# Maximum Efficiency of a turbine on a moving vessel

Discussion in 'Hydrodynamics and Aerodynamics' started by MicMac, Nov 16, 2022.

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1. Joined: Oct 2020
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### seandepagnierJunior Member

I guess the military doesnt use solar panels.. oh wait
You think a diesel engine is more reliable than sails or oars. My sails are 50 years old, older than the 30 year old engine. My oars can last even longer. Compared to a less reliable system with dozens of moving parts that frequently break down. Everywhere I go I meet people who have broken engines: they break all the time. Then you have to contend with people who actively sabotage engines: very very easy to do.

As for walking away for 30 years... nice idea but you have to change the oil and stuff, and having to supply fuel for 30 years is too much fuel. Starting and stopping it all the time and it will break even sooner.
It is not at all what anyone cares about for a tow generator. The OP specifically is interested in power output vs drag, or power output vs speed reduction.

This has absolutely nothing to do with the betz limit or available energy given a specific diameter what you are posting about is irrelevant. The diameter of turbine is not really limited in a tow generator: it can be huge. It can have overall very little drag but extract a small fraction of the energy available too it and be very useful

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### gonzoSenior Member

The motorized world is evidence that you are wrong.

Even easier to sabotage sails; it only takes scissors.

The oil is not necessary to change. I don't know what "stuff" is in an engine. As for fuel, in 30 years of not running the engine will use approximately zero gallons.
That is the definition of efficiency; what the OP is asking

The Betz limit is a physical reality, so it is the most important parameter in the calculation of energy available for a fluid flow.

That is absolute nonsense. Claiming that a huge turbine can have overall little drag is in the realm of fantasy. Are you really claiming that a sailboat can drag, say, a 100 foot generator? And then that it will have less drag than a 3 foot one?

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### seandepagnierJunior Member

the nonmotorized world is evidence that I am right.

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### gonzoSenior Member

"Quote" I only state the betz limit is irrelevant in the comparison of drag vs power generation, and for this you can achieve better than 90% efficiency, same as propellers can exceed 90% efficiency."
That is nonsense. Unfounded claims do not change the laws of physics.
"Quote" The turbine will only extract a tiny fraction of total power available no where near the betz limit. Because the hydro turbine is not working anywhere near betz limit it becomes irrelevant to even discuss it further."
Then you claim that you are not using 90%, which contradicts you previous absurd claim. Either it is a magical turbine that defies the laws of physics, or a very inefficient turbine that extracts a tiny percentage of energy. Make up your mind.

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### jehardimanSenior Member

Guys, guys, guys....you are arguing two different perspectives, neither of which is correct in the OP's scenario... a "generator" towed by a sailboat cannot extract power from the water as there is no "power" to extract. The water is effectively not moving and the turbine "generator" causes the water to move in the same direction as the turbine...i.e. putting energy into the water.

You can not address the "efficiency" of the towed turbine, only the inefficiency. It is what power (torque*omega) you get out for what power (drag*velocity) you put in. The only way it will ever be close to "100% efficiency" is if the drag is zero and the output torque is zero (very similar to marine propeller efficiencies) because the work done is always negative. You are never "making" power by towing a turbine because you are putting energy into the water. This is very different from a wind powered vessel or vehicle where there is energy to extract from the flowing air even given the velocity of the vehicle.

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### gonzoSenior Member

I agree that the generator is not extracting power from the water, if the ground is the reference. However, it is the velocity between the boat and the water that is of interest. Efficiency is not too important since the wind is free and the boat is not in a race where fractions of a knot are crucial. I think that a better metric is how many watts per dollar invested can the generator output.

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### BlueBell. . . _ _ _ . . . _ _ _

Hello MicMac,

Welcome to the Forum, as contemptuous as it may be.

I'm going to go out on a limb and say 50 - 75% efficient.

Generator losses 15% at best.
Then you've got all your line losses from mechanical drive losses to electrical wiring losses.
That would be the end of your question I believe.

Then you've got your battery charging and storage losses, unless you're going to feed it directly to an electrical appliance.
Even then it's going to have to go through a regulator/controller, 10% losses at best.

One place you may look is Leo's S/V TALLY HO project going on in Port Townsend, Washington, USA.
Sampson Boat Co is his company name I believe.
He is utilizing a hybrid diesel / electric with recovery while sailing.
I believe it is a new set-up from the UK!
I'll see if I can find you a link as I'm sure they give an efficiency value...

Cheers, BB

EDIT: check out 25:20 on this video:

Last edited: Dec 16, 2022
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### seandepagnierJunior Member

The betz limit does not apply in this situation because hydro generators on boats generally are no where near to approaching this limit anyway. How many ways does this have to be told? Why are you going on about the betz limit?

turbine losses can be as low as 3-5% generator can be as efficient as 98%, conversion electronics (99%) and battery can be better than 95% It is possible to extract 90% of the energy from boat speed lost in drag into a battery but realistically 70-80% would be considered "very good"

Now, using a propeller instead of a generating turbine, and the efficiency is already terrible because it is cambered the wrong way. Couple this with gears, other forms of friction, brushed motors, or a controller that cant match load (allow generator to run at mppt voltage) and so forth and typical alternator driven prop shaft is way below 50% efficiency and creates significant drag. Generating maybe 100 watts, from more than 100 times this much energy it may not slow the boat a noticeable amount. It would be possible to get more than twice the electricity from the same drag, but not using a propeller and an alternator, it needs a purposefully designed turbine for that particular speed and power output and high efficiency generator.

I 3d printed my turbines for different speeds, as well as used a brushless drone motor i rewound to make a generator. I use synchronous rectification to convert 3 phase ac to dc to eliminate diode losses, and a controller modulates the phases (dynamic regen) to adjust power output to maximize efficiency with slight speed changes.

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### jehardimanSenior Member

I wonder what would happen if you entered "Maximum Efficiency of a turbine on a moving vessel" in to two instances of ChatGPT and let them talk to each other.....questions, questions....

philSweet and baeckmo like this.
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### gonzoSenior Member

I go on about it in response to you mentioning it. Further, it is relevant as any other physical limits are. If efficiency is a factor on the design, then limits must be considered.

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### seandepagnierJunior Member

even when the betz limit is completely irrelevant... ok

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