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?

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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Spray's cruising power - Part 2

 

I've occasionally wondered : If Spray's diesel engine ever died (not likely but you never know), rather than spend $20k for a replacement diesel, would I want to consider electric propulsion? Rip out the massive diesel engine and the huge fuel tanks and replace with a small electric motor and appropriate bank of batteries. Imagine the benefits :

 

- No smelly fuel or diesel exhaust. No chance of a fuel leak.

- Nearly silent and vibration free operation.

- No FNR transmission needed.

- Much less waste heat means probably no seawater heat exchanger is needed.

- Much less maintenance. No oil changes. No fuel filter changes.

- Smaller carbon footprint. Motor would be 90% efficient rather than 39% efficient (see last blog entry).

- Could recharge batteries from the sun? No fuel expense?

 

That's a lot of benefit! Is it for real? Let's look at the realities.

 

The toughest reality to face is the fact that we must store the energy we use to push Spray along. Diesel's biggest asset is how densely it stores chemical energy : 136 million joules per gallon! Even with its piddling 39% efficiency Spray's 240 gallon diesel capacity yields a cruising range of about 1,000 miles. Even though we usually fill up before the tanks are half empty, we go weeks without even checking the fuel level, knowing that only after 400 or 500 miles of cruising will we want to top off the tanks.

 

In the electrical world the storing of the needed energy is much tougher. The best solution (today) is the electrochemical battery, either the traditional lead-acid type or a newer lithium-ion type. Lets see how many batteries we would need. To help out the electric solution we'll only require a cruising range of 100 miles (3 days typical cruise or 2 days if we push it).

 

100 (nautical) miles at a speed of 6.5 knots. That's 100 / 6.5 = 15.4 hours of cruising. Recall that Spray needs 30 HP of engine output for cruising, and that equals 22.4 kW of power. Since our electic drive motor is 90% efficient it will need 22.4 / 0.90 = 25 kW of input power from the batteries. Multiply the 25 kW by the 15.4 hours (energy = power x time) and you yield a needed battery capacity of 385 kwh.

 

Well, not quite. Unfortunately, as a battery spits out amps (especially at high rates), it heats up, and that heat is wasted energy. Let's assume that our battery is 90% efficient while discharging, ie. that 90% of its stored energy comes out as usable juice and 10% is lost to heat. Now our needed battery capacity is 385 / 0.90 = 428 kwh.

 

How many 4D marine wet-cell (basic lead-acid) batteries do we need for our 428 kwh capacity? A typical such battery might have a rated capacity of 200 amp-hours, but the battery will live much longer if we only half-discharge it, using 100 amp-hours. Since electrical power in Watts is current x voltage we get 100 amp-hours x 12 volts = 1200 watt-hours = 1.2 kwh capacity per 4D battery. It'll take 428 / 1.2 = 357 of these bad boys to give the needed capacity. Each 4D battery weighs about 100 lb so we are looking at 35,700 lb of total battery weight.

 

Houston, we have a problem! Spray will need to tow a barge to hold the 357 heavy batteries needed to move her a mere 100 miles. Towing that barge will seriously impact our 6.5 knot cruising speed. Not only that but at around $200 each, we'll need to shell out over $70,000 to buy those batteries, and they won't last forever.

 

So wet-cell marine batteries are obviously NOT a viable solution. What about more modern battery technology? Check out Part 3 of this analysis to find out!

 

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Spray's cruising power - Part 1

If you have read some recent blog postings you know that we cruise Spray at 1600 rpm, yielding a speed of 6.5 knots (in still water), and burning 1.5 gallons of diesel fuel per hour.  How much power does it take to do this?

There would be many ways to calculate that but the easiest (and probably most accurate) is to use a rule of thumb provided by my friend and boat guru David Wyman, who tells me that a typical marine Diesel engine outputs 20 HP for every gal/hr it burns.  So for Spray this means the engine outputs 30 HP as we cruise.  Doesn't seem like much considering Spray weighs 18,000 lb and contains everything we need to live almost comfortably.

To put that into metric units (SI units to you techies), Spray's diesel engine outputs 22.4 kiloWatts (kW) as we cruise (1 HP = 0.746 KW).

[Some technobabble : When we speak of the diesel engine's output power we mean the power of the rotating shaft of the engine.  Unfortunately, some of that power is then lost in Spray's transmission, prop shaft bearings, and especially the propellor.  No matter, we'll just hold all those losses as a constant and only compare shaft power of various drive systems.]

----- Warning !!! ----- Tedious Physics Calculations Ahead !!! ------

[How efficient is Spray's diesel?  Well, a gallon of diesel fuel contains about 136 million joules of chemically stored energy (that's a lot!).  So a 100% efficient diesel engine burning 1.5 gal/hr would produce 204 million joules of energy per hour, which is 57,000 joules per second.  As you know, a joule per second is a Watt, so our 100% efficient engine outputs 57 kW.  But Spray's diesel only outputs 22.4 kW, so instead of being 100% efficient its only 22.4 / 57 = .39 or 39% efficient.  Doesn't sound very good but I'll bet its better than your car engine does.]

