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Optimal propulsion system

11K views 77 replies 34 participants last post by  Brewgyver 
#1 ·
I have what may be a slightly unusual problem. My wife loves to sail but hates it when I have to put on the engine. I mean really hates it. She would rather sail at 2 to 3 knots than motor at 5 knots.

For coastal sailing however motoring seems to be a pretty common necessity.
The alternative is to get into a strange harbor after dark and miss dinner and a slip assignment.

I'm thinking that an optimal system might be an electric drive system with battery capacity for about 5 hours. This battery bank would be mated to a gen-set that had the horsepower to drive the boat at max throttle.

The system would be reasonably simple.
Genset with charger connected to battery bank.
Batteries connected to electric motor.

During the day if the wind was light I could motor sail for a few hours. The motor would be so quiet no one would know.
In the afternoon while we were off the boat I would run the genset to charge the batteries (The genset would be insulated so its running would not be noticed by any other boats)
If I really had to run the genset to run the motor it would be big enough to do the job and I would just have to deal with the noise.

  • I know this is not a cheap solution. Probably double what a re power would cost.
  • It is also pretty complicated with lots of controllers and electronic controls.
  • It will be somewhat inefficient converting gen-set horsepower to electric instead of shaft power. I'm hoping that the built in inefficiency will be off-set somewhat by the fact that the the gen-set can be set to the most efficient rpm as opposed to a shaft driven motor where the rpm is used control the boat speed.

Weather I would be willing to pay for and actually build such a systems is another question but assuming the above usage what do you think?
 
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#43 ·
Their is a very active electric yachts yahoo forum where this has been discussed a great deal.

The consensus is that for most boats, even in light usage you would not have the room to fit enough panels to make that work.

Panels are used often to supply led lighting and to top off, the last 10 percent, charge all the time.
There was a showcase boat however recently that looked like an aircraft carrier, power only, that used only solar. It was a several million dollar experiment however.
 
#44 ·
What if you put a motor well in take out the desial. put a motor well in and use a 15hp or 20-25hp 4 stroke plenty of power and if it's in a well you won't hear it at all and will be good on gas, no noise. and will still keep the look of a inboard.
 
#45 ·
There was an good article in Good Old Boat not to long ago about a guy from Maine that that did this very thing. I believe it cost him around 2K to do this. He only putted out of the harbor so he recharged the batteries during the week w/ a small solar panel. I think that any long range "motoring" would be impractical using electric. You'd have no juice left to power up the blender:)

The generator thing, what's up with that? If you're going to buy a generator to recharge the batteries, might as well just upgrade the motor & call it a day. Even the DC generators cost around 7K.

For reasons that escape me, I always thought sail was the "optimal propulsion system"
 
#46 ·
The generator thing, what's up with that? If you're going to buy a generator to recharge the batteries, might as well just upgrade the motor & call it a day. Even the DC generators cost around 7K.
In case you missed reading the earlier posts, the biggest advantage (and one well worth the extra $$$ and effort) is that you can put the diesel generator anywhere you like - it doesn't have to be in the stern of the boat and in line with the prop.

For reasons that escape me, I always thought sail was the "optimal propulsion system"
I don't think anyone here would disagree..
 
#47 ·
David,

Your problem is a solved one. Up until now there were basically a bunch of startup electrical propulsion companies and there were DIY versions using floor waxer motors or golf cart motors (and batteries).

Now there is a mature looking system that has been picked up by Beta marine and Yanmar. I've emailed the designer of the engine/generator part and I've spoken with a salesman at Beta and the system seems really good.

Without getting into the whole diesel-vs-electric debate there are defiantly some good reasons to have a hybrid on a sailboat. Motor sailing is the main reason. You can be sailing along at 3 knots and with a silent push of the throttle you can now be going 5-6 knots and with very little amp hour expense. Personally, if I had the coin, I'd have one of these on my boat right now.

6 months ago when I contacted Beta and the motor designer you could get a brand new beta with the hybrid electric motor all set to go but Yanmar was not quite ready to ship. The electric motor could be bought by itself, or with a transmission, ready to mate to your existing engine (Volvo) shipped from the UK. I can dig up the prices and info if you like.

