Interesting thread....
My 21.25 ft. LWL with a calculated hull speed using 1.34 x square root LWL (which in itself is a moving target - but we have to agree on a baseline) is 6.2 Knots. My bunkered and victualled displacement in long tons (2,240 Lbs) is 3.33 tons. At 60% hull speed = 3.72 Knots I use around 600 Watts. So for that speed I'm using 171.43 Watts per ton.
And, as said elsewhere one can choose EP motor power ratings based on the size of the diesel that was originally fitted, however that assumes the appropriate diesel was fitted in the first place, which is often not the case. Still the 1/3rd rule works for me. My old diesel was a 7.5kW and my continuous electric is 2.5kW. I consider either to be a sailing auxiliary engine, not a prime mover.
Personally I'm happy with a top continuous speed (duration not currently specified) in flat calm salt water
Conditions, with no wind, no tide, no funny swirls and currents of 85% of hull speed and a cruise of 65%. Cruise speed to coincide with the peak efficiency rating of the motor where possible. This all being analogous to using a high revving light weight non continuous use light duty cycle diesel at 75% of peak RPM which is normally fitted to small yachts. I exclude the low revving more continuous use heavy duty cycle diesels fitted to some larger sail boats, whose 'continuous' use may be around 85 - 90% of peak RPM.
In some ways I don't understand the seeming obsession with peak continuous power, top speed and 90% hull speed. It is all very American ;) To me it is more important to agree what the cruise speed is. I suggest 2/3rds hull speed at or around the peak efficiency of your motor, as this is where we spend most of our time. Over that and as long as we have a reserve of power be it high torque low revs/shaft speed or high revs/shaft speed and low torque giving the same kW (you decide how you want to achieve your peak power) then as long as I can get off a lee shore (with the help of sails) I'm happy.
If I want a higher continuous cruise speed, then I should have a diesel or a hybrid, not pure EP (with solar or a battery assist petrol/diesel small generator). If enough electricity came out of the sky (mega efficient solar
built into sails with enough power to cruise) then the goal posts would move and I'd be looking at higher speeds and more efficient motor/controller materials tech and non-air cooling, if one must. Analogous to that is why cool a diesel? We have to for power reasons, as the most efficient diesel would be one that was insulated and used any other heat not converted to thrust to do something else/useful with it. But as we don't have fancy ceramic pistons and affordable materials technology to convert the energy more efficiently to thrust, we're stuck with cooling.
So that all brings me back to standards (STCs). As most small sail boats by definition are ones under 24m, as over that we are into large yacht coding (non Merchant Navy stuff). Below 24M we are either RCD (recreational craft directive in Europe) for pleasure boats and ABYC US Coastguard in the USA? If commercial under 24m we are invariably MCA (Maritime and Coastguard Agency) coded or a derivative, subject to the flag.
Realistically though for pure EP and staying with 48V and under systems (as I think safety specs change after that) we could define a test boat as being a sail boat 10m and under with an agreed light ship displacement. Power boats and canal barges have different requirements not to mention prop size variations that are not ideal for sail boats. i.e. the best/fastest sail boat is one without a prop or in our case it is a low power EP electric/electro sailing boat.
Any test would be independent of hull design/shape and prop type in fact as the standard (now I think more about it) is to test different manufacturers and assemblers of EP systems on a dyno? Measuring such things as duration, temperatures, amps, volts, torque, RPM, efficiency, time to achieve RPMs etc. and plotting it. This should sort the wheat from the chaff.
Maybe though we are not ready as the industry is small and really the only systems actually out there, suitable for 10M and under yachts are Motoenergy and Lynch motor derivatives. i.e. mostly traction low rpm/volt, high torque/amp motors.
We'll leave higher RPM motors for bigger boats to the likes of YASA etc. http://www.yasamotors.com/products/yasa- 400/, however I think some of the motorcycle designed motors can plug the intermediate gap. I'll leave Elco type motors out of this. As pretty as they are and well marketed, I don't see them as a realistic option due to weight and form factor for sail boats around 10m.
The bottom line here as many seem to agree though is "What Watts at what knots?"
In that case I shall take my 8M sail boat to the canal this year and put the tests online in real time for all to
see. This assumes my 3G dongle will get a signal. For a demo of how this works see https://vrm.victronenergy.com/ and click demo. Look for the OceanVolt Test and choose the Advanced tab where you will see such data as:
Motor Summary
Last update: less than a minute ago
Motor RPM
0
Controller DC Power
0.0 W
Voltage and Current
53.4 V
0.0 A
Motor temperature
35 C
Controller Temperature
28 C
I guess they have not long stopped the motor as temps are still up. You'll also note battery info and GPS info.
My own real time online system with additional spreadsheet logging is hidden at present, as I'm testing beta
software.
If you want to look at real time hybrid (the outcome of the HYMAR project) see
http://www.triskelmarine.co.uk/hybrid
Anyone else out there ready for the "What Watts at what knots?" real time or post real time logging challenge?
John
https://www.facebook.com/ElektraYachts & http://John.Rushworth.com
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