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Old 19-12-2012, 03:13   #46
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Re: Permanent Magnet Generator on Propshaft

It looks to me like you have plenty of room for a reasonably easy solar instalation. All boats get some shading, yours is a bit more than most, but you have a large area and solar is very cheap these days, more panels will make up for shading, some will always be producing and even those in shade will provide some contribution.
Solar will decrease your generator run times, allow you to get your batteries closer to 100% charged ( rather than 80% accecieved with generator). This will extend your battery life and effectively increase your battery capacity.

However running a high capacity watermaker from green energy is difficult. You will probably need all three sources of propshaft, wind and solar to get close.

I would rethink your strategy and rely on the green energy sources allowing you to greatly reduce your generator run times, but use the watermaker when running the generator or main engine. Given your frequent motor sailing I would think it would be worthwhile having a large enough inverter ( my preference), or a DC motor on the watermaker so that water could be made while motor sailing.
Seawater away from harbours is much cleaner and better for the watermaker.
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Old 19-12-2012, 03:16   #47
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Re: Permanent Magnet Generator on Propshaft

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Originally Posted by Pelagic View Post
Dave, how many amps were you getting at about 7 knots?
from memory, about 7-10 amps peaking 12. The main issue was it( the spinner) jumped from wave to wave and causing the output to pulse. But it could be put out for days on end. knocked about .4 knots of the speed of the boat
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Old 19-12-2012, 05:37   #48
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Quote:
Originally Posted by twistedtree

OK, I think you are taking my statement too literally. My point is that if you load up the prop such that it generates N kw of power, you are creating N kw of drag on the boat. Actually more when you consider losses. That drag slows down the boat just like any other form of equivalent drag. To dramatize my point, I was trying to describe N kw of drag in the form of a propulsion motor pushing in an opposing direction to the boat's movement.

If you don't mind slowing down your boat, then by all means hook up a generator and let the amps flow. I just worry that a lot of people think there is "free" power there to be harnessed, and there isn't. It's not opinion, it's physics.

Perhaps the more important question is "how much will the boat slow down?" It wouldn't be too hard to estimate it, and doing so is probably a good starting point. There are lots of calculators that will tell you how many HP or kw are required to push you boat at some speed, say 5kts. I'd suggest doing the calculations in kw, not HP so it's easier to translate into electrical power. Then run the calculations again for 05.kts less speed and see what the difference in kw is? Take the difference and now you know how many kw are associated with the 0.5kt speed difference. Conversely, it tells you how many kw you can generate with a 0.5kt slow down.

But keep in mind that what I've described assumes a perfect, loss-free system which of course you are not dealing with. Between generator losses, prop slippage, even more slippage due to the prop effectively being run backwards, and you are probably down to something like 50%-60% efficiency, so you will generate that much less power for the 0.5 kt slow down.

If you run the numbers, I'd love to hear the answer. Maybe it's enough to be a useful tradeoff of speed for electricity. And I'd think an estimate of how much you can generate is a pretty important thing to know early on in the design process. It's also an essential starting point for sizing your generator, gear ratios, regulator sizing, etc. Generating 50 watts is very different than generating 2kw.
Your point is well stated, and I will run the numbers and post them when I get closer to this particular project.
In my case I have just bought a project with an outboard hanging off the back. I first need to get it home for a complete restore an
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Old 19-12-2012, 05:41   #49
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Darn phone!
And repower. I am considering electric as it suits my sailing. The fact that it would already be set up for recharging is just gravy.
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Old 19-12-2012, 09:20   #50
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Re: Permanent Magnet generator on propshaft

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Originally Posted by sailr69 View Post
...there are tow behind generators commercially. when there is good wind, it will not slow down a boat at all...
Don't know if you meant this literally, but the whole point about the first law of thermodynamics is that, yes, it WILL slow the boat down. It HAS to!

Now, will it slow the boat down enough to matter? That's a whole different question. If you slow down from 6.5 kts to 6.3 kts, and in exchange you get 5 amps flowing into your batteries... Well, I'd call that a tradeoff that is darned well worth it. In fact, for most passages I would happily give up a half a knot in exchange for 3-4 amps, 24 hours per day, rain or shine.

