assumptions that are not accurate for my boat.
My simple question was if Bill S. had a single data point to validate this question. What is the observed energy load on Barbara Ann's electric drive at 7.2kts?
It is physics, not magic, so I know that there is an explanation.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "\"hanermo\" - CNC 6-axis Designs" <gcode.fi@...> wrote:
>
> To me this is obvious. I am quite certain but might be wrong. (No, not
> really).
>
> 1. The ICE system is quite inefficient....
>
> 2. At low speeds, the efficiency is much lower, as small ICE systems are
> optimised for top speed running....
>
> 3. The efficiency numbers are also consistent - practically all are
> within 10%-15% as Eric said....
>
> 4. Given that the trends are all very close, these also poitn out to the
> well-known fact that most hulls are very close to each other in efficiency....
>
> 5. The power of the early 200-400 ton trading ships, in the early 1900s,
> was about 80-150 hp.
> When the age of sail transitioned to steam.
>
> 6. As a conclusion LWL is the best way to increase effiency and reduce
> overall costs and speed made good (if marina cost is not taken into
> account)....
>
> 7. Bill Southworth -
> As you have excellent data, measurement systems and experience, could we
> see that is the actual consumption of power at 0.8 and 0.9 hull speed
> (hull speed) with the 100 kW conversion ?...
>
> Best,
> hanermo
>
> > 3kts = 2.97kW or 3.98hp
> > 4kts = 4.37kW or 5.86hp
> > 5kts = 6.81kW or 9.13hp
> > 5.5kts = 8.74kW or 11.72hp
> >
> > 3kts = 9.54A at 52.4V = 500W or 0.67hp
> > 3.9kts = 19.2A at 52V = 999W or 1.33hp
> > 4.4kts = 28.9A at 51.5V = 1488W or 1.98hp
> > 5kts = 48.5A at 50.7V = 2459W or 3.28hp
> > 5.7kts = 78.8A at 49.6V = 3908W or 5.21hp
> >
> >
> > What do you think? Feel free to share this info with the rest of the
> > brain trust behind your boat. It does make one wonder...
> >
>
No comments:
Post a Comment