Generating electricity - water around the hull/keel

[Carogan]

I'm a newb when it comes to sailing (although I've been reading a tremendous amount for the past 5 years) so the answer may be obvious to some of you old salts. However I'm hoping some of you can help me with a question I have.

Obviously generating electricity for a boat, specifically without using a combustion engine, is one of the great challenges that boaters and manufacturers have been stymied with for decades.

I've often wondered why it is that manufacturers have not yet figured out how to harness the energy of the water that flows around the hull - and most specifically around the keel - in order to generate and store energy while the boat is under sail.

Yes, we have wind generators and solar panels, and we've seen prop generators that can be towed behind the boat. However it seems to me that the most kinetic power available in the environment surrounding a sailboat at any given time would be generated from the forces of the water as it flows around the keel while the boat is under a stiff breeze above water.

I do not know the physical calculations at all, but I have to believe that the power availed by water flowing over the keel must be significant when there is enough breeze to move a 15 tonne sailboat. How is it that this cannot be harnessed? Even if it is at the expense of boat performance (at least when energy production is induced) does it matter - the boat can only travel at a certain hull speed in most instances anyways. And surely if we can generate power its a sacrifice some sailors who are cruising would happily trade wouldn't it?

Could some of you smarter people out there help me with this question I've been struggling with?

Thank you all in advance!

Rob

[DeepFrz]

Yes, it can and is being done.

[SV DestinyAscen]

It's a sailboat not a tidal generator with sails.

A towed/prop generator works because the drag induced is relatively small - and most are designed to better function when youre 6 knots or more.

Solar is way more promising. Works when youre not moving. Doesn't foul with growth. Doesn't mess with the boats primary function.

[Carogan]

Wow. This was the type of unuseful remark I had hoped not to see in reply to this thread.

Originally Posted by SV DestinyAscen

It's a sailboat not a tidal generator with sails.

Clearly they are not. That's why I asked the question and is something that I'm interested in exploring (although nobody brought in the topic of "tides" into this discussion)

Originally Posted by SV DestinyAscen

A towed/prop generator works because the drag induced is relatively small - and most are designed to better function when youre 6 knots or more.

Apparently they don't work very well because they don't generate much electricity and they need to function at 6 knots or more. The flow rate of water through a towed or prop generator won't be nearly as high as water flowing around, say, the leeward side of a keel.

Originally Posted by SV DestinyAscen

Solar is way more promising. Works when youre not moving. Doesn't foul with growth. Doesn't mess with the boats primary function.

I'm not sure I suggested that solar shouldn't or even couldn't be used in conjunction with alternative sources. Do you think one should only stick to one technology use for generating electricity - or even for a sailboat? Because I don't.

And the one interesting drawback about solar is - it only generates electricity when the sun is out. The interesting thing about generating electricity using the power of the wind...is that wind is typically created in inclement weather when the sun is often not available.

Originally Posted by SV DestinyAscen

Doesn't mess with the boats primary function.

Why don't you tell me what the "primary function" of a sailboat is? Because maybe the primary function of a sailboat for you is different for other people. In addition, some of us might think it is important for a sailboat to have more than just ONE function. Maybe you don't - good for you. Then find another thread to go contribute something useful to and don't buy a boat that might utilize other technologies other than what YOU consider a sailboat should have as YOUR primary function.

As I said in my opening comment - I figure there are some people who would be interested in exploring a tradeoff of efficiency in return for generating power. After all - alot of people who are cruising do rather enjoy the benefit and comforts of electric power and might find it useful to be able to acquire without having to have to go through the inconvenience of earning the money, travelling and finding a petrol station, and consuming a scarce in supply, polluting fossil fuel.

So how about this - if you have a scientific or practical reason to contribute to this discussion - feel free to share it. But please keep your dismissive comments to yourself because I for one don't welcome the negativity.

After all - there was a time where people like you believed the world was flat and no one could travel to the moon either.

Thank you very much.