One nice thing about this engine is that you could also run the diesel engine without turning the prop, and it would turn the electric motor, creating a 10KW genset. So you had the option of running the diesel engine as you normally would for propulsion, running the diesel engine without turnint the prop and getting 10kw, turning the prop and using reserve HP to make electricity (up to 10kw) at the same time, or using the electric motor without the diesel engine as an electric drive. IT also had some nice features like an ability to sense when you were giving your engine near full throttle and it would automaticlally cut out the electric generator component to make sure you got full HP.

Here is the link:
Seagoing Hybrids - Hybrid Electric Marine Propulsion
For some reason the betamarine hybrid links are down today. Google shows them as an active link but it's not working today.

MedSailor
 
#51 ·
OK..This sounds like something I've been daydreaming about. I have a 15 Kw/55 amp propane 4 cyl Onan gen set. Total weigh w/o gas is #850. There is no out shaft, as the gen and motor are interconnected. I'd like ta see what it would take, motorwise??? .75 X 15 formula comes up to 11.25 hp. Size a boat to that hp and see if it could handle the weight of motor and gen set as ballast????? Figger mebbe 1200# with a couple batts and motor controller. If ya could mount the weight low enuff........... ;)
 
#52 ·
delta, propane has a very low energy density compared to diesel. Which is why ships and submarines and even locomotives use diesel-electric propulsion very successfully, but never use propane-electric.

Propane will work if you can get it very frequently, very conveniently, very cheaply, and you've got lots of storage space. But unless you've got a rare boat operating n a rare environment, forget about it.
 
#55 ·
Hellosailor;
re: density/BTU; Yes, too true. The only advantage to propane is that it stores indefinitely. My musings were more about the drive-train issues and the other applications you mentioned. Since a freight train uses diesel to run gens to power electric motors; would the same reasons/logic/engineering apply? I suppose not,; as a sailboat does not need to shift hundreds of tons of weight. :D
 
#56 ·
In the automotive world, Tesla seems to be on the cutting edge of battery technology. Their cars run about 300 miles before needing a charge and, if my memory serves me correctly, requires only an hour or so to recharge, maybe even less. And their cars are lightning fast! But their cars aren't cheap.

But how would the best battery technology we have today work in a marine environment, propelling a much heavier object through a much denser media? Would it reduce the load on the generator? Would it burn the batteries out? Would they need constant recharging? Maybe Tesla can answer that.

When I see what Tesla is doing, I have to ask myself why the major auto companies aren't doing the same thing. I think the Chevy Volt is only good for about 35 miles on battery power alone. Why don't the big three make a more expensive model that can compete with Tesla? We gotta get the Tesla guy to work on marine propulsion. He'd figure it out.
 
#57 ·
Julie,

Tesla is using lithium ion batteries last I looked. Figure an energy density on them of about 2% by weight compared to gas. So to replace a gallon of gas you would need roughly 325lbs of batteries. The Tesla battery pack weighs around 900lbs, so figure the energy equivalent of 3 gallons of diesel. And they aren't cheap, the battery pack which is good for a nominal 4 years cost roughly $36,000. So even assuming electricity is free it's going to cost you $8,500 a year in battery packs.

At today's fuel prices you would have to be doing a lot of motoring to come anywhere close to justifying it from a cost advantage.
 
#58 ·
Julie,

Tesla is using lithium ion batteries last I looked. Figure an energy density on them of about 2% by weight compared to gas. So to replace a gallon of gas you would need roughly 325lbs of batteries. The Tesla battery pack weighs around 900lbs, so figure the energy equivalent of 3 gallons of diesel. And they aren't cheap, the battery pack which is good for a nominal 4 years cost roughly $36,000. So even assuming electricity is free it's going to cost you $8,500 a year in battery packs.

At today's fuel prices you would have to be doing a lot of motoring to come anywhere close to justifying it from a cost advantage.
To you good sir, I concede that you win this argument, and you will win for a LONG time to come. This isn't about just energy density and cost though. The cheapest way to get from point A to B on the water is by ferry. Next best, is powerboat. We can't possibly justify all the design and space losses of our sailboat hulls, the weight of our keels, and the massive expense of our rigs and sails compared with a simple small engine that can push us at displacement speed.

In fact, most of us already have that engine aboard but prefer to use our cost-ineffective method of propulsion (sails) even though they usually make us go slower, we can't pick our direction and they require constant attention. Curious isn't it? We crazy sailor types must like things in our propulsion systems besides cost and efficiency then. See my post a page back or so on motor-sailing and why we like electric.

