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Discussion Starter #1
I have a buddy with a 21 ft. 1974 Pen Yan Explorer. It's a typical 'motorboat'... had an inboard gas v-8 engine.

Design goals are just to cruise an hour or two under power at moderate speeds, no need to pull waterskiers or travel for hours under power.

What I am wondering is if anybody has any ideas what kind of power is needed to hold a boat like this at a moderate cruising speed. I'm looking for some realistic numbers so I can have a decent idea for battery capacity needed per hour of cruising time.

Does anybody have any charts showing power vs speed for a typical v hull motorboat?

Preliminary design would use:
ADC 8" series wound motor
zilla 1k controller
120vDC x 100ah battery pack

I know the 8" motors can air-cool at continuous output around 15-20hp, so I might have to consider a water jacket to help cool the motor for higher continuous loading.... but I would hope to avoid that in any case because it would drain the battery pack too fast.

I'm basing this on my experience with this equipment in a Suzuki Swift.... where the motor is pretty happy pulling 200amps or less to keep the car at 60mph. I just don't have much information on what it takes to cruise a boat like this at a moderate speed.
 

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