The Offshore Voyaging Reference Site

Hydro Power

So far we’ve looked at how to deliver supplementary power when at rest, a difficult enough challenge with today’s complicated boats, but one which pales into insignificance when underway 24 hours a day, when many other ‘hungry’ devices are factored in – the autopilot, navigation lights, instruments, etc.

This was our stumbling block, as whilst we’ve always been able to meet our daily needs with wind and solar when at anchor, we’ve struggled when passage making, and had, on occasion, to run the engine for a couple of hours a day to generate electricity. And this despite the fact that we use our windvane over our autopilot, use only an LED navigation light, and shut down our navigation system to the bare essentials when offshore. Solar can only do so much, and wind generators are at their least effective downwind – and who likes to go upwind?

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Dick Stevenson

We wintered over in Barcelona in 2010-11 and a couple of docks away the crews for the around the world double handed were preparing their boats. We spent months walking past the boats (and talking with the crews) and I believe that most had a Watts & Sea generator. I believe most were happy with its functioning after the race. It was certainly a slick design looking well thought through and executed.
Dick Stevenson, s/v Alchemy

Dave Benjamin

I think over time that units like the Watt & Sea will come down in price. Our Amel originally was outfitted with a shaft alternator. Many of our fellow Amel owners still use them. Personally I don’t like putting a speed brake on the boat. Drag from a fixed prop is bad enough without letting it spin.

As for a towed generator, I have concerns about a curious shark nailing it. I’ve never used one though so it would be great to hear some real world experiences.

To those celebrating the holiday, Jean and I wish you a Happy Thanksgiving.

John Harries

Hi Dave,

I’m with you on the drag issue. I guess it is my racing background, but I just can’t stand sailing with the drag of a fixed prop. The boat just feels sluggish and it makes me crazy(er) in very short order.


Hey Dave,

I can understand someone saying he does not stand the drag from a fixed prop.
BUT the difference between draging a fixed prop blocked or spinning is minimal.
So if you choose for a fixed prop because they are so much cheaper or in order to reduce the risk of breaking down to the minimum there is no reasonnot to have an alernator on the shaft.

Dave Benjamin


As a former airline pilot who spent his formative years flying various propeller driven craft, I can tell you that a spinning prop, particularly one that is extracting energy from the environment creates far more drag and performance penalty than a stationary prop when the engine is not producing power. Air is a liquid, albeit one with a fraction of the density of water. It all boils down to physics and hydrodynamics along with the principle of noting comes for free.


Hey Dave,

Thank you for your response.
I will not assert anything about aircraft propellers in the air (the speeds are so different) but concerning three blades propellers in water I cannot agree with you…

In the past, there have been two university studies (one by MIT in 1994) done.
Both studies agreed on the fact that a spinning/ freewheeling propeller drags less than a locked on.
The only thing you could argue against those conclusions is to say that the test were done in a test bassin.
But later on (but I cannot find it) there was a guy on Sailnet who did an emperic test. His conclusions were identical.

Moreover what we say is that the difference in speed on a 12,8 T cruising boat even at an average speed of 6.5 or 7 knot is is unsignificant.
We have tested and measured that on several occasions.

I’m afraid this has been a “misinterpretation” or “wrong rumour” for a long time…

PS; As always, this are our measurements, our conclusions of our hydrodynmaic engineer and naval architect.


Dave Benjamin


Certainly there is a distinction between a freewheeling propeller and one extracting power to turn an alternator, is there not? I am not a naval architect or a hydrodynamicist, but I took physics at university and don’t comprehend how this energy exchange can take place without additional drag being created.

I will grant you that the difference in drag between a freewheeling propeller and static one may not be significant but as soon as the shaft alternator is engaged, there is inevitably a decrease in speed. We’ve had several discussions on this topic in the Amel group on Yahoo, and if I’m not mistaken, the consensus was about a half knot reduction in speed when the shaft alternator is engaged.

John Harries

Hi Dave and Jean-Francois,

On the prop drag, spinning or stopped question, we looked at that at length in the comments to this post.

