Electric Dinghy

A dinghy is a useful, and in some cases, necessary accessory for a cruising boat.  I haven't yet started cruising, but it seemed that it might be fun to have, and could certainly be useful for short trips where we might anchor out.

The Dinghy

I ended up buying a West Marine RIB 310, single floor, PVC dinghy.  Comparatively speaking, this was a readily accessible model with decent features, at a competitive price (I got it on sale - though this summer it went on sale for even less).   

 We also looked at the 8' Takacat.  The Takacat is fully inflatable - so that you can deflate it, roll it up, and put it in storage (out of sunlight) when not in use!  That was a huge selling point.  The marketing claims that it handles waves really well.  Unfortunately, we found the 8' version it to be just too small to comfortably fit two "full sized adults".  We also found that we couldn't comfortably row the 8' Takacat with two people in the boat.  In hindsight, it would be interesting to comare the 10' Takacat to the West Marine RIB 310.  At the time, the 10' Takacat was not considered, as it was thought to be too large to comfortably fit on Sunset Dream's dinghy davits.

We have a friend that had an 8' hard plastic "Portland Pudgy".  Neat boat, very distinctive.  The pudgy has many accessories that allow it to be configured to motor, row, sail, and even serve as a life raft.  Unfortunately, the Portland Pudgies are very expensive, and not readily available in my location (Texas).  After testing our friend's boat, we came to the conclusion that an 8' dinghy was just too small for two "full sized" adults.

9-4-22  Tested Portland Pudgy with 12V trolling motor and 150Ah lithium battery going around Clear Lake Shores Island.  Out for about 2 hours, after having rowed quite a bit of the way back, due to concern about the battery running low.  Had to run the trolling motor at top speed (5) to make progress in the Clear Lake channel against a strong tide.

9-24-22 Tested Portland Pudgy with 24V Torqeedo, 50Ah 24V Lithium battery.  Motored to Schaffer's at low to med speed, about 3mph.  Did great - out for about 1.5 hours with power still left.

We finally settled on the West Marine RIB 310.  The Vee bottom should provide more stability and wave handling than the flat bottom, fully inflatable versions.   (There are tradeoffs with everything!)  So, we won't be able to store it easily, but at least it will hopefully be sea worthy.

So far, our experience has been than the West Marine RIB 310 holds two people very well (our first trip was with 3 "full sized" adults, and that was fine).  The RIB 310 is reasonably balanced, and very stable.  We will probably never have enough horse power on board to get it up on plane.  But it should easily get us from boat to shore. 

The single hull version is very deep inside (more of a challenge for a "vertically challenged Admiral" than the captain...), but is lighter and cheaper than the same model with the flat interior (though the false bottom version has a nice storage locker at the front that would be really helpful for stuff that could stay on the boat).

So far  -  We've been very happy with the RIB 310.

6-9-23  Dinghy experiments with Torqeedo 2T 24V vs. Hangkai 48V.

7-25-23 Dinghy to Shaffer's with Hangkai 48V 50Ah battery, about 30 min to get there, about 45 min to return.  Plenty of power left.

Need to understand why the trolling motor seemed to perform so poorly.  Need to monitor power draw from trolling motor at various RPM and speed over ground.  Is the Portland Pudgy that much harder to push with two adults, compared to the 10' RIB?  Or is the trolling motor that much less efficient than the 24V Torqeedo and 48V Hangkai?

A Tale of Two Motors

The next part of the equation was:  Which motor to power the dinghy.  After my prior experiments with electric power, an electric dinghy seemed natural.  It's small enough to be powered by available batteries for a reasonable run time.  The motors are less than 50 pounds, making them FAR easier to manage.  The lithium batteries are reasonably light.  While a motor, a battery, a backup battery, and a box including the power switch and battery monitor - will all add up to the same weight as a gas outboard, you can at least break it up into pieces that are easier to move on and off the dinghy.

The real beauty of an electric dinghy is that you simply turn on the switch and go.  No mixing gas.  No carburators to gum up.  It just runs.