[Carogan]

Originally Posted by DeepFrz

Yes, it can and is being done.

Do you have any sources or links you can share? I have only been able to find info on tow generators - it would be nice to read up on any developments!

[SV DestinyAscen]

Mate if you're going to lob personal insults because someone told you that fouling hulls don't make sense for sailboats, and ignore how tidal generator are basically what you're suggesting, ok.

fwiw, no one struggles with cruising or sailing because of electrical generationdifficulties . It's easy and people have solved this problem in various ways without messing with hydrodynamic flow of hulls or increase fouling.

[senormechanico]

Any "harnessing" of the water moving past a boat is going to SLOW IT DOWN.
Solar panels don't except for slight weight and windage.

Your hopes of somehow overcoming the laws of conservation of energy are entertaining, but unfortunately not possible.

[Stumble]

You will never see a generator placed along a foil or keel I feel anyone with an engineering degree can help it. The speed of the water flow is exactly the same as at the transom, or what atowed behind generator seems, but the drageffećts on interrupting the laminar flow of the water passing over the keel would be disasterous. Even a fraction of inch of soft biofouling will noticeably effect the speed of a boat thru the water, adding any sort of contraption there would be awful.

As for why towed generators perform poorly at low speeds... it takes a bit of physics, but it isn't hard to grasp. The formula for the amount of power available in a flowing liquid is

W=.5 * rAv^3

Where:
W is power
r is the density of the liquid
A is the area of the propeller
v is the velocity

so running some numbers.

W (1kn) =.5* 1029kg/m^2 * .5m^2 * 1kn^3 = 257.25 units
W (2kn) = 2058 units
W (4kn) = 16,464 units
W (5kn) = 32,156 units
W (6kn) = 101,871 units

You can see I dropped the units for the results, but it doesn't matter since we are just worried about scale here. It's pretty easy to see why towed generators need 5kn or so to operate. There is 125 times more power at 5kn than at 1kn.

If you want to design a system to generate the same power at 1kn as existing systems do at 5kn then all youhave to do is increase the swept area to compensate. Now i used a prop with a r of 16" initially which I thinkis the largest tow behind generator on the market.

Since the area of a 16" prop is 804in^2 multiply that by 125 to get our new area of 100,531. So all we need to do to generate the same power at 1kn as a 16" prop generates at 5kn is use a 178.86 inch (or 14.9 feet) prop.

I am not sure how to mount it, but there you go.


Now these numbers are theoretical, and don't take into account the drag on the boat from the prop slowing it down, or bearing losses, or skin friction. All of which favor smaller props. But it doesn't matter much, any reasonable prop size simply won't generate enough power at low speeds to matter much, at moderate speeds there is a lot of power and it's just a question of how much you want to extract.

[a64pilot]

I don't understand your premise, why would there be more power in the water near the keel?
Water isn't compressible

[dlymn]

As said before the hull surface is travelling at the same speed all over. Newton tells us that if I extract energy from a process, then that has an opposite effect which sailors call drag. Even towing a dinghy creates drag. Sailors try to keep hulls as smooth as possible. Every time water has to be diverted, it induces drag. As you say, you see that your idea might only work when the boat is operating at hull speed. This is not a frequent occurrence for cruising sailors.

Although sailors might want more electrical energy, they also accept that they will rarely generate enough to replicate land based power. For a cruising sailor, most of their time is tied up to the dock, touring, dining out or just chilling and usually hooked into power.

sailors also know that any moving parts permanently under water are subject to many destructive forces and so usually avoid them if possible.

I reckon it's more important to learn to sail by going sailing than it is to provide advice to old salts about improving the sailing experience

[valhalla360]

If you have a boat that regularly travels at hull speed and you regularly reduce sail so as not to overpower the boat and you are regularly under sail...it will work well.


This basically describes some racing boats racing in the roaring 40's. The depart and are under sail at high speeds for a few months.


