Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 18-12-2012, 12:04   #31
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,150
Re: Permanent Magnet Generator on Propshaft

Another issue is the noise of a free wheeling prop. If it does not bother you then great.
__________________

__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Old 18-12-2012, 12:15   #32
Registered User
 
nimblemotors's Avatar

Join Date: Apr 2011
Location: Sacramento, California
Boat: Solar 40ft Cat :)
Posts: 1,557
Re: Permanent Magnet Generator on Propshaft

Quote:
Originally Posted by jeanathon View Post
... When you are pushing up against hull speed I doubt the drag from a spinning prop would be noticed. But I am guessing here.
So the hull speed therefore will limit the amount of power available from the prop. With a wind generator, there is no such limit.
If you already have the prop connected to an electric motor, then the regen can be mostly "free" as in cost of components to make it happen, of course it slows down the boat.

Traditional engine alternators have very poor efficiency.

Selecting all the components to yield high efficiency is not an easy task,
and probably why engineered solutions are very expensive, they take a lot of research and testing.

JackB
__________________

__________________
nimblemotors is offline   Reply With Quote
Old 18-12-2012, 13:01   #33
Registered User

Join Date: Mar 2010
Location: Barcelona
Boat: Cheoy Lee Clipper 42
Posts: 2
Re: Permanent Magnet Generator on Propshaft

hello from Barcelona,
a spanish guy has mounted a PMA 520 in a Endurance 35. Shaft pulley have 12" diameter.
At 5 knots produce 6 amps and 9 at 7 knots.
You can see the video here:



A little noisy without the floor but builder think is not too much when know that amps are going to his batt for free.
regards
cheers an beers...
__________________
perecaracol is offline   Reply With Quote
Old 18-12-2012, 15:31   #34
Registered User
 
nimblemotors's Avatar

Join Date: Apr 2011
Location: Sacramento, California
Boat: Solar 40ft Cat :)
Posts: 1,557
Re: Permanent Magnet Generator on Propshaft

Quote:
Originally Posted by nimblemotors View Post
Selecting all the components to yield high efficiency is not an easy task,
and probably why engineered solutions are very expensive, they take a lot of research and testing.
JackB
Ok, just to try and be a little more helpful..the generator puts out voltage depending on its rpm, different windings generate different voltage profiles.
Power remains the same, more voltage, less amps.

The most general method is to use a DC-DC that takes whatever voltage the generator generates and converts it to your load voltage (e.g. 12v, 24v)
When you draw power (the load), generator will slow down, and thus the voltage drops, but the DC-DC keeps it constant.

A simpler method is to regulate the load to keep the voltage constant.
Select the generator to give the voltage at lowest rpm.
Now you need a way to regulate the load, once the bats are full, where do the amps go to keep the voltage constant? This can be very wasteful, or not a big deal depending on the bats, etc. With full bats and a fast boat,
the generator is wearing itself out for no good reason.

You can make the pulley so it can disengage, but now YOU are the charge controller, whereas with a DC-DC, it will just let it spin without a load.

hope that helps.

JackB
__________________
nimblemotors is offline   Reply With Quote
Old 18-12-2012, 15:44   #35
cruiser

Join Date: Sep 2007
Location: Tampa to New York
Boat: Morgan 33 OutIsland, Magic and 33' offshore scott design "Cutting Edge"
Posts: 1,594
Re: Permanent Magnet Generator on Propshaft

If I'm not mistaken the stator is custom wound to suit the application and magnets are added or subtracted to further allow mods. The finer the wire used the more power it will put out at low rpms but the easier it is to burn out. Adding or subtracting magnets will have the same effect. For this application I could see havIng windings of quite fine wire as the speed will have limited range(while sailing). I could see burning it up if the engine were run with it engaged.
__________________
forsailbyowner is offline   Reply With Quote
Old 18-12-2012, 15:57   #36
Registered User