The obvious down side of the electric dinghy is run time.   While I am setting mine up to have at least 3 hours of run time between the primary and backup battery - if I use that up, I'm simply done.  There is no "just pour more gas into it and go".   Gas engines unquestionably allow for far longer range, easy refills, and far greater power, if you need to go fast.  But if you just want to putter around a little bit, or just get from boat to shore from time to time, an electric motor is a very capable solution - and easy for all crew members to handle.

From my prior experiments, I still had the 24V Torqeedo 2T (tiller model version).  I had a single 60AH 24V lithium battery that I could use to power the Torqeedo.  The issues with this are:

• The Torqeedo is about a $4000 motor.  All electric motors in a salt environment can expect to fail sooner or later, and replacing a $4000 motor would give anyone pause.

• 24V lithium batteries are expensive, not as plentiful as 48V batteries, and are hard to find in sizes that will fit standard battery boxes.  A second 24V lithium battery (because you always do want to get home after you somehow drain the first battery...) cost more than a cheap Chinese 48V electric outboard - so it was compelling to buy the 48V outboard instead of a backup battery (especially considering I already had two 48V eBike batteries).

In terms of performance, the Torqeedo powers the dinghy with authority.  The Torqeedo has a relatively large 3 blade prop that provides what feels like a lot of torque.  The Torqeedo feels solid, and is relatively quiet.   No - electric motors are not "silent".  That's a myth.  But it is FAR quieter than any kind of gas engine.   

After trying the Torqeedo on the dinghy, and finding it to be entirely adequate for my needs (other than the expense of a backup battery, and the high cost of a replacement motor), I decided to try a 48V Chinese motor.

I purchased the 48V Hangkai 1000W motor for under $350, delivered.  Less than 10% of the cost of the Torqeedo.  Initial impressions:

• MUCH LOUDER than the Torqeedo.   The Hangkai obviously has the electric motor in the head unit, driving a shaft, like a traditional gas engine.

• The Hangkai has a tiny 3 blade prop.  I strongly suspect the Hangkai would do better with a larger prop.  Maybe not quite as large as the Torqeedo prop, but at least bigger than the existing one.

• The model I bought has no reverse!  Be aware of this  The advertisement in Amazon said there was a "reversing gear" - but this is not true!  You have to turn the motor 180 degrees to achieve reverse.  Look for the version (at least 1200W?) that has TWO buttons - one if for holding the existing speed, the second is for reverse.

• As mentioned in the Amazon reviews, the throttle is too sensitive.  It needs more physical range of the throttle to allow finer control of the power level of the motor.  The current throttle makes it extremely difficult to pick a desired range.  However, they do provide a button to allow locking of the speed -  at whatever power level you currently have the throttle at.  Though holding the throttle, hitting the button, and watching forward all at the same time is quite a challenge!!

• There is no kill switch for this motor!   The throttle is spring loaded.  So, if you take your hand off the throttle - AS LONG AS YOU HAVEN'T LOCKED THE THROTTLE SETTING - the motor will stop.  But most people will lock the power level if they are going any distance.  If you lock the throttle setting, and  manage to fall overboard, there is nothing to stop the motor.  My solution to this is to wire a 48V relay between the battery and motor, and use an outboard kill switch to operate the relay.  I'm wiring a volt/amp power meter in the same box with the relay and kill switch.

Impressions of the Hangkai "performance":

• Yes, the motor "feels cheaper" than the Torqeedo.  But for 1/10th the price, one must be willing to accept that.  The reality is that (so far, and I don't have much experience with the motor yet!) - there has been no indication that it is poorly built.

• It takes a bit more power to achieve the same speed as the Torqeedo.  I suspect this is due to the prop being so small.

• BOTTOM LINE - as long as you can accept putting around at 3-4mph, the Hangkai is an inexpensive, simple solution to driving the dinghy.  Using eBike batteries, I think I can reasonably expect a range of over 2 hours on a 30Ah battery, with an extra hour of backup (emergency / get home) from a second 20Ah battery.  The Hangkai 48V system with a 48V ebike battery is a much more cost effective option than the Torqeedo.