So we conclude that Spray"s diesel engine is about 39% efficient as it cranks out 22 kW of power while we cruise.  In Part 2 of this analysis we'll look at the option of using an electric motor that is 90% efficient to power Spray.  Wouldn't that be great?

NE through SC

Hello from Southport NC. Since the last blog entry we had a 3 day stay in Charleston SC, then 3 days of travel through SC until yesterday we crossed into NC.

 

It's spring, so the weather has been varied, including rainy, windy, and cold conditions. We've been piloting Spray from her enclosed salon rather than from the flying bridge. While the salon is noisier, it sure is warmer. As you can see above, each crew member helps in piloting.

 

Our visit to Charleston from last Sunday to Wednesday was very enjoyable. We were again at Charleston Maritime Center and again it was a bit bouncy in the marina, mostly from boat wakes. Katie and Riggs were regulars playing ball/frisbee on the lawn area, and we made good use of the showers and free laundry.

 

On Monday we deployed our bicycles and rode downtown for lunch at 'Fast & French'. That evening we walked to a great seafood restaurant where we met up with my Uncle Larry and his wife Bobbe. They live in Stone Mountain GA and had driven down to Charleston for a few days to visit with us and do the city, and we had a great time with them.

 

Tuesday had nasty weather. Larry and Bobbe came to the marina and met the pooches and we stayed in Spray's cozy salon while it rained outside. Sheila cooked a nice lunch and we had fun catching up on family activities. The rainy afternoon was perfect for a museum visit so Larry drove us to the Charleston Museum, where we spent a few hours learning local history, of which there is a lot! It was all very well presented but by the time I got up to the Civil War era I was saturated with information. Here's a photo Sheila took of Larry, Bobbe, and some boat bum in front of the museum.

They had to chase us out of the museum at 5 pm and Larry then dropped Sheila and I off at the fancy Harris Teeter grocery store so we could add to Spray's pantry.

Wednesday was our departure day with strong NE winds but we didn't have to say goodbye to Larry and Bobbe just yet as Larry came aboard for our 5 hour cruise up the ICW to McClellanville SC. It was a bit bouncy in Charleston Harbor but not bad once we got behind Sullivan Island in the protected ICW, although as mentioned we stayed below in the salon. Larry got to savor the pleasures of slow cruising and enjoyed the sights of marshland, dolphins, etc. Bobbe relaxed in Charleston and then drove to McClellanville to meet us at 3 pm. We walked the beautiful town for an hour or so and then bid farewell as they returned to enjoy more of Charlestown.

 

We spent Wednesday night docked at Leland Oil co., just as we did back in November. A nice guy on a neighboring boat had purchased a bushel of local oysters and offered us all we wanted plus training on cleaning the muck off them and grilling them (on his grill) until they popped open and the very fresh meat could be easily removed. Amazing to me how something that lives in nasty, gritty muck can be so clean and grit free inside. After much effort I presented Sheila with a small bowl of oyster meats which she scarfed down and declared excellent.

 

Thursday morning we shoved off into the incessant NE winds (but sunny at least) for a 25 mile run up to Georgetown SC, another of our favorite ICW towns. The Nina and Pinta replicas where there awaiting us (we toured them in Beaufort SC) as we tied up for a quick pumpout of our holding tank, then dropped anchor right off the Pinta.

As you can see, in Georgetown's protected harbor, it was a nice afternoon. We walked the town some, and had a snack and drinks in front of this restaurant, and did some work aboard Spray, including changing her oil and oil filter.

 

On Friday we continued for a longish 45 mile run through a very scenic section of the ICW, where it goes inland through forested land that was in spring splendor. The rivers were high with Spring runoff so it was mostly an uphill ride into the current. As you see below, it was gray and raw outside but we were comfy in the salon.

When we reached Myrtle Beach we came across a unique golf course, with its parking on the east side of the ICW (where the road is) but the golf course on the west side. How do the golfers get back and forth? Cable car, of course!

Friday's destination was Barefoot Marina in North Myrtle Beach. We just had time for some dog play before it started raining. We had supper at the restaurant there and then retired to Spray as the rain poured down. I had lowered the dinghy to the water and in the morning it had several inches of rain in it.

 

Yesterday we had another longish run of 40 miles, again into NE winds but thankfully in protected waters. Eventually we crossed into NC, and at times we could peer out of an inlet to the big bad ocean and the mountainous waves made us appreciate the barrier islands we travel behind. Larger boats throughout the southeast, who need to travel on the 'outside', have been stuck for quite a while awaiting calmer weather.

 

At one point I saw a couple dolphin dorsal fins ahead, but as we got closer I could see I was wrong:

Deer can swim quite well.

For the last hour of travel the wind was screaming so we were relieved when we pulled into Southport Marina and tied to the inside of their long face-dock. After dog walks we attended a daily presentation here on upcoming weather and possible ICW hazards, which is a unique service and well appreciated. Then off to a nice dinner at Fishy Fishy.

