# Ideas on how much power reqd for 21' motorboat?



## piotrsko (Dec 9, 2007)

Obvious thought: how much throttle at what rpm are you intending. That should give you a HP starting point for base calcs


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## boekel (Nov 10, 2010)

What you could do is measure gasoline use while traveling different speeds.

I don't know for gasoline, but a diesel engine uses about 0.2 liters per hour per hp.


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## brian_ (Feb 7, 2017)

boekel said:


> What you could do is measure gasoline use while traveling different speeds.
> 
> I don't know for gasoline, but a diesel engine uses about 0.2 liters per hour per hp.


That sounds like a good approach.

Typical values for brake specific fuel consumption (in both lb/hp-hr and g/kWh) are listed in a table in the Wikipedia page for BSFC.

a litre of gasoline is about 0.755 kg
a litre of diesel is about 0832 kg
a US gallon of gasoline is about 6.2 pounds
a US gallon of diesel is about 6.94 pounds

Depending on the volume units chosen, and using the appropriate consumption rate from the table, you can calculate what the engine must be producing, in kilowatts or horsepower. The electric motor needs to match that in continuous use.


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## brian_ (Feb 7, 2017)

boekel said:


> ... a diesel engine uses about 0.2 liters per hour per hp.





brian_ said:


> Typical values for brake specific fuel consumption (in both lb/hp-hr and g/kWh) are listed in a table in the Wikipedia page for BSFC.
> 
> a litre of gasoline is about 0.755 kg
> a litre of diesel is about 0.832 kg...




0.2 L/hp-hr is 0.166 kg/hp-hr or 223 g/kW-h (or 0.37 lb/hp-hr)... not great for a diesel, but reasonable.


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