Minna Kota Motors

[Michael Rogers]

Thanks again, Peter.

Your (experienced) thinking chimes with my naive attempts to make some sense of the lack of relevant information available. All I can say is, it's a funny way to sell things!

I'll report back when I've got something to report.

Another (feeble?) attempt to drag Jeremy back into the discussion. I hope the eco-house is a success, Jeremy. If you're there and you've got a moment or two, could you join in? - you know more about this subject than all/most of us.

Michael

[Peter Taylor]

To give a bit more perspective, it would be good to know how much power the Torqeedo consumes when producing 12kg of pull, the same as your Minn Kota at maximum. It would then be possible to calculate its range and measure noise levels at that speed.

Well, I tried - but I'm not sure if I succeeded! At around 240W I measured (no more like estimated) between 10 to 15kg pull from the Torqueedo. Before I could get a really steady reading the rain got into the electronic balance and it stopped working...  if the balance recovers I'll try again some time!  If these readings were correct then it suggests that the Torqeedo does have more thrust at a practical level of power than the Minn Kota - but I wouldn't like to quantify the difference.

Peter

ps it wasn't raining when I started!

[Peter Taylor]

Finally, some bollard pull values for the Torqeedo 1003S mounted on my Seafly:
         100W = 5 to 6 kg
         200W = 10 to 11 kg
         300W  = 13 to 14 kg
         400W = 15 to 16 kg
         500W = 18 to 19 kg
These are plotted in the graphs attached. The bollard pull v power relationship is nearly linear, which surprised me. The duration plot assumes you can get 500Wh out of the Torqeedo battery. The duration for the Minn-Kota RipTide 40 is fairly arbitrary since it's easier to add another 12V battery, whereas its expensive to add another battery for the Torqeedo.

I think the plots confirm the Torqeedo is better but not by the amount you might expect given the cost difference.

Other things learnt: (1) it's remarkably difficult to steer a dinghy using an outboard when the dinghy is attached to a jetty via a rope and electronic balance! (2) you get lots of comments like "you'd go faster if you untied it!  Peter.

[Michael Rogers]

Peter, you're a hero to put yourself through that ridicule in the name of 'science'! (I suppose that, if onlookers have no idea about the what and why, you can slightly see their point of view?).

It all remains a bit confusing in terms of what to do (buy) and on what basis. And I had no idea, until I went into it, how many different trolling-motor-type electric outboards there are out there. My latest discovery is Flover, which is Korean, seemingly straightforward and with a minimum of bells and whistles. Flover claim that comparative tests show one of their outboards to be more 'efficient' than the equivalent Minn Kota (they do not quote chapter and verse): and they're certainly cheaper. I confess to a bit of a bias in favour of Korean engineering, in that we have a Hyundai 4x4 which is rugged and reliable. Flover seem to be an offshoot of the Korean equivalent of Bosch

Anyway, I'm crossing Torqeedo off my list, on the basis of the less than brilliant experience of Peter, Graham, and a couple of other people I've spoken to. The biggest Flover, or the equivalent Mariner, plus a 100Ah LiFeSO4 battery pack (weight 15 kg) would cost me about £450 less than the Torqeedo 503S.

[garethrow]

Michael - as a thought

15kg is I think heavier than the Honda 4 stroke 2.3hp petrol engine which is around 13kg, where 1 tank of fuel (approx1 litre) will last about 1.25hrs and drive you at your maximum hull displacement speed.

Regards

Gareth

[Michael Rogers]

Nice try, Gareth! I'm possibly being cussed on this one, but stinky, noisy, might-not-start (yes, all right, they usually do) IC outboards are a no-no for me. You can come back to me on that if/when the RNLI pluck me off the rocks somewhere.

I plan, actually, to have two 50Ah (or thereabouts) batteries stowed well forward, one each side. Yes, Peter, I will ensure that the leads will take full whack: 50A should do it?

I'm already into winter planning/dreaming mode, partly because we are moving house this side of Christmas, and I have to have something to look forward to while I fill a skip with loft clear-outs etc. No, not to the coast, sadly, but nearer than we are here. So hope to see Julian around Poole/Studland next season, as early as I can possibly make it (I'm accumulating brownie points for good behaviour meanwhile).

Michael

[Peter Taylor]

I will ensure that the leads will take full whack: 50A should do it?

Well today I did some measurements with the Minn-Kota Rip Tide 40 on the back of my Seafly which was moored to the jetty. Power was from a fully charged, fairly new, 100Ah Yuasa "Marine" battery which has deep-discharge and traction capability (whatever the latter means).  The results are shown in the table below where:

"Gear" is the speed setting (1 to 5).

"Amps Measured"  were read using an electronic  Watt meter in series with the motor.  These decreased during the relatively short test as the supply volts dropped from 12.6V to around 11.7V.  This was no doubt due to pulling up to 52A from the battery at full power; on the second run it had dropped to 42A, the figures shown are a rough average. 

"Quoted hours" are as printed on the top of the motor control pod.

