# Drag Coefficient versus Size

Discussion in 'Hydrodynamics and Aerodynamics' started by Inquisitor, Sep 15, 2020.

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### InquisitorBIG ENGINES: Silos today... Barn Door tomorrow!

I have a question that's kind of academic and kind of practical. Say I'm going dead down wind, wing on wing with a 155% Genoa. I have a telescoping whisker pole that can "near" flatten out the sail. Somewhere between...

1) flatten (Cd = 1.0) with and maximized sail area
2) parachute shape Cd (maximized) yet smaller cross-sectional area

... there is a maximum drag on the Genoa and thus thrust to the boat.

Even if I had access to a 3D-CFD program, I don't really think I could calculate this. There are just too many variable... triangular shape, indeterminate cut, head-stay angle, influence of the main... etc, etc.

So, I'm thinking historical experience is far better toward a practical, real-world answer. So... what percentage of the flattened length of the whisker-pole do you think would result in the most speed?

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### BlueBell. . . . .

100%

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### bajansailorMarine Surveyor

I would agree with Bluebell, but I must admit that I do not know much about sail theory - 100% seems logical though.

Rather than going dead downwind, I would sail at 140 degrees apparent to the wind, with the 155% genoa flying loose and the mainsail to leeward, and a much smaller high cut jib poled out to windward.
We did this the whole way on a transatlantic passage from Tenerife to Antigua, gybing every couple of days or so, and it worked very well.
This was on a 54' yawl; we took 18 days for perhaps 3,000 miles sailed through the water (a bit more than the rhumb line distance which I think is about 2,700 miles).

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### BlueknarrSenior Member

I would treat it like a spinikar. Pole in until the luff breaks. Then pole out just a bit.

Lift + drag will out run drag alone.

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### InquisitorBIG ENGINES: Silos today... Barn Door tomorrow!

I've been away from sailing for some time due to extenuating circumstances and am now arm-chair sailing why I get a boat back in order. I completely agree about the off-wind. That's one of the reasons I said this thread is kind-of academic. My inquisitive nature drilled down to China about the trade-off between larger area versus higher drag coefficient. Another reason I started this, I priced a 16' whisker pole and it hurt. Thing costs more than my boat is worth. I'm weighing attempts at making one versus really needing the full length of the genoa.

Years ago, a buddy and I were in a race using McGregor motorsailers (imagine snails racing). We tried some questionable things back then... one was slightly off-wind flying the cruising spinnaker leeward, 155% Genoa poled windward and trying the main both leeward and windward using the soft boom-Vang tied to the rail windward to prevent crash jibes. In our youthful insanity, we even tried emptying the water ballast. This required us to stay more DDW for obvious reasons... kind-of like balancing a yardstick on your nose while riding a unicycle. We didn't have any modern electronics to weigh benefit for VMG. It was all seat-of-the-pants. I look forward to doing some trials in the future.

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### Will GilmoreSenior Member

This is the Alligator Amphibious Tank. It was originally developed to reach remote and stranded victims of hurricanes in the swampland of Florida. During its development, they originally tried flat tread flanges to propel her through the water. They couldn't get any reasonable speed. They then cupped the flanges, as you can see in the photograph, and the results, as I understand it, was the only patent to come out of the Alligator's development. They got more grip on the water with cupped tread than flat tread.

I would assume the same for a sail. A flat board would have less pressure from the air than a cupped sail. In the case of DDW. A broad reach usually translates to a greater VMG than DDW. Boat and rig, of course, will have something to say about its reality.

-Will (Dragonfly)

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### InquisitorBIG ENGINES: Silos today... Barn Door tomorrow!

A more fitting example could not be found!

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### Tedd McHenryJunior Member

Does a parachute shape actually have a lower Cd than a flat plate? That doesn't seem right, to me. The curved back side of the "parachute" shape should help the flow "close up" behind the object, leading to less wake turbulence, shouldn't it? As a practical matter, in the Genoa example, is there even enough curve to significantly affect Cd? I would think area would be the dominant factor, even if I'm wrong about the Cd.

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### Will GilmoreSenior Member

Your logic is exactly how I would see it, as well, except...

I know the eddy effect can increase the appearance of surface area as well as displacement. The bulbs on the bow of some ships pushes a column of water out before the boat and that acts as an extended waterline. It also would act as more surface area.

This is just my hypothetical thinking here, but imagine the surface tension of water against water where you were dragging a big old heaping bowl of water through the water. The still water in the bowl would add to the surface area to increase your Cd.