The conclusion being, backed up by my own experience as a PHRF handicapper, is that it is simply, not that simple! 🙂 It depends.

Having said that. I would expect that the drag of a spinning prop would, as Dave says, be a bit more if one is taking power out of the water.

John Harries

Hi Jean-François,

That makes sense. I think that a fixed prop and a shaft generator are perfectly sensible solutions.

My position against them is purely a personal preference and a hang over from my racing days. I can’t stand having any scum or weed on the bottom either, a prejudice that costs us a lot of money in paint and more frequent haul outs!

Dick Stevenson

Dear Colin,
There are readers who do a great deal of passage making, but, when it comes to towed generators, I would suggest skippers take a cool un-romantic look at his/her cruising habits and where you get the most bang for your charging buck (euro, pound etc.). Longer offshore passages in waters with dependable winds and with a minimum of debris are where these units will shine: hence the offshore races being proving grounds.
Alchemy, for example, wandered the British Isles for 7 months last season covering 2400 miles (1700 under sail). We had 75 stops and did 4 trips of greater than 24 hours, none greater than 40. The previous season was similar with one longer trip of 4 days. Generally leaving fully charged (engine use or shore power), we are good for 24-36 hrs. before the batteries need charging. Our seasons in the Med and in the Caribbean were similar. So, assuming one of the passages to have fluky winds and we off and on motor, we have at most had 3-4 occasions a season where a towed generator might have been a use to us. At most, how many amps would that be per season? (I would not use a towed generator for day hops because I am likely fully charged and the chance of damage with close to shore debris etc.)
I would suggest that, for most of us, even those who wander widely, longer offshore passages are occasional events and our charging systems are best designed to provide power at anchor and at sea.
My best to all, Dick Stevenson, s/v Alchemy

John Harries

Hi Dick,

Colin is at sea, so I will pinch hit on the comments to this post.

I think you are absolutely right that for many cruisers with usage patterns like yours that the complication and expense of a hydro- generator would simply not be worth it.

In our own case, on our 10,000 mile cruise to the arctic and back last year we would have only used a hydro-generator for about 5 days and it would have only saved about 7 hours of generator run time over eight months—hardly a great return.

On the other hand, if I were contemplating a long trade winds passage, as Colin is, I think we would look at adding a Watt and Sea, rather than a towed generator, because our power requirements are larger than Colin’s because MC is a bit big for a vane gear.

The key take away here, as you point out so well, is that we voyagers should not just add gear to our boats because a particular gadget is the latest thing, but only after long and detailed thought about our own use patterns.

Colin’s posts in this series have done a great job giving us the real world information to make those acquisition decisions from a well informed position and your comments have provided a really useful second scenario, thanks.


Interesting topic. I think a lot of this depends on three factors. Your designated cruising grounds, length of passages and average boat speed.

If you look at the numbers for all these systems for an average rate of travel of 6 knots, the most I can do on my boat, then the outlay in cash does not warrant in the power input capacity..:) So a quick glance suggests that these systems are designed for vessels over 35 ft that can maintain an average cruising speed of 7 knotts or more. Thats when the real power curve starts to kick in. For those of us with vessels under that length, all of these system do not appear optimal and other solutions offer more value in terms of costs and max output. It really comes down to cost/amp and reliability.


Robert Sapp

Would there be a way to design a transom-mounted hydrogenerator that could incorporate windvane-driven self-steering and could also serve as a backup emergency rudder? Such a device may offer decent value for the price, as it would be three essential cruising systems in one.

John Harries

Hi Robert,

I guess it might be theoretically possible, however, in my experience, the track-record of devices that try to do too many different things at once has not been good.


Dear Colin,

There is in France a new brand of hydrogenerators on the market : Cristec. It very similar to Watt & Sea.
It has been tested during the last Vendée Globe on the boat of the winner Michel Desjoyaux…
Because of competition prices might go down.

On Boréal we will have very soon feedback on hwo it works with the Featherstream from darglow as we are installing two in the spring.