MedSailor
 
#59 ·
"We gotta get the Tesla guy to work on marine propulsion. He'd figure it out. "
Someone hasn't read up on what the Tesla cars are. It would be very easy to build a boat the same way. Make it a two seater, tack an extra fifty grand on the price to cover the battery system, and Bob's your uncle.

There's no magic in the Tesla cars, just a lot of EXPENSIVE stuff.

They were supposed to be partnering with Toyota for the RAV4 EV this year, but Toyota has also made some major quite announcement about getting out of the EV market (except for a handful of pre-committed cars as a test) completely now. They see it as being totally uneconomical.

Got a boat? Get a diesel, Mr. Herr Professor Doktor whatever Diesel designed them to run on peanut oil, not petroleum products. Of course, he had no idea the particulates in the exhaust would cause asthma and cancers, but the fuel (vegetable oil, unprocessed) is certainly cheap enough.
 
#62 ·
Right. The high-end sports car is ideal for electric in many ways. The car itself can be tiny so the batteries are practically carrying nothing but themselves and it benefits from the incredibly high low-end torque of electric motors. Plus it's expensive. Tesla was smart going in that direction but none-the-less I think the company still had a lot of close calls. Designing that car still wasn't easy.

As for boats, what I'd love to see someone try is to properly design in batteries as balast. An encapsulated full keel has lots of potential space if a boat builder designed it from the outset to hold batteries with a robust rack system.

If you could stack them 2 or 3 layers deep that could be significant capacity and since it's replacing ballast anyway there would be little penalty - the main one being the lower weight density of batteries compared to lead (you could still have lead at the bottom). The challenge would be to make it safe and serviceable.
 
#61 ·
OK, this is going to sound stupid: If noise and vibration are the issue; what about changing to an Atomic 4. My A4 is very smooth and quiet. If it happened that your diesel died, it would probably be cheaper to re-power with a rebuilt A4 than another diesel. Gas doesn't have the range of diesel, but, you are not crossing oceans. Just a thought.
 
#63 ·
Asdf,

Again we get to an energy density problem. Lets assume that we design a keel so that the entire mass is one big battery. And let's assume that the design will keep the same density as raw lead (we know this isn't true, but it makes the math easier).

Taking a Beneteau 31 for our comparison (because they use iron keels, so the density might be closer to correct)

1) the 31 carries 2,200lbs in ballast.
2) lead batteries have an energy density roughly equal to 1/650 of diesel.
3) converting the entire mass of the ballast gives the same energy potential as about 4 gallons of diesel.

So we suspend a huge amount of money designing the battery pack, figuring out the electrical issues, have a relatively short lifespan for the ballast, ect... And still only have 1/10th the motoring range of a comperable Diesel engine. But we can't carry jerry cans to full up on route. If we run out of power, we either have to find a marine with power, or be able to generate power on board.

So if we want to reasonably extend the range, we have to put a pretty large generator on board, which is going to require adding back the fuel tanks, thru-hulls, ect that we avoided with the electric drive in the first place. How big of a generator? Figure 15kw would be sized to the engine.

Rit now there just isn't a storage device anywhere close to the same density of diesel. For all it's problems, fossil fuels are insainly energy dense compared to anything else we have come up with, and while battery technology is getting better, there isn't anything on the drawing board that even comes close to fossil fuels.

So how much do batteries need to improve?

Diesel.......................45.4. Mj/kg
Gunpowder.................3.0
Lithium ion batteries ...0.72
Lead acid battery........0.1
 
#65 ·
Asdf,

Again we get to an energy density problem. Lets assume that we design a keel so that the entire mass is one big battery. And let's assume that the design will keep the same density as raw lead (we know this isn't true, but it makes the math easier).
Some may a drifted this thread a bit but the original proposition was diesel / electric hybrid.
So we are talking about the energy density of diesel.

The trade offs are that it will cost more as you need more equipment.

It will not be as efficient as their are conversion losses but maybe not as much as might be at first supposed due to being able to run the diesel at optimal rpm and run the electric with optimal torque.

The only advantage, and for some of us it might be worth the extra money, is the quality of life advantage. IE for most passages you may not need to run the diesel at all but if you have to you can.

Also you possibly get the advantages of a genset without a separate diesel for that purpose.
 
#64 ·
Stumble, I'd like to spend more time running the numbers because this scenario, replacing balast with batteries, is as good as it's going to get for electric boats. But a couple things jump out quickly.

