Cruising Catamarans and Hydrogenerators


Last Updated on May 3, 2021 by Amy

We tackled a slew of projects in Cape Town, and one of them was installing a new hydrogenerator. Since then, we’ve sailed over 6,000 nm on three long passages. Let’s talk about how our hydrogenerator has worked out.

What is a Hydrogenerator?

A hydrogenerator is a unit that sits on the back of our boat. When deployed, the propellor of the hydrogenerator is fully submerged in the water. The propellor spins and generates power which gets pumped into our battery bank.

Why Use a Hydrogenerator?

Prior to installing our hydrogenerator, we had three options for charging our batteries:

  • solar panels
  • generator
  • alternators on both engines

That set up has been working fine for at anchor, but while underway we were needing to use options two or three nearly every day. The addition of the autopilot, chart plotter, radar, AIS, etc means that we are using nearly double the power when we are underway. The solar panels just can’t keep up.

Plus, after running out of fuel in the Indian Ocean, we were looking for more ways to reduce our diesel usage.

Selecting a Hydrogenerator

We purchased a Watt & Sea Cruising 600. Watt & Sea is probably the biggest brand of hydrogenerators on the market. Many of our friends have Watt & Seas.


David had talked a lot with Sailing Yacht Eight, an FP Saona, regarding their hydrogenerator install, and he had a plan all set out to use a track to slide the hydrogenerator up and down to deploy or retract it. Unfortunately, when we got our hydrogenerator, we realized that our bridge deck transom was not tall enough to get the hydrogenerator completely out of the water when retracted. We had to scramble over the holidays to come up with another plan.

There were three major aspects of the install.

The hydrogenerator itself is removable, but it needs a bracket to be permanently installed. Typically the bracket is attached to the transom which is a bit tricky for us as the lower transom on Starry Horizons has a slight curve to it. A machine shop guy came by and talked to us about what we needed. David had something in mind, and the guy took measurements and made the bracket to our specifications. It came out perfect!

David installed the bracket by drilling four holes in the transom just off of the swim ladder. The machine shop had also made two backing plates for us. In order to give those backing plates a flat surface, David mixed up some epoxy to fit between the hull and the plates (coating the plates in wax in the hope we will be able to remove them again someday!) then through-bolted all of this together and used 3M 4200 to seal the assembly.

The second part was installing the plug nearby. It needed a vertical space to be mounted, so we installed it on the wall by the outdoor shower.

The third part was running the wires all the way from the plug to the battery bank. Running wires is always a major pain, as there’s not much empty space and the wires always tend to get jammed in the pipes. Plus, our batteries are all the way in our main salon, so the wire has to be extra thick and long.

Attaching the Hydrogenerator to the Bracket

Since the hydrogenerator is removeable, before each passage we pull it out of storage (aka the forward guest bunk) and attach it to the bracket. The hydrogenerator attaches using two locking pins. We work together to get it onto the bracket – definitely a two person job.

Deploying and Retrieving the Hydrogenerator

The hydrogenerator comes with a cam cleat at the top. The line to raise the hydrogenerator feeds through this cam cleat. There’s another line that comes through a hole just below the clam cleat. That’s the line that you pull to drop the hydrogenerator.

Unfortunately, you have to do quite a bit of additional set up to get the raising and lowering to work properly. You are deploying the hydrogenerator underway, and there’s a lot of force necessary to get the hydrogenerator completely down.

We installed two pulley systems, one to raise and one to lower the hydrogenerator. It takes a bit of coordination and practice to get it right. We had to increase our pulley ratios for me to be able to fully lift the hydrogenerator, and I really have to put my back into it.

I had done less research than David had, so I didn’t realize going in how much work the hydrogenerator would be. We have to raise it if we sail too fast, if the batteries are full, if we have to motor or run the generator. We are learning to always keep an ear out for any troublesome noises, espcially sailing in the Caribbean with sargassum seaweed getting caught on the propeller.

