Hi John,
The problem is that these "power required" calculators don't actually work for many of our boats. We had a long discussion last year about this, I even discussed the problem with Dave Gerr himself. Dave admitted that his formulas won't work at the lower speeds and powers that we typically operate at.
Here's my proof. The caculator that you suggested says that my 30' ketch with a 24' LWL and 10,200 lb displacement would need more than 6.81kW or 9.13hp. But I know that the boat only draws 2500W from the batteries to motor at 5kts, less than 37% of the predicted value. How do I know this? Here's an excerpt from post #18842 on June 21, 2011:
"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, first 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."
The bottom line is that you can't really take the power prediction calculators as gospel, in fact my boat uses about 1/3 the power that the Gerr caluculator predicts at 5 knots. At 3 knots, it predicts that I should need 5 times the power that I actually use.
This shows the importance of checking real world performance, at least to validate the theoretical predictions. In this case, the available calculators are way off.
fair winds,
Eric
1964 Cheoy Lee Bermuda 30, 5.5kW Propulsion Marine drive, 8kWh lithium batteries
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "john_casperson" <john_casperson@...> wrote:
>
>
> This is the speed calculator I use.
>
> For a cat, calculate for a single displacement hull. Then double the power requirement it spits out for the twin hull requirement.
>
> http://www.psychosnail.com/boatspeedcalculator.aspx
>
> John Casperson
>
No comments:
Post a Comment