Enjoy your crossing.


C. Dan

I have read that for hybrid-electric engine installations (or full electric drive) the shaft-driven option becomes much more sensible.

While I am sure there is still some noise generated by the prop/shaft turning, I bet it is nothing compared to running the engine or a generator!

RDE (Richard Elder)

If you’ve been following the Globe you’ve noticed that water generators have been the single most troublesome piece of kit onboard. Bernard Stamm has been particularly snake-bit by the problem.

After looking at this mounting bracket would you consider it adequate for a Hunter intended for island hopping in the Caribbean to say nothing of a new multi-million dollar Global single hand race boat built from 100% carbon and unobtanium?

Any boat is only as good as its weakest link, and no fancy architect’s brand name is a substitute for common sense.


Hey Richard,
I had the chance to be on the pontoon of the Vendée the day before the start and when they left…
I took pictures of all mechanism.
As you can see on the picture of the link,Bernard Stamm has his own system to lift vertically the Watt & see unit out of water. So, as lot of others did, they have their own home-made system. It is nothing standard.
(I believe) The main reason is that with the standard mechanism it is difficult to swivel the generator out of water at high speed (the forces are then too big on the bracket).
On the Transquadra (signle or double handed transatlantic race on standard cruiser/racer) seem to have had the same problem… (sailing downwind under spi at high speed and having to raise the generator out of water because there were producing to much…)
BUT for standard cruising ships the bracket as you see here on the picture seems strong enough… I believe they work without any problems on a lot of boats. (and when you hit something they swivel which is not the case on Stamm’s boat)

I hope this helps to make up your mind.

Pete Rasmussen

Hi John, interesting discussion. I’m wondering if the Boreal alternator will also give current being driven by the engine? the Boreal is interesting as it would be the least problematic device to run, being below. If it lives up to the graphed output I think the best choice if a bit of speed loss isn’t too big an issue, probably only a problem in light air anyway.


Hi Pete,
For the moment the alternator on the shaft is not adding up its current with that generated by the alternator of the running engine. Is is indeed ashame… We have not found (yet ???) the solution to do so…

I’m just back from two weeks sailing in the islands of Cabo Verde.
During those two weeks we have sailed from one island to the other and except for entering and getting out of moorings we have not used the engine at all. Combined with our windgenerator (Superwind) we have never had any energy issues. Certainly not when sailing where we had more then enough power (running the fridge “full trottle”)…
BUT you are right to say the problem is in light airs. Especially downwind… (Windwards the real wind added to the boat speed makes the apparent wind quickly important enough to generate power trhough the windgenerator)…

John Harries

Hi Jean-François,

Now there is a very good point I had never thought of: Use the shaft generator when sailing on a broad reach or downwind when the drag is not so important, and use the wind generator when sailing upwind when it is more efficient and prop drag hurts a lot. Add a feathering prop, as Colin suggests above, and maybe you have the perfect system?

RDE (Richard Elder)

Hi Jean-Francois,
Thanks for the on-site information. I’ve only had a chance to look at details for the Watt & Sea mounting on the photos of Bernard Stamm’s boat. Are other boats using some type of kick up mount with rope fuse weak link like on the rudders? Would this type of mount solve the problem if it was designed into the boats from the start? Certainly need to prioritize protecting the rudder drag link from the results of a kick-up so as to not repeat Alex Thompson’s adventure in mid-ocean repair! Seems like if the generator had a shaft brake incorporated it would create enough drag to self-pivot most of the way out of the water when you wanted to retract it if that is a problem.

Mike Golding just added his name to the list of hydro generator failures—in his case a burned out controller, no spare on board, and questionable fuel reserves to make the rest of the voyage.

re shaft generators: I spent some time on a ULDB with an electric drive. It had a relatively large three blade prop connected to a motor/generator with a DC diesel generator as the hydrocarbon power source when motoring and at anchor. When under sail you either made electricity when you had enough wind or used electricity to supplement light wind areas, all controlled by a simple joy stick that worked like an automatic transmission. In “neutral” you had no prop drag and only miniscule power consumption. People were always amazed when you “sailed” off an anchor with only the wing mast up—! The system was far from perfect, but with Li batteries and a large solar array it certainly has promise. Certainly the ultimate expression of a shaft drive generator!

