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Shaft & Tow Generators

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.

Typical water generator set-up on the long-distance Australian cutter GEMINI.
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:

  •  Noise:  How noisy is your freewheeling prop shaft?  WHOOSH has a V-drive arrangement in order to keep the engine out of the bilge but also far enough aft to allow for its aft cabin/aft cockpit layout.  This mechanical cast of characters - transmission to jack shaft to V-drive to prop shaft' - is not quiet and gets downright noticeable above 5 knots when the prop free-wheels.  It's also located a few feet from the off-watch berth in the aft cabin.  Living with our freewheeling shaft at sea, which we had to do for a bit while getting acquainted with our new MaxProp, didn't appeal.  In addition, some transmission manufacturers warn against unlimited freewheeling (although this turns out to be a hotly debated issue) so check out your transmissions limitations, if any.

  • Space:  What physical limits does your shaft alley present?  The shaft pulley-to-alternator pulley ratio will govern the rpm range the alternator will turn, thereby determining a given alternator's output.  At the low shaft rpm speeds one can expect, that suggests the shaft alley will need to accommodate a relatively large shaft pulley so the alternator can be productive.  In addition, one may want to consider installing an external regulator for the alternator to improve output.  And then maybe  an 'off/on' switch for the alternator's excitation...and come to think of it, perhaps a breaker or fuse as well.  (There is no end to such things when tinkering, it seems).  Just how much room does your boat have in this area?

  • Sturdy Set-up:  The alternator mounting will need to be fairly robust as we're talking about significant forces, although belt tension may be mitigated somewhat by using a toothed or notched belt if the pulley will accommodate it.  Regardless, a belt-tensioning method needs to exist as well, just as with a regular engine-mounted alternator.  And one might want to remember putting a set of spare belts around the shaft, pulled aside until they are needed, so a broken belt doesn't require backing out the shaft to put a fresh belt on the shaft pulley.

  • Cost:  Unless one is an excellent scrounger, the shaft pulley and the alternator mount are going to require some custom metal fabrication.  Add in the costs of an alternator, the belts, the electrical bits and perhaps a regulator, and the “free” nature of the electricity will start to get expensive.

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.

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:

- at the SSCA Discussion Board is a useful start

- is a more in-depth discussion on both installed shaft-driven systems and also towing generators

- 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:

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.

 Even assuming the crew could tolerate the noise, WHOOSH does not offer the space in its shaft alley and engine box to make the mounting of a sizable shaft pulley possible.  Therefore, our search turned to:

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.

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 .  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 . 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 .  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.

For more information:  For a general discussion on towed generators and also problem areas with the Hamilton-Ferris product, see SSCA thread . is a more in-depth discussion on both installed shaft-driven systems and also towing generators.

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 , 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.


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.

  • 'Sharks Ate My Prop!”:  Good friend Ross on GEMINI researched the 'fish eat props' dock talk extensively before he purchased an Aquair and couldn't find a single first-hand report where the sea monster (and that's about what it would take) was actually seen to chomp the heavy 3' metal shaft and weighted prop.  However, there are many accounts of chafe on the line where it's terminated to the generator and the shaft.  And beyond this, let's not forget that nothing prevents the crew from deploying the tow line in waters that are still relatively shallow or offer  coral heads and uneven seabeds to snare the shaft/prop unit.  My conclusion is that, no matter what you choose to believe about the 'Fish Ate Prop' debate, you should routinely inspect the towing line terminations for chafe – perhaps even if it means pulling in the line while on a long passage -  until you can anticipate just how much chafe you are experiencing.

  • Deploying the prop shaft and line:  The same basic guidelines exist as for deploying a drogue off the stern.  Insure the towing line is flaked out carefully, insure there are no obstructions or loose things that can be snared as the  line runs out, and mind your fingers and hands as you deploy the shaft/prop unit.

  • Retrieving the prop shaft and line:  There are two interesting issues related to this topic.  The first is that there seems to be a variety of opinions on how to get the shaft prop & line back aboard – which suggests to me there's no one obvious, easy option.  Heaving the vessel to (reducing speed through the water), sending a funnel down the tow line to interrupt the water flow past the prop, and clipping a fender onto the tow line and letting it race down the line to both interrupt some water flow and bring the shaft/prop unit closer to the surface are all mentioned.  The universal observation – offered by owners – is that retrieving the unit takes a well-proven technique as otherwise the spinning line and shaft/prop drag simply defeat any attempts to bring it aboard by hand.

  • Using a Funnel:  This is commonly heard advice, which leaves me to wonder why the Ampair/Aquair folks don't offer one they know will work with their unit.  (The H-F folks do include what they believe is a suitable funnel with their unit).  I shopped funnels at a commercial marine supplier – the light-gauge, small size funnels I found at Ace Hardware, West Marine and Home Depot didn't strike me as up to the task – and found a 12” funnel was going to cost $60-70 USD, depending on the one selected.  W-a-y too pricey, it seemed to me.  In the end, we purchased a large, shallow funnel sold by NAPA Auto Parts (Oil Pan Drain replacement funnel, Item #DK 015 6002, price ~ $25 USD) because it was more than big enough, the plastic was very heavy gauge, and we could cut it down to what seemed like a workable size.  Of course, one has to be able to add the funnel to the tow line while the line is spinning busily.  Consequently, one cuts a slot cut into it and uses small stuff to lace the two sides back together after it's been placed around the line.  If all this sounds pretty fiddly, that's what I think as well.  We'll see how it works in practice.  (We're also hoping to trial the use of a small fender, clipped onto the line using a carbiner).

The Napa Auto Parts 'replacement drain funnel', before surgery.
  • So this is where we ended up after looking at the various water generation options.  But before closing, let me just mention two final observations.  The first is not to underestimate the solid, heavy nature of these systems.  Today much yacht gear we handle is made fundamentally of plastic (winch handles, helm autopilots, parts of all electronics) or has been put on a weight-reducing diet.  One of my initial surprises was just how beefy our generator is and how strong and heavy the tow line/prop shaft combination is.  You need to keep this in mind when thinking of mounting and retrieving these units.  The second observation, and a pleasing one to experience from users, is how they often view these units with a fondness not unlike that assigned to wind vanes and autopilots.  In part it's no doubt because one has almost unlimited power offshore, which is when most boats' energy budgets are highest.  But my sense is that it's also because these are usually so problem free.  No tinkering or spares needed, normally unobtrusive while in use, and only some monitoring of line chafe is required.  I wouldn't be surprised if some of these end up with names assigned by the crew.

Trimmed and laced, the funnel's now ready for use.


What We've Learned So Far...


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

© Jack Tyler – December, 2009

WHOOSH, currently cruising along Florida's East Coast