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The Engineer
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New Warships Hit 45 knots (52 mph)

10/22/2009 1:48 PM

I saw this article on yahoo. Really awesome stuff. It got me thinking about the golf ball car episode of mythbusters (Here). Dimpling was extremely successful on those full body swim suits at the olympics (story here) to the point where they are being banned. Why not dimple the bottom of boats to try to increase speed (if they aren't doing that already)? Why not planes? