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Review of Watt and Sea Cruising 600

Kevin and Carla Nash

Beneteau Oceanis 50
October 16th, 2021

Over the past few years we have covered nearly 20,000 nautical miles as a couple, and we thought it was time to share what we have learned. We hope you find these articles interesting and informative. This article focuses on power requirements and generation on passage, as well as how we addressed our own demands by installing a Watt and Sea Cruising 600 hydrogenerator on Gargoyle.

Gargoyle is a Beneteau Oceanis 50, well equipped for life at sea but she relies on electricity to drive her many systems. These include a deep freeze, refrigerator, cockpit secondary refrigerator, a Spectra watermaker, electric flush toilets, laptops, phones, tablets and a full suite of navigational electronics, as well as an autopilot. All these systems require electricity.  During the day our 1,500 watt solar array can keep up with that demand without an issue in virtually all situations; however, once the sun sets we are reliant fully on the power we have managed to store in our batteries, and that is when the demand of a passage reaches a point where an addition to solar is required.

Your first question may be why is this an issue on a passage, but not at anchor? On passage, in addition to the usual load of appliances, we are also running all our instruments and most critically, our autopilot. These instruments utilize approximately 10 amps, increasing our demand from ~20 to 25 amps at anchor to ~30 to 35 amps while underway.

Power on Gargoyle comes from our house bank of four AGM 8D batteries, which has a capacity of ~1,000 amp hours. While that may sound like a lot, one must also factor in the depth of discharge impact on AGMs. The lifespan of batteries, as shown below, is dependent on the Depth of Discharge (DOD) and the difference between discharging AGMs 50% vs. only 25% can mean years of life. With a house bank of AGMs costing $5,000 or more, this is a substantial cost factor on a cruising boat’s budget. We have decided that a depth of discharge of ~25% is our target to increase longevity.

AGM battery life cycles vs. depth of discharge

Over the last two years, 60 nights at sea have shown us that on a typical passage we reach the 75% remaining charge target by 0300h, and then we would need to run our engine about three hours to keep our batteries above that state of discharge. Our engine has a secondary alternator specifically dedicated to charging our house bank. This alternator delivers approximately 40 amps to our batteries; however, running our diesel on passage costs us money, interrupts our sleep, adds hours to our engine and most importantly, burns fuel we would prefer to use to make miles when conditions demand. It is not uncommon to have hours or even days of no winds on a long passage and rather than spend days waiting for wind, we prefer to motor across those dead zones, such as the Intertropical Convergence Zone or ITCZ, which we encountered on our passage to the Galapagos and will again cross on the way to the Marquesas.

To illustrate, let us look at our upcoming passage to the Marquesas, a 3,500 nautical mile trip. At our average cruising speed of seven knots, this passage will take 21 days or 500 hours. Gargoyle holds 124 gallons of diesel. Using a conservative fuel burn rate of 1.5 gallons/hour means we can motor for approximately 83 hours. This gives us a range under power of around 600 NM or a power generation capability of over 3,300 AH. Breaking down our trip to the Marquesas and running the engine for three hours a day for power, we would consume over 80% of our fuel just to generate electricity, leaving us with only an approximate 120 NM of motoring range. That is simply not enough. Our solution? We opted to go for a Watt and Sea Hydrogenerator.

How does the Watt and Sea address our power issue? It generates electricity by using the force of the water to turn a propeller in the same manner as the wind spins a wind vane. The faster you go, the amount of electricity you generate grows exponentially, as illustrated below. At six knots, it generates 150 watts or 12 amps and at our projected average speed of seven knots, 242 watts or 20 amps. Note that to generate our Watt and Sea’s rated 600 watts of output, we would need to be sailing at 12 knots. While we regularly see nine knots plus (440 watts / 37 amps), for the sake of this discussion we base everything on a conservative seven knot average. Our calculations are based on the standard 240 mm propeller; we also have the 200 mm aboard, but have not deployed this propeller.

Watt & amp – sea output curve

If you have been following along here with the math, you will observe that at seven knots we are only generating a bit more than half of what we are consuming, or 20 amps against a demand of 30 to 35 amps. That is OK as that rate keeps us well above our target Depth of Discharge of no more than 25%. In the real world, we have been seeing a DOD closer to 20% before our old friend the sun really kicks in so our solar panels supply our demand plus quickly top off our house bank. In cruising the Pacific and Caribbean, we usually have a full house bank by 1300h and we do not start to draw against it until shortly before sunset. At that point, the Watt and Sea is dropped back into the water and it takes over as we cruise through the night.

You may ask how can you tell how much power you are outputting? On the newer models, the power converter has a Bluetooth connection that provides this data to an app on your smartphone or tablet. Ours is a secondhand unit and the older models operate in a more visually pleasing manner. The converter has an LED that glows in different colors to indicate power output. Green is on, purple is zero to 60 watts, dark blue is 120 watts, light blue is 240 watts and then it brightens to pure white as power output increases. Our converter is mounted under our sink in the aft head and the glow lights up the drain in the sink beautifully, providing a welcome night light when using the head on watch. You can upgrade the older converter to the Bluetooth functionality as well, but we just feel that this is way cooler.

