10-20a alternator for sailing prop-power

[Al - s/v Persephone]

Since I have a 3-blade prop, which (unlike a 2-blade prop) offers the least drag when free-wheeling, and since my Yanmar transmission's cone clutch will stand it, I have a spinning prop while sailing.(No postings about prop drag theories, please. I have personally measured the drag differences, confirming the USN studies, and Yanmar says OK on my transmission.) The possibility of supplying my approx 5 amps of instruments/radar power from a pulley-plus-generator system on the prop shaft has led me to look for something that will only increase drag to the degree necessary for that load plus a small margin.Obviously, running a 60a or 80a alternator is power overkill and would add a lot more drag than a minimally sized alternator.However, searching for any self-contained 13.8v alternator that only puts out 10-20a has been an exercise in frustration.The best pricing (and most well-tested designs) are automotive alternators. However, they are much too high-capacity - even for something like a Geo Metro. The 60a on my Yanmar seems to be their lowest capacity for marine diesels.Lawnmower engines and outboards that have electric starting seem to have the right capacities, but are usually built-in alternators. I need a standalone.Bicycle alternators are too low power.Has anyone successfully tackled this problem? Are there any motorcycle engine alternators that would be a workable possibility (the ones I've seen are built-in, like on lawnmowers and outboards)?Fair winds,Al

 

[Ed Schenck]

Old Hitachi?

My old Yanmar 2QM20 had a 35amp Hitachi alternator as standard equipment. Would that work for you? Should be able to find one from a dealer on a trade up. Try an E-Bay search for "alternator xx" where xx is the amperage. Might give you some more ideas.

 

[Ross]

Al if you have no load on an alternator it doesn't matter what its

load capacity is as far as the power it takes to turn it. So you can use a 35 amp alternator and limit the output and thus limit the torque needed to turn it. Fifty amps will take about one horse power but five amps will demand a tenth of that. Will you have a clutch included in this to disconnect when motoring? Have you checked the shaft speed while sailing? You may find that the slow speed of the shaft while sailing limits the output of the alternator and further as the output increases the alternator will act as a brake on the shaft.

 

[ruedi ross]

how about looking for a wind turbine alternator?

they come in all sizes and are more effecient at slower speeds than an automotive version. There are even a few plans for homemade ones. There is also a generator made to tow behind boats, see the link.

 

[Stu Jackson]

It's the output at a certain rpm of the alternator

pulley, Al, so the description of what you are looking for needs to be addressed.Your concept has great merit and was written up years ago (early 1970s) by Don Street in his "Seawise" and "Ocean Cruising Yacht Design" books.I suggest you do some more research online about the concept which you describe well before you get into the hardware, because I'm sure people are doing it and have already figured out the equipment. Also try some cruising websites, since most of us are coastal cruisers, and this application is more suited (in general, but I hear ya) to long distance cruisers, or at least has been employed by them more often.You do not need a 13.8V output alternator that only puts out 10-20 amps. You need an alternator that's "belted" to your shaft in such a manner as to put out 10 - 20 amps. Subtle, but VERY different. That's why you haven't been able to find one.

 

[ruedi ross]

Another idea,

you can use a surplus DC permanent magnet motor as a generator. There are lots of articles on the web describing this. Google "permanent magnet dc generator" and there are several home made applications from using a bicycle to power your TV set to building wind generators. Surplus motors can be found very cheap.

 

[Ross]

This link may be helpful.

http://www.autoshop101.com/trainmodules/alternator/alt135.html

 

[Paul Akers]

Why Bother?

Al, you will have to run your engine to pull into port (probably). So if you do, you will most likely replace the lost amps that you used while you are motoring to your home destination. Or, just leave it running for a few more minutes once you arrive. Rather than going thru the expense of your alternative solution.

 

[Joseph Shirley]

Marine shaft alternators are available

and the literature on the max prop specifically addresses the subject. If you run the prop in reverse while your boat is going forward and disengage the shaft, the prop will remain locked and will not feather: Ideally intended to turn a shaft alternator. I don't know any more about it than that, so good luck.

 

[Al - s/v Persephone]

Thanks, guys.

I appreciate all your responses.It looks like I'll have to keep digging for what I want.As Stu pointed out, for constant shaft rotation speed I could use any alternator whose power-vs-rpm curve had my desired point in it. However, I want at least a 2-to-1 rpm range, so in order to avoid complex control circuits I'd prefer to be up on the flat part of the curve, not down towards its origin. As Ross pointed out, keeping the load low will minimize drag. However, with a low capacity limit, I don't have to have any circuitry manage that problem. Another factor is the higher low-/no-power minimum torque of a bigger capacity alternator.As Ross mentioned, I want to disconnect the alternator, but my solution was to be just to switch off the field excitation. Simple is best, and mechanical clutches and linkages are not what I want to get into.(I plan a simple two-wheel pulley-and-belt arrangement that fits my available space. Hence I need to keep in the size-ratio range of no more than 2.62 [gear ratio] times the size-ratio of the pulley pair on the front of my 3YM30.)Hence my desire for an alternator with much lower max power rating than typical automotive (or even marine diesel) alternators. So far that has meant built-in ones, rather than mount-on ones.I'll look into the wind generator idea, since that British one with a towed propeller option originally got me thinking about my present concept.I did look into Ruedi's bicycle generator idea, but found mainly projects to make DC generators and I really want the lower rpm range of an alternator and the simple, proven (store-bought) control and regulation circuits.If worse comes to worst (no or only expensive alternator options) I'll have to try the smallest auto alternator I can find and develop the appropriate control loop circuitry. Thanks again, for all your inputs. You guys are as usual a great source.Fair winds,Al

 

[Ross]

Al I noted it the alternators 101 link that you can get

alternators connected either delta or wye. With the low speed advantage going to the delta windings for current and the advantage to the wye windings for low speed voltage output. I think that I would set this whole thing up on the bench with a variable speed motor to drive the system and see how it works. A battery, a light or three for a load and a handful of meters and you can learn all manner of good stuff.