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sailak

Member Since 21 Oct 2007
Offline Last Active Yesterday, 02:50 PM
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Posts I've Made

In Topic: Battery water level down - what's the result

16 April 2016 - 03:36 PM

I just replaced some golf cart batteries on a scissor lift that had no visible electrolyte in them at all.  Amazing thing was the machine was still working and after adding a few gallons it seemed to hold a charge.  These batteries would have to be classified as severely abused! I would have to agree that 1" below the plates for any period of time will cause permanent damage resulting in significantly reduced capacity.  Total failure?  Not so sure after what I just saw...  Poor things must have had just a few inches in the bottom of the cells!


In Topic: Propulsion: Electric horses are bigger!!!11

12 April 2016 - 03:03 PM

What type of DC motor are you are talking about?  At max speed?  They have a rated speed.  At rated speed they pull rated amps at rated volts.  They can achieve rated power(which is at the shaft, i.e. a 5kW motor can do 5kW of mechanical work, obviously consuming some degree more than 5kW of electrical power), or nearly rated power over a range of RPM's which a diesel can not.  A series DC motor develops crazy amounts of low speed torque and can achieve destructive RPM's, but compound motor designs can be more or less tailored to the job at hand and develop good torque at rated speed.  

Why use a DC motor anyway?  I would look to use an induction motor and inverter or a brushless motor and drive.  Electronics have come a long ways in the last few years, especially in price.  Many of the newer ships including navy vessels are using large scale low rpm motors in lieu of a shaft and reduction gear setup.  This technology scales easily and costs are pretty low.  Go buy a cordless drill and many are using very similar technology to a freaking Tesla, which is remarkably similar to one of our large ice breakers or a modern diesel-electric railroad locomotive.  

 

I like diesels for their simplicity, reliability, and I even like the sound.    But they do have many, many drawbacks.  There is no reason we shouldn't be seeing a higher percent of pleasure craft being electrically driven, or even hybrid drives.


In Topic: Propulsion: Electric horses are bigger!!!11

11 April 2016 - 03:21 PM

Common motors are not rated by input power.  NEMA standard motors will have rated amps and volts, but the HP or kW rating stamped on the machine is mechanical power, shaft power.. how much work the motor can do.  Electrical power requirement is higher.  Of course there are many non-standard motors sold and lots of fancy marketing.  Love the 6.5hp shop vacs!  At 120 volts and "rated" power that would mean the motor is pulling around 40 amps on a 15 amp rated receptacle.  Cool shit.  


In Topic: Propulsion: Electric horses are bigger!!!11

10 April 2016 - 04:16 PM

Internal combustion engine horespower ratings are misleading.  You only get peak power at one specific RPM, and you only get to use it if your prop size and pitch, transmission gear ratio, hull type are all perfectly matched, and Mars is in the 5th house of Aquarius.  Otherwise you are only getting a fraction of what your engine could potentially make on a bench dyno.  Some curves are worse then others.  Power is a product of torque and RPM, so if you can produce gobs of torque at lower RPM like an electric motor can (sometimes 2-3 times what they produce at rated speed) you still get the power.  Basically, an electric motor can produce much more usable power.    Just because you are turning RPM's that coincide with the peak on the power curve does not mean you are getting full horsepower.   In fact, you wouldn't necessarily want to be getting peak power...  Gensets are designed to deliver something around 80% to help extend engine life.  A similar overpowering is likely to occur in boats, or these days it seems like boats are getting delivered with twice the power they used to.  You also need about 3hp reserve for your alternator. So for a 20hp engine it would go something like 20hp * 80% = 16hp. 16hp-3hp = 13hp.  Then say your prop is around 50% efficient (high number) and your gearbox and driveline is 80% efficient.  13hp * 50% * 80% = 5.2hp or 3.9 kW.

 

Power ~ torque x RPM

 

In metric units Power(watts) = torque(Nm) * radians/second

 

Electric motors have a variety of torque/power curves to draw from.  Induction motors are flatter, and DC motors depending on their specific design can be just about anything you want them to be.  


In Topic: electric vs manual head

10 April 2016 - 05:23 AM

Fishing charters swear by electric heads here.  Seems its marginally more difficult for touristas to clog them.  I'd strongly consider one.  The electrical requirements are more than likely going to go unnoticed.