A REVIEW OF OUR WATT AND SEA CRUISING 600

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 interesting and informative. This article as well as the  accompanying YouTube episode, 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 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 and 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 AGM’s. The lifespan of batteries, as shown in figure 1, is dependent on DOD and the  difference between discharging AGMs 50% vs only 25% can mean years of life. With a house bank of  AGM’s 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.  

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 3:00 AM and then we would need to run our engine ~ 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 Inter Tropical 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 ~ 600nm 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 120nm 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 in figure 2 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. Note that we based our calculations on the  standard 240 mm propeller. We also have the 200 mm aboard but have not deployed this propeller.  

If you have been following along with the math and I will assume you are as you are still reading,  you will note 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 and 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 1:00 PM 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 in the older models it was more “French” in the way it operated. 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 to 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 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, in our opinion. 

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 series 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 ~ 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 AGM’s. 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 k eep 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 a custom  backing plate be made for your boat. Ours uses two separate aluminum plates that we had custom  machined to which the mounting bracket bolts through the stern. Professional installation and  manufacturing of the plate cost us ~$500. The electrical installation is relatively straight forward 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 derig and stow after passage. A large duffle/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 noise a bit irritating. Again, like that of a wind generator but both agree it is a whole lot  better than listening to the Yanmar run at 1,600 rpm.  

Drag 

That 300KG of added drag mentioned above does impact speed, reducing it by ¼ of a knot/hour. 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. ¼ 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! 

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.