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#8555
Swiebertje
Participant

Time for some highschool physics:

According to the Torqeedo web site one horsepower from an electrical engine is the same as 2 hp generated by a combustion engine. This makes my toes curl inside my shoes! One HP is one HP no matter how it is generated. 1 HP is 745 Watts PERIOD! 👿

After getting up to speed with my two HP Honda, I can throttle back to half throttle. Getting to that speed at half throttle is possible but would take longer. So for discussion sake let’s assume 1 hp (745 watts) is sufficient to push a Wayfarer to its hull speed. The Torqeedo closest to 1 HP is the Travel 801 model which is a 800 watt motor. (It has 350 watts [=0.5 HP] propulsion power according to Torqeedo 😕 ) At 12 volts it needs 800 / 12 = 66 Amps. If you want to use it for an hour a battery of 66 Ah is needed. Oh no, hold on, we can only drain a battery 80% otherwise we may damage it. So in real life we need a 66 / 0.80 = 83 Ah battery for using the Torqeedo for one hour. In fact it is even worse as standard battery capacities are given at C/10 (10 hour discharge current). At 66 amps the capacity is far less. From a table (found on the internet) I read that we need at least a 105 Ah type battery for 57 minutes of power.

The weight of such a battery is around 30 kg. Add to that the considerable weight of the Torqeedo motor of 8 kg, the total package weighs a whopping 38 kg. My Honda 2 HP weighs 12 kg and for an hour at half throttle it needs 1 liter of gasoline. Total weight: 13 kg.

The suggested (build in) battery would give you 15 minutes of power at best but I would not be surprised if you run out of power in less then ten minutes. The weight of that combination is 11.5 kg, just under the 13 kg of my 2.2 Honda with a full fuel tank. By the way, if I would like to run my engine for five hours I would still only have 12 + 4 = 16 kg on board. (Assumed that gasoline weighs 800 gr / liter).

The only advantage I see is that some of the weight could be shifted forward. The battery could be placed near the centerboard but still I rather have my 13 kg at the transom then the 38 kg of an electric motor.

Having seen the Torqeedo web site I have the impression that it is typical American, they don’t tell you the whole truth but they don’t lie either. They like to juggle with power figures. The web site does not explain why a combustion HP is less then an electrical HP. Obviously it is the same, a HP is a HP regardless how it is generated 🙄 . Fortunately you can use a little highschool physics on the given figures and that should get you on track for a good descision.

Or to make it easy: For the same weight you get a fraction of the power and for only 10 minutes.

Ohw, did I mention you need some serious copper wire for 66 Amp’s? Like two times 10 square mm, four times thicker then the thickest wire used in your home, for example by your washing machine.

Now, for cost…..