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Old 23-07-2011, 22:25   #91
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Re: Electric Propulsion

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Originally Posted by Zonker View Post
Why not use Li batteries ? And would it make sense to combine house and propulsion batteries ?
In my opinion, yes, I don't really understand why there is a need to separate the two banks. I would plan on having a single, but very substantial battery bank that would serve both house needs and the propulsion needs.

As to the use of Li vs. Pb batteries, some of that depends on your budget and the type of boat you have. If you have a largeish monohul, you could potentially get by with flooded Pb's, but on a catamaran the weight of Pb cells is a deal-breaker.
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Old 23-07-2011, 22:32   #92
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Re: Electric Propulsion

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Originally Posted by smallyachts View Post
I can just imagine what the content of this thread would be if it was around when folks first started talking about putting steam or internal combustion engines in boats.

It'll never work because:

Fuel bunkers will take up cargo space.

What if it breaks when you are out to sea.

It's an explosion hazard.

It's never been done before.


Thank goodness someone was foolish enough to ignore the nay sayers back then.
On that same token, it's amazing anyone sailed anywhere before the addition of internal combustion engines.

It was so.... unsafe... sailboats are clearly meant to be motored at least half the time...
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Old 23-07-2011, 22:44   #93
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Re: Electric Propulsion

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Another words, if your boat is equipped with a 58 HP diesel, a minimum replacement electric motor should be 58*0.40, or a 23.2 HP electric motor, which is about 17 KW.
This is where you lose the rational engineers here. Unsubstantiated claims. I say if it takes a 58hp diesel to successfully drive a cruising boat with a properly matched propeller, then it will require an electric motor of like 58hp rating to successfully drive the same boat. There is no way to talk around that. And not surprisingly nobody has successfully done so. Statements like "I'm really happy with my electric motor" don't convince this skeptic. Sorry that I don't have any ideology to make be believe.
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Old 23-07-2011, 23:31   #94
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Re: Electric Propulsion

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This is where you lose the rational engineers here. Unsubstantiated claims.
I don't think so. The rational engineer (BTW, is there any other kind?) will realize that in my previous post I raised no new argument, I simply pointed to examples of electric propulsion implementations that have produced speeds comparable to the diesel powered versions.

Now, you may chose to ignore those real-life examples and be dismissive of the apparent empirical relationship between diesel and electric power, but some people might consider that type of an attitude to not be very rational in itself.

If you consider the first-hand results provided to this very forum by cruisers such as Mbianka, Fascat, Hyperdrv, etc., as "unsubstantiated claims", then there is probably (and unfortunately) no need for any further discussion.
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Old 23-07-2011, 23:59   #95
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Re: Electric Propulsion

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Originally Posted by Adamante View Post
if your boat is equipped with a 58 HP diesel, a minimum replacement electric motor should be 58*0.40, or a 23.2 HP electric motor, which is about 17 KW.
Prove it.
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Old 24-07-2011, 00:40   #96
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Re: Electric Propulsion

Now, let me see. Lots of people are describing their electric powered boats and, at the same time, others are indulging in arcane discussions about how many electric horsepower it takes to replace how many diesel horsepower. If you really want to contribute to the discussion stop talking about how many angels can dance on the head of a pin and address the real problem.

FACT: A propellor doesn't where the power comes from.

FACT: Whatever the ratio between electric and mechanical power sources, there is plenty of both. GE built a 5000 HP electric locomotive. HP or KW, that's way more than enough to drive even a very large boat.

The real problem is not does electric propulsion work. Clearly it does. The question is does it well enough under the conditions where I will use it. For some of us the answer to that question is, obviously, yes. For others, me included, the answer is not yet. In this thread we are discussing probably the worst possible application for electric power. High demand for long periods of time, small sources of replacement power and almost complete absence of supporting infrastructure. Electric locomotives have almost infinite available power. Electric cars, however limited, can plug in. There are no catenary wires and electric outlets at sea. We are, therefore, stuck with internally generated power, solar or wind. Both are intermitten and unreliable. No sun, no power. No wind, no power. So we need to decide what question we are asking.

Does electric power work? Yes.

Under what circumstances is it practical for me. That depends on me. You need to answer that for yourself, recognizing the advantages and drawbacks and how they relate to your particular case.

Dick
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Old 24-07-2011, 00:43   #97
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Re: Electric Propulsion

horse power does,nt move a boat it is the torque of a motor petrol or diesel or electric torque is the twisting power that moves you . electric motors have full torque at 1 rpm other motors have to rev to obtain 1000 plus rev to have any torque.
I love this discussion and think we are at a turning piont
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Old 24-07-2011, 01:21   #98
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Re: Electric Propulsion

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Originally Posted by Dick Pluta View Post
Now, let me see. Lots of people are describing their electric powered boats and, at the same time, others are indulging in arcane discussions about how many electric horsepower it takes to replace how many diesel horsepower. If you really want to contribute to the discussion stop talking about how many angels can dance on the head of a pin and address the real problem.

FACT: A propellor doesn't where the power comes from.

FACT: Whatever the ratio between electric and mechanical power sources, there is plenty of both. GE built a 5000 HP electric locomotive. HP or KW, that's way more than enough to drive even a very large boat.

