Spray's cruising power - Part 4

If you've read the first 3 parts of this analysis of Spray's propulsion system, and future options, then you deserve a treat.  The treat is that this Part 4 will complete the series!

We've looked at the option of repowering Spray with an electric propulsion motor and a bank of batteries.  We only need 30 HP to cruise so it seems a 50 HP motor (37 kW) would do the trick, giving a margin of extra power when needed.  We need a lot of battery capacity, even for the moderate goal of a 100 nm range.  We found that five 85 kwh batteries from the Tesla S sedan might do the trick, although they may be a bit too heavy and are definitely too expensive (around $150,000 today but less in the future).

There is one more major issue which is how to recharge the depleted batteries after our 100 nm of cruising.  The ideal scenario would be an array of solar panels, such as shown above.  Free green energy.  The photo above looks to be around a 1,000 Watt installation, so lets use that.  Unfortunately solar panels only produce their rated power under very ideal conditions : sun directly overhead and very clear air.  Mr. Google tells me that a real 1,000 W system, mounted horizontally on a completely cloudless summer day, might produce 4 kwh of energy.  And that's best case.

Recall from Part 2 that we need 428 kwh of energy from our batteries to move Spray the 100 nm.  Sadly we'll need to obtain even more energy than that to refill the batteries.  Remember how the batteries get warm as they discharge, losing 10% of their energy to waste heat?  Well, roughly the same thing happens when recharging, the batteries again get warm and waste (we'll assume) 10% of the charging energy to waste heat.  So to put 428 kwh into our batteries the charging system will need to output 428 / 0.90 = 475 kwh of energy.

Back to our solar array providing 4 kwh of charging energy on a perfect day.  It'll take that array 475 / 4 = 119 days to recharge our batteries.  So we cruise for 2 or 3 days, then hang at anchor until 119 perfectly sunny days occur to recharge.  Not gonna work, is it?

If not solar panels, then what?  Maybe the plug!  When we stay a a marina we usually pay about $5 to plug into a 30 amp., 120 volt supply, so we can run Spray's systems (primarily the fridge) and also recharge her house battery.  For us that $5 is a ripoff as we only use maybe 25 cents worth of electricity overnight.

But if we install honker rechargers for our gigunda batteries, how much energy could we pull from that marina plug in a 24 hour day?  Well it would be 30 amps x 120 volts x 24 hours / 1,000 = 86.4 kwh.  (Now that's getting our $5 worth!)  Much better than the solar panels but we'll need to stay for 5 days to get the 475 kwh we need.

Boats larger than Spray have a different plug they use.  It provides up to 50 amps @ 240 volts and usually costs around $10 per night.  Let's convert to that plug and now in a day we draw 50 amps x 240 volts x 24 hours / 1,000 = 288 kwh.  So now we only need to stay on the plug for 40 hours to recharge.

We've solved it!  All we need is $150,000 for batteries (call it an even $200 K to include the propulsion motor, control electronics, big honking battery chargers, etc.) and we can cruise for 2 or 3 days, travel 100 nm, and then hit a marina for 2 days, paying only about $20 to refuel (right now it costs us about $100 in diesel to cruise 100 nm).

And this isn't just for Spray.  All cruisers will want this.  When a fleet of new electric powered trawlers pulls into a marina and plugs in, the adjacent town will go dim from the power draw!  2 days later the boats all continue on their clean, quiet, low cost way.

So who wants to invest?