Your average 35-40' cruiser will have a hull speed of 7-8kts and be happy if they can average 5-6kts over 24hrs and frequently will be much lower. At 5-6kts, the loss of a 1/2kt is a 10% loss in speed. On a 3 week passage, that's an extra 2 days at sea. At lower speeds, the power production is minimal and speed loss is a greater factor.


Then you have the fact that the vast majority of cruising boats are in port over 90% of all days. So on 90% of days, there is zero energy production.


Reality is energy production for modest house loads is relatively cheap and reliable with solar and works pretty much anywhere. The problem has already been solved.


If you are trying to run heavy loads like air/con or propulsion, even at hull speed with a large towed generator, you will have difficulty producing enough energy (and for propulsion it's a net loss as you need more power to tow the generator than the generator produces).


FYI - towed generators have an advantage in that they operate in undisturbed water. On many boats, the keel in front of the prop disturbs the water and reduces the efficiency that could be obtained. Also, props are designed to produce thrust not extract energy, so it's an issue of not being able to have both.

[Telesail]

Several have already given key parts of the answer - that the amount of energy available to harness is related to the speed of the water flowing over or through the device that collects it and the collection area. This does not differ as a result of where you mount it.

But in the spirit of learning (and assuming your thread is not because you think you have come up with a game-changing product for which you are doing a bit of consumer research): It would make little difference in terms of the available power whether one dragged a towed generator behind the boat or attached the same generator pod to the keel or elsewhere on the hull. However, it would probably increase the drag of the composite system (i.e. the combination of hull and attachment unless, perhaps it had been specifically engineered like the intake cowl of a turbine on a fighter jet) and would probably be a lot more expensive than any of the currently available options.

Perhaps the bigger issue is the overall maintenance - I prefer being able to retrieve and service any moving equipment above water as easily as possible rather than fixing it in a hostile environment that includes marine growth.

The defensiveness with which you refuted the, admittedly slightly sarcastic, replies you received is at odds with your initial statement that you are asking the experts because you are a Noob......Although I like the idea of caterpillar drives from "Red October" running in reverse to harness power rather than propel the boat. Remember they had iPads on Star Trek 30 years before the real thing and "Goo-lex-tan-iri" has been around on space ships for generations but was called "Hal" as that seemed snappier.

[masonc]

If you think you have an idea that will work, build a model and test it. You'll soon get an idea how feasible it is. I suspect a towed generator will do the same thing but all ideas are worth exploring.

[guyrj33]

Rob;
If you're truly a newcomer to sailing, I suggest you learn to sail and build an appreciation for the finesse and skill required to keep a sailboat driving at hull speed. It's only when traveling at hull speed that you have any extra energy.
Then I think you'll better understand the comments that have been offered. I'd also suggest you study hydrodynamics and aerodynamics to make sure you understand lift and foils.
If you're a TROLL then please just go away.

[Shrew]

What a weird post. Nothing like arguing with everyone trying to discuss your topic.

I've often wondered why it is that manufacturers have not yet figured out how to harness the energy of the water that flows around the hull - and most specifically around the keel - in order to generate and store energy while the boat is under sail.

When you say 'manufacturer's what do you mean (e.g. Boat manufacturers, alternative energy business?) Boat manufacturers are not in the business of making generators. They can provide options for installing 3rd party Diesel or gas generators, solar panels or wind generators. It would not be cost effective for a boat manufacturer to allocate R&D when there are companies out there dedicated solely to this type of manufacturing.

The amount of energy yielded would need to be at least comparable to other options out there. If the cost is high, and the energy yield is low, it will not be marketable. It's tough to justify cost of R&D for something as yet unmarketable.

Your point was to the water forces around the keel. These forces would only occur as a result of the hull moving through the water, or the water moving around a hull at rest. Thus the parallels to tide generators were made.

However anything that can harvest the water forces moving around a hull will need to capture those forces through movement of their own. That method of capturing hydrodynamic forces can't itself impede sailing performance.