Shaft & Tow Generators |
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It isn’t just a recent ‘Green Conversion' among cruising sailors that motivates them to turn the water passing by their hull into electricity…which as we all know really means turning it into things like ice cubes or potable water. Sailors have been belting alternators off their prop shafts and towing old outboard props connected to a modified alternator for perhaps five decades now. We too wanted to make some “free” electricity while passagemaking aboard WHOOSH. This is a discussion on how we looked at the choices. |
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Typical water generator set-up on the long-distance Australian cutter GEMINI. |
NB: This article's title uses the word 'generator' generically. Both alternators and generators have been used when harnessing a boat's motion through the water to make electricity. I'll standardize on the term 'alternator' when describing the generating hardware to make the writing simpler and because it's the most frequent choice for water-powered electrical generation. I'll use the term 'towing generator', simply because it's a commonly used term, even tho' the commercial systems are essentially alternators generating rectified DC current. |
Using the Main Engine's Prop Shaft | |
Whether the engine has a hydraulic or mechanical transmission, it’s possible to have the boat’s fixed-pitch propeller free-wheel. This is even possible with some of the self-feathering props now in use on cruising boats. Just thinking about that rotating shaft invites one to consider installing a pulley on it. Then it’s 'only' a matter – or so the thinking goes – of belting a remotely mounted alternator to the pulley, energizing the alternator field and piping lots of free DC to the battery bank(s) when sailing. Here are some things we considered when looking at this option: |
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A good example of this approach is the Belgium yacht we berthed next to while in Horta. A mid-40's aluminum cutter, these folks did a lot of long distance sailing, did not like to run their engine, and so relied on shaft power generation – using a really hefty DC generator – for their electricity. Ironically, they had gobs of power at sea but would immediately power down on arrival in a port since they had no installed AC battery charger, hoping to make their battery bank last for the duration of a stay. In Horta, they purchased a multi-voltage battery charger/inverter and began using their DC battery bank for both modest AC and all their DC systems. But for many years, their cruising lifestyle was supported primarily by shaft-driven power generation while in the Pacific, Caribbean and while crossing the Atlantic. |
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For more information: Sailing-related on-line discussion boards offer personal experiences and issues one must keep in mind if considering this idea, so 'Search' is your friend. A couple of references to get you started are: - http://forum.ssca.org/phpBB3/viewtopic.php?f=4&t=8415&hilit=shaft+generator at the SSCA Discussion Board is a useful start - http://ssca.org/phpBB3/viewtopic.php?f=6&t=5801 is a more in-depth discussion on both installed shaft-driven systems and also towing generators - http://www.sailnet.com/forums/miscellaneous/22098-propshaft-alternator.html offers some thoughts about the electrical side of the installation – which can get a bit complicated - and a sample wiring schematic. - a good picture of a pulley/generator installation can be found at Eric Forsyth's website and it gives you a feel for the kind of pulley ratio you'll need to consider: http://www.yachtfiona.com/hauloutnewsletter8.html |
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Our Conclusions: Pretty obviously, this option's most inviting aspect is that it's “built in” (and so no towing of things behind the boat). This may also be one of its biggest drawbacks, since this requires the installation of multiple mechanical bits where, on many boats, space and access is already challenging – or simply not available. Anecdotal reports I've seen suggest the electrical generation is mostly a function of boat speed through the water (assuming an efficient installation). So in rough terms, this seems to be – electrically – comparable to a towed generator in output. And that boils down to roughly 1 amp DC for every knot of boat speed above 4 kts. Below 4 kts, shaft-generated power production apparently falls off quickly to negligible amounts, just as with a towed generator. |
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Using a Tow Generator | |
While using the propeller shaft to generate electricity might be simpler insofar as 'using' the installed system, purchasing and installing a tow generator for use under sail is certainly the easier, simpler (and possibly cheaper) choice. And that probably sums things up: Mechanically and electrically matching a shaft generator with the rest of the boat's systems can be more complex to build and install; using a tow generator is simpler to purchase and install but requires more attention to technique in use. |
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Tow Generator Choices: There were three commercial products we considered and we didn't consider – though it may be worth doing so – a home-built system. One choice is a “Water-powered Generator”, built in the USA by Hamilton Ferris, which can be found at http://ferrispowerproducts.com/water-power.html . Hamilton Ferris offers a variety of 'alternative energy' products. Another choice, branded as an Aquair, is built by the UK company Ampair and can be found at http://www.ampair.com/ampair/waterpower.asp . Ampair produces a variety of wind, water and stream generators. And a third product is a pole-mounted combo water generator/wind generator known as a Duogen and built by Eclectic Energy Ltd. in the UK. It can be found at www.duogen.co.uk . The Ampair and Duogen products use a fixed-in-place and sealed alternator that rectifies its output into 12V DC power; the H-F unit is described as a generator. Both the H-F and Aquair tow generators drag a length of relatively tightly woven double-braid line, on the short shaft of which is mounted a fixed pitch prop. Their methods for attaching the tow line to the prop shaft and alternator differ and, because chafe at these ends can be a problem, considering their differences might prove worthwhile. The Duogen is a different kind of choice because it is pole-mounted. It is not unlike Ampair's UW water generator, which could be a simple-to-use alternative except that it lacks a mount. |
