T-Pain Is On A Boat: Re: [Electric Boats] Trolling motors ==> electric outboards

Wednesday, May 29, 2013

Re: [Electric Boats] Trolling motors ==> electric outboards

Hi Oakbox (?),

Yes, that amount of power should put you in the ballpark. Unfortunately, trolling motors are made for ummm.... trolling, so they are rarely propped for the speed that you are looking for. Making this work better may be as easy as finding the right propeller, but some have found that even then, the motor isn't built for the particular loading that you would be imposing.

So a Torqeedo 1003 at 1kW (40A @ 25V) would probably be just a bit underpowered, but propped way better for what you want. I would predict that a Torqeedo 1003 would be at least 1 mph faster than the Minnkota while using 30% less energy. A Torqeedo Cruise 2.0 would have a bit extra power (but probably not enough to push you all the way to hull speed). Though I haven't performed the experiment myself, I would guess that Torqeedo motors use half the energy as the Minnkota that you're looking at to push the same boat the same speed.

And Minnkota's range estimates are very optimistic. A 24V 100Ah battery has 2400Wh of rated capacity. That's at the 20 hour discharge rate of 5A. But as you raise your discharge rate, the amount of usable energy falls off. Do a quick search on Peukert's Effect to find out how that really works. Once you can accept the losses due to Peukert's Effect exist, I can tell you a bit more about what your proposed battery can do.

With a 2.4kWh 24V flooded lead acid (FLA) battery, a 56A load will drain it 100% in 58 minutes, assuming a Peukert's exponent of 1.25. We also know that draining a battery completely is not good for it, so most boaters will limit their depth of discharge (DoD) on FLAs to 60%, actually most cruisers will stop at 50%. Using the same 56A, you'll hit 60% DoD in less than 35 minutes. You would need 155Ah of rated capacity to run a 24V FLA battery down to 60% DoD at 56A for 60 minutes.

But AGM batteries are better due to their lower internal resistance. You can run them down to 70% DoD on a regular basis. A typical AGM Peukert's exponent (PE) is 1.1 and now you only need 105Ah of rated capacity to run the motor at 56A for 60 minutes. So in my opinion, AGM are well worth the money for electric drive traction batteries.

What about lithium? They discharge to 80% and have a PE of 1.03. You only need 77Ah of rated capacity to support a 56A load down to 80% DoD in 60 minutes. They're pricey, but for some, well worth the premium. Let's summarize the results.

We want 60 minutes of 56A at 24V:
FLA to 60% DoD = 155Ah = 165 pounds
AGM to 70% DoD = 105Ah = 125 pounds
LiFePO4 to 80% DoD = 77Ah = 50 pounds

That's right, the lithium pack can do the same work with half the rated capacity and less than 1/3 the weight of FLA.

I said earlier that a Torqeedo can probably push your boat at the same speed using half the energy as the Minnkota. If you could cruise at 28A instead of 56A, this is what it looks like.

We now want 60 minutes of 28A at 24V:
FLA to 60% DoD = 77Ah = 82 pounds
AGM to 70% DoD = 52Ah = 62 pounds
LiFePO4 to 80% DOD = 38Ah = 25 pounds

How much effect do you think that 140 pounds of un-needed weight (Minnkota with FLA compared to Torqeedo with LiFePO4) will have on your boat? How much does a Minnkota with two 155Ah FLA batteries cost compared to a Torqeedo with a 20Ah battery built in and an extra 20Ah battery to bring your range up? What is your sailing performance worth? My first boat was a Catalina 22, I would probably step up and buy a Cruise 2.0 and a separate 40A 24V LiFePO4 battery, but that's just me (see my signature).

We can see that there is more to converting a boat to electric drive than simply wiring batteries to a motor to drive a prop, regardless of what James was thinking in his other thread.

If you have any additional questions, feel free to ask.

Fair winds,
Eric
1964 Bermuda 30 ketch, 5.5kW Propulsion Marine drive, 8kWh Lithium batteries
Marina del Rey, CA

PS I'm not a professional electric boat guy, just a hobbyist. I've always thought that this is the kind of conversation that this group excels at.

--- In electricboats@yahoogroups.com, oak <oak_box@...> wrote:
>
> Ok, so at 50 amps * 24V, I'd be in the ball park for a 2300# Catalina 22.
>
> From a youtube review of the Traxxis MinnKota 80# motor, they report:
>
> slow5A20hr
> med28A3.6hr
> top speed56A1.8hr
>
> The above assumes a 24V, 100AH battery.
>
> If I could get ~5mph out of a $600 motor for an hour, I'd be THRILLED!!
>
>
> ________________________________
> From: Eric <ewdysar@...>
> To: electricboats@yahoogroups.com
> Sent: Wednesday, May 29, 2013 5:00 PM
> Subject: Re: [Electric Boats] Trolling motors ==> electric outboards
>
>
>
> Â
> Hmmm. If there was only some way that one could easily estimate how much electric drive would be needed to move their boat at 90-95% of displacement hull speed....
>
> Oh yeah, back when we used to discuss boat conversions, the rule of thumb of 1kW of power for each ton of displacement has proven to be correct for boats ranging from 18 feet past 40 feet.
>
> Of course, one can move the boat with much less power (search this list for "Jack LaLanne" for low power examples), but for safe operations in all but the most extreme cases, this "rule" works surprisingly well.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>