Are you factoring in conversion losses? Diesel engines have an efficiency of about 45% and Electric motors are >70% (70-99.99 apparently) based on some quick googling. So that close to doubles your 4 gallons to 8.

Second, 2200lbs? My Contessa has more ballast than that. However I'm not saying 100% of balast should be replaced but many boats that size have far more.

And really, if we're talking 8 gallons of gas that's a lot. I've gone 2 seasons on less, at the least that's a significant passage even for a large boat.
 
#67 ·
" a 10kW generator is going to recharge the batteries mighty quickly! ..and give you plenty of AC power (enough to run a small house!) whenever you want it. "

Yep! ;)
I could run the whole (little) marina on mine!
I don;t remember the name; but there's some kinda motor controller that lets ya run from 0 rpm up to max without loss of torque. "Variac" comes to mind; but I could be disremembering?!


OK.. new twist....
What about air cooled engines??
I got's an old 2 cyl Honda 350 that would be a willing donor!!!!!!;)
 
#68 · (Edited)
I don;t remember the name; but there's some kinda motor controller that lets ya run from 0 rpm up to max without loss of torque. "Variac" comes to mind; but I could be disremembering?!
They're called "variable speed drives" or "VVVF motor controllers" or (whatever else they can think of) in industry. There are a host of manufacturers out there: Toshiba, Mitsubishi, ABB and Danfoss being the most common for marine use. Now cheap and off-the-shelf..

OK.. new twist....
What about air cooled engines??
I got's an old 2 cyl Honda 350 that would be a willing donor!!!!!!;)
Too noisy. You need to get air in to cool it (obviously) and that means plenty of holes to let the noise out. There's plenty of nice cool water in the ocean that does a better job. :)
 
#69 ·
Asdf,

Those numbers are what I remember from an engineer on another forum. I believe he calculated in conversion efficiency, but I could be wrong. But don't forget you also have to calculate in the usable energy portion. Lead acid batteries can only reliably use about 50% of their nominal power, without damaging the cells lifespan.

Hartley,

I actually think diesel/electric is a reasonable way to go. At least under some conditions.

1) there is or needs to be a generator anyway
2) reduced power range is acceptable
3) the size of the house loads correlate reasonably to the propulsion demands (we don't want a 20kw generator running to power one AC)
4) little or no range expected from the batteries
5) propulsion requirements are minimal

In short I think it is possible, but doesn't meet many boats requirements since 3 and 5 are conflicting. The conversion is roughly 1hp=.75kw. So if your boat has a 20hp engine, you would need a 15kw generator to have the same power available at the throttle (ignoring conversion inefficiency). If you have a 40hp engine, which is pretty common on 40is footers you would be in the 30kw range.

Frankly I don't know of many boats that would install a fraction of this size generator for house loads. And it would be massively inefficient to operate it just for house loads. Even if you were willing to sacrifice some available power you would have to make a pretty massive reduction in available power.


The only way electric power is if you can accept a pretty minimal range. Like just in and out of a harbor. For many day sailors this would work fine. But you would also loose the capability to make longer trips under power. I for instance would be fine with this system 99% of the time, but for a week or so a year we take the boat about 100 miles away for a distance race. I don't know that I would be willing to give up the capability to do this even if the rest of the time electric power would be fine.
 
#70 · (Edited)
Hartley,

I actually think diesel/electric is a reasonable way to go. At least under some conditions.

1) there is or needs to be a generator anyway
2) reduced power range is acceptable
3) the size of the house loads correlate reasonably to the propulsion demands (we don't want a 20kw generator running to power one AC)
4) little or no range expected from the batteries
5) propulsion requirements are minimal

In short I think it is possible, but doesn't meet many boats requirements since 3 and 5 are conflicting. The conversion is roughly 1hp=.75kw. So if your boat has a 20hp engine, you would need a 15kw generator to have the same power available at the throttle (ignoring conversion inefficiency). If you have a 40hp engine, which is pretty common on 40is footers you would be in the 30kw range.
First off, I've seen these systems in operation on motor cruisers (with large beer fridges!) and they certainly work and work very well - providing the added advantage of very short range propulsion to get to to anchor or out of the way should you happen to run out of fuel (don't ask!).

Your numbers seem about right to me - you'd have to expect that the diesel geneator would be about one or two sizes smaller than the diesel you pulled out. Don't forget that the generator control system will run the generator at it's optimum speed constantly regardless of boat-speed - there's no need for the "peak power/extra revs" capacity you might normally have with a diesel installation because that can be supplied better by the batteries.

