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Old 22-03-2020, 12:15   #1
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Will a catamaran produce more wind generator power than a monohull with the same unit

As proof that my screen name doesn't bias my engineering approach :-) Doesn't the monohull wind generator depower it's blades when heeling in the same way the sails do? Do Momo's spend more time going to windward and that more than compensates?

My thought started with realizing that I could mount a wind generator on the 2" stainless tube frame of my radar pole. By rotating a rod grip, you can adjust the radar platforms angle from 90 in both directions so that it can be made horizontal when heeling. The structure is sound enough to exceed wind pole required strength and I would think benefit from facing the wind rather than dipping it's head when going upwind.

So, thoughts people? I'm curious about this, not looking to start a mono/cat fight.
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Old 22-03-2020, 12:59   #2
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Re: Will a catamaran produce more wind generator power than a monohull with the same

There would be a modest amount of reduction of the swept surface area presented to the wind. One could gimble the genny if you wished to have it remain in fuller presentation, but since the gennies derive modest energy I believe if would be for little absolute gain and no gain once the wind velocity reaches the maximum rating of the machine which is likely to be achieved when the vessel would be heeling over at its maximum.

The energy content of the wind varies with the cube (the third power) of the average wind speed.
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Old 22-03-2020, 13:36   #3
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Re: Will a catamaran produce more wind generator power than a monohull with the same

I'm going to say they are equal for all but a rounding error since boats spend most of their time at anchor.
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Old 22-03-2020, 18:18   #4
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Re: Will a catamaran produce more wind generator power than a monohull with the same

The answer completely depends on if you ask the question in the monohull or the catamaran forum.

But seriously... the difference--if any--is totally trivial.
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Old 22-03-2020, 18:36   #5
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Re: Will a catamaran produce more wind generator power than a monohull with the same

Which produces more apparent wind wins?
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Old 23-03-2020, 11:38   #6
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Re: Will a catamaran produce more wind generator power than a monohull with the same

Wind generators always point directly into the wind. Unless you change the pitch of the pole it's mounted on, fore and aft, it will always present the same amount of blade area to the wind, regardless of heel angle. If the pole leaned aft 10 degrees, you would lose 10 percent of the area and power.
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Old 23-03-2020, 11:55   #7
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Re: Will a catamaran produce more wind generator power than a monohull with the same

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Originally Posted by SteveSadler View Post
Wind generators always point directly into the wind. Unless you change the pitch of the pole it's mounted on, fore and aft, it will always present the same amount of blade area to the wind, regardless of heel angle. If the pole leaned aft 10 degrees, you would lose 10 percent of the area and power.
That was my initial thought when I read the OP's post. But, as a boat heels the tail of the generator, that keeps it pointed into the wind, is no longer vertical. And the generator itself now starts to feel gravity a bit differently, so depending on the weight of the "front" vs. "back" of the generator it could end up being pulled in a direction that is not directly into the wind.

As a result, I don't think you can say that the generator will remain pointed directly into the wind as it would when the boat is flat and the mount is vertical. It's an interesting though experiment in 3D visualization and forces, but I suspect the measured output would be negligibly different.
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Old 23-03-2020, 12:58   #8
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Re: Will a catamaran produce more wind generator power than a monohull with the same

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There would be a modest amount of reduction of the swept surface area presented to the wind. One could gimble the genny if you wished to have it remain in fuller presentation, but since the gennies derive modest energy I believe if would be for little absolute gain and no gain once the wind velocity reaches the maximum rating of the machine which is likely to be achieved when the vessel would be heeling over at its maximum.

The energy content of the wind varies with the cube (the third power) of the average wind speed.
Ummm...that would be the square of wind velocity. A doubling of wind velocity (or any mass in motion) = 4X the energy, not 8X.
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Old 23-03-2020, 13:13   #9
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Re: Will a catamaran produce more wind generator power than a monohull with the same

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Ummm...that would be the square of wind velocity. A doubling of wind velocity (or any mass in motion) = 4X the energy, not 8X.
Reference:
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Old 23-03-2020, 13:35   #10
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Re: Will a catamaran produce more wind generator power than a monohull with the same

A lot of power is lost by the boat rocking from side to side and in doing so causing the wind generator to swing around. Ive noted that mine loses a lot of output in even moderate seas. I suspect that a cat would not have the same problem.

Also, I believe it would be much easier to get clear air flow to a wind generator on a cat, or, to do as one cat owner on this forum has done and mount one generator at the stern of each hull.
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Old 23-03-2020, 13:41   #11
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Re: Will a catamaran produce more wind generator power than a monohull with the same

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Originally Posted by OneHullPaul View Post
As proof that my screen name doesn't bias my engineering approach :-) Doesn't the monohull wind generator depower it's blades when heeling in the same way the sails do? Do Momo's spend more time going to windward and that more than compensates?

My thought started with realizing that I could mount a wind generator on the 2" stainless tube frame of my radar pole. By rotating a rod grip, you can adjust the radar platforms angle from 90 in both directions so that it can be made horizontal when heeling. The structure is sound enough to exceed wind pole required strength and I would think benefit from facing the wind rather than dipping it's head when going upwind.

