# I can't calculate engine power requirements, can anyone help?

Discussion in 'Electric Propulsion' started by serdarbas, Sep 2, 2024.

1. Joined: Nov 2023
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### serdarbasJunior Member

Hello; I'm building a wooden slipper launch but I'm having difficulties to select an engine power and batter size.

- Wooden, slipperlaunch.
-Waterline 24foot
- Weight: 1.3 tons.
- Capacity: 6 person
- Hullspeed 6.6knots.
- Desired cruising time: 2 hours at min 6 knots.

Now this is the confusing part.

I have two options 5kw vs 11kw engines.
Steerable POD Archieven - Rim Drive Technology https://rimdrivetechnology.nl/product-categorie/maritime/azimuth-motors-maritime/

And a single option for battery: 48vx200ah
48V batteries boat - LiFePO4 48V 200Ah - Rim Drive Technology https://rimdrivetechnology.nl/product/lifepo4-48v-200ah/

Some calculations I saw online for displacement boats around 25foot is, 5hp=6knots and 10hp=6.5 knots.
Engines for Yachts with Displacement Hulls - Beta Marine Propulsion Engines https://betamarine.co.uk/engine-selection-for-yachts/

Option one: 5kw engine + 1 battery:
5kw=6.7hp.
If we run at 80% , 144 minutes of runtime on single battery.
Travel speed at 80% is 6.0 knots with an option to speed up to 6.2 knots.

Option two: 2x 48v 200ah batteries with 11kw engine.
This option is around 10.000 USD more expensive.

Is my math correct? I tried to understand advices from this forum for similar questions but the answers and calculations were seriously rocket science for me. I don't know if 5kw engine is simply too weak for my boat or ideal? Or 11kw is overkill and waste of money..

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### BlueBell. . . _ _ _ . . . _ _ _

Don't know.

But if you borrow a 9.9 outboard you could see if you're happy with the performance.
I strongly suspect you're going to want the more expensive set-up,
especially if you have any seas or headwinds.

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### SolGatoSenior Member

A few things.

First, besides the difference in power the two motors also have different size “props” which can make a big difference in performance.

As BlueBell mentioned, the decision should really be based on typical environmental conditions you’ll be operating in plus having some reserve power for those occasions when it is needed.

Based on what you have stated and not knowing all the variables, I would recommend the larger motor. With its bigger prop and 2X the power (assuming efficiency is on par with smaller motor) you will be able to operate it at a slower RPM which should result in greater efficiency (range) if the ESC is of a Pulse Width Modulating design.

One of the most common mistakes made is rating performance of an electric motor based on its Input wattage when the Output wattage (at the prop) is what you really need to know in order to compare it to an ICE. Electric motors are rated based on input not output like an outboard motor.

And some motor controllers are very in inefficient, while others are much better, and then some by design have a sweet spot of peak efficiency which should coincide with your target cruising speed with the rest left for reserve power.

I did not see any documentation relating to the efficiency curve of those motors, nor any specs relating to the controllers efficiency.

Last edited: Sep 2, 2024

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