Thrust is not power. Comparing systems based on thrust ratings is comparing apples to oranges. Trolling motors are sold to people who want to troll, i.e. motor at 1 to 2 mph. Electric power in the context here presumes 4 to 6 mph and on up. The best way to compare motors is to use the watts consumed, watts are physically the same as hp. 746 watts = 1 hp.
To harness the power you need a prop matched to your hull and motor. Trolling motor props have too little pitch to be effective at 4 or 5 mph.
Torqeedos work because they have a very high rpm motor geared down approx 10:1 so they can turn a large dia, large pitch prop. Trolling motors are direct drive, simple and cheap but not efficient outside their intended use parameters.
In experiments I did a few years ago I was able to double the efficiency of a trolling motor pushing a light 18' sailboat by changing the prop and fairing the round tube.
Denny Wolfe
www.wolfeboats.com
On 5/30/2013 2:05 PM, John Francis wrote:
I'm going to go out on a limb and pretend I think I know what I'm talking about, simply becuase the thrust and HP is too confusing.It appears to me that electric motors, while having a HP rating(of the actual motor), in a boat propulsion system ... is rated in thrust.Evidently THRUST already computes all the prop data and revolutions to achieve a specific THRUST ... like a packaged, computed value.I've heard that if a person changes over a MinnKota prop to a Kipawa-type prop, the THRUST increases, while the motor in the buld unit remains the same.So THRUST is the value of pushing power off of the propeller and whatever speed is predetermined as the base-speed(motor).A 10 HP I.C., is an engine that delivers 10HP of work at some point between the cylinders and the propeller shaft. You can put a double sized prop with a double deep bite, and the motor remains a 10HP motor. If you put a smaller prop on a 10HP motor, it remains a 10HP motor.So many variations that all come down to pairing up different components.Personally, I think THUST is the most descriptive value of what the motor can do for us.This is not that electric mootors need to have available HP ratings, but rather, that IC engines should be computed for THRUST ratings. After all it is THRUST that pushes water astern of you boat.Probably, none of this is important . . . bigger is better . . . until a group of "green", sailors with displacement hull sailboats, who work in that magic marginal land of decifering THRUST, HP, matching propellers, prop speed and torque.Very confusing in deed.Thankfully, I tend to be a minimalist and I'm willing to error on the short side of the computation.JohnNewport 28 Port Clinton, Ohio
On Thu, May 30, 2013 at 10:22 AM, oak <oak_box@yahoo.com> wrote:
John (other John...),
Thanks for your response - it's good to hear that the Torqeedo 2 will work well for that size boat!!
Eric or someone had explained in previous a previous thread how a 48V system would be more efficient and better on battery life than a 24V system. But given the doubling of effort on the battery side, I think I'd compromise the power efficiency to be able to afford better batteries and charging.
If I can get some data on the Catalina 22 with the Torqeedo 4, it would be really neat to see if we could correlate that to data on you Capri 22 with the Torqeedo 2. (which would also be really neat to compare to trolling motor data, if anyone had one...)Wouldn't it be nice if you could RENT these things for a week and try them out before putting down the $$!!
Thanks again for the encouragement.John (in Texas)
From: John <johnpomer@mac.com>
To: electricboats@yahoogroups.com
Sent: Thursday, May 30, 2013 7:35 AM
Subject: [Electric Boats] Re: Trolling motors ==> electric outboards
I have been using a Torqeedo Cruise 2.0 on my Capri 22 for the past two seasons - the last season was on a mooring with only solar recharging. I had tried a Minn Kota Endura 50. With a 4 inch pitch, it was insufficient to maneuver the Capri in wind. (It worked fine for my O'Day 19 which has half the weight.)
When looking at the power output, Torqeedo uses "power at propeller" (including all of the drive train losses), not shaft power (which neglects power losses after the engine shaft).
The Torqeedo performed as well as the Nissan 5 HP (2-cycle) that came with the boat. Low speed and reverse are much better using the Toqeedo. I can reach hull speed with the Torqeedo.
Battery system is 4 GC2 lead acid batteries with a 100 watt solar panel and a Genasun GV Boost MPPT charge controller (takes the nominal 12 vdc from the solar panel and intelligently charges the nominal 24 volt battery bank. Always had a full charge when leaving the mooring twice a week.
John
(NOT aka "oak_box" <grin>)
--- In electricboats@yahoogroups.com, oak <oak_box@...> wrote:
>
> Eric,
>
> While I can "intellectually" appreciate the wisdom of all that you're saying, my pocketbook doesn't want to hear it...<grin>
>
> Seriously, I get what you're saying, and it makes a lot of sense.
> You make a good point about the sizing of the props on the trolling motors. I can tell you that the prop on my Torqeedo 4R is BIG compared to any trolling motor, and definitely even any kind of outboard up to the same size. If you believe the Torqeedo literature, they specifically designed the prop on that motor for the electric outboard application, and it seems to do a good job (though I'm anxious to get more experience with it).
>
> This afternoon, I came across the Minn-Kota Riptide that has dual motor units and is rated at 160 pounds thrust - close to what the Torqeedo 4R is, and at 24V (I didn't see anything that listed how many amps it would draw).
>
> At 1/2 to 1/3 the price - it's very appealing.
> But again, your points about prop size, motor speed, etc, are all well taken.
>
> I happen to already have the Torqeedo 4R that I will experiment with on the C-22. But if I could afford to get down to a 24V system instead of a 48V system, it becomes much more reasonable to get a single 24V lithium pack than to have to buy two. If only money weren't an object.
>
> The unfortunate reality there is that I can easily spend 3x the price of the boat on the outboard and batteries!
> So yes, there are tradeoffs to be made And fortunately - in MY case - I don't have to go terribly far on battery power. It would be really nice to be able to cruise 2-3 hours. Typical case (at least if I was sailing) would probably be 30 min. to less than an hour, and a good amount of that could be at reasonably low power.
>
> I really wish I had the spare disposible cash to buy one of each of several and try them all out! If the 80 foot pound motor was enough to push the boat around a little bit, it would be fun to have the Torqeedo 2.0 as the main engine, and the trolling motor in a locker as a backup. Then it would be fun to have the 24V lithium as the main battery, probably with two cheap FLA batteries as a standby bank that I knew would get me at least 30-60 min at low speed to get me home if I exhausted the primary bank...
>
> I need to buy more lottery tickets...
>
> John
> (aka "oak_box", but not really - just John.)
>
>
> ________________________________
> From: Eric <ewdysar@...>
> To: electricboats@yahoogroups.com
> Sent: Wednesday, May 29, 2013 8:28 PM
> Subject: Re: [Electric Boats] Trolling motors ==> electric outboards
>
>
> Â
>
> 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 mailto:electricboats%40yahoogroups.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: mailto:electricboats%40yahoogroups.com
> > Sent: Wednesday, May 29, 2013 5:00 PM
> > Subject: Re: [Electric Boats] Trolling motors ==> electric outboards
> >
> >
> >
> > ÂÂ
> > 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
> >
>
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