Battery powered paddle mixer to propell a light weight tender?

Hi, like many people I am looking for a cost effective electric drive for a lightweight tender to get me 200m to and from my swinging mooring. Speed is not important but must be able to cope with 4 or so knots of tide. I have been looking at various threads and I wonder if this might work....

M18 FUEL Mud Mixer https://www.milwaukeetool.com/Search/2810-20
This 18v cement mixer seems like it would have enough torque (no problem mixing thick concrete and mortar) according to the manufacture it uses a POWERSTATE™ Brushless motor with a variable speed dial, apparently with the M18 REDLITHIUM™ battery the user can mix up to 15, five gallon buckets on one 5.0Ah battery you can also buy larger capacity batteries that will fit.
I can't find details on the power (kw) of the motor, but the speed is around 600rpm.
Was thinking of using it like a longtail engine but not sure what prop would be suitable?
These are about £200 anyone think it's worth buying one for a test, or am I going mad during lockdown? All comments welcome...

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Would a small used trolling motor be less expensive?

 

Yobarnacal, thanks for your reply, you could be right. But I get the impression that unless you fork out for a bigger torqueedo style affair you end up with something that is a bit underpowered when using in tidal waters, that is without the problems associated with using a heavy 12v lead acid battery. Would a trolling motor be able to push 4kns of tide, I have no first hand experience with them?

 

In the olden days, seamen with oar power and sail power only, timed arrivals and departures relevant to the tides and weather.
you may need to copy those tactics. I suspect even the smallest of trolling motors will deliver more thrust than an electric paint mixer paddle.tool.

The heavy battery isn't necessary. Electric bikes use lighter more modern batteries, like lithium.

If you research the top of the linev most powerful trolling motors, you will discover they increase in voltage in the last few elite models.
Instead of 12 volt, it's 24 volt. The next more powerful model, is 36 volt, and the queen of the fleet and most powerful of all is 48 volts!

Same motor, just heftier control circuit to handle higher voltage. You can safely overvolt any DC motor for short periods of time. Gives you more RPMs for more thrust. Try it with an inexpensive toy motor. Like those ineffective AAA battery powered handheld personal fans. Overvolt one and observe!

 

Can your dinghy achieve speeds over 4kn under oars?
If yes you are good to try small electric outboards.
If not you are better with a gas outboard.

 

Gas and diesel is currently the most compact portable stored energy we have.
How much energy is in a gallon of gasoline?
Refinery efficiency is about 90 percent and the energy content of a gallon of gasoline is about 132,000 Btu. Put that together and you have about 13,000 Btu of energy cost per gallon of gasoline produced, which is equivalent in energy terms to 4 kilowatt hours.Oct 26, 2011

How many kWh is a gallon of diesel?
According to this source , a small generator at full load would generate about 12.5 KWH of electricity from a gallon of diesel. A larger one would generate about 14 KWH of electricity from a gallon.

How many kilowatt hours does a car battery hold?
A battery rated for 100 amp hours will provide 5 amps for 20 hours. If we have a 12 volt battery, we multiply 100 by 12 and determine that the battery will provide 1200 watt hours. To apply the metric 'kilo' prefix, we divide the result by 1000 and determine that the battery can supply the 1.2 KW hours.

So, a gal of gas = four charged 100 amp 12 v batteries and a gal diesel 12 batteries

 

Of course these liquid fuel outboards aren't cheap, and a cheap one is probably a problem.

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The battery for the tool you linked is 18V 5Ah, wich translates to 90Wh. Now this 90Wh can be supplied at a constant rate of 5A for an hour or at higher A rates for less time. Good tool batteries are capable of 10-20C continuously without problems, that means you can draw 50-100A or 900-1800W but only for a very short time.
You have to go 200m, 1kn is 0,51m/s so you need 102s or roughly speaking 2 minutes for the trip. The problem is the tide, to make 1kn SOG you need to make 5kn trough the water. So the question becomes how much power does your dinghy need to make 5kn. If you can row it the mixer will also make it (after you find the right prop) for sure.

 

It is "Up to 15 charges". That means that if you mix drywall mud, you get 15. For concrete is will be considerably less. A bucket of drywall mud may take one minute to mix and is not using maximum power. In short, for the proposed used it may run 8 minutes. However, this is not a trolling or outboard motor. You will need to fabricate a bracket, a steering system, a propeller adapter, and some kind of shield for the salt water spray. You will also have to buy a propeller, batteries and charger.