Re: [electricboats] Sizing motor for 45-50’ boats

Thanks Dan, this is helpful! 

I wonder, are electric yacht using a differential or just two pulleys on the same output shaft? The positioning of the motors makes it look like they are using a differential. 

Any reason that a single Sevcon Gen4 could not drive the two 10kw in parallel?

I think if one failed, and the gearbox doesn't have a way for idle input to disconnect, the result would be current fed back into the system - and a doubling of the torque the other motor is required to produce? 

On Mar 14, 2021, at 11:05, Dan Pfeiffer <dan@pfeiffer.net> wrote:



I have been looking at this as well though not for a larger boat but for what it takes for sustained cruising at 6+ knots as I could easily do with my 23hp diesel in my 33 ft boat (12,500 lbs, 28.3' LWL).  That's about 85% of hull speed.  I don't think the 10KW motor (ME1115) is really up to that once you get past 11,000 lbs displacement?  The continuous rating is something like 150 amps or 7200 watts at 48V.  That should get you into that 5.5 to 6 kts range but should you really be running the motor at 100% sustained?  I also wonder how much prop optimization has been done on the examples I have seen data for.   I suspect that in general most are running smaller props at higher speeds.  But I don't know. 

We need more data.  More, more...

A motor like the Motenergy ME1616 is rated at a peak of 600 amps.   But the continuous rating is 250 amps with liquid cooling (150 amps without) which at 72V gets you the rated 18kW.  If you plan to run at sustained speeds you need to account for the continuous capacity of the motor.  That is your limit.  Peak is unusable for anything more than a minute at a time.   You can't run continuously at 600 or even 300 amps even if you have the capacity to deliver that amount of current continuously (like with a big genset).  But if you can get 6kts at 7000 watts that's 60% for the ME1616 and that seems reasonable as far as the motor.  And with an 8kW genset you can do that for as long as you have fuel.   But without the big genset forget it.  You're looking at 90 minutes with a 300AH bank at 80% DoD. 

Running two 10kW motors parallel (ME1115 x 2) is exactly what ElectricYacht does with their QT20.   And they run two 12kW (ME1616 x 2) in parallel for the QT30.  I like how they put a water pump on the jack shaft of the QT30.  Both of these use dual belt drives and dual controllers. 
https://electricyacht.com/electric-sailboat-motors/

And there are some examples of installations on the youtubes. 
rigging doctor has a QT20 (though I don't think he is a real doctor)
Spoondrifter has a QT30 (not launched yet)

Search on youtube and add the word motor to see a bunch of videos regarding these installations.  These are both in 40-45ish foot boats?  I wonder with these setups what happens if one motor fails.  Will it freewheel or will you have to go in and remove the belt.  I know rigging doctor has run on just one motor when the other failed but I don't know if they removed they removed a belt.   I also know they spend a lot of time waiting for wind (and that works fine for them).  But I want to be able to motor up the Detroit river and transit the Welland and Erie canals.  That requires sustained speed.  Not so much in the Erie but in the Welland and Detroit river you are dealing with real ship traffic all day long.  That's no time to be puttering along at 3-1/2 kts. 

Spoondrifters has a genset and that is basically going to be  a series hybrid drive.  But I think the genset is on the order of 6-9kW and that won't get them a lot of sustained speed?  The battery bank might for as long as it holds up (not long).   Regardless, you won't be drawing 600A with anything for very long as the motor won't take it.  Even the ElectricYacht QT30 with two 18KW ME1616 motors will be rated for 500 amps continuous.   But that's running at 100% and look at what you would need for supply power to sustain that for anything more than 20 minutes.   Maybe that's getting more practical on a 45 foot boat with the current lithium battery options like EVE 280AH cells.  48 of those would get you a 72V 560 amp bank for $6,000.  But that's only going to run you at 500 amps for an hour and at that you're abusing the batteries.  And the genset to keep up would need to be 30kW? 

I think I might spend more time looking at parallel hybrid setups in a bigger boat like that.   Beta Marine is getting into that now. 
https://betamarine.co.uk/kc-hybrid-propulsion/

Look at Nigel Calder's work on parallel hybrids too. 

Dan Pfeiffer

 

On 2021-03-14 11:35 am, Ryan Sweet wrote:

So it seems clear that the motoenergy 10kw + sevcon gen4 combo (thunderstruck kit) works great for boats in the 30'ish range, more or less replacing motors around 20hp or hull speeds from 5-7kt.  

Now I'm interested in skilling-up to take on conversions of larger boats.

Those of you with 40-50' boats, 60-150HP diesels, what are you using to re-power?   At higher voltages, what are the power curves like? Will a 72v motor draw 600a at top speed (and thus need a *much* larger bank than our 48v systems)?  Anyone experimenting with multi-input/single output transmissions and multiple smaller motors (eg run two 10kw 48v side by side)?  What are the best chargers for 72v/96v systems?

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