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PDQ 39' sailing catamaran FOR SALE
On Wed, 08 Jul 2009 09:09:30 -0500, Rick Morel
wrote: On Wed, 08 Jul 2009 19:22:34 +0700, Bruce In Bangkok wrote: Your preliminary comments are correct :-) I'm interested in your reasoning for the electric drive as it seems to present no advantages to have a diesel fueled generator which charge a set of batteries to provide power to an electric motor to turn a propeller. Given that any mechanical or electrical device will have some losses it would seem that there will be less efficiency then a system that connects the diesel engine directly to the propeller rather then going all round Robin Hood's barn, as it were. Cheers, Bruce Be glad to explain my reasoning. First, you are correct in that is is less efficient, maybe by as much as 15%. Don't forget though, that the clutch is going to give some losses, more so with a hydralic one. But. In my case at least I think overall it would be better. Not a major thing, but it would give me a bit more of stowage space because the generator would be moved back from where the engine is and the electric motor would take little room and be back where the clutch is now. I've built a couple electric cars, bikes and trikes and a motorcycle so I'm quite familiar with the beast :-) The electric on battery only would give 3.5 to 4 kts for several hours, assuming I don't add any batteries. It would be used mostly for less than an hour at a time, probably more like a few minutes to 15 or 20 minutes. The solar/wind should be able to put this back in a day or two or three in most cases. If not, there's the generator. Assuming the generator is not needed, a plus is no heat to make its way into the cabin. Efficiency is really not a factor. You might as well say it's infinity percent or totally free sine the solar/wind is eventually putting it all back. For longer runs, which do happen occasionally, the electric motor would be run directly off the generator, like a train diesel locomotive or hybrid car. This would give 5 to 6 kts, depending on generator size, for as long as there's fuel. This mode would be where the efficiency loss comes into play, but I feel it would be worth it for the other battery mode. Again, this would be a fairly small part of "normal" operation. So there would be two motor circuits - one for pure battery and one for generator. The pure battery would be simple, just inexpensive "continuous duty solenoids" and maybe a couple big diodes for a "rectactor" controller to series/parellel the batteries for 2 speeds. You don't really need a "throttle" type controller for these lower powers. The generator ciruit would be simpler and cheaper as well. Instead of a complex DC controller, SCR's can be used on the AC output, then rectifiers to convert to pulsating DC for a true variable "throttle". The trick is to match the prop to the motor. It would probably be better to gear down to turn the prop slower. This could be a fairly simple chain or notched belt drive. So... Mode 1, battery only, would be simple to use. No warming up the engine, moving, cooling down and stopping the engine. Just flip a switch and go. Most of the time the energy gets put back "free". Mode 2, generator only, would be pretty much like now except burning a little more fuel. Maybe. Maybe not. There are factors that could make that up. Clutch efficiency on a "straight engine drive" and the higher torque of an electic motor come to mind. Okay, there's got to be some loss overall, but again I think it would be worth it having Mode 1. The above is my opinion based on the calculations. It would work for my cruising style, but maybe not for yours. Rick I can see what you are doing but I still don't see an advantage - this is not to say that "I wanna do it that way" is not a valid reason. But it seems to me: I don't know what sort of electric motor you plan on using but the one boat installation I saw used raw water cooled motors. There was a fairly extensive cooling system with at least one over temp warning system per motor. The one or two water cooled generators I have seen exhibited severe corrosion in only a few years of use. will water cooled motors have the same service life and a low speed, small, diesel engine? I wonder about the high power/low power control systems. What is the advantage of this? with the diesel auxiliary I might use idle to back out of a slip or to creep up on the anchor rode but that is generally a very small percent of the engine's operating time. In addition, especially inside a marina, I have frequently seen a need for almost full power in maneuvering, particularly when the current is running 3 or 4 knts through the marina. It seems to me that saving space is the biggest advantage of a diesel-electric drive. You do get rid of the gearbox but do you need a speed change between the motor and the propeller? In short, is there a distinct advantage in the diesel-electric drive? I'm not trying to start an argument here, I know what the advantages are for a diesel-electric drilling rig, for example, and I'm trying to figure out if there is one for a small engined sailing boat. Cheers, Bruce (bruceinbangkokatgmaildotcom) |
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