The only way electric power is if you can accept a pretty minimal range. Like just in and out of a harbor. For many day sailors this would work fine. But you would also loose the capability to make longer trips under power. I for instance would be fine with this system 99% of the time, but for a week or so a year we take the boat about 100 miles away for a distance race. I don't know that I would be willing to give up the capability to do this even if the rest of the time electric power would be fine.
Not necessarily. One thing is for sure, as your list indicates: The power system and all components need to be correctly selected, designed, installed and commissioned by an experienced electrical engineer so that everything works together properly to meets the needs of a particular boat+owner - a diesel hybrid isn't something your average Joe Boatie (or even his regular Marine Mechanic) can put together without something going horribly wrong.

At the end of the day, similar to the diesel-hybrid system in a car, a diesel-hybrid system in a boat will cost more than a straight diesel but will also have a few advantages to make up for the added cost - depending entirely upon the selected installation and the way it's used.

Just imagine using thrusters instead of a conventional propeller/shaft? Or water jet?? For the first time since the propeller was invented, stern glands are no longer an essential part! With a hybrid system the possibilities are endless..

But for most people, in most places, with technology as it stands now, a hybrid is simply too complex to be worth the added cost/benefits - but that doesn't mean that, just like hybrid cars, it isn't the way of the future. :cool:
 
#72 ·
Ok I wanted to run some numbers and see how things looked in more detail. Hopefully I don't make a mistake.

Diesel fuel is listed at 45MJ/kilogram which is 20.45MJ/lb or 36MJ/liter.

A Group 27 from Deka weighs 53 lbs and has 90Ah. This is 90Ah*12V=1080Wh.
A joule is a Watt-Second so 1080*3600 (seconds per hour) is 3.888MJ.
So per pound this is 3.888MJ/53lbs = 73kJ /lb
or by volume it's 3.888MJ/13 liters = 0.3MJ/l

This puts diesel at 280X as dense as the lead acid by weight or 120X as dense by volume. Or another way, one Group 27 is 0.0285306 gallons..

So to compare this to the example from earlier 2200lbs gets you 42 batteries which nets about 1.2 gallons...

As pointed out earler this gets better (by about 75%) when factoring in conversion efficiency (electric is more efficient than diesel) but then worse (by a factor of about 2) when you figure usable capacity of a bank.

Man that is pretty bleak for electric. Lithium is several times more dense than lead acid, at least about 5x according to some quick numbers but that's still not too good.

My only last thought is that the 45% number for the desel engine seems a bit high, perhaps that number is a train engine or a power plant operating at optimum RPM? What's a small boat engine get when running at 1/2 or 2/3 throttle?

http://www.dekabatteries.com/assets/base/0194.pdf
 
#73 · (Edited)
My only last thought is that the 45% number for the desel engine seems a bit high, perhaps that number is a train engine or a power plant operating at optimum RPM? What's a small boat engine get when running at 1/2 or 2/3 throttle?
Train engines tend to be diesel-electric (i.e. Hybrid minus the batteries) and this would also have some benefit on a yacht - a bigger one at any rate.. There are a few ocean racing yachts still around from the 70's and 80's that used hydraulic drives for all the same reasons.

The figures are beyond me, but I'm not sure you're comparing apples with apples..:

1. A diesel engine in a boat gets throttled anywhere from idle to 100% with different fuel consumption at each throttle setting ("efficiency" if you like - at some throttle setting it will be "most efficient", but Murphy's Law states that max efficiency won't occur at cruising rpm.. ;) )

2. A diesel generator in a marine Hybrid installation runs at "max efficiency" all the time i.e. the ECM selects the optimum diesel engine speed to match the load, which will be either (a) running the motor or (b) charging the batteries or (c) both.

This means that, in terms of "fuel efficiency"/"energy consumption"/whatever, the Hybrid will be far more fuel efficient under nearly all conditions than a straight diesel engine - offset by the cost of the batteries, etc, etc.

If you take a Hybrid car for a spin, you'll quickly see how it works.. :)
 
#74 · (Edited)
One problem with all these energy density numbers is that they only show how a boat would use the electric motor ONLY or the diesel engine ONLY and compare them. Many of the advantaged to a hybrid, or even pure electric involve ways that you can use then engine other than motoring along at hull speed from point a to point b.

