Interestingly there is a set of power consumption curves on boat design.net showing a shaft power of .5 to .6 KW/ton at 5 knots with a 7 meter WL which is reasonably close to 2.5 KW for 5 tons at 5 knots. Hopefully this is the correct web address - www.boatdesign.net/forums/attachments/diesel-engines/10143d1163311033. Thanks. Steve S.
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Hi Redu,
>
> Without trying to sound pretentious, this was more analysis than speculation. The exercise was to validate the theoretical predictions with quantitative data collected with primary research.
>
> I have absolute confidence in my data. The current measurements were done with a TBS E-Xpert Pro HV (no pre-scaler required) using a 500A 50mV shunt. One end of the shunt is wired to the battery bank, everything else is wired to the far end of the shunt. In addition, the current measurements match the capacity of the batteries themselves, a measured 107Ah used took just about 4 hours to recharge at 27.5-28A from the Elcon PFC 2000+ charger.
>
> The amps load readings were also duplicated by the Sevcon Gen4 controller, with readings supplied by the ClearView display that was provided in my drive system. So the current was measured simultaneously by two independent, high-precision meters giving the same readings.
>
> As for the speed measurement, a GPS was used during 2 sessions on different days over a controlled course. The 5 knot measurements and current readings were averaged over 8 separate passes of more than 400 yards as described in previous posts.
>
> Given the effort that I went through to get consistant, repeatable measurements, I have to believe that my data is more than 98% accurate.
>
> We also know that a Catalina 30 with a 25' LWL and displacement of 10,200 lbs generated 100 lbs of drag at 5 knots, measured with a towing test with a scale in the tow rope. So a 5kt, 170 lb drag prediction is way off (too high) for a Catalina 30 or for my boat.
>
> You provided the formula with unit conversion constants, I provided the speed, current and driveline efficiency constants and the only 2 variables left are drag and prop efficiency. Given the formula, these variables are directly proportional, if the drag number is higher, then it requires a higher prop efficiency to go the same speed with the same energy.
>
> I proved mathematically that if my boat has 100 lbs of drag at 5kts, then my propeller efficiency must be 56% at that speed. Likewise, if the drag at 5kts is 170 lbs, then my prop efficiency must be greater than 96%, and nobody here believes that. If my propeller efficiency is less than 56% at 5kts, then my boat's drag at 5kts must be less than 100 lbs. At this point it is down to simple math.
>
> You see, I used to believe the Gerr power/speed predictions, until I could measure the real-world performance of my conversion. The Gerr formulas prdicted my power requirement at 5kts to be 300% of my actual measured power used, at 3kts it was 500% of my power used. I was hoping that your calculations would be better, and they were better, but a 70% error in the prediction still isn't close enough for me. I'm sure that if you bought a car that said it would get 34mpg and it only delivered 20mpg, you wouldn't be pleased.
>
> Anyway, thanks for taking the time to work through this with me.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
>
> --- In electricboats@yahoogroups.com, redu <reino.urala@> wrote:
> >
> > Eric,
> > thank you for interesting speculations! I think 170lb and 50% prop
> > efficiency are optimistic figures? What about your example kWatts and
> > speed figures? Are you absolutely sure?
> >
> > My mistake 3 years ago: I measured currents to el-motor using a "hook
> > ammeter". Don't know the correct instrument name in english? But the
> > sharp and high controller spikes spoiled my measured data completely.
> > Since then I only use current shunts and RC-filters to get reliable data.
> >
> > redu
> >
>
No comments:
Post a Comment