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post #71 of 78 Old 10-17-2012
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Re: Optimal propulsion system

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I read recently that the technology exists for much better batteries but that the oil companies own the patents. They didn't cite anything that actually proves that but it sounds plausible or even likely. If so, it's just sad.
Yeah, just another conspiracy theory, and total BS. If such technology existed our military would be using it, and they're not.

The fact is, higher tech batteries DO exist, and can be made/bundled with the ampacity for such application. They're just prohibitively expensive. You probably have one on your person.

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post #72 of 78 Old 10-17-2012
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Re: Optimal propulsion system

Ok I wanted to run some numbers and see how things looked in more detail. Hopefully I don't make a mistake.


Diesel fuel is listed at 45MJ/kilogram which is 20.45MJ/lb or 36MJ/liter.

A Group 27 from Deka weighs 53 lbs and has 90Ah. This is 90Ah*12V=1080Wh.
A joule is a Watt-Second so 1080*3600 (seconds per hour) is 3.888MJ.
So per pound this is 3.888MJ/53lbs = 73kJ /lb
or by volume it's 3.888MJ/13 liters = 0.3MJ/l

This puts diesel at 280X as dense as the lead acid by weight or 120X as dense by volume. Or another way, one Group 27 is 0.0285306 gallons..

So to compare this to the example from earlier 2200lbs gets you 42 batteries which nets about 1.2 gallons...

As pointed out earler this gets better (by about 75%) when factoring in conversion efficiency (electric is more efficient than diesel) but then worse (by a factor of about 2) when you figure usable capacity of a bank.

Man that is pretty bleak for electric. Lithium is several times more dense than lead acid, at least about 5x according to some quick numbers but that's still not too good.

My only last thought is that the 45% number for the desel engine seems a bit high, perhaps that number is a train engine or a power plant operating at optimum RPM? What's a small boat engine get when running at 1/2 or 2/3 throttle?

http://www.dekabatteries.com/assets/base/0194.pdf
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post #73 of 78 Old 10-17-2012
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Re: Optimal propulsion system

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My only last thought is that the 45% number for the desel engine seems a bit high, perhaps that number is a train engine or a power plant operating at optimum RPM? What's a small boat engine get when running at 1/2 or 2/3 throttle?
Train engines tend to be diesel-electric (i.e. Hybrid minus the batteries) and this would also have some benefit on a yacht - a bigger one at any rate.. There are a few ocean racing yachts still around from the 70's and 80's that used hydraulic drives for all the same reasons.

The figures are beyond me, but I'm not sure you're comparing apples with apples..:

1. A diesel engine in a boat gets throttled anywhere from idle to 100% with different fuel consumption at each throttle setting ("efficiency" if you like - at some throttle setting it will be "most efficient", but Murphy's Law states that max efficiency won't occur at cruising rpm.. )

2. A diesel generator in a marine Hybrid installation runs at "max efficiency" all the time i.e. the ECM selects the optimum diesel engine speed to match the load, which will be either (a) running the motor or (b) charging the batteries or (c) both.

This means that, in terms of "fuel efficiency"/"energy consumption"/whatever, the Hybrid will be far more fuel efficient under nearly all conditions than a straight diesel engine - offset by the cost of the batteries, etc, etc.

If you take a Hybrid car for a spin, you'll quickly see how it works..

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Last edited by Classic30; 10-17-2012 at 08:31 PM.
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post #74 of 78 Old 10-17-2012
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Re: Optimal propulsion system

One problem with all these energy density numbers is that they only show how a boat would use the electric motor ONLY or the diesel engine ONLY and compare them. Many of the advantaged to a hybrid, or even pure electric involve ways that you can use then engine other than motoring along at hull speed from point a to point b.

I think pure electric systems could be very practical for small boats who sail well and only use their engines to get in and out of harbor anyway. I've been on many a T-bird that departed the dock with only a gallon or two of gas. Pure electric wouldn't be much different.

But pure electric isn't for most. And it's applications are varied and it's merits many. Part of this though is you have to imagine how you would use it differently to your regular propulsion system. Personally I would kill for the ability to have a hybrid system such as the beta marine system I listed at the beginning of this thread. The advantages for me with just a very modest battery band would be the following:

1: I could add several knots of speed to my light air sailing with very few amps used. This would mean much more sailing and less time with the diesel on instead.

