Hi Myles,
You seem to know the concepts and numbers pretty well. So, my next question is about solar vs wind. How many panels would I need to recharge a 48 volt battery bank? Does a wind turbine create drag against what your propeller is doing. Also, since my 3.1HP motor is too small to push a 40' x 14' houseboat around the Great Loop, what size electric motor would be appropriate for the taskSpeed is not a major concern. I am more interested in range. I have been spending hours on the web searching for answers only to find sales info rather than answers. I appreciate having people like you who are willing and patient with helping newbies like me? Please forgive my ignorance.
William A. Garrison
--- On Thu, 2/10/11, Myles Twete <matwete@comcast.net> wrote:
From: Myles Twete <matwete@comcast.net>
Subject: RE: [Electric Boats] Cruising with EP
To: electricboats@yahoogroups.com
Date: Thursday, February 10, 2011, 8:51 AMGNH offered: "You're last question about recharging the battery is the best one."
And then also: "http://www.pkys.com/alternator_installation.htm Contact Peter Kennedy, he'll be able to explain the In's & Outs."
I don't think these comments help.
Let's look back at what Mr. Garrison asked: "Will I be able to produce enough DC from the alternator to charge the bank?"
Now let's look at what he wanted to do with his houseboat:
"I would like to attach one axle to a propeller via a pulley configuration. I think running a generator off of the other axle could turn a generator, which is turning anyway. The generator could then be recharging the battery bank as we are cruising."
SURE, you could attach an alternator to the drive train of an electrically-driven boat. But what's the point?
If your intent is to generate power at some other voltage (e.g. 12v) to power boat accessories, that might be jusitified.
But the way this was worded, it reads to me as if the expectation is that somehow an alternator or other generator can deliver more power back to the battery bank than the motor used to power the drive train. As someone else noted, that is expecting perpetual motion and that violates some serious laws of physics. But hey, give it a try and let us know how that works out for ya'! But first consider a hypothetical on the power losses in the process: 100% power IN * 0.96 (controller) * 0.85 (motor) * 0.95 (tranny) * 0.7 (generator, at best) = 0.54 or 54% efficiency
So, say you're "cruising" and using 3.5kw of power just to drive the boat.
Now you think "hey, why not engage the generator to put that 3.5kw back into the batteries?"
So you do that.
Here's what happens if you actually delivered 3.5kw back to the batteries:
1) The motor needs 6.48kw EXTRA power in order for the generator to deliver 3.5kw to the batteries
2) The total motor power is now 10.0kw
3) Net power from the batteries: 10-3.5 = 6.5kw
In other words, all your hard work to add a generator has been worthless.
Now, what CAN you do with the other leg of the axle if you choose to go that way?
Maybe you could optionally "drive" that axle with a small gas or diesel motor, making your boat a plug-in-hybrid and now having the ability to indeed "generate" power to the battery bank, only now it would be power derived from burning fossil fuels and using the motor as a generator. Conceivably, you could be generating power while underway if the ICE engine were delivering enough power and speed.
In case this makes sense-
-Myles Twete, Portland
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