HP vs Lb thrust on a trolling motor

[LadyAnn]

Hi, My Wilderness 21 displaces ~ 1 ton and it needs an auxiliary motor. The last owner had a 2 Hp gas outboard and I'm wanting to go with an electric trolling motor. Wondering if anyone has experience powering a small sailboat with one. Oh, we will be in saltwater.

[SailingHarmonie]

There is no direct way to convert thrust to HP. Thrust is a static measurement and HP requires a force through a distance in a given time. Im most cases fisherman (the primary users of this kind of motor) are not actually using them for trolling, but rather to hold a steady position. So in this case the thrust rating is actually a useful comparison.

But... VERY roughly you can estimate the HP of an electric motor at full throttle by taking its rating in Watts, and dividing by 746.

So a 1000 W motor will generate ROUGHLY the same performance at full throttle as a 1000/746 = 1.3 HP gas engine at full throttle.

To ROUGHLY duplicate the performance of a 2 HP gas kicker at full throttle, you would need an electric motor rated at 1500W, or 124A @ 12V or 62A at 24V, or 31A @ 48V.

That's a lot of battery...

[donradcliffe]

We have a basic fishing trolling motor on our 3000 lb Santa Cruz 27. It moves us at 3 knots in a dead calm. It draws less than 50 amps, so should run for 2 hours on our 100 ahr LiFePo4 battery. If we have to go into wind and seas, we sail.

[Cruiser2B]

I had an Epropulsion Spirit 1.0 Evo on a Cape Dory 25. 4000lb boat. It pushed the boat well in all conditions except head on into a steep chop. When in steep chop I would motor sail with just jib or genny and tack my way upwind. The Cape Dory 25 is at least twice the displacement of the Wilderness 21. The CD25 also has a lot more wetted surface. I would think the Spirit 1.0 would suit you well in most situations. I posted a few short videos of the electric setup on my YouTube channel.... Cruiser2b. Check them out to get a better idea of the type of setup and boat I'm comparing.
Good Luck....nothing like the silence of electric propulsion. My current boat is an Alberg 30 with 6hp gas outboard in a well. it does well....but it is SOOO noisy...wish I had an extra $8k for a Epropulsion Navy 6.0 setup.

[Jammer]

The trolling motor manufacturers like to promote the narrative that HP for trolling motors can't be compared to HP for gasoline motors. This is nonsense, of course. They say that because the HP for even very large trolling motors is, well, smaller than that of the smallest gasoline outboards presently on the market. Makers of small electric outboards like the Torqueedo say the same thing, and use made up terms like "5 hp-equivalent."

Gasoline outboards are certified as to HP by their manufacturer in accordance with NMMA and EPA guidelines. I understand that there's a +/- 10% tolerance. In the early years of the outboard motor industry there was widespread mislabeling of outboards as to HP for various marketing purposes. For example in the 1930s the "Elto Ace" was labeled as a 3/4 hp motor even though it had the same output as the more expensive but nearly identical Evinrude 1.5hp motor.

Anyway, a trolling motor with 55 pounds thrust (a fairly typical size) will draw roughly 55 amps at 12 volts, and produce roughly 3/4 hp at the propshaft.

I think that might be enough for your Wilderness 21 depending on your situation. It would work well in benign conditions though you will not achieve hull speed, probably only 2-3 kts. It may not be enough to deal with a strong current or to go upwind in stronger winds.

[Lepke]

I have a 55 lb. thrust trolling motor. It's a little less powerful than the 2hp Merc outboard I use to use.
The big difference, the electric motor always starts and I don't have to go for gas.

[NorthCoastJoe]

In dead calm on my Pearson 30 I can paddle at 1 knot continuous with a whitewater guide paddle from the bow. I can get 2 knots for a short burst.

As long as it's calm you should be able to do it with a trolling motor. I would not want to try against the current.

[KTP]

Originally Posted by SailingHarmonie

That's a lot of battery...

It used to be, not so much anymore.

The 22 pound Li-time group 24 LFP battery with a bunch of safety features, bluetooth monitoring and rated at 12.8V 100AH is slightly less than $300.

Four of those gives you 5kWh at 88 pounds on a 48V system for $1200. Call it 100 pounds with straps, wiring, etc. You can place these in the bow for good weight distribution and then put the 20 pound electric motor on the stern. You then have 5 hours of full throttle motoring on something like a Spirit class motor, which for our 17 foot sailboat, pushes you at 4.5 knots. Going up to a Navy 3.0 would only give you about 1.5 hours at full throttle but you would reach hull speed (5.3kts or so). At the throttle where most people run and where they claim their gas outboard "sips gas", you likely would be traveling at 3.5kts and have 10 hours of battery life.