I'm kind of looking at this differently regarding the KW of a system being compared to hull speed when "motoring". Based upon the weight of my boat, rule of thumb would dictate 5 KW. It's not achieving hull speed that concerns me. It's having the power to punch thru stuff if the needs arises. Of course hitting hull speed under sail, that works for me:)
Great thread!
Thanks
Bob
On Tuesday, March 25, 2014 11:16 AM, "ewdysar@yahoo.com" <ewdysar@yahoo.com> wrote:
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 to 15hp. 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,
Eric
Marina del Rey, CA
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