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For more information: For a general discussion on towed generators and also problem areas with the Hamilton-Ferris product, see SSCA thread http://www.ssca.org/phpBB3/viewtopic.php?f=6&t=8685 . http://ssca.org/phpBB3/viewtopic.php?f=6&t=5801 is a more in-depth discussion on both installed shaft-driven systems and also towing generators. |
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We chose the Aquair product. We had been cruising in Europe for five years, had seen a lot of Aquair units mounted on boats, and rarely heard of any problems in their use over extended periods of time. Because we purchased the Aquair unit while in the USA, we found their USA distributor, Marine Warehouse http://www.marinewarehouse.net , to be a convenient vendor and the price. With shipping its cost was just less than $1,000. |
Our Ampair unit's components after mounting. The connector used was one intended for use with battery powered outboards: large contact surfaces, an o-ring seal ,and assuring polarity between the battery bank and the generator. |
Installation: Installing a tow generator is quite simple. Mechanically, one fixes the generator to the aft end of the boat in a fashion that permits some amount of vertical and horizontal rotation so the generator can align itself with the tow line. Like almost every Aquair unit we've seen, we opted to secure its surrounding stainless ring with small lengths of line, in our case to the stern pulpit (top) and a mooring cleat (bottom). Electrically, one runs the 12V DC wires as instructed, perhaps via a connector as we did, which allows us to remove and stow the generator. We connected the positive lead to our 12V house bank's battery post and the negative lead to the front side of the shunt for our Link 2000R – the same point where the negative lead from our wind generator is mounted – so we can read the actual in-use output of both units using the Link. Because of the reports of chafe, we terminated both ends of the tow line around stainless thimbles and then shackled the line to the generator and prop shaft. |
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Using a Tow Generator: As I write this, we've yet to use our unit at sea...but I have talked with a number of users who've described their experiences. Here are a few anecdotes that I put some faith in, and in time we'll offer an update on our impressions from our own use. |
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The Napa Auto Parts 'replacement drain funnel', before surgery. |
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Trimmed and laced, the funnel's now ready for use. |
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Using the Aquair Tow Generator We've learned several things while first using our tow generator, and we think they are worth passing on. In the order we've learned them, they are: 1. The concept itself seems sound and is about as dead simple as one could wish. Once off soundings, one deploys the shaft/prop unit (carefully – that line goes out FAST at 6 kts!) and then needs do nothing further. Amps go into the batteries as a function of boat speed, and there's rarely a need to retrieve the shaft/prop until the destination is approached. (Once in the ITCZ we pulled it in when we were becalmed and current led it near the rudder. Very rare event). If the goal is to have the batteries charged at a low rate for an unending period of time, this is a very easy option. And since we average 5-6 kts, that's over 100 amp/hrs/day – for us quite possibly 150 amp/hrs/day – that is available for running the radar at night, making water with offshore water quality, feeding the SSB radio, and keeping the iPod charged. 2. The 'retrieval' process required a learning curve, as the first two slit, laced up funnels (to smother the shaft/prop and kill the rotation) were very difficult to easily assemble around the spinning line. Our mistake was trying high-quality funnels made with thicker, more rigid plastic. We finally found an el cheapo funnel in Colon, Panama – at 9” diameter, it doesn't even totally cloak the prop's 10” - and that has worked well. (Why Ampair does not provide such a funnel as an optional purchase item is a mystery; what a profit margin they'd have). The one snag with this approach is when sailing downwind in a good breeze, where the light weight funnel's wide mouth catches the wind and comes BACK up the spinning line rather than sliding down it. Change course for a brief moment and down it goes. 3. Ampair's support, when we had our one bad experience with the unit, has been excellent. They ended up shipping 3 sets of replacement rectifiers to us free of charge (first to the Galapagos and then French Polynesia), as that's what it took for one set to reach WHOOSH. Why new rectifiers? A combination of a dull-thinking sailor who mistakenly unplugged the power cord before retrieving the shaft/prop and Ampair assuming all boats will install these units hard-wired to the battery bank. I doubt most sailors who have experienced the Caribbean would like to hang a $1,000 piece of equipment off their boats without any way to stow it away, and with only wire cutters needed to 'liberate' the generator...which is why we wanted to be able to plug it in (or unplug and stow it). Consequently, there's no mention in the brief owner's manual to the effect that you PLUG IN FIRST, DEPLOY SECOND, RETRIEVE THIRD and UNPLUG LAST. Unplugging before retrieving just once caused one of the rectifiers to blow; thus, the need for a replacement pair. So here's what I'd suggest based on that experience, if you too want to make the unit removable. In addition to remembering the sequence above, make sure the connector/receptacle hardware you choose 'locks' the connector in place. This will make bumping and unintentionally unseating it unlikely. (The connector we're using does do this). And order a spare set of rectifiers to carry with you. You too may one day become sleep deprived and fuzzy headed, and unplug unintentionally. Final Note: On our last overnight run, from Moorea to Hauhine, we were seeing a steady 10-12 amps going into the house bank with the tow generator and mast-head mounted wind generator whirling away. That means we were feeding the ship's electrical needs while concurrently recharging the battery bank that had lost some of its charge while we lazed away in Moorea. Not bad for 15-18 kts of wind and 6 knots of boat speed. © Jack Tyler – July, 2010 WHOOSH, currently cruising the Leeward Society Is., French Polynesia |
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© Jack Tyler – December, 2009 WHOOSH, currently cruising along Florida's East Coast |