2: If the diesel died, I could motor right up to the breakwater of my marina and use my small battery bank to drive her in to the slip using pure electric.

3: More torque at low speeds means better handling in close quarters.

4: I would have a huge genset (not that I really need it)

5: Regenerative power while sailing at hull speed.

and all this could be done with your regular, or slightly enlarged, house bank. No need to replace your keel with lead acid batteries or spend 20L on Lithium ion batteries. What this system would not do is be the only force driving me long distances. For that, I have my diesel engine, and these funny triangular white things....

The link below outlines the pros and cons of how a hybrid system (of sorts- actually its a pure electric and an outboard) can be used in many different ways. Here is an excerpt from the link below:
Epods in Use

3. Light wind sailing was a pleasure for once. Typically the Gemini is not great in light winds due to her low aspect sail plan and small control surfaces ineffectiveness at slow speeds. However with the e-pods providing just a little push (5 amps) the boat was able to create its own wind to keep the sails pulling and the boat moving. The e-pods providing enough water flow over the rudders to allow you to steer. We sailed past the bow of a 40 foot mono who was hardly moving at maybe 1 knot mostly due to the tide; they were greatly impressed with our speed under sail; they had no idea there was anything helping us along as the outboard was clearly off and up out of the water at the time. With only a breath of wind she was doing between 2-3 knots; just enough to keep me from resorting to the outboard. Keep in mind that this was with 14 people aboard so the boat was heavy and we had been expecting normal heavy summer winds and thus were using the small 100% heavy working jib and full mainsail; not at all the right setup for light winds so the speed was even more impressive given these facts. I am definitely more likely to sail more or should I say motor-sail with the e-pods.

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Last edited by MedSailor; 10-17-2012 at 10:44 PM.
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post #75 of 78 Old 10-17-2012
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Re: Optimal propulsion system

Those e-pods look cool!

I agree with all that, except this might need some tossing around:

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5: Regenerative power while sailing at hull speed.
There are indeed a few "towable generators" on the market, but I'm not sure how well using your normal drive propellor to regenerate actually works in practice - since I'm led to believe different shaped blades are required for generating power from the water flowing over them without creating too much drag in the process.

I'd think you'd be better advised to stick with a folding/feathering prop and use wind/solar generation instead... Just my 2c worth.

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Last edited by Classic30; 10-17-2012 at 11:03 PM.
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post #76 of 78 Old 10-18-2012
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Re: Optimal propulsion system

I wonder how much drag would be introduced by one of those 'drop-in the stream' hydro-gens? IIRC, they produce some tremendous amounts of power without costing huge dollars or taking up a lot of space.

A couple of fins ta stabilize and a tow cable added to the wiring set and just throw it over the stern! Nah! Mebbe a faired stem and clamp arrangement to lower it off the stern quarter?

Would it be worth it to have the amperage vs the loss of a few tenths of a knot in cruising speed?

OK.. I see Aquair has that towed thang down already Howzabout a MInn-Kota trailed over the side and reversed to generate??
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Re: Optimal propulsion system

The problem with towed and shaft generators is they require a boat that can maintain high average speeds thru the water over long periods of time. Most of these generators require a boat speed of 5kn to start working and only become really powerful past 6. That doesn't sound that bad, but they will also cost you roughly 1/2 a kn of speed while operating.

So unless you are consistently cranking out 130+ days you just aren't going fast enough to use them. And while I know many cruisers have hit that mark once or twice, not many can do it day after day reliably enough to count on them for power generation.

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post #78 of 78 Old 10-18-2012
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Re: Optimal propulsion system

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I wonder how much drag would be introduced by one of those 'drop-in the stream' hydro-gens? IIRC, they produce some tremendous amounts of power without costing huge dollars or taking up a lot of space.
They produce very little power, typically 100 watts, and that's at 7.8 knotts through the water, which is more than hull speed for a typical cruising sailboat. A more realistic 5 knotts, the output is down to about 50 watts - you can get that from a single solar panel.

With the micro-hydro, it would take several days of continuous operation to recharge the batteries from the use of an electric drive for an hour.

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