dc generator are they available?

See this site www.siemens.com/index.jsp

If you power an automotive alternator with a small gasoline or diesel engine, you can get outputs in that range of voltages. You might need to replace the alternator's diodes with ones of slightly higher voltage rating, and you could not use the voltage regulator (which would prevent the output from going above about 14.5 volts). Given a constant field current, the output of the alternator will vary in a nearly linear way with engine rpm. If you poke around hobbyist wind power sites you'll find more info. Some of these places sell permanent magnet alternators, which would produces those voltages. That complete range might be a little difficult to attain with the permanent magnet route, because if you produced 72V at 3600 rpm, to get 12 volts you need to run at 500 rpm -- at which speed the engine would be on the verge of stalling.

Also any permanent magnet DC motor can be easily used as a generator, again, with a near linear relationship between rpm and voltage.

There may be something pre built along these lines, (like a typical small home generator, but with variable DC output) but I am not aware of a source.

There may be something pre built along these lines, (like a typical small home generator, but with variable DC output) but I am not aware of a source.

Actually, as I think about it, I am aware of a source. Any typical marine diesel generator could generate either 12 volts for the starting and house batteries, or higher voltage for the propulsion batteries. Possibly, the cheapest way to do this would be with two alternators, (one with regulation to "12" V [14.5, typically] and another with regulation to your propulsion battery bank) driven from the same diesel. Another possibility would be a single alternator with regulator which could be set for different voltages. But then you'd have to switch between banks, and the contactors to do that may cost more than the second alternator. The power required to drive whichever alternator is off load would be negligible.

Re voltage: 50 volts and above is considered dangerous by OSHA. Although virtually any voltage can kill you (because the lethal current is only milliamps) I think there is some value is keeping voltages low if, when you work around them, you are apt to be in wet, cramped quarters. If there is a compelling reason to go higher, then do so, but if you can keep voltage to nominally 48, you might be a little safer. Of course you might very well have 120 service on board, but in my view, the fewer opportunities for electrocution, the better.

I had a racy 33 foot ultralight some years ago, and it had been set up for long single-handed ocean races, with transferable water ballast, solar cells, etc. I used it mainly for daysailing and some overnight races, and was impressed by how well the solar cells kept the batteries charged. At the time, I thought that I could use an electric motor for my auxiliary, and (because I rarely used the boat more than once a week) have the solar cells keep the batteries topped up. If I had more than a couple miles to go under power, I'd be out of luck -- but for the way I used the boat, I (almost) never did (although, with other boats, there have been times when I've had to motor 100 miles, becalmed all the way.) If I wanted to be able to go long distances, then having a generator on board would provide for that -- although at a loss in efficiency vs driving a prop directly.

Other random thoughts:

Any DC motor is also a DC generator. Most are about equally efficient in either role.

Automotive and marine alternators are usually of low efficiency (65%). A good permanent magnet motor can have efficiency of 90% or better, in either role.

An alternator can be used as a motor, although it requires a three phase AC supply (and the rectifiers must be bypassed).

A brushless DC motor is often called PMAC (permanent magnet AC) because the commutation is handled by supplying AC (typically three phase). The controllers for these motors can take a DC source (e.g. a battery bank) and convert it to the right type of current for the motor. For a clean, simple, low parts count installation, a PMAC motor in sailboat would work well. Typically, the controllers for these motors are four quadrant, meaning that they can control forward and reverse in propulsion and braking, and generally do not need reversing contactors to do so. You could generate current while sailing (although you'd go slower*) to charge up your battery bank.

*Although if you were at hull speed, with excess wind, there'd be no appreciable difference. As you may know, there are some "hybrid" catamarans which generate current while sailing, and the "wind hp" required to drive these floating hotels to near hull speed is so great that the 2-5 hp sucked up in generating current for the battery bank is probably of little concern.

If you want to control the speed of the DC motor then go for WARD LEONARD System OR use DC Drive. The former one is the cheap option.

Try WW2 surplus aircraft generators, there is a big range and with combination of speed and field excitation you should be able to reach 72Volts.

I would recommend finding a generator/alternator that will supply a constant 72volts. You can always reduce your voltage with a speed control to your desired setting.

I think that you should post more accurate infos about the size of the motor, the generator, the powers you have calculated that you need, sailing boat? or not etc etc.

That way we could probably help better......

I agree with Andy! Else we all can tend go on tangents and maybe oblivion.

** Think it was my 10th grade Chemistry teacher who said it well "back it up with facts and figures".

I find that many of the posts lack specific info/known parameters as to really what the originator wants to do.

Hi Bill,

thanks for your comments.

It worries me that seemingly sane people, who have (probably) bought a boat (in this particular example), now start to plan changes to that boat THAT THEY DO NOT HAVE A CLUE ABOUT HOW TO DO IT, now approach CR4 for an answer!! But give us precious little info, why, because they have so little knowledge themselves, they cannot even guess what we need for a sane and sensible answer......

The reason that I personally do not need CR4 for my problems is that I plan ahead and make myself clever by reading books or buying magazines, so that BEFORE I get involved financially, I know how to go about what I need to do......

Sometimes, when reading the requests for help on CR4, I wonder who is the crazy one.......I am not sure anymore.....but at least it keeps me off the streets.....the crazy ones too....

Your question is just too general I feel, for a proper answer at this time, which is why I commented as I did before.....

For example, if your boat is tiny, the river or lake is not large, then the answer you had with regard to using car alternators is well thought out.

If your boat is large or you want to go to sea (not just lakes and rivers), then that will probably not work.

If you have a motor in mind already, what current does it require at various voltages using the prop size that you want to put on it?

Furthermore, the answer you received about the Ward Leonard system, was a good well thought out answer and not too difficult for someone to build themselves....and it allows complete control of speed and direction of a DC motor.

As yet, I have not seen your answers to such excellent ideas, so please be so kind as to comment and if necessary, give us fuller details to allow us to help you further.

I am researching the options suggested and I did post the type of motor I would like to use. Others using this motor (the wheel) have large battery banks and I am looking to avoid battery weight, instead a generator which would supply electricity for propulsion, refrigeration, air conditioning, charging of a house battery bank. I have received information here that different revolutions will produce different voltage using permanent magnet motor as generator. That was key to evaluating permanent magnet motor to use as generator. I am well on way finding a suitable motor, and computing power requirements of all the systems. Much thanks for everyone help.

It worries me that seemingly sane people, who have (probably) bought a boat (in this particular example), now start to plan changes to that boat THAT THEY DO NOT HAVE A CLUE ABOUT HOW TO DO IT, now approach CR4 for an answer!!

Of all the things to worry about, you pick this!!??

I'd find it very nearly impossible to conclude from the original post that the poster has does "not have a clue about how to do it." What purpose is served in insulting the original poster?

We get some phenomenally broad questions here, and many people will answer even the broadest patiently and politely. Why not?

I am "worried" about this situation as it relates to all power to be used by the boat for electrical systems I will be installing. I don't talk like an engineer because I am not one. I am a technician. I often encounter Engineers and Educated people, who ask me questions relating to fiberglass fabrication and repair. And some of them think because they are able to understand complexity in one area, only need a bit of info to master another area. I am obtaining great help from many here and seeking to avoid moving in wrong direction during initial planning phase.

An automobile alternator will do just fine. Without the voltage regulator, the output voltage of an alternator becomes a speed dependent variable. Most alternators can definitlly generate at least 120 volts at around 3600 rpm and the diodes have sufficient voltage rating for in excess of 120 Volts.