So, it WILL slow you down some, though not necessarily a significant amount. Just exactly how much involves so many different variables that you just have to try it and see. Those who use towed generators usually report somewhere from 1/3rd to 3/4ths of a knot lost, from what I've seen. I would expect about the same loss from connecting a generator to the prop shaft.
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Old 19-12-2012, 18:27   #51
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Re: Permanent Magnet Generator on Propshaft

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Originally Posted by noelex 77 View Post
It looks to me like you have plenty of room for a reasonably easy solar instalation. All boats get some shading, yours is a bit more than most, but you have a large area and solar is very cheap these days, more panels will make up for shading, some will always be producing and even those in shade will provide some contribution..
Hi Nolex, your advice is very well taken and has been the other voice in my head when deciding strategy and if I can rely on large Solar Panels.

Problem is that I keep hearing conflicting reports about shading effects from both users and warnings from suppliers, so I have been hesitant to commit.

Now that we plan to head out to Micronesia next year, time to commit to a balanced DC generation strategy with what I have on board.

I will start another Thread on Solar Bimini Installation to help to quantify the issues. Thanks for the confidence vote on Solar
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Old 19-12-2012, 20:50   #52
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Re: Permanent Magnet Generator on Propshaft

I would also maximize solar capacity before adding a PMA for regenerative power. If I were going to add a PMA for regenerative power, I would probably go all the way to electric propulsion.
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Old 19-12-2012, 21:19   #53
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Re: Permanent Magnet Generator on Propshaft

The problem with the PMA on a big prop shaft is that the shaft may turn at 600 rpm under power, and 200 rpm when sailing. A PMA designed for a X rpm shaft is going to overheat when you are powering at 3X.

The solution is to put clutch or belt untensioner in the system so that you only turn the PMA when you are sailing. You will still need to monitor the battery voltage.

The other solution is to use an automotive-type alternator. They have the advantages of being designed for a wide range in RPM and voltage regulation is cheap and easy with the field coil. They are not as efficient, and do consume a lot of power, but that is not a problem for bigger boats.
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Old 19-12-2012, 21:51   #54
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Re: Permanent Magnet Generator on Propshaft

6ftx5ft equates to around 500w of solar. email independentpowersolutions@gmail.com
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Old 20-12-2012, 00:39   #55
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Re: Permanent Magnet Generator on Propshaft

If I'm reading Pelagic's diagram correctly, he appears to have more than 15 square meters on which to mount solar panels. If we conservatively assume 12 square meters (due to the shape) of panels can be fitted, then it should be possible to install a nominal capacity of 1500 to 2000W. Depending on latitude, season, weather, shading, etc., such an installation should generate between 50 and 400AH per day at 24V. A 1hp 24V DC motor for the RO watermaker should draw about 30A. Depending on where you are, the solar might generate enough electricity to run your watermaker for about four hours per day in the winter and about eight hours per day in the summer -- assuming a large battery capacity. I would want a lithium phosphate battery bank of 400 to 1000AH to go with that much solar capacity.
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Old 20-12-2012, 05:36   #56
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Re: Permanent Magnet Generator on Propshaft

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Originally Posted by donradcliffe View Post
The problem with the PMA on a big prop shaft is that the shaft may turn at 600 rpm under power, and 200 rpm when sailing. A PMA designed for a X rpm shaft is going to overheat when you are powering at 3X..
Thanks Don, I had not considered the difference might be that big and will need to confirm shaft speeds with seas trials and a speed gun for various conditions:
Under power only at 1600Rpm.
Motor Sail… at 1000 RPM (10 to 14 knots T-wind)
Sailing… no engine.. (15 and above)

I am studying PMA uses/Solar/Wind over the Xmas break to find out what I can safely do
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Old 20-12-2012, 05:40   #57
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Re: Permanent Magnet Generator on Propshaft

Thanks mcarling and familycruiser,
so as not to clog up the PMA discussion,I shifted my solar study over to
Solar Panel Mounting on Bimini
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Old 20-12-2012, 08:09   #58
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Re: Permanent Magnet generator on propshaft

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Don't know if you meant this literally, but the whole point about the first law of thermodynamics is that, yes, it WILL slow the boat down. It HAS to!

Now, will it slow the boat down enough to matter? That's a whole different question. If you slow down from 6.5 kts to 6.3 kts, and in exchange you get 5 amps flowing into your batteries... Well, I'd call that a tradeoff that is darned well worth it. In fact, for most passages I would happily give up a half a knot in exchange for 3-4 amps, 24 hours per day, rain or shine.