C Dan

As someone who hates the smell and noise of combustion engines, I am thrilled to see such optimistic comments regarding electric drive from RDE.

RDE (Richard Elder)

Hi C Dan,
We have built our entire technological civilization around the existence of cheap, energy-dense fossil fuels. What that means is that given today’s level of technology development you have the choice of the smell of burning diesel or burning money.

In order for the particular boat that I described to deliver a level of performance and convenience equal to a conventional diesel engine it would need a motor/generator twice as powerful, a $10,000 Li battery pack, $5,000 in solar panels & mounting, two high performance wind generators, and an extra 2kw of generator size. If it were a heavier conventional 44′ design those numbers would be even more unfavorable.

C. Dan

Well, the good news is that the electronics industry is focused on bringing down the cost of Li Ion batteries and is spending a lot of time and money on new technologies aimed at solving this problem.

Also, the cost per installed watt of solar photo-voltaic energy systems is falling dramatically… close to $1/watt now versus over $3 just a couple years ago. And although the cost of installation is higher in a “marine environment”, the savings are MUCH higher when compared to people plugged into the grid (or shore power in a marina).

In terms of our civilizations’ technological inertia, I will just say that going from 1% alt energy to 3% in just a couple of years (the path we’re on) is actually quite impressive, and the expansion of the manufacturing base to allow it is resulting in ever-lower prices.

I would not be surprised if, by the time the A-40 prototype is done, the cost of an electric-only system is 30-50% less than today. Call me an optimist!

John Harries

Hi All,

Since we are on the subject of electric drive, I can strongly recommend Matt Marsh’s very clear summation of the types of electric drive, the state of the technological, and the practicality of its application in various scenarios.

For those that don’t know him, Matt is an engineer who has worked on the bleeding edge of this technology.


good article


Hi, I can partially comment on the Darglow feathering prop locked in reverse + Boreal shaft alternator. My main concern was that a free-wheeling prop with the blades at their reverse angle wouldn’t rotate nearly as well as a standard (or folding but blades at forward angle) prop with the boat in forward motion. I still think, intuitively, that the efficiency is lesser, and I don’t have a numerical comparison (hard to do unless you can sail two boats with different props side by side at identical speeds, and Darglow didn’t have the answer for me), but…as I was just able to test again over the past 3 dowwind days from Mustique to Curacao, with the wind generator turned off, the Darglow feathering prop + shaft alternator, past a 7 knots speed threshold, generates enough energy to cover fridge/freezer (SMALL freezer, nothing like yours), a fairly hard-working NKE autopilot, VHF and Vesper Watchmate AIS. Add chartplotter, nav and or interior lights (even LED), and the equation turns slightly negative again up to 8.5 kts average speed. In my limited experience so far, the 7 kts threshold is key though: below it, and the prop just doesn’t rotate enough to come close to competing with the wind generator (or in this region and by day, solar panels).

John Harries

Hi Matt,

Great information, thank you very much.


Aboard Adjo Akama we have what I believe was a tape drive motor from an old mainframe computer that is powered by a small outboard prop (from about a 2hp outboard).

The prop is fitted on a 1.5m long stainless shaft with 10m of stiff double braidline to the boat. Just in front of the prop is a small lead weighted delta shaped paravane on a 150mm deep strut that is free to rotate on the shaft. This holds the shaft and prop well down into the water and we’ve never had problems with skip despite the short line.

The motor/dynamo itself is mounted low on the transom (actually on the boarding platform) in a watertight housing and simple mount that allows for horizontal and vertical alignment. The braidline has a shackle that fits (quite securely) onto a hook on the motor for rapid removal when retrieving.
This setup generates 15 amps at 7 knots.