So, thoughts people? I'm curious about this, not looking to start a mono/cat fight.

The area reduction happens only with the wind abeam and heeling, so the overall loss will be marginal.
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Old 23-03-2020, 17:18   #12
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Re: Will a catamaran produce more wind generator power than a monohull with the same

Splitting hairs for minimal gain.
Same often done with solar installations.
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Old 23-03-2020, 21:00   #13
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Re: Will a catamaran produce more wind generator power than a monohull with the same

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Originally Posted by Montanan View Post
Reference:
"The energy content of the wind varies with the cube (the third power) of the average wind speed."

You seem to be confusing the POWER (kinetic energy over time), which varies with the cube of velocity, with the KINETIC ENERGY (transferred force of wind exerted against a surface) - which, of course, varies with the square of velocity:

E (kinetic) = 0.5 X (M x V-squared)
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Old 24-03-2020, 09:30   #14
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Re: Will a catamaran produce more wind generator power than a monohull with the same

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Originally Posted by PineyWoodsPete View Post
"The energy content of the wind varies with the cube (the third power) of the average wind speed."

You seem to be confusing the POWER (kinetic energy over time), which varies with the cube of velocity, with the KINETIC ENERGY (transferred force of wind exerted against a surface) - which, of course, varies with the square of velocity:

E (kinetic) = 0.5 X (M x V-squared)
Okay, let me be of further assistance. I will recommend these references so as to provide clarity of the output power being a factor of the cube of the velocity of the wind.

The Power available from the generator is 1/2pAv^3(Cp)

p = Density of the air
A = Swept area of the turbine
v = Velocity of the air [which velocity is a function taken at the third power]
Cp = Coefficient of power [i.e., the power conversion efficiency of the turbine system]

https://www.raeng.org.uk/publication...3-wind-turbine

Copied below is an explanation in rather layman's terms and an exemplary output power graph thereof is attached below:

The Power of the Wind: Cube of Wind Speed

The Power of the Wind: Cube of Wind Speed

The wind speed is extremely important for the amount of energy a wind turbine can convert to electricity: The energy content of the wind varies with the cube (the third power) of the average wind speed, e.g. if the wind speed is twice as high it contains 2^3 = 2 x 2 x 2 = eight times as much energy.
Now, why does the energy in the wind vary with the third power of wind speed? Well, from everyday knowledge you may be aware that if you double the speed of a car, it takes four times as much energy to brake it down to a standstill. (Essentially this is Newton's second law of motion) Similar to your conception of kinetic energy above.

Power Content of the Wind

But in the case of the wind turbine we use the energy from braking the wind, and if we double the wind speed, we get twice as many slices of wind moving through the rotor every second, and each of those slices contains four times as much energy, as we learned from the example of braking a car.
The graph shows that at a wind speed of 8 metres per second we get a power (amount of energy per second) of 314 Watts per square metre exposed to the wind (the wind is coming from a direction perpendicular to the swept rotor area).
At 16 m/s we get eight times as much power, i.e. 2509 W/m 2

Because the power output increases by the cube of the velocity of the wind and a boat heels over significantly only when the wind speed is high [and the sails are large, not reefed] one can see that the generator machine typically reaches its thermal limits for power far before the tilting of the support structure will have any significant effect. When the boat tilts over the genny is lowered towards the water surface a small amount and wind speeds nearer the surface are generally slower than they are higher up, so if the boat heels the generator will see less apparent wind then if it was held further up. A catamaran will tilt less than a monohull so a catamaran may avail marginally superior power output potential, but the real limiter will be the power rating of the genny that is attained quickly as wind velocity increases. Cubic function is quite amazing in that regard.

About 20 years ago, here in Montana, I participated in the prototype design and building of a novel 3 megawatt, permanent magnet generator for a wind turbine. The generator operated at direct drive, couple to and rotating at the same speed as the blades [no expensive and heavy gear box utilized to reduce torque and increase speed to the genny]. The generator machine's rated speed was only 13 rpm. The generator machine was in one dimension, large at 12 meters [40 feet] in diameter, the turbine blades diameter was 100+ meters, and the nacelle would in turn be located on top of a tower 100+ meters above the ground. While the generator machine was large in diameter at 12 meters, it was axially quite short at about 6 feet at the junction of its rotor hub at the axle, and the rotor narrowed to less than 2 feet in axial length at the outer circumference of the 12 meter diameter. The stator of the generator was unique in architecture and materials as there was No Iron nor Wires in the construction of the Stator. The stator was only about 2 inches in axial length [cross thickness in the magnetic field] and only about 18 inches in radial length, extremely lightweight. The stator was not rigid axially, it could flop about a bit axially but obviously had tremendous torque applied to the stator. Not many motor or generator engineers can even imagine a stator that has no iron nor wires in its architecture or bill of materials; they will draw a blank in their minds, kind of like as if talking to an architect and telling them about a building that has no floors, walls or ceilings. There be no frame of reference or prior experience with such architecture of electrical machine.

I hope this helps in understanding. All the best.
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