Interchangeable Propellers

Depending on how fast you can go, you will have to change out the propeller so that your hydrogenerator doesn’t get overloaded. Our model is quite sensitive to high voltages generated when the prop spins too fast. We have propellers in two different sizes: 240 mm and 280 mm. This is the medium and large props offered by the manufacturer. We also bought two spares in case they get any damage.

Hydrogenerator Performance

We’ve sailing three long passages (1,000+ nm) with our hydrogenerator so far.

On the first passage, sailing from Cape Town to Saint Helena, we discovered that the wire from the hydrogenerator to the batteries was not big enough. Basically, when we started sailing faster, the wires weren’t large enough to handle the amount of power flowing through them. This “traffic jam” caused a large enough voltage drop that our hydrogenerator thought the batteries were full. This makes the hydrogenerator go into absorption mode, and the propeller starts freewheeling (which makes a lot of noise).

While we were in Saint Helena David undertook a pretty massive rewiring project. Relocating some components of our electrical system to be closer to our batteries (and thus needing smaller wires – which we had) and re-purposing the larger wire from those components for the hydrogenerator. Our second passage, sailing to Recife was much better. We had very light winds, so we used our largest propeller the whole time.

Sail to Brazil – Using Primarily Large Propeller

  • Avg Speed: 6.43 knots
  • Avg Amps Generated/Hour: 12.83
  • Avg Amps Generated in 1 day: 308.02
  • Max Amps/Hour in 1 day: 13.28 @ 6.38 knots

Our third passage had more variable wind speed and boat speed. We were sailing from Recife to Antigua, through the doldrums and into the tradewinds.

Sail to Antigua – Total Mix of Large/Medium Prop 

  • Avg Speed: 6.73 knots
  • Avg Amps Generated/Hour: 10.34
  • Avg Amps Generated in 1 day: 248.07
  • Max Amps/Hour in 1 day: 13.64 – large prop day at 6.42 knots

Sail to Antigua – Medium Prop Days

  • Avg Speed: 7.2 knots
  • Avg Amps Generated/Hour: 10.3
  • Avg Amps Generated in 1 day: 247.2
  • Max Amp/Hour in 1 day: 12.67 @ 7.46 knots

So does our hydrogenerator perform as advertised? Short answer: no.

From the Cruising 600 webpage and the interactive output curves graph, with the 280 mm diameter propeller sailing at 6.4 knots, we should be seeing nearly 22 amps. On our sail to Brazil, we only used the 280 mm diameter propeller and averaged 6.43 knots. We generated, on average, 12.83 amps.

This difference probably has to do with our wiring. To run all the way back to the main salon from the engine room is quite a long run and requires a thick wire. David has plans to replace the wire yet again with a thicker one.


On our last passage sailing from Recife to Antigua, we only used our generator for 38 hours. This was always to top up our water tanks, not to top up our batteries. Combined with 19 hours of motoring, we only used 34 gallons of diesel, a major reduction from our previous usage.

Despite not getting output close to the output curves published by Watt & Sea, we are happy with the performance. Once we replace the wire again, we should be producing more amps. If we could get to the output advertised by Watt & Sea, or at least closer to it, it is possible that we could have enough power going in to run the watermaker off of our inverter and top up the water tanks without having to run the generator.

Pin it!


  1. I’m getting a FP Helia 44 and considering WattNSea as well. Did you notice any slowdown in sailing speed when deploying it?

    I’m considering the “sail drive” format of the device vs having to raise it all the time. One for each hull, just worried about sailing performance loss.

  2. Hi guys – did you buy the short or long shaft? We’re onboard Uno, a Helia 44 (hull #1!) and about to buy a Watt & Sea 600 to overcome power issues on our Atlantic Crossing from Canaries to Caribbean in 5 weeks. Also, any reason why you installed yours on the starboard sugarscoop and not port? Cheers, Nicole

    1. Hi Nicole, thanks for reaching out! We went with the long shaft version as I was worried the short shaft wouldn’t get deep enough to meet the Watt&Sea guidelines: “The recommended depth between the surface and the propeller axis is 300 mm (12 in)”. The deeper the prop the more efficient it’ll be as it’s further away from the wake, but it also increases the force on the mounting so make sure it’s nice and strong.

