Re: Can Electric Marine Propulsion Be A Reality (in pleasure craft)?
The 30Hp was picked because it takes about 1.77Hp to produce 1000Kw (slightly under the rule of thumb, 2 Hp per 1000Kw).
That would be a pretty good conversion!! 1000KW is about 1360 HP.
You might want to get your decimals straight!
1000KW is 1 megawatt........... 1HP = 746W or 0.746 KW......so ................1.77hp is about 1.3KW (1300W)
You did get the 30HP to 22KW conversion right however!
1. Assume that all the parts will fit into the boat?s engine compartment
2. Assume that specific adjustments to various parts would inevitably be needed based on many needed calculations that are not a part of this discussion
3. Assume that weight is not an issue
4. Speed and efficiency is not the immediate objective. Forward and reverse propulsion is the objective
1. Remove engine and transmission from the engine compartment leaving the stern drive mounted.
2. Install a variable speed electric motor with a spline shaft that will marry up to the stern drive
3. Install a small engine (of say 30 Hp) with a through hull exhaust system
4. Install a 10k watt generator
You cannot just "assume weight is not an issue".
Weight is a HUGE issue. Too much weight in the extreme case, will just sink the boat!
Too much in any case will make the boat so inefficient that you will not be able to carry a load.
Speed and efficiency is not the immediate objective.
You could probably eliminate speed, but you cannot eliminate efficiency.
Here's the problem. A boat has to operate in a medium we call WATER!
The hull design we commonly use in water, depending on the speed of the hull through the water encounters drag which is a FIXED quantity depending on the amount of the hull contacting the water and the velocity through the water.
The force (Lbs or Newtons) required to make the hull go a certain speed must be sufficient to overcome that drag.
It doesn't matter what is used to produce that force (jet engine, reciprocating engine, electric motor, "nucular" power, star drive, di-lithium crystal hyperdrive, squirrels, lions, tigers & bears (oh my!) :facepalm:
1 gallon of gasoline has a potential energy of approx 115,000 BTU's (British Thermal Units)
So if you have a boat engine that is burning 5 gallons per hour at a "cruising speed", You're using the equivalent of 115,000 x 5 =575,000 BTU/hr
Convert that to KW and you get about 168kw or approx 225hp.
Now we know that reciprocating engines are not 100% efficient. they're more like 25% efficient. (25% of the above 225hp is actually driving the prop)
So in round numbers, we're talking around 55 hp is required to get your theoretical boat to "cruise" to stay on plane.
Electric motors are probably somewhere around 70-90% efficient. (depending on whether AC(3ph) , DC and how efficient your control system is etc)
BUT if you need 55hp connected to your stern-drive for cruising, you'll need more than that to get it up on plane.
Even if you assume a 90% efficient motor, and you need say, 25% MORE power to get up on a plane, then you'll need around 70hp to get it up on plane.
70hp is about 52KW
let say you have 48V batteries (like 4 BIG 12v truck batteries connected in series) driving your boat.
To supply 52,000 W of power to drive the motor to get you up on plane, those batteries (at 48V) would need to produce approx 1100A
Even if the motor operated at 100V DC, you would need more than 500A to produce that 50kw.
Of course, even HUGE HEAVY truck batteries wouldn't be capable of producing 500A for very long. (we're talking MINUTES here!!!!)
Now remember, the above 52KW was needed to get up on plane, so you wouldn't be at that power setting for more than a min or so.
BUT you WOULD operate at the lower (55HP setting or 41KW) for continuous "cruise".
at 48V, 41KW would still require nearly 850 AMPS! those 4 HUGE 12v truck batteries would not last long (we're still talking minutes)
At 100v 41KW is still over 400A
Right now the battery technology is not quite there.......but it is getting better.....
Electric boats are also not new at all........
http://en.wikipedia.org/wiki/Electric_boat
Submarines have been electric since WWII. And they used HUGE batteries.
I am not saying it's no "do-able".
But for our kind of boating (water skiing, cruising at planing speeds, etc) it might be a while before they're affordable and we see them at the boat store........