• I REALLY WISH the Hangkai had the following 3 features:

  • Reverse!   (apparently this is switch selectable on other models.  I did a bad job of ordering...)

  • Throttle needs to "stay put" instead of being spring loaded to cut off if released.  The "push to hold speed" button is very difficult to operate, and almost impossible to do with one hand while holding the speed, and watching where one is going...

  • Kill switch - Instead of turning off if throttle is released (which most of the time, will probably be set to "push to hold speed" mode, which then won't shut off!), there needs to be a separate kill switch with a lanyard.

DATA  

On 6/9/23, a "side by side" test was conducted of the 24V Torqeedo vs. the 48V Hangkai electric outboards.

Both motors were tested on the West Marine 10' RIB with fiberglass hull.  Two adults.  Speed measured by Navionics.  Power (watts, amps, volts) measured using a Victron Battery Monitor smart shunt.

Torqeedo Cruise 2T - 24V, 2400W max

WATTS Amps Volts Speed (knots)

200W 7.2A 26.5V 2.8k

500W 19.8A 25.77V 4.0k

800W 31.7A 25V 4.0k

1200W 47.5A 24.8V 4.8k

Hangkai 48V, rated at 1000W

Watts Amps Volts Speed (knots)

200W 3.4A 58.3V 1.5-2k

580W 9.9A 57.8V 3k

660W  11A 57.15V 3.3k

840W 14.7A 57.49V 3.7k

1000W 14.7A 57.49V 3.8k

1000W 17.6A 56.6V 3.4k   (into wind?)

Trolling Motor Revisited...

I'm still not excited about trying to use a Torqeedo in a salt environment.  Just doesn't seem like a good idea at $4000, and very expensive 24V lithium batteries.

Last weekend (4/14/24), I hooked up the Hangkai to the 48 volt lithium batteries.  Both batteries were dead.  I recharged the batteries, but so far have not gotten them to run the motor.  Not sure if this is a motor issue, or if leaving ebike batteries exposed (in battery boxes) to the elements over the winter was too much.   We had a couple of freezes, but nothing too extreme - and the batteries were completely disconnected.  The batteries had not been charged - so maybe the BMS ran them down?  So far, it seems that both ebike batteries are shot.  They will charge, but don't seem to be able to deliver any appreciable current.   Or maybe the motor is shot?  Or maybe both??

Anyway, I had also brought the 12V trolling motor back to the boat for additional testing.   From my previous notes, the trolling motor on the Portland Pudgy didn't get us to Schaefer's and back without significant rowing...  The 150Ah lithium battery should have been able to run the trolling motor plenty long enough to make that trip, with plenty of margin.

So - first experiment was to fully charge the 150Ah battery, and do a static battery torture test at the dock.

The trolling motor (Minn-Kota 35 pound thrust, 12V) was mounted on the dinghy.  The dinghy was kept tethered to the dinghy davits.  The motor was fired up to top speed ("5"), and let run.

The 150Ah battery was able to power the trolling motor at top speed for 3 hours.

From what I can tell, using the Victron Battery Monitor, with a 20% floor set, this performance was a little short of what I would expect from a 150ah lithium battery.  However - it is PLENTY sufficient for operating the dinghy - at least from a "runtime" perspective.

150Ah Eco-Worthy Lithium 12V battery Test

4-15-24

Minn-Kota 35 pound trolling motor - Motor was run at full speed ("level 5") for the duration of the test.

The motor was mounted on a West Marine 10' RIB 310, that was secured to the trawler in the marina (static run test to just test the battery performance under a real electric trolling motor load).

The test was monitored on a Victron bluetooth 150A "SmartShunt" monitor.  The "floor" was set to 20%, battery capacity set to 150%.  Battery was fully charged @ 14.6V prior to test.  Percentage in the table below is the Victron "% capacity remaining" estimate.  Voltage, current, and watts are as reported on the Victron app.