Now down to the details. It is important to know that a Watt and Sea is not an inexpensive option. With a price of 6,000 USD for the unit, brackets and shipping as well as an installation cost of ~$500, this is a solution best suited for serious passage makers. It is also not a static power generation system, meaning that when you are at anchor, you will need another option such as solar or wind. As I have said, our choice for a primary power supply was solar, but you may be interested in why we did not use a wind-based solution to meet both needs.

Wind power, like hydro, is dependent on speed. Unfortunately, when sailing we are not able to get the required output to feed our house bank. Using a popular model of wind generator, I calculated the output that would be produced cruising at seven knots. First, to get Gargoyle moving at seven knots consistently on a broad reach, our preferred point of trade wind sailing, we need an apparent wind speed of around 13 knots. At this apparent wind speed, the wind generator would get an output of approximately 5 amps or roughly 1/3rd the output of the Watt and Sea. Even with two mounted on our arch, we still would not be at the same level as the Watt and Sea.

Other options we considered were to switch to lithium ion batteries, or to install a generator. The generator was an easy “no” for us as the last thing we want to do is to burn more diesel. The lithium ion option, however, is a bit more complicated.

Having an ability to manage a much greater depth of discharge, coupled with fast recharge times, a large bank of lithium ion batteries would have been a viable option when paired with our large solar array. That said, we have another three years of expected life in our AGMs. Couple that with the high initial cost of a lithium ion installation, as well as the environmental and social impacts of their production, and we felt it made sense for us to wait for our next battery replacement cycle and re-evaluate at that time.


When considering a Watt and Sea you should keep the following in mind:

  • Installation: Installation goes a bit beyond basic DIY levels. A hydrogenerator creates significant drag against the stern of the boat, producing ~300kg of force on the stern. This amount of pressure requires that a custom backing plate be made for your boat. Our mounting bracket is bolted through two custom-machined aluminum plates that we have attached to the stern. Professional installation and manufacturing of the plate cost us ~$500. The electrical installation is relatively straightforward and well documented.
  • In Use: Our Watt and Sea has the long shaft and is a bit bulky to handle. It really needs two people to mount and dismount, especially when you consider a slip would result in dropping the unit into the water. Ours was purchased second hand and has a custom mounting bracket that allows for other stern mount options such as an emergency rudder. On Gargoyle it takes us approximately 30 minutes to rig the unit prior to a passage and another 30 minutes to de-rig and stow after passage. A large duffel/storage bag would be a good option to add to protect the unit if stored in a lazarette. Note that it is also best to have two people manage it when retracting or deploying in the water.
  • Noise/Vibration: In operation, it does make a bit of noise. We call ours The Witcher, as it makes a supernatural moaning noise as the boat speeds up. We are split on the annoyance factor. Kevin does not notice it when sleeping in the aft cabin and finds it soothing, like road noise on a road trip. Carla on the other hand does find the sound a bit irritating.  So although, like a wind generator, there is noise, both agree it is a whole lot better than listening to the Yanmar run at 1,600 rpm.
  • Drag: That 300 kg of added drag mentioned above does impact speed, reducing it by ¼ of a knot. Due to this, we always pull ours up when motoring, as well during daylight hours, to reduce wear on the unit, save fuel and increase speed. One-quarter of a knot does not sound like much, but on longer passages it adds up. During our trip to the Marquesas, pulling up the Watt and Sea during daylight hours means we will arrive ~10 hours earlier than if we left it deployed. I do not know about you, but at the end of a passage there is nothing better than cutting 10 hours off the trip!

You can watch our review of the Watt and Sea below:

The Bottom Line

After our recent 1,000 NM crossing from the Turks and Caicos to Panama, we give the Watt and Sea Cruising 600 two enthusiastic thumbs up. If you are planning for long passages offshore, a hydrogenerator is a great option.


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  1. David B. Zaharik says:

    Hi… I am glad you had such a good experience with the Watt & Sea… mine wasn’t so much. Brand new from the factory, our sea trials appeared to be totally fine. Unfortunately the further away from civilization we sailed the more issues developed until, on our Atlantic crossing, we had to literally unplug and replug the unit about ever 20 minutes to reset the unit. Frustrating? lol….

    The good news is, Watt & Sea were fabulous about the problem. They suspected the interface control module which I took off and sent back to France. They found it faulty and sent a brand new one back… no questions.

    I have not tested the new unit yet thanks to the pandemic and then a hip replacement…. perhaps this coming season…

  2. Eric says:

    Great writeup guys thank you

  3. Rod Morris says:

    Wow, what a fabulous review. I have exactly the same power consumption vs. generation issues on Oh! and have long felt that hydro generation was the answer. While at anchor my 570 watts of solar and 600 AH AGM bank are more than enough for daily needs while in the Caribbean. However, the added demands of power on passages 24 per day means engine time to generate power. Your article is the best overall review of the problem, possible solutions, and practical considerations I have seen so far…and it is thoroughly readable and comprehensible for my very low level of electrical knowledge. Thank You, THANK YOU for taking the time to share your experience and knowledge. All the best to you on your journey.

  4. Arvind says:

    An excellent write-up!
    You covered not only the “what” (action), but also the “why” (rationale), of each aspect.
    And well written too.

    Keep safe.
    Regards from Perth, Western Australia.