With the relatively short line retrieval is fairly easy but it’s good to be fast. Slow down if practical, grab and unhook in one go and pay out the loose end while hauling in on the prop.

I’ve thought about having a funnel system to reduce drag and spinning for retrieval but haven’t really found it necessary.

No regulator fitted – I simply monitor the battery state and pull it in when I reckon they’ve had enough charge.

Simple and cheap – I built it 20 years ago and think it cost less than $100 then. I probably swapped and salvaged bits. I can’t remember the details. i remember getting the motor off an American cruiser who had a boat full of useful junk.

I’m not sure if tape drive motors are still available but I imagine that an old tractor dynamo would do the job and they are available (new) for about $100 on ebay. I expect it would be necessary to fit a thrust bearing in the housing to take the drag load.

Drag isn’t an issue with such a small prop and short line. In light winds (when it might be relevant) we don’t use it (we are often motoring anyway if unable to do more than 4 knots under sail) and once over 5 knots I doubt if the loss of speed is measurable.

John Harries

Hi Patrick,

Welcome to membership at AAC and thanks for really interesting comment that shows what ingenuity can do.


Human power? How do cruisers stay physically fit? Is there a case for an exercise machine on board – hooked up to a generator? Want a cold beer? Spend half an hour on the exercise bike to cool the beer fridge first. 🙂

Dick Stevenson

Paul, I was worried, needlessly it turns out, about weight gain when I moved aboard 12 years ago. I believe the last decade to have been the healthiest of my life. However, after a 13 day passage to Flores in the Azores, the climb (1-200 meters if that) to Paula’s bar was a real effort. A couple days later all was fine. My wife and I think that all the constant adjusting to being on a moving vessel, even at anchor, accounts for a lot of general fitness.
Dick Stevenson, s/v Alchemy


Collin or others!
I have built a hydro generator using cots parts and I am pleased with the results so far. How do I combine multiple sources of power i.e. solar, hydro and eventually maybe wind? If my controllers use the battery voltage as a reference for charging what can I do to keep the pwr sources inputting?


John Harries

Hi Bob,

As long as all of the input sources have regulators set to the same voltage they will all supply the battery with current (amperage) up to the maximum that they can supply, or the maximum that the battery can accept (depends on state of charge) whichever is higher.


Thanks John.
That is what I thought but it is nice to have confirmation.

Markus Moser

I’ve just been sailing around 1200 nm with a wattandsea cruising 600 on my 45 feet boat stern. It’s an absolute amazing experience how this thing is working. It covers above 7.5 knots all my electrical needs. In my opinion this is the future of gaining energy on long haul cruises. The mounting procedure is a little bit tricky but after that it’s just amazing.



I’d love the Watt’s and Sea shaft generator, but prohibitely expensive. Heard from fellow Swedish sailors that the Aquagen is no good, then read at your site that the Ampair might do the job. When seeing the revolving rope I get sceptical, but the main thing is to get lots of Amps from it, so function precedes estetics.

Anyway, our battery budget is 250 Ah consumption versus 150 Ah production without engine running. So the Ampair would be much welcome.

Regarding solar panels, we could meet our energy demands if we begin mounting them just about everywhere on our deck, but what about non-skid?! WIth the decreasing prices of solar panels – if somebody knows of a non-skid solution I wouldn’t mind filling the deck with flexible solar panels you can walk on.

Biggest problem with the towing Generator Ampair or similar, would be the fact that we have a Windpilot Pacific windvane mounted. Or will they not interfer with each other?

Our plan was fishing during the Las Palmas -> S:t Lucia Passage. Guess we have to bail out there (sadly!). Or run the towing generator during night, we won’t fish then when one person at the helm only.

Cheers and thanks for great site!

Tobias, member
Norlin 37


HI all,
We did complete our circumnavigation in July last year. We have been using a 500W Watt & Sea hydrogenerator since we left Patagonia in 2012. We sailed some 35000 NM with, and it did cover all our energy needs.
In 2014 we fitted an Hydrovane self steering system which share the transom without any issue. Both have done a perfect job together along more than 20000 NM.