5:40pm - start - battery at 100%

5:45pm 12.78V 98% 36.7A 470W

6:19pm 12.73V 83% 34.3A 438W

6:56pm 12.66V 68% 33.9A 429W 44.3Ah consumed

7:39pm 12.62V 51% 33.49A 422W 68.4Ah consumed

8:16pm 12.5V 36% 33.32A 417W 89.3Ah consumed

8:40pm 12.42V 26% 32.99A 409W 102.6Ah consumed (reported by Victron)

Victron estimated 16 min remaining to floor.

Test stopped at 8:40pm.   At this point, the motor had been running constantly at "level 5" (max speed) for 3 hours.

Assuming 150Ah at avg 12.5V, and an average load of ~440W, and a 20% floor (only using 80%), I calculated that I should get about 3.4 hours.  This experiment showed about3.2 hours of run life.  So not bad...  Not great, but close enough to the estimate to be satisfied.   In this case, 3 hours of run time at full speed should be plenty for typical dinghy use.

Minn-Kota 35 pound thrust Trolling Motor Performance Test

Data collected on 4/20/24.  overcast skies, about 75-80F, breezy.  The motor was mounted on a 10' West Marine RIB  310, single floor RIB with fiberglass hull.   Due to wind conditions, most of the data collected below was running back and forth in the fairway behind the trawler in a reasonably protected area.

Speed was recorded using Navionics - speed recorded in knots.  Motor power used was monitored with a Victron 150A SmartShunt.  The battery was the above Eco-Worthy 150Ah 12V battery, that was fully charged before this and experiment.

Motor Level Speed (Knots) Power used

5 2.3kn 410W

4 1.5kn 230W

3 1-1.3kn 160W

Newport 55 pound thrust Canoe Style Motor 

This motor was purchased new on Amazon. $216 including tax.  12V, 5 power settings, 50A max.

Motor Level Speed (Knots) Power Used

5 3kn 550W

4 2.1kn 290W

3 1.7kn 245W

General comments:   

The  Newport "Canoe  Style" trolling motor  has a  3  blade prop instead of the 2 blade on the Minn-Kota.  At 55 pounds thrust, it  does seem a bit more authoritative than the 35# Minn-Kota.

The Newport motor was very quite, very smooth - nice to use!  The 24" shaft lenght fits the dinghy much  better than the 36" shaft length of traditional trolling motors.

While the recorded speed difference doesn't look like much on paper - it did "feel" a bit faster.

MOTOR SUMMARY:

Motor Voltage Max Speed Max Power
Torqeedo 2T 24V 4.8Kn 1200W**   (Motor testing limited at 1200W due to battery limits)

About $4000, rated at 2400W

Hangkai 48V 3.8Kn 1000W About $330, Rated 1000W - VERY Noisy

Minn-Kota 35# 12V 2.3Kn 410W

Newport 55# 12V 3Kn 550W About $200, Rated 50A@12V

Summary thoughts:

For speed and power, the Torqueedo 2T was clearly on top.  Unfortunately, the 24V Torqeedo is about $4000+.   Finding 24V batteries that fit in a standard battery box is a bit of a challenge, but do-able.  Currently, they are running about $300 for a 60Ah battery.   Torqeedo offers other smaller motors with integrated batteries - but they are still have  about a $2000-3000 price tag, and spare batteries are VERY expensive.

The Hangkai is a VERY cost effective answer for a 48V outboard electrict motor.  However, this motor has a VERY touchy throttle, is extremely noisy, and didn't perform significantly better than the 55# Newport trolling motor.

While the Newport 55# won't get you anywhere fast - it does seem to do a decent job, and should be sufficient for getting the dinghy from boat to shore (as long as there isn't a strong/fast current).  12V lithium batteries are easy to obtain, and a very common voltage to work with.  Overall bang for the buck in terms of a nicely performing, low cost motor with an easy to purchase / replace battery seems to be the Newport.  Just don't be in a hurry.