Hi Mick,
About 10 years ago, the members of this list came up with the following "rule of thumb" to size drive systems for electric conversions. What we came up with back then is "1kW of drive power for each ton of displacement" We found that this estimate worked pretty well for a broad range of conversions. In fact, a couple of years ago, I saw that WestMarine published that same guideline in their catalog, in the section on electric conversions, but we should be proud to recognize that it appeared here first.
Here is a post that I made about conversion sizing suggestions back in 2014.
Hi Myles,
I agree. When I first showed up here, the only conversion suggestions were to use an electric motor that was rated 1/3 to 1/2 the ICE that you were replacing. But regular boat engine sizing is all over the map, so using the previously installed motor as a basis wasn't as consistent as I preferred. Take the venerable Catalina 30 as an example. They came from the factory with engines that varied from 13hp to 30hp, same hull, same weight, same rig. Obviously the appropriate electric motor would be the same regardless of how the boat was originally delivered. My boat, also 30' and 5.1 tons displacement was offered with an Atomic 30 (virtually never configured to deliver 30hp in any boat) or a variety of small diesels. My boat happened to have a Yanmar 12hp installed from a previous repower. Basing my choice on motors available in my design or in a Catalina 30 could result in electric systems ranging from 4 (3kW) to 15hp (11.3kW). That was too vague for me.
So I looked at any objective performance data that I could find (still a rare piece of data, even today) and discovered that displacement was the most consistent single design attribute that influenced power demands. Of course, every other aspect of the hull and boat design influences the same power demand, but from looking at the data, to an increasingly lesser extent. I first noticed that 1kW/ton fit many common sailing auxiliaries using the engine replacement guidelines. Going to the real world electric performance data, the displacement rule of thumb has proven to hold up for all but extreme designs, outliers from a statistical perspective. Of course there are assumptions, once a boat leaves displacement mode, the power requirements do not progress consistently, but those power levels are usually out the reach of battery powered vessels (as Myles noted).
Since the displacement rule of thumb was conceived, more data has been collected, James at ElectroProp wad his boat towed with a strain gauge to measure real world drag (anyone else?), and those figures varied from the online formula predictions by more than you would think. Working backwards from the measured performance of my conversion, I found that D. Gerr's power formulas predicted more that 3 times the power needed at the prop than my boat pulls from the batteries ( so my power at the prop is even lower still). So I encourage all of you to measure and post your actual performance results, with more data, we can develop better models and more accurately predict performance. Don't sit back in your armchair and figure that someone else will do this for you, participate and contribute. As they say, "a rising tide lifts all boats" (weirdly on topic).
Fair winds,EricMarina del Rey, CA
So does this conversion guideline work? My performance measurements certainly fall right in line. The few other members that have posted objective performance observations have provided additional data that supports this "rule of thumb".
Here's one of my posts about measured performance from 4/04/2013
Another post from 3/28/2011Last weekend I realized that it had been about 2 years since my last performance test, and I wondered if anything had changed over time. I was at the marina on Easter morning and the conditions appeared to be ideal for another performance test. I ran through the same process listed below in about 100 minutes, covering a little over 7nm in total.
Here's this weeks results:
497W = 3.1kts
1000W = 4.0kts
1500W = 4.5kts
2500W = 5.2kts
3975W = 5.7kts
5000W = 5.9kts
So there have been pretty minor changes, well within testing errors. Like before, the max winding temp was 72C and that temperature had dropped to 53C by the end of the last slow speed pass. 2 years later, the drive, prop, hull, batteries, etc. seem to delivering consistent performance.
If I average the three different testing sessions, the numbers look like this...
Here's the overall average results:
499W = 3.0kts
1000W = 3.9kts
1500W = 4.4kts
2470W = 5.0kts
3935W = 5.7kts
5115W = 5.9kts
This data collectively represents more than 3nm at each speed, in calm conditions, in multiple directions.
On another note, I did a full throttle run at the dock. Average amp draw was 158.8A DC at 47.24V or 7.5kW! Compare that to the 5.1kW noted above at full throttle in open water. Since I was starting with 80Ah already used of my 128Ah of usable capacity, the 160A load was rapidly draining my battery pack. After more than 5 minutes at full throttle, the motor windings had climbed to 85C, well under the manufacturer's rated limit of 150C, although the temp had not stabilized yet. I decided to stop the experiment as I quickly approached having only 30% of my usable capacity left and I didn't have too much time to get some electrons back into the pack before I had to leave. Perhaps I can do this again with a fully charged pack to see where the ME0913 motor temps will stabilize while pulling 7.5kW in the bilge of my boat. In spite of what some of the component vendors might say, I'm pretty sure that running that motor in boat at 10.5kW will cause it to overheat in relatively short order. I appreciate that my vendor conservatively rated my entire drive system, I'm sure that it will continue to perform well for many years.