Join Date: Sep 2010
Posts: 793
Re: Permanent Magnet generator on propshaft

Quote:
Originally Posted by noelex 77 View Post
Prop shaft generators will slow the boat down.
However generating X power from the propshaft is not the same as additional drag as as applying X power in reverse. This does not not violate any any of the thermodynamic laws, so the world will not come crashing down. Well not until the 21st anyway
How is it not the same?
__________________
twistedtree is offline   Reply With Quote
Old 18-12-2012, 20:06   #37
Registered User
 
jeanathon's Avatar

Join Date: Mar 2012
Location: WNC mountains U.S.
Boat: 1968 Hinterhoel Redwing
Posts: 513
Re: Permanent Magnet generator on propshaft

Quote:
Originally Posted by twistedtree View Post
How is it not the same?
First of all because the prop is not turned the right direction to go in reverse. A prop spinning in reverse will never make as much thrust as forward because it is not designed to. Hydrofoils work best when used as designed.

Second is because the generator will not actively be turning. At 0 knots the generator will not pull you backwards it will be well 0. As your speed increases the drag from the generator will be a percentage of the boats speed not an active opposing force. 1/10 of 5 knots is .5 penalty, 1/10 of 6 knots would be .6 penalty. No matter how you look at it it is additional drag not opposing lift.
Now I am confused.
__________________
jeanathon is offline   Reply With Quote
Old 18-12-2012, 20:36   #38
Registered User

Join Date: Sep 2010
Posts: 793
Re: Permanent Magnet generator on propshaft

Quote:
Originally Posted by jeanathon View Post
First of all because the prop is not turned the right direction to go in reverse. A prop spinning in reverse will never make as much thrust as forward because it is not designed to. Hydrofoils work best when used as designed.

Second is because the generator will not actively be turning. At 0 knots the generator will not pull you backwards it will be well 0. As your speed increases the drag from the generator will be a percentage of the boats speed not an active opposing force. 1/10 of 5 knots is .5 penalty, 1/10 of 6 knots would be .6 penalty. No matter how you look at it it is additional drag not opposing lift.
Now I am confused.
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.
__________________
twistedtree is offline   Reply With Quote
Old 18-12-2012, 21:34   #39
Registered User

Join Date: Oct 2010
Location: michigan
Boat: CORBIN 39
Posts: 301
Re: Permanent Magnet Generator on Propshaft

Quote:
Originally Posted by Paul Elliott View Post
I've got no practical advice regarding the PM alternator, but people have indeed been able to extract a useful amount of energy from a free-wheeling prop.

The typical issues are wear on the transmission, and the additional drag of the spinning prop (it's more than a locked prop). I recall once seeing something that Steve Dashew wrote about an auxiliary prop set up for power generation, and I know that others have used their existing props.

a couple of things here. my ZF transmission states: if you lock the transmission, lock it in reverse. but, we recommend letting it freewheel in neutral. I tried it both ways and the boat is quicker in the freewheeling state. now; my transmission is a V-drive, I have an extended shaft with a 14 " v-groove pulley running a small 25 amp altenator, I can turn this on or off at will. works fine guys!
__________________
sailr69 is offline   Reply With Quote
Old 18-12-2012, 21:41   #40
Registered User

Join Date: Oct 2010
Location: michigan
Boat: CORBIN 39
Posts: 301
Re: Permanent Magnet generator on propshaft

Quote:
Originally Posted by twistedtree View Post
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.
I think we are getting all wrapped up in our underware here, there are tow behind generators commercially. when there is good wind, it will not slow down a boat at all unless you are trying to cook on your electric stove.
__________________
sailr69 is offline   Reply With Quote
Old 19-12-2012, 01:54   #41
CF Adviser
 
Pelagic's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Boat: Van Helleman Schooner 65ft StarGazer
Posts: 6,892
Re: Permanent Magnet Generator on Propshaft

Quote:
Originally Posted by David M View Post
Another issue is the noise of a free wheeling prop. If it does not bother you then great.
Just to clarify, my prop and shaft always freewheels when I sail.
Being inside a sealed oil bath tube for the first 5 feet before cutlass bearing and then 2 intermediate bearings before transmission, all I hear when I stand above it is a reassuring rumble.
Heavy stuff and overbuilt as is the usual Dutch way.

