Hi Steve,
I don't know if this is the "right" way, but I used the following process to measure my boat's performance under electric power alone. This process should provide pretty consistant data since the process only took about 90 minutes.
First, I picked an easily identified course, in my case, I picked two bouys inside our marina that are about 400 yards apart. This distance means that the boat can stabilize at its natural speed for that power setting. Each power setting was run in opposite directions, fisrt from north to south, then turn around (without touching the throttle) and run the same course from south to north. This should reduce any influence that a consistant wind or tide may have on readings for a pair of passes. After completing a pass in each direction for a given power setting, I increased the power to the next target while turning around and repeated the process. After making two passes at 6 power settings, I reversed the process starting with 2 more passes at the top setting. After each pair of passes, I reduced the throttle and jiggled it until I was close to my target power setting. So for me, I made a total of 24 trips between the bouys, 2 x 6 working my way up and 2 x 6 working my way back down through the power settings. My power setting targets were 10A, 20A, 30A, 50A, 80A and full throttle (106A).
As for data collection, I recorded pack voltage, motor RPM, southbound speed and average amperage, northbound speed and average amperage. Then I averaged the results for the 6 major power settings. The pack voltage and current was measured with a E-xpert Pro HV battery monitor, and the speed was measured with a Garmin 60CSx GPS. The pack voltage reading was usually taken during the turn between the southbound and northbound passes at the same setting, the motor RPM was taken about halfway down the first leg. The amperage was observed over the entire course, the normal instantaneous flucuations were averaged by me to give reading to 1/10 of an amp that I believe was close enough.
I've done the whole process twice and posted the results here within a few days. Here's the "watts to knots" from both of those sessions.
3/13/2011 - 11:00am, wind calm, water smooth
500W = 3.0kts
1000W = 3.8kts
1475W = 4.3kts
2460W = 5.0kts
3910W = 5.7kts
5170W = 6.0kts
3/27/2011 - 12:00 noon, wind WSW 4-8kts, water smooth
495W = 3.0kts
1000W = 3.9kts
1500W = 4.5kts
2450W = 5.0kts
3905W = 5.7kts
5165W = 5.9kts
You can see that the results are pretty consistant even though the conditions were different.
If you would like to see the spreadsheet of raw data that I compiled, I've uploaded it to the file section of this Yahoo group in a folder called "Eric's files - Serenity". The Excel 2007 spreadsheet is called "Measured performance - Serenity.xlsx". Feel free to use the format if you like, although to be honest my first day of data collection was on the back of an envelope, and the second day was on a work tag left by my bottom cleaning service. I put all the figures into excel later.
I know that this is more than most people want to do, but I can state that my performance numbers are consistant and repeatable. I'm probably going to do this again, the next time the wind is too light for decent sailing (I don't want to sacrifice a good day under sail).
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Mike <biankablog@...> wrote:
>
> Steve:
> Â
> A good question. I'm still trying to figure that out myself. I did a few test runs in the harbor just after launch recently to check things out and I got some strange readings in one of the runs as I think Eric has mentioned too in some of his tests. It was not a windy day but, I suspect the harbor currents were affecting the test. I'm thinking I need to do the test in a bigger and deeper body of water where subsurface currents are not as strong as in the harbor. A lake would be great but, those of us in salt water need to take in the currents as well as wind when testing. Slack tide on a windless day would be about the best time to do the tests but, I suspect perfect test conditions are hard to achieve in some areas.
> Â
> Capt. Mike
> http://biankablog.blogspot.com
>
>
> --- On Tue, 6/21/11, Steve Dolan <sdolan@...> wrote:
>
>
> From: Steve Dolan <sdolan@...>
> Subject: RE: [Electric Boats] Re: Sea trials and data gathering
> To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
> Date: Tuesday, June 21, 2011, 10:13 AM
>
> Could someone put together the right way to gather all the data everyone is looking for when testing out their Electric Conversion? I know it’s been posted here in pieces but maybe it needs to be gathered and put into one place. I guess what I’m looking for is a form or at least some direction to get the data down including any formulas for conversion so I can get it right. As some of you know I have a 41’ Catamaran with a 144V system which goes into the larger category for conversion but I want to have the data be usable for others.
> Â
> Thanks,
> Steve in Solomons MD
> 410 Se Lagoon Electra Glide
>
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