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Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance

Last week we published a video interview I did with Ed, a cruiser who installed a Watt & Sea Hydrogenerator.

Now let’s analyze Ed’s experience, together with Andy’s earlier review, to come up with a way to decide if a Watt & Sea is right for each of us.

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More Articles From Online Book: Electrical Systems For Cruising Boats:

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  22. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  23. Efficient Generator-Based Electrical Systems For Yachts
  24. Battery Bank Size and Generator Run Time, A Case Study
  25. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  26. Eight Steps to Get Ready For Lithium Batteries
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Black Outs
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Batteries Buyer’s Guide—Part 1, BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Part 2, Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Part 3, Current (Amps) Requirements and Optimal Voltage
  33. Lithium Battery Buyer’s Guide—Part 4, Fusing
  34. 11 Steps To Better Lead Acid Battery Life
  35. How Hard Can We Charge Our Lead-Acid Batteries?
  36. How Lead Acid Batteries Get Wrecked and What To Do About It
  37. Equalizing Batteries, The Reality
  38. Renewable Power
  39. Wind Generators
  40. Solar Power
  41. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  42. Battery Monitors, Part 1—Which Type Is Right For You?
  43. Battery Monitors, Part 2—Recommended Unit
  44. Battery Monitors, Part 3—Calibration and Use
  45. Battery Containment—Part 1
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Matt Marsh

I think John’s conclusions here are generally correct.

On a first back-of-napkin estimate, not accounting for any nuances of turbine blade design, I’d guess you are looking at — for a 6 knot cruise speed — perhaps 100 N of drag from the little wheel and 150 N from the big one. (Those figures could be off by 20-30% in either direction but probably not by much more than that.)

This is a device that will work very well on a fast boat, one with sail thrust to spare, and that spends a lot of time underway. If you’re regularly pulling 7+ knots on passage, and do passages regularly, the W&S might make a lot of sense.

It’ll be marginal at best, and probably uneconomical, on a boat whose combination of waterline length and sail-carrying power leaves it with little thrust to spare. A boat that can’t consistently average 6+ knots should, I think, not fit one of these gadgets.

It’ll be pretty much useless on a boat that spends most of its time anchored or motoring.

A two-year factory service interval is worrying, for a technology whose fundamentals have been well understood for over 100 years and whose large-scale commercial relatives normally go several decades between overhauls. The weak mount is also worrying. Are complete service & overhaul manuals, and parts, available to customers on fair terms? If so, that’d go a fair way to increasing confidence and acceptance.

Whitall Stokes

I concur with all of the above. After 12,000 miles with a W&S on an Open 50, I would say they are for a pretty specialized use case: fast boats on long passages. The unit was part of an energy generation mix of solar, alternator, and W&S.

As my speed varied I would need to adjust the prop or pull it out. The bracket I used was the same as Ed’s, and it indeed bent. The bracket eventually failed off Brazil when the leg struck something, and it was trailing by the generator wiring before I pulled it back aboard.

Talking to Randall Reeves he had trouble with vibration and had to tighten the bolts holding the blade to the bracket. I only had to do this once, so I assume his prop was not quite balanced but I’m guessing. His transom design also made access to his unit more difficult than mine.

My understanding is 10,000 mile maintenance requires shipping the entire blade unit to France. They are large. I’m not aware of user-serviceable parts for the generator unit. Perhaps Bruce Schwab can chime in.

Programming the charge controller for LiFePO4 was awkward, requiring a laptop with a small windows program and a mini-usb (who uses those?) cable.

Attempting to adjust the prop (yes I was hanging off the stern), I lost a critical bolt overboard to pull the prop off. Luckily I ran across Jon Schwartz on the Boreal 44 Zephyrus was in Puerto Williams at the time and had a spare.

I’m glad I had it, but it was fussy requiring attention a few times per day. When it failed I didn’t miss it as I had enough diesel fuel. I’m glad I had it, but it was fussy requiring attention a few times per day. This is an anecdote, not data and just reflects my experience with one unit.

I plan to travel far and wide with my new 50’+ cruising boat. Would I put it one my (new to me) M&R 56? No. I do not believe they are trouble-free enough for my intended high-mileage use to remote locations.

Whitall Stokes

Thanks John! Love your advice on the charging solutions, very valuable. I’m finally heading to Cheticamp to go through the systems and transit to Maine via Bras d’Or Lake.