I can easily turn prop with one hand when diving and cleaning it but I once
tried using a 4x4 piece of lumber to try and stop it (Out of curiosity)
Just burnt the wood!
__________________
Pelagic is online now   Reply With Quote
Old 19-12-2012, 02:40   #42
CF Adviser
 
Pelagic's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Boat: Van Helleman Schooner 65ft StarGazer
Posts: 6,892
Re: Permanent Magnet Generator on Propshaft

Wow.. Thanks guys for all your input!
Interesting stuff.
Lots of things to consider and I would be lying if I said I was not a bit confused!

The catalyst for this is to replace the power needed to run a 1hp R/O unit under sail only, without running my big AC Gen which is mounted to the side and heels under sail.

That means either:
Making sure my Victron 3000W Invertor can easily power an AC 1hp under start up conditions. Or, ..Switch to a DC 1 HP motor for water maker.

Either way my 24v House bank system needs lots of DC Generation to make water and when you add other large consumers underway like my Autopilot which spikes to 12amp DC on Rudder action,....
.....it becomes a challenge of seeing if I can come up with more green DC generation?

My shaft Gen, using a standard 24v automotive alternator never put out much.
I have room to put a much larger PMA than the 520 in the available bilge space, but issues like heat generated and torque are deciding issues as well as would that really provide enough Green DC generation for all that bother?

Idea of combining Solar and Wind Hybrid system is more attractive because that does not need me to be underway.

But my schooner rig has a lot of shading and my aft boom overhangs the dingy davit where I could place solar panels away from rigging

I extended strong stainless Bimini as shown right out to sides, but again I think that might be too much shading for Solar panels? (I have no experience with Solar).... Opinions????

Also thought of mounting Wind Gen on Aft Mast, but I believe output size is limited for practical installation limits..... Opinions????


My decision to choose a big, heavy comfortable home over speed and efficiency did come with trade-offs.

Reality is that I economically motor sail at 1000 to 1200 engine rpm most of the time if wind is under 15 knots. That gives me about 7 knots speed

Maybe I need to admit that sufficient DC generation will need to come from either my Main Engine or Diesel generator if I want to make water underway?
Attached Thumbnails
Click image for larger version

Name:	Final Stainless Bimini.jpg
Views:	105
Size:	45.0 KB
ID:	51434   Click image for larger version

Name:	SG Wind Gen em.jpg
Views:	96
Size:	108.9 KB
ID:	51435  

__________________
Pelagic is online now   Reply With Quote
Old 19-12-2012, 02:53   #43
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Permanent Magnet Generator on Propshaft

I cant see why a simple permanent magnet towed generator wouldnt do the same thing easier and possibly cheaper. Ive used them, they work "OK"

Dave
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 19-12-2012, 02:54   #44
Registered User

Join Date: Dec 2012
Posts: 650
Re: Permanent Magnet Generator on Propshaft

Quote:
Originally Posted by Pelagic View Post
Or, ..Switch to a DC 1 HP motor for water maker.
Given that you are trying to make as much use of wind and solar power as possible, replacing AC motors with DC motors is a good idea -- if you can reasonably do so and the price is right.

I would prefer everything on the boat to be DC, but I would still want an inverter for consumer appliances, even if I won't be using them myself.
__________________
mcarling is offline   Reply With Quote
Old 19-12-2012, 02:55   #45
CF Adviser
 
Pelagic's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Boat: Van Helleman Schooner 65ft StarGazer
Posts: 6,892
Re: Permanent Magnet Generator on Propshaft

Quote:
Originally Posted by goboatingnow View Post
I cant see why a simple permanent magnet towed generator wouldnt do the same thing easier and possibly cheaper. Ive used them, they work "OK"

Dave
Dave, how many amps were you getting at about 7 knots?
__________________

__________________
Pelagic is online now   Reply With Quote
Reply

Tags
generator

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 13:36.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.