Anyway, I collected the data and I thought that some people here might find it interesting...
Fair winds and smooth seas,
Eric
1964 Bermuda 30 ketch, 5.5kW Propulsion Marine drive, 8kWh lithium batteries
Marina del Rey, CA
Hi everybody,
I did another set of performance trials yesterday. I started at 12:00 noon, after the last rain shower passed. The process took about 90 minutes and I covered 6.35nm during the tests. I ran the same process as before, 1 pass in either direction at a particular throttle setting, progressively climbing through 6 settings and then repeat the process descending through the same settings until done. There was some wind by the end of the trials, but that should be offset through the pass in each direction. The results were more consistant than the last trials which had apparent tsunami surge that skewed some of the numbers.
Here's this weeks results:
495W = 3.0kts
1000W = 3.9kts
1500W = 4.5kts
2450W = 5.0kts
3905W = 5.7kts
5165W = 5.9kts
Slightly better at the low end and a little worse at the top end, but fairly close (+/- 0.2kts) to the previous tests.
Because we had a conversation in the interim about motor temperatures, I recorded winding temps during the second half of the trials to see how quickly the motor would recover during the lower power settings. With a max observed winding temp of 74C at the end of the fourth pass at full throttle, the motor dropped to 52C by the end of the last 500W pass.
Right after the trials, I set the throttle to about 2500W and headed out of the marina, around the detached breakwater in open seas and then back in. 45 minutes later, at a constant power setting, the motor was stable at 60C and I had covered an additional 3.85nm. Apparently, the speed boost downwind is greater than the penalty going upwind resulting in an average speed of just over 5.1kts.
After messing about for 30 minutes or so at slower speeds, I brought her back to the slip. The entire day was 10.9nm with a 4.0kt average. The battery meter showed 107.7Ah consumed. After 15 minutes rest, the battery bank was resting at 52.35V. 4 hours on the charger, and she was fully recharged.
In a couple of weeks, I'm going to try a range run on the open ocean at about 1400-1500W to see how the batteries match up against their specs. I'm predicting about 18nm at 4.3kts out of 6.4kWh of usable capacity, that should take a little more than 4 hours.
I know that all this info is more interesting to me, so thank you for your time...
Fair winds,
Eric
Marina del Rey, CA
So I finally got a chance to do some controlled performance trials over the weekend. I headed into the main channel of Marina del Rey at about 11:00 AM to measure system data while travelling a 400 yard course in opposite directions. Unfortunately, there appears to have been some residual tsunami surge that skewed some of the runs. From nearly no difference between the 2 runs to 1kt faster speed southbound, 1.4kt faster northbound 10 minutes later and 0.5kt faster southbound 15 minutes after that. So I'll have to repeat the tests sometime in the next few weeks.
The boat is a 1964 Bermuda 30 ketch, 8'9" beam, 24' LWL and 10,200# displacement. The conservatively rated 5.5kW electric drive was purchased from Propulsion Marine in Santa Barbara. I assembled my own battery pack from 16 Thundersky 160AH LiFePO4 3.2V lithium cells for a pack rating of 8kWh @ 50V nominal.
I measured pack voltage, motor RPM, southbound speed and average amperage, northbound speed and average amperage. Then I averaged the results for the 6 major speed 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.
But here's the quick recap of my boat at 6 different speed settings, 4 runs per setting.
500W = 3kts
1000W = 3.8kts
1475W = 4.3kts
2460W = 5kts
3910W = 5.7kts
5170W = 6kts
The funny thing is that my old diesel maxed out around 5.5kts at full throttle. While I don't intend to motor places at 5kw, it's nice to know that I've got the extra power if I get into a tight spot.
The system worked great and now I get to re-work my range estimates for the better. This system exceeds my expectations, and I want to thank James for guiding me through the conversion. It has been a real partnership, again more than I expected. The only negative that I can see so far is that the new prop has more prop walk in reverse than the old one, so I've got to spend some time practicing backing down so that it becomes second nature again.
That's it for now....
Fair winds,
Eric
Marina del Rey, CA
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