      We did actually install ours on the port side but there’s no reason you can’t do either side. Our reasoning was that our swim ladder is on the port side which means we typically board from the dinghy on the starboard side. Going with a port side installation kept all the mounting hardware away from where the dinghy normally sits when tied up.

      Hope you have a great crossing!


  3. No generator will put out more power than the batteries will take. Since you almost never ran your generator to top up the batteries, but only to run the water maker, I have to wonder if your low generated amp hours were because the battery was full? Or in the case of lead acid, full enough that it won’t take the available power.

    On my engine, I have 200 amps of alternators, but can spend hours putting out less than 30 amps as I reach full charge.

    My Watt & Sea has the 24 cm prop, and I can routinely hit 15 to 20 amps of power. But only if the batteries can take it.

    1. Hi Harry,

      I certainly agree that if the batteries are in absorption (more for lead acid) or float (for lithium) charge stage, then the input amps will drop down quite significantly. However, that is definitely not the problem we have with our hydrogenerator when we’re out on passage.

      When we run the generator to make water, it also will get our lithium batteries up to float at the same time, but after turning our generator off, we would wait to redeploy the hydrogenerator until after the batteries had been drawn down. We have two different battery monitoring systems on our boat and when we’re sailing with electronics on, AIS, radar, autopilot, fridge, freezer etc, we can easily tell when the batteries are ready for an additional charge source. Our lithium batteries can take almost a full charge load all the way up to float status so they should be able to take anything the hydrogenerator can put out.

      That was a good thought on a potential problem though. Thanks for mentioning it.



  4. Best one i saw was an underflow chevron paddle wheel driving and off the shelf tractor alternator via a belt. Very little drag, no complex seals, low expense.

  5. We recently installed a Cruising 600 as well but seem to be having better luck on the output side. Is yours the long shaft model? We’re an Oceanis 50 and we mounted ours the recommended distance from the center line. That seems to keep us churning our power without issue even when heeled. Ours was second hand and had already been installed and wiring tweaked to match demands so we didn’t have any issue there. One note is that we simple point up to stall the boat speed to make raising and lowering easier. Last tip, we use a boat hook slid down the back of the unit to clear the accumulated sargasso grass. Works like a charm. Thanks for the post, very interested in your output numbers with the different props.

    1. Hey yall! Thanks for the comment. It’s been a while since we’ve used our hydrogen, so we will have to hold off on further reporting until we get some more long passages under our belt!

  6. Very helpful. Energy issues are always my top concern when sailing on the sea. Hydro generators is probably an effective solution to this problem; it helps to save energy and also contributes to environmental protection.

  7. Hydrogenerators are really nice to have. I especially like your beautiful installation. They can be had by just about anyone, I made one for free. All you need is an appropriate voltage permanent magnet motor a couple of shaft fasteners, a long rope, and a small lightweight propeller, some fairly heavy wire(enough to handle around 10 or 15 amps continuous at the appropriate voltage…oversize by 50% if possible. I found my motor in a junk heap, the rope in a thrown out dockbox, and a prop off a sunken outboard. I had to buy the fasteners which were a couple of stainless nuts, and about 20ft of ten guage marine wire. All in I put in about $12.85. I painted the motor and prop, then tied the rope around the motor to make an adjustable bracket of sorts. Having done that, I then attached the locknut to the shaft after some special shenanigans to make the rope line up better. Then tied the prop to the end of the long rope. The final step was hooking it up to the batteries. Whereas I don’t have a charge controller, I just watch it on passage and I know it will always put in over 12-14.4 volts and about 5 amps at speeds over 4 knots. Since I use more power on passage than it puts in a charge controller isn’t really in the mix. It isn’t as nice as the Watt and Sea, and definitely isn’t as capable, but it helps my autopilot live long enough for me to get to places on short hops. I really like the super nice installation on Starry Horizons, and love the blog post and all the effort which obviously went into it!

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