I think pure electric systems could be very practical for small boats who sail well and only use their engines to get in and out of harbor anyway. I've been on many a T-bird that departed the dock with only a gallon or two of gas. Pure electric wouldn't be much different.

But pure electric isn't for most. And it's applications are varied and it's merits many. Part of this though is you have to imagine how you would use it differently to your regular propulsion system. Personally I would kill for the ability to have a hybrid system such as the beta marine system I listed at the beginning of this thread. The advantages for me with just a very modest battery band would be the following:

1: I could add several knots of speed to my light air sailing with very few amps used. This would mean much more sailing and less time with the diesel on instead.

2: If the diesel died, I could motor right up to the breakwater of my marina and use my small battery bank to drive her in to the slip using pure electric.

3: More torque at low speeds means better handling in close quarters.

4: I would have a huge genset (not that I really need it)

5: Regenerative power while sailing at hull speed.

and all this could be done with your regular, or slightly enlarged, house bank. No need to replace your keel with lead acid batteries or spend 20L on Lithium ion batteries. What this system would not do is be the only force driving me long distances. For that, I have my diesel engine, and these funny triangular white things....

The link below outlines the pros and cons of how a hybrid system (of sorts- actually its a pure electric and an outboard) can be used in many different ways. Here is an excerpt from the link below:
Epods in Use

3. Light wind sailing was a pleasure for once. Typically the Gemini is not great in light winds due to her low aspect sail plan and small control surfaces ineffectiveness at slow speeds. However with the e-pods providing just a little push (5 amps) the boat was able to create its own wind to keep the sails pulling and the boat moving. The e-pods providing enough water flow over the rudders to allow you to steer. We sailed past the bow of a 40 foot mono who was hardly moving at maybe 1 knot mostly due to the tide; they were greatly impressed with our speed under sail; they had no idea there was anything helping us along as the outboard was clearly off and up out of the water at the time. With only a breath of wind she was doing between 2-3 knots; just enough to keep me from resorting to the outboard. Keep in mind that this was with 14 people aboard so the boat was heavy and we had been expecting normal heavy summer winds and thus were using the small 100% heavy working jib and full mainsail; not at all the right setup for light winds so the speed was even more impressive given these facts. I am definitely more likely to sail more or should I say motor-sail with the e-pods.

MedSailor
 
#75 · (Edited)
Those e-pods look cool!

I agree with all that, except this might need some tossing around:

5: Regenerative power while sailing at hull speed.
There are indeed a few "towable generators" on the market, but I'm not sure how well using your normal drive propellor to regenerate actually works in practice - since I'm led to believe different shaped blades are required for generating power from the water flowing over them without creating too much drag in the process.

I'd think you'd be better advised to stick with a folding/feathering prop and use wind/solar generation instead... Just my 2c worth.
 
#76 ·
I wonder how much drag would be introduced by one of those 'drop-in the stream' hydro-gens? IIRC, they produce some tremendous amounts of power without costing huge dollars or taking up a lot of space.

A couple of fins ta stabilize and a tow cable added to the wiring set and just throw it over the stern! ;) Nah! Mebbe a faired stem and clamp arrangement to lower it off the stern quarter?

Would it be worth it to have the amperage vs the loss of a few tenths of a knot in cruising speed?

OK.. I see Aquair has that towed thang down already :D Howzabout a MInn-Kota trailed over the side and reversed to generate?? ;)
 
#78 ·
I wonder how much drag would be introduced by one of those 'drop-in the stream' hydro-gens? IIRC, they produce some tremendous amounts of power without costing huge dollars or taking up a lot of space.
They produce very little power, typically 100 watts, and that's at 7.8 knotts through the water, which is more than hull speed for a typical cruising sailboat. A more realistic 5 knotts, the output is down to about 50 watts - you can get that from a single solar panel.

With the micro-hydro, it would take several days of continuous operation to recharge the batteries from the use of an electric drive for an hour.
 
#77 ·
The problem with towed and shaft generators is they require a boat that can maintain high average speeds thru the water over long periods of time. Most of these generators require a boat speed of 5kn to start working and only become really powerful past 6. That doesn't sound that bad, but they will also cost you roughly 1/2 a kn of speed while operating.

So unless you are consistently cranking out 130+ days you just aren't going fast enough to use them. And while I know many cruisers have hit that mark once or twice, not many can do it day after day reliably enough to count on them for power generation.
 
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