The real problem is not does electric propulsion work. Clearly it does. The question is does it well enough under the conditions where I will use it. For some of us the answer to that question is, obviously, yes. For others, me included, the answer is not yet. In this thread we are discussing probably the worst possible application for electric power. High demand for long periods of time, small sources of replacement power and almost complete absence of supporting infrastructure. Electric locomotives have almost infinite available power. Electric cars, however limited, can plug in. There are no catenary wires and electric outlets at sea. We are, therefore, stuck with internally generated power, solar or wind. Both are intermitten and unreliable. No sun, no power. No wind, no power. So we need to decide what question we are asking.

Does electric power work? Yes.

Under what circumstances is it practical for me. That depends on me. You need to answer that for yourself, recognizing the advantages and drawbacks and how they relate to your particular case.

Dick

I think that pretty much summed up this whole thread. It's been great entertainment though .

- Nick
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Old 24-07-2011, 02:43   #99
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Re: Electric Propulsion

Given the apparent confusion about electric horsepower, torque, etc. I think it might be useful to explore this subject in a bit more detailfor the non engineers. The folks on this site aren't alone. It really not a terribly clear subject but, as any drag racer will tell you, it's all about torque. Everything else is just a matter of gearing.

First, it is necessary to define a few terms.

Work, in engineering terms, is moving a weight through a distance. Normally expressed in foot pounds.

Force is pressure exerted against a mass. Normally expressed in pounds, kilograms, etc.

Torque is a force applied through a lever to produce rotation. Normally expressed in foot pounds.

Torque and force are static measurements.

Horsepower introduces the element of time. Work done over a period of time. It is defined as 550 foot pounds per second.

An electric motor rated at 100 foot pounds of torque produces that torque at 0 RPM. It is measured by device called a Prony brake that stalls the motor and measures the force it applies at stall.
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Old 24-07-2011, 02:53   #100
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Re: Electric Propulsion

Oops. I pressed the wrong button. Here's the rest and why it matters.

An internal combustion engine has a torque curve. At idle it produces only enough torque to keep itself running. As you increase speed, you increase torque until, at some speed, you produce the rated horsepower of the engine. The reason you use gearboxes is to keep the engine running at or near that peak in the torque curve.

An electric motor produces maximum torque instantly and keeps it there until it reaches it's rated speed. In other words, unlike an I/C engine, it twists just as hard from .00001 RPM all the way to full speed. There are lots of kinds of electric motors that are beyond the scope of this discussion, but one kind is a constant torque motor. It's top speed is limited only by the load it is applied to and, with no load, will simply keep going until it destroys itself.

The practical effect of this is this. An electric motor will accelerate more quickly that an I/C engine and requires no gearing to stay at the peak of the power curve. An ideal power source, assuming you can get enough power to run it.

Dick
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Old 24-07-2011, 07:43   #101
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Re: Electric Propulsion

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Originally Posted by daddle View Post
Prove it.
See this post: Electric Propulsion

It's not engine HP that drives the boat, it's prop rpms. The prop sets the "demand" for the HP and the engine provides it.

Someone earlier wondered if a boat could really make 3 knots with only 1HP. For a 28-30-ish boat you can turn the prop about 1500 rpm for only 1kW. If 1500 prop rpms can drive the boat 3 knots, then that's really all the energy you need at that speed.
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Old 24-07-2011, 08:03   #102
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Re: Electric Propulsion

Realistically 58 hp diesel is plenty of power for a catamaran. I use twin 18hp diesels, many use 20 or 25hp twin diesels. The idea then would be to divide the energy and drive an electric generator, off the same engine, using the energy to power the other prop. Seems very easy. You could have the best of both worlds, powering both hulls for any distance (a disadvantage of electric) or running for short periods on stored energy (an advantage of electrics).

One advantage for small twin diesels is spare parts. You don't have to carry any. You make the repairs when you get to port. Important weight savings for a cat. You won't accomplish this with differing systems.
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Old 24-07-2011, 08:05   #103
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Re: Electric Propulsion

Even though the math is wrong, there's nothing to prove. It's a definition which is, by definition, a definition. One horsepower = 746 watts. You could look it up. One horsepower is one horsepower whether it comes from an electric motor, a diesel or even.....a horse which is, of course, precisely where Mr. Watt got the number in the first place. (58 horsepower x 746) / 1000 = 43.27 KW BTW, buy a car in Europe, even a diesel, and you'll get the engine rating in KW. A distinction without a difference.

Dick
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Old 24-07-2011, 10:38   #104
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Re: Electric Propulsion

Okay, thanks everyone. Especially those who patiently explained torque and work. I have misunderstood it for so many years. I am off to buy a 1 horsepower electric motor. Okay maybe 2 horsepower so I can go 4 knots. I'll add that useless dirty 40 horsepower diesel to my mooring.

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Old 24-07-2011, 11:14   #105
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Re: Electric Propulsion

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Prove it.
You probably lose some top speed, but not that much. The power needed to push an object through a fluid increases as the cube of the speed. With half the power you get 79% of the speed (third root of 0.5 = 0.79) or, you lose 21% of the top speed. Then you probably recover some of the loss through combined efficiency of the slow turning high diameter propeller and zero mechanical loss of the absent gearbox.

You end up with some 15-20% loss of the top speed with electric. You probably never go for those last 15-20% anyway because of noise and vibration of a diesel at full throttle. On the other hand, you can drive electric at its full power in virtual silence, provided for adequate air cooling.

Stan
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