Really looking forward to getting to know her.

Joshua Marieholm

Great article… not all the gold is for everyone

Jean-Louis Alixant

We are all in agreement regarding the long passage / “fast” boat but let us not scare potential users away. I use the W&S starting at 5 kts, and a “long” passage is one where my service bank and the solar panels will not suffice to meet our power requirements, simply because I do not want to turn the engine on. This could be a 24-hour passage. That said, I never use it for day-sailing.

On a monohull, you’ll probably need to move the device from one side to the other as you tack / gybe to ensure that the W&S propeller is sufficiently submerged: during coastal cruising, with shorter tacks, this can become a nuisance. Note that this also means you’ll generally need to have two brackets installed on the transom (I did not see a second bracket on Ed’s transom). Mooring stern-to in the Med, I wouldn’t leave the device on during port maneuvers, so more handling if you coastal-cruise from port to port.
I own a long-shaft Cruising 600, which I fitted 18 months ago on our “fast” 43’ monohull cruiser. We’ve used it for some 4000 NM, of which two 1000 NM passages. We sail at 6 to 9 kts, and I almost always have the largest prop on (280 mm). If we are faster, the W&S comes out of the water anyway, given that we have a lot on our (usually short) hands anyhow.
I have the U-shaped brackets on KaliX. They are bolted through the transom, into a laminated backing plate, positioned low enough to have the propeller deep in the water away from hull disturbances, and they are inclined roughly 9° outboard for optimum orientation in water when heeled. These installation points are important to allow the device to generate all the power it can.
Some users have complained about vibrations and fitting a sound insulation between the bracket and the transom worked for them; we didn’t need that.
We’ve had the propeller fouled by jelly fish once; it can be quite messy and time-consuming to clean up. As soon as we spot them now, the device comes out of the water.
Once all is in place (unit programmed for your battery bank, the six-braid purchase to lift and lower the device, a set up that allows you to switch sides easily, a safe storage place when off the transom), operation is rather smooth. Watch out for the wear of the locking pin, chafe of the control lines, and do carry a few spares (propellers with screws and spacers). It is prudent to secure the device with a line tied on the pushpit if anything just in case of poor handling during manipulation.
The Bluetooth connection can drop occasionally and the best way to get it back is to turn the MPPT off completely and restart it; be sure to install a switch allowing you to do that, otherwise you’ll need to access the supply cable and disconnect it physically.
The USB interface is indeed no fun to access either (under the waterproof cover of the MPPT), so some users have drilled a permanent access and fitted a small plug to close it when not in use, others have left a USB cable permanently fitted. Now that all is set up, I just use the Bluetooth interface.
I have thoroughly checked W&S’s production claims: sporadically with the 240 and 200 mm propellers, rather extensively with the 280 mm one. During these tests, I have used Boat Speed on Water (meticulously calibrated 😉 ), not SOG. The use of SOG and approximate Boat Speed calibration (usually too high) can explain the discrepancies reported by users, although I did observe lower production (up to 20%) than claimed between 5.5 and 6.5 kts with the 280 mm prop. Below and above that bracket, the production data match closely. I have not exceeded their published curves. W&S checked they production data and stand by them, I didn’t check further, happy with the output we obtain.
In spite of the significant force exerted on the device, I have not been able to measure any reduction in Boat Speed.
Finally, it is worth re-emphasizing that Watt&Sea do provide a remarkable service, both before and after sale. I have been in direct contact with them since 2017, when the Pod600 was released. They have always been very open and forthcoming in our discussions, and prompt to offer suggestions and help when raising an issue. Also, they make good use of the feedback they receive to improve their product.

Key to success is proper installation, both mechanical and electrical, which John will cover next.

Jim Rankin

This is why a friend of mine has developed a charging system at less than 6 knots for cruising people