 

Oh yeah, last night we watched this movie on Netflix. The movie is so-so but sure makes Southport look good.

 

We have a gale warning today and the wind has been pressing us onto the dock such that our fenders look like pancakes. So instead of the planned 1 night stop we are staying at least a 2nd night. We have an electric heater going and are cozy aboard Spray. A good time to crank out a blog entry.

 

 

Next stop will be Wrightsville Beach NC.

 

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Walking in our own footprints

When we were traveling southwards last November and December, we chose what we considered to be the best possible anchorages or marinas to stop at each night, based on several factors, not least of which was how dog-friendly each stop was. Now as we head northwards through the same areas and apply the same decision criteria, its no surprise that we come up with pretty much the same list of places to overnight.

 

For example:

 

- Last Tuesday we left the Marina at St Simons Island and cruised up to the southern end of Sapelo Island (GA), anchoring in the Duplin River near the ferry dock that serves Sapelo, where we had anchored on Dec. 8. We had a short walk Tuesday evening, then a longer walk Wed. morning to the U of GA Marine Institute. Again the island was beautiful and now that its spring there where many birds to enjoy. Here is Riggs checking out a mansion now used by the Marine Institute:

As we prepared to pull anchor at mid-day we were surprised when a smallish cruise ship squeezed by us and dropped anchor. Apparently one can take a cruise to anywhere these days.

 

- Wednesday afternoon we cruised farther north to anchor in Kilkenny Creek Landing (GA), where we had stopped on Dec. 7. Again we had a nice supper at Marker 107, and used their dock for shore access.

- Thursday we continued northwards to cross the Savannah River and entered South Carolina. Just as we did on Dec. 5, we tied to the county dock near the south end of Daufuskie Island and had supper at Marshside Mama's. We also did some walking around the island.

 

- Friday we pushed on to anchor off Beaufort SC, one of our favorite towns on the ICW and where we had stayed Dec. 3 & 4. They are now setting up a mooring field so we anchored just outside of that area. We had time for ball play with the dogs at their waterside park and had supper at the Old Bull Tavern. Saturday morning we had time for more ball play and then a quick tour of the Nina and Pinta replica ships which are touring the east coast (we'll see them again in a few days).

- On Saturday we deviated from repeating our southbound itinerary by anchoring in Fishing Creek, about half way between Beaufort and Charleston. It was very remote and beautiful (low country grassy marshland) but was near a boat ramp allowing important shore access for emptying the dogs.

 

- On Sunday (yesterday) we continued up to Charleston where we again are staying at the Charleston Maritime Center, as we did over Thanksgiving week. This time we plan to stay 3 nights and we are excited as my Uncle Larry and his wife will be visiting. Larry has been armchair cruising with us and we haven't seen him for years.

 

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Georgia's Heavenly Isles

We've spent the past week enjoying 3 of Georgia's coastal islands - Cumberland, Jekyll, and St Simons.

 

Cumberland Island is indeed heavenly. We anchored at the Cumberland Sound anchorage for 4 nights (last Mon - Thurs). The national park has a dock and pavilion there, and its where the ferry from St Mary's lands with day trippers and campers. Less than a mile to the beach and lots of trails and great scenery.

We had a wonderful surprise on Tuesday when sailboat Shine dropped anchor near us with Castine/New Bern friends Scott and Lynn aboard, whom we had last seen in Oriental NC. We spent much of the rest of the week walking beaches with them and sharing drinks/snacks on each others boats. Great to see familiar faces and swap cruising stories. They are headed further south for a springtime cruise.

 

Saw many wild horses (actually quite tame) on Cumberland, a few deer, and many birds:

 

On Friday we pulled anchor and made a short 16 mile run up to Jekyll Harbor Marina, where we had stopped on our southbound trip. Jekyll island is developed but not too crowded, and has the best system of bike trails:

Many of the trails don't run along a road but instead cut through the woods, with wooden bridges across marshy areas. Really nice. The dogs enjoyed walks on the 2 mile loop between the marina and the historic district. We stayed 2 nights to give ourselves time for some biking.

 

On Sunday we left mid-day for a short 8 mile hop up to Golden Isles Marina here on St Simons Island. This island is more developed, which is the point as we needed to borrow the marina's courtesy car and run to the grocery store to replenish Spray's pantry. They limit car use to 1 hour which equates to $200 at the fancy Harris Teeter store. We are now well supplied.

 

Other than the grocery store we haven't seen much of St Simons, although we are enjoying this deluxe marina. We happened to arrive during a cruisers potluck party, with beer supplied by the marina. I'm also a sucker for the newspaper and muffins delivered to the boat every morning.

 

We had planned on staying only 1 night here but nasty weather prompted us to stay a 2nd night. Strong thunderstorms were predicted for last night, and there was a tornado watch, but what we ended up getting was just a lot of rain without much other drama. It's still raining this morning but should stop soon. We'll shove off at noon for a 27 mile run to another heavenly isle : Sapelo.

 

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