So, it WILL slow you down some, though not necessarily a significant amount. Just exactly how much involves so many different variables that you just have to try it and see. Those who use towed generators usually report somewhere from 1/3rd to 3/4ths of a knot lost, from what I've seen. I would expect about the same loss from connecting a generator to the prop shaft.
Yes as I said in a previous post, we saw 0.4 knots average lost when towing the generator. We generated about 120-150 Ah per 24 hours on average.
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Old 20-12-2012, 09:30   #59
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Re: Permanent Magnet Generator on Propshaft

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from memory, about 7-10 amps peaking 12. The main issue was it( the spinner) jumped from wave to wave and causing the output to pulse. But it could be put out for days on end. knocked about .4 knots of the speed of the boat
Somewhere in the Atlantic, there is a BIG Blue Marlin with at least 1 Walker rotor in his belly. Lost it in '71.
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Old 20-12-2012, 11:12   #60
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Re: Permanent Magnet Generator on Propshaft

Pelagic,
I applaud your energy system considerations.....and if looking for watermaker choices...have a look at my post here....
Which Watermaker To Choose


1) I have experience with PM towed-water-generators and small alternator-based prop-shaft generators.....

a) The towed-water-gen (Hamilton Ferris) is my curret set-up and I like it very much for those cloudy days on passage.....but my 520 watts of solar / MPPT controllers do most of my energy genration on-board....
I get about 1.2 - 1.5 amps per knot of boat speed from 3kts - 7kts...and a bit more (2amps/kt) above 7 kts.....

In heavy seas when at boat speeds 8 - 9 kts, it does break free from the front of one swell before digging into the next.....so power output becomes limited....although Hamilton Ferris does make some nice bronze weights that are supposed to solve the problem, I haven't used them (yet)......

Since My solar keeps me well supplied with power, I usually deploy my water-gen only on cloudy days on passage, and then usually at boat speeds above 4 kts...but it DOES work VERY well!!!
See an article/photos of my set-up here...
Towed-Water-Generator



b) Back in the 1970's we had a small (35-amp Motorola) alternator w/ its simple regulator attached to the outside of the case, spinning from a larger pulley on the prop shaft......which back them was called a "sailing generator".....
It produced about 1.5 to 2 amps per knot of boat speed undersail.....and a bit more as we sailed near hull speed (8.5 kts).....took it off the boat 20,000+ miles and many years later....
(note this back in the day of radar draing 18-20 amps at 12vdc...plus refrigeration, etc...and solar was pretty expensive as well, so got our "green energy" anyway we could....)
{And, a funny note....I still own this exact Motorola alternator, as it is mounted on the front of a 400+ hp Chevy V-8 engine in a '23-T Rodster (Hot Rod) I built years ago...}





2) Depending on "where" and "how" you are sailing/cruising, the advice / recommendations will vary....
But, for "most" cruising in the tropics or temperate latitudes, between 30* N and 30* S (as well as summertime up thru 40* latitude), adding as much UNSHADED solar as you can fit, before wind-gen or water-gen is my usual advice as well as most of us who've done so!!!

In cases where there will be shade on the panels, it is best to figure daily energy output conservatively, and make calculations based oin the total amount of solar (in watts) that will be unshaded for most of the day's highest production hours (+/- 3 hours from local noon).....

Note that depending on your exact layout and what shading you have, this number may be taken from panels in one central group....or different panels (such as some on one side working well before noon and others on the other side of the vessel working well after noon)
{as an example, if you had 600 watts total, and in the moring you had 200 watts on one side shaded, and in the afternoon had 200 watts on the other side of the vessel shaded....you should figure daily average energy generation form the array based on approx 400 watts....and whatever extra output you get is just an added luxury...}

As for calculating that energy generation for planning purposes....
Depending on "where" and "when" you're sailing/cruising, the values do change....but for the most popular areas within 30* of the equator, you can calculate a daily average over an entire years...(having higher production in summertime with higher sun angles and lower production in witer with lower sun angles)

Based on your 24 volt system, and using MPPT controllers, you can take the total amount of unshaded solar (in watts) X 20% = average daily A/H's generated by the solar array....
{Using the above example of a 600 watt array, with an average of 400 watts unshaded for the highest producing hours, that gives you 400 x 20% = 80 amp / hours generated per day.....

Have a look at an article/photos of my solar set-up here....
Solar Panels


3) For my article/photos on my watermaker choice and installation have a look here....
Watermaker


4) In a nutshell, I recommend a large solar array....or at least the largest you fit without substantial shading.....
BEFORE spending much time/effort on water-powered generators....
Only after going the solar route, then look into the water-gens...


Fair winds and sunny skies....

John
s/v Annie Laurie
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