Colin Palmer

I am developing a towed generator for my Ovni 345 and I have been doing some calculations that may be of interest.
Waterline length is 9.1m, so I have selected 5.5 knots as a design speed (this is based on speed length ratio of 1.0 in imperial units). At 5.5 knots the hull resistance will be around 700N.
The power output of a “water windmill” is proportional to the area and the cube (third power) of water speed. So, yes, speed through the water has a dramatic impact on power output. At 6.5knots the power available will be more than twice that at 5 knots, but this is basic physics and we have to live with it. (The same goes for windmills in the air, which is why they are so variable in performance on a yacht.)
I calculate that a 0.2m diameter prop will produce around 64W (pretty close to the Watt&Sea 300 power curves).
The drag of different sized props will be:
0.2m 64N, which is 9% of hull drag
0.25m 100N, which is 15% hull drag
0.3m 143N, which is 20% hull drag
These are not trivial numbers, but they are better expressed as a speed reduction, and represent a speed loss of roughly 0.17, 0.27 and 0.38kts respectively.
Which begs the question of whether the extra diameter is worthwhile. Does the speed loss offset the extra power due to the increased area? With the 0.2m prop, the speed loss is 0.17kts, so the speed without the generator would be 5.67kts. The power output is 64W at 5.5kts. At the same speed, the 0.3m diameter would give 144W, but actually the speed will be reduced by 0.38knots, to 5.29kts, so the power output will only be 128W. Thus the extra area (and extra drag) does produce more power, albeit at a slower boat speed.
I have skipped over a lot of detail behind these numbers, but happy to explain more if anyone is interested. The generator I am building is based on the Thai longtail concept – a direct drive to the generator and the whole thing tilts up to get it out of the water. If it works, I plan to make the design open source for anyone to copy/develop.

Philip Aston

I’m a happy W&S user. We have a Boréal 47 and plan on 150NM days when doing ocean passages. It provided the bulk of the power on a 6 day Falmouth to Lagos passage in October. The 360W solar panel didn’t do anything of note until we passed Cap St Vincent. The Superwind wind generator gave us the rest, but we we’re downwind most of the way. And of course, the windgen and the W&S are both good for the night hours.

We used the W&S on shorter hops when cruising the Western Isles of Scotland. We have no generator, and in two years of full-time cruising I have only run the engine on two occasions for charging. Both times when we were at anchor for several days. If we’re moving every day, the W&S and minimal use of engine propulsion are enough, even with the tepid solar output in Scotland.

However, it can be unwieldy and fragile on the stern, and it has to be lowered so E can deploy the dinghy from the arch lift. We’re now bay hopping in the med, the W&S is safely stowed, and the (increased) 560W of solar is the king. We’ll get it out again in October when we have our next multinight passage.

Stein Varjord

Hi John,

I agree with all your comments. The core point in my opinion too is that an actually fast boat is needed. There needs to be enough speed through the water to power up the propeller, and there needs to be an overflow of driving power from the rig. Power that is already being wasted on making waves. Free power. If there’s no excess of power, we steal from our speed and reduce the responsiveness of the boat. Not ok.

I’ve not sailed enough with W&S units to build my own options. In about 2003 (?) I sailed a catamaran from the Caribbean back to Europe with a prototype version of what became Duogen.

The cat was a 12metre (40 foot) cruiser, but a very fast one. Carbon mast etc. Even in loaded down and conservative cruising mode we mostly stayed above 10 knots. (Well over 20 was fine in racing mode.) Our Duogen prototype has the configuration mentioned in another comment, the Thai longtail. It wasn’t very long, though.

The generator was quite large and around 10 kilos (20 pounds). It was on a swivel mount on the middle of the aft beam. Straight on its axle was a shaft to the propeller, via a joint, so it kept the prop axle horizontal. A tiny foil kept the angle of the assembly right. Worked well. This was very easy and quick to employ or lift and store.

The issues we had were reliability, on most elements of the unit, probably related to this being a prototype, but also that it wasn’t made for the speeds we did. (We told them, but they thought we exaggerated.) they supplied us with a smaller propeller, a bit less than 20 cm diameter. It worked most of the crossing, with quite a bit of repairs needed. At 6 knots the charging started, but was negligible. Probably due to the small propeller and some non ideal bearings. At around 8 knots it shot up to around 10 Amps. From there it went up steep. The highest charging current we had for extended times was just above 30 Amps at 14,4 Volt, measured at the battery terminals. We reached that level from around 12 knots speed. Above that we lifted it.

I think the concept is a good idea. The really big generator that is above water and a very light axle and propeller. We never could notice any reduction in speed. Even when repeatedly tilting it in and out of the water. Ther must be some, of course, but negligible on that powerful boat.

Duo in the name is because it also works as a wind generator. Tilt the axle straight up. Remove the small prop assembly and put on a wind prop. It actually worked quite well at anchor. With good wind (meaning poor anchorage) we saw over 10 Amps 14,4 V on the battery poles.

