Small budget E-Outboard and LiFe battery ?

[fredrussell]

…The reason I read this post is that I'm looking for an LFP battery to increase range even more or grunt for those occasions when leisurely isn't quite what one wants.

I’m sure you’re aware of this but work out what max amp draw is of your outboard and then check that against max output amps of LiFePo battery BMS. My friend bought a 50a/h LiFePo, then found his leccy outboard required more amps at full throttle than the BMS would allow.

 

[Sea Change]

Is there a formula or formulas to help relate kicker thrust in pounds - to Watts as in e-propulsion display etc - to KWh battery power taken - to square metres of underwater area - and shape water displacement - to calculate a possible speed, and whether a kicker will be sufficient for a particular purpose.

180W on the e-lite gives a gentle jog around when not in a hurry, but the 500W is useful when there is a bit of wind and tide - so it will be interesting what Refueller finds.

Very roughly the current at 12v should be close to the lbs thrust.
My 68lb Bison motor would draw about 58lbs.

I don't know if there's a scientific way of sizing the motor to the dinghy, but if it's any help that motor would happily take my 11ft ply/epoxy sailing tender up to about 5kt with a decent load. I think the prop was limiting it as it didn't go any faster with a lighter load.
For comparison, a 3.5hp 2T would get the same dinghy up to 5.5kt loaded and in best possible conditions 6.5kt with just me aboard.

 

[toothless]

i usually run my torqeedo on my dinghy at about 200w on the hamble, even against tide. it is fine, if a bit slow. I can give it the beans if i like, but why bother when its only a 5 min ride anyway and i quite like the journey? 180w would work for me ok, so I guess it depends on the prop etc that is on your motor. I don't see why it wouldn't work in principle.

 

[nestawayboats]

i usually run my torqeedo on my dinghy at about 200w on the hamble, even against tide. it is fine, if a bit slow. I can give it the beans if i like, but why bother when its only a 5 min ride anyway and i quite like the journey? 180w would work for me ok, so I guess it depends on the prop etc that is on your motor. I don't see why it wouldn't work in principle.

This is the reason lower-powered, integrated battery motors like the (500W) ePropulsion eLite now exist. Partly because we suggested it, knowing that many - not all - dinghy/tender users were finding that they used their 1kW (1000W) class motors (Torqeedo 903, 1003, 1103 etc, ePropulsion Spirit) at 500W or less nearly all the time.

500W will drive a typical small inflatable at 3.5 to 4 knots (it depends on many factors including hull shape and load), which many people are happy with, whilst 1000W might only be a knot (and quite possibly only half a knot) faster.

Speeding up requires exponentially more power than the percentage increase in speed, especially as you get near displacement hull speed limits... and 4 knots is about the maximum displacement speed for a boat with an 8ft waterline (many dinghies are of course shorter than that).

Equally, if you slow down you will need much less (disproportionately less) power. For the dinghy above, if it would do ~4 knots at 500W it would probably still do ~2.5 knots at 180W (60% of the speed for 35% of the power). As toothless says, if it's only a short journey and the sun's shining, why not enjoy it?

The electric outboard manufacturers know all this (you'd hope!) so the propellers they fit are optimised to make use of the low down torque that's available from electric motors. That means they use quite big propellers, that are relatively slow turning. This is great for pushing surprisingly heavy boats up to or near their displacement hull speed limits, but not much good for getting boats faster than that (ie on the plane).

The above is a simplification but reasonable working "rule of thumb". One additional consideration being a 200W motor at 200W won't make as much torque as a 500W motor at 200W, so will be running a smaller propeller.

So the conclusion: 180W will probably be enough if you're happy with 2 to 2.5 knots. But I would point out that speed will be lower in chop/headwind, and 2 knot tides are not unknown in UK waters. That's why the eLite is 500W not less, and boostable to 750W in brief bursts eg to get across river to the Folly Inn when the river Medina is in full ebb.

Ian, Nestaway Boats

 

[Chiara’s slave]

That is all perfectly true. The time you need more than 180w is when the current is more than 2kn, cos you’ll be out there a very long time.

 

[PetiteFleur]

I bought an eLite last year and it's transformed our trip in our dinghy to our mooring. It's lightweight, and Quiet making conversation easier not shouting above the Honda 2.3.

 

[mogmog2]

I’m sure you’re aware of this but work out what max amp draw is of your outboard and then check that against max output amps of LiFePo battery BMS. My friend bought a 50a/h LiFePo, then found his leccy outboard required more amps at full throttle than the BMS would allow.

Thanks, that's a good point. Something I would know, but might forget about

 

[webcraft]

I have a Talamax TM30 on an Avon Redcrest (floppy floor). 30lbs thrust, about a quarter of a horsepower. It is powered by a LifePo4 60AH battery with a Victron smart shunt to monitor state of charge. I usually run it in 4th gear, which uses 19A, so expect to get three hours out of a full charge.

It is NOT fast, but it will make progress against a 2kt tide and 20kts of wind. You just need to accept that slow and steady wins the race. I use a Firefly 100W folding solar panel to charge the battery, or charge it from shorepower when in a marina.

We love it.