In another thread about the shape of the stern where the discussion touched on the detachment while planing, it seems the more abrupt detachment of a square stern is more slippery than the rounded stern where water will adhere to the curve and cause more turbulence. Perhaps a flat disk will also drag an elongated eddy behind it giving more surface area, but also reducing the Cd by giving it a more trapered shape. It gains mass and surface area, but reduces the Cd overall.

Just a thought.

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### InquisitorBIG ENGINES: Silos today... Barn Door tomorrow!

A quick Google search gave back Cd's from 1.4 to 1.7... which if the area was reduced (by as much as 0.7 to 0.6 respectively) the net drag (thrust for the boat) would be the same. If the area reduction was not as bad as 0.7 then one should gain a thrust advantage by adding belly. Which was the point of my question. I would have thought that adding more belly to the Genoa (approaching a parachute or spinnaker) would actually give more thrust than a tight boom vang. But the opinions above were NO.

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### DCockeySenior Member

If the flow separates cleanly from the edges of a parachute the shape of the parachute will have very small effect on the pressure on the upper surface of the parachute. If the flow separates but not cleanly from the edges of a parachute the drag will probably be higher, but the wake will be less stable which could lead to unwanted motion or intermitent partial collapse of the parachute. A standard technique to reduce the drag of automobiles with rounder rear ends is to add something, perhaps a small ridge, which will cause the flow to separate cleanly and reduce drag.

Eliminating instability of a parachute or spinnaker is more important than increasing drag coefficient.

Waves generated by boats are caused by gravity and the different densities of water and air not surface tension. Surface tension of water has an almost infinitely small effect on the wave making of boats and associated resistance. Surface tension effects become insignificant for waves larger than an inch or so, perhaps smaller.

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### Tedd McHenryJunior Member

@Will Gilmore :

My old fluid dynamics textbook lists Cd for various shapes. At the kind of Reynolds numbers a sail running downwind experiences (10^5 ish), a flat plate has more drag than a cupped shape. I think this is the case that's most relevant to a sail on a downwind run. If the whisker pole is short (or there is no whisker pole) then the sail will be more like the cup. If the whisker pole makes the sail relatively taught then it will be more like a flat plate. My suspicion is that the cupped shape influences the flow after separation so that, even though separation occurs at the same position, the total energy in the turbulent flow behind separation is lower, meaning less drag.

However, the difference in Cd is very small, about 1.1 or 1.2 compared to about 1.4. Probably more important is that a Genoa might never be cupped enough to have a significant effect on the downstream flow, the way a parachute or inverted a bowl does. The real-world Cd of a Genoa is probably essentially the same, regardless of its shape (within reason). So, I'm pretty sure that maximizing the area of the sail perpendicular to the relative wind direction will give the most thrust, when running in pure drag.

Tedd

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### Will GilmoreSenior Member

Thank you for that explanation. I think I used the wrong term. Capillary Effect, maybe, but I was not imagining the effects between the layers of two different fluids, not that I was anything close to right. I'm just thinking out loud to see what others might have to say about it.

Now that's interesting. Maybe, in the case of the cupped treads in an amphibious vehicle, there also needs to be an attack angle that adds a lifting force, as well as drag, to make them more efficient.

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### Tedd McHenryJunior Member

I don't really know what's going on with the cupped treads on an amphibious vehicle, but several thoughts come to mind. One is that snowmobile treads seem to work reasonably well in that application, suggesting that the cupping might not be critical. Also, the tread has to move forward on the other side of the belt, and it's possible that the cupping is more important there, where it presumably reduces drag, than in the driving part of the belt. Also, the cups on an amphib's tread aren't rigid, and they may flex significantly under load. A flat plate that deflected would present a smaller cross section whereas a cupped section that deflects will present a larger cross section.

There could be loads of other things going on, too, now that I think of it. That kind of amphibious propulsion seems horribly inefficient, strictly from a power versus speed perspective. It's throwing a lot of water all over the place, only a small amount of which seems to be contributing to thrust in the direction of travel. But I imagine that thrust efficiency isn't the driving factor behind those designs. Efficient packaging and finding a good trade off between performance in the water and performance on land are probably both far more important that thrust efficiency. So it may be that flat plates would actually produce more thrust than cups, but that cups are better for the full range of conditions that the vehicle has to operate in.

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### Will GilmoreSenior Member

In the case I posted above, of the development of the Alligator amphibious tank, they changed from a flat tread to a cupped tread because they got no speed through the water with the flat tread.
That may be the secret. By cupping the treads, perhaps it focuses more water aft than to the side. There is a volume or mass of water associated with each tread and cupping would inherently increase the mass being thrust aft.

I don't know, I saw an analogy between the shape of something like a tank tread moving through water and a sail shape when sailing down wind.

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