I never tried the finished product and I don’t know if it’s still in production, but it was for some years at least and it did get some awards.

Stein Varjord

Hi John,

I googled them and they are indeed at, as Eclectic Energy.

I agree that different functions in one device often means compromises have been made, that reduce performance too much. In this case, however, the propeller is exchanged, so it’s only the generator that is actually the same. The wind propeller is larger than most standard wind generators and it did give very nice power, when there was enough wind. The noise when giving good power was a fairly loud hiss, but no howling or whining, like some wind generators produce.

My main gripe with wind generators is that they are too low down to do much useful, most of the time, so the cost/benefit evaluation gives it a clear no, for most boats. The DuoGen (3 626 Pounds), however, gives the wind generator as a lower added cost to the (for fast boats) much more important water generator. It seems like a worthwhile concept. I’d want to try a currently available product before I say it’s good, but I do think it might be.

They also have pure wind generators (1 718 Pounds) and pure water generators (2 532 Pounds), based on similar thinking and parts, and in general seem to have much lower prices than W&S. Perhaps it’s worth looking at? Their home page and product availability seem to have issues, though, so it needs a bit of research.

Klaus Bonde Christensen

We crossed the Atlantic January 2023 and had severe problems with Saragossa weed, although to a lesser extent, it continued all the way up through Bahamas. It fouled our main rudder (J46) and would be very difficult to remove. It fouled our Hydrovane rudder as well and it created vibrations and strain on the fittings. Some days we sailed through football fields of weed that would slow the boat with more than 50 %. There was a lot of weed every day!
I can only imagine what impact that would make on a Watt &Sea- another aspect to consider, depending on your cruising ground and time of the year.

Lynn Li

The most important single issue with W/S is that they offer no support for self servicing the unit. We had bearing issues after upgrading to the 280 mm prop and surfing at 10 knots + on the back of waves . As we were in Patagonia at the time about to cross the pacific, I had to open and replace the bearing myself. Not a difficult task in itself, but having no exploded diagrams and assembly instruction were a pain and I damaged a thread in the process. At the time it was totally impractical to send the unit back to France for servicing.

My rule of thumb is do not rely on a critical piece of gear you can’t service yourself if you sail remote places…. Save your money and put it toward better solar panels or fuel for the genset….


Shaen Tarter

Hi John- We purchased the 300 watt version of this unit for our trip from Hawaii to French Polynesian in May. We have an Island Packet 40, which actually achieves decent speed, but yeah, yeah, we know the reputation 🙂 Anyway, about 5 days out of Oahu, we experienced a catastrophic loss of our aux engine. We continued on to Tahiti in energy and water conservation mode, and arrived 23 days after departing Honolulu with half our potable water and the batteries happy and healthy. This was possible only because of solar, wind vane steering, and the W&S. Our 500 watts of solar probably averaged 40% of its rated output for 5 hours each day. Not great. What saved us was the little hydro generator making 6 amps per hour, 24 HOURS PER DAY, every day that we could sail. If I had purchased the 600 watt version, we probably could have used our water maker a few times and kept both fridge compressors running. The W&S is strangely designed in some ways. The hold down line is much too small for the load (I replaced with high strength, low stretch), the thing can swing wildly when pulled up in rolly seas before being tied down, and the power output cable can get caught in the main bracket. Otherwise, it works very well. It’s controller has programmable features that are similar to Victron’s MPPTs, different prop sizes are available, and I received excellent support from the Hydro Vane people in BC. One more thing….it is very sensitive to the “quality” of water driving the propeller. It needs smooth, clean water to work at its best.


Shaen Tarter
SV Arctic Tern

Ronald Langeler

We sailed 30.000 nm with the W&S 600 long. Mixed feelings. Mechanical failure attachment white banana-looking plastic (broken resulting in loosening underwater part from rest). Lots of quarter back waves reducing waterspeed at transom each wave. Electrical output shifting from 0 to 10 amps continuously, leaving MPPT confused (red leds flashing). Serious problem with sea weed. Not simple to clean when sailing. Unit is hard to raise under power since securing pin is heavely loaded and can not be unlocked whilst sailing. Prop damaged and replaced by spare. When battery full W&S goes into idle giving pulsing noise and vibration.
Since we run a generator daily, the W&S not our prime source but a backup when generator down. For that is was ok.
Our unit was bought in 2016, maybe current design is improved.