Spray's cruising power - Part 3

In Parts 1 and 2 of this analysis we found the following :

- Spray needs 22.4 kW of power from her propulsion engine as she cruises at 6.5 knots.

- To cruise 100 nautical miles with an electric propulsion system, Spray will need battery capacity of 428 kwh.

- To yield that capacity using 4D type wet-cell lead-acid batteries will require 357 such batteries, weighing around 35,000 lb and costing around $70,000. That would sink Spray and so is not viable.

 

Can high technology provide a solution? A good example of the state of the art of proven battery technology is the Tesla S sedan, which can be ordered with a lithium-ion battery package (photo above) that has a useable capacity of 85 kwh, has a well developed cooling/safety system, and that they warrant for 8 years of everyday use in their car. Five of those will yield 425 kwh of capacity. Close enough!

 

So we have a solution, except for 2 little problems :

 

1) Each 85 kwh battery assembly, with its cooling system, weighs about 1200 lb. That's why it's built as a big flat plate so it can fit on the bottom of the car to reduce handling issues. Spray will need 6,000 lb of these battery units. Even if we place them low in Spray's hull to act as ballast for stability, they are about 3,000 lb heavier than Spray's diesel engine and filled fuel tanks. Its only a sixth the weight of the required lead-acid batteries but we will need a naval architect to tell us whether Spray could handle the extra weight.

 

2) Its estimated that each 85 kwh Tesla battery costs about $30,000. So we need $150,000 worth of batteries. This explains why the few electric powered cruise boats you can find are expensive science projects and that diesel still completely dominates for yacht propulsion.

 

Maybe I just need to be patient. Even today you Tesla customers can buy a replacement 85 kwh battery for your car for only $12,000, deliverable when the battery warranty expires in 8 years. So Tesla has faith that lithium-ion battery prices will decline in time. Maybe they will be able to lighten the batteries too.

 

As for me, I'm glad I just changed the oil on Spray's diesel. That engine is gonna need to last for many more years.

 

You readers aren't off the hook just yet. In Part 4 of this analysis we will try to figure out how we would recharge our 5 Tesla batteries. Remember that we'll need to do that every 2 or 3 days of cruising.

 

 

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