"Ah assumed" is the implied practical capacity of a 12V 100Ah battery,for example at speed setting 1 Minn-Kota are assuming that 80% of the nominal battery capacity is actually usable.

These Minn-Kota quoted hours, or duration values, seem fairly reasonable except maybe for the higher speed settings. Drawing 40 to 50 A from a 100aH lead acid battery is not going to do it much good. However, as observed in the test, the result is to lower the battery voltage and hence the current drawn. So maybe it would go on for 1.5 hours at full power but at the expense of ruining the battery. This also implies that my estimate of 12kg bollard pull at full power would not be available for the full 1.5 hours - it is an over estimate. Of course a Lithium chemistry battery pack should be able to supply large currents more easily so in that case full performance would be maintained for longer.

I should mention that my Minn-Kota was bought a few years ago and has been used quite a bit. I don't know how it's performance compares to a brand new up to date motor (however they seem in the same ball park as those quoted on the Flover web site).

Is 50A enough? Well for my Minn-Kota 40 you would need a larger fuse than that and the wires should be rated for more current than the fuse. Looking at the Flover web site they say the 55 model draws max 45A to produce 25kg thrust "in bench tests" and 47A to produce 14kg in a "field test" so 50A is marginal.  Also these figures again confirm that the thrust quoted for trolling motors is an over-estimate by 150% or more compared to what is achieved.

Sadly, and with much regret, I'm becoming more and more convinced that small electric outboards don't have the capabilities needed for use on the sea - or anywhere where there is a significant wind or current. Flover say their largest outboard will propel a 20' yacht at 3mph (2.6 kt)... that's not enough! So I'm stuck with my stinky, noisy, starts if you're not in a panic,  Tohatsu!

Peter
p.s. I had't heard of Flover before - they look remarkably similar to the Minn-Kota!

[david]

Great job testing  the motors. Way above my skills. I am getting ready to upgrade from a 2 hp Honda, to a 6th tohatsu. I sail mainly on the ocean. Your tests seem? to confirm my thoughts  that the  electric motors were really  only good for inland waters. I do need a little more umph to get home. The reverse gear will also help make coming into the dock a little less exciting! More power to make the dock Vs less power and miss? Depending on the wind, this calculation can be a lot of fun. I have learned  to always have an oar handy lol.

[Julian Swindell]

Michael
It would be great to see you round the Dorset coast next year. Let me knwo when you are around.

[Michael Rogers]

Julian
I'll keep you informed. It would be great to fire the odd broadside at each other, or something equally friendly. A basic problem may be that, I believe anyway, you are a weekend sailor whereas I, as one of the idle retired (never been busier in my life, actually), and with a tendency to w/e demands on my time
('in choirs and places where they sing', to quote the BCP) will probably take to the water more often during the week.

Details not yet settled, but I'll probably be sailing out of Studland.

Michael

[EDWIN DAVIES]

Perhaps a note of caution, When testing a motor in a static situation you may get spurious results. This may be more evident with a coarse pitch propeller. I remember flying control line speed model aircraft when the motor could be heard accelerating as the speed enabled the coarse pitch to become usable. Only a thought.

[Peter Taylor]

Perhaps a note of caution, When testing a motor in a static situation you may get spurious results.

Edwin, I agree entirely.

My tests are described in more detail on my Seatern blog and the 22nd October entry in particular notes the artificial situation of being tethered. The flow past the hull is not the same as when motoring freely. For both Minn Kota and Torqeedo there were times when spirals of air were drawn down from the surface to the propeller causing a significant alteration in motor pitch. I did not take readings at those times but that does not guarantee that at other times the results were representative of free motoring.

However, apart from the 11th October attempt when I got rained off, I did manage to get reasonably reproducible results. The main difference observed between repeated runs could be ascribed to the changes in battery voltage due to the high current drawn by the Minn Kota. I therefore think that the results provide some sort of comparative assessment between the Minn Kota and the Torqeedo. The results are also broadly in line with other tests (e.g. PBO October 2014) and the field test figures quoted by e.g. Flover.

By the way, one motor tested by PBO was the Haswing Proturar 2hp. Looking at the Haswing web site  ( http://www.haswingmarine.com.au ) that motor is fresh water only, as is their very intriguing Comax 55lb motor. Their Osapian 80lbs motor is salt water use and has specifications of thrust: 36.3 kg (is that 22kg in reality?!) and current draw of 35A at 24V. Going to 24V means more power at lower current draw and hopefully better range. The down side is that in addition to the motor weight of 12.8 kg you would need 2 100Ah 12V lead acid batteries. However Lithium-ion might get round that (at a cost).

However a warning! the Osapian 80lbs and some other trolling motors (and the Torqeedo 503) use 3 bladed propellers. One would need to check if the motor can be tilted up given the width of a SB outboard well opening. That rules out the Torqeedo 503 for example. Those trolling motors with a twin blade propeller and circular shaft might be OK if slid up vertically to shorten the shaft before being tilted. But check the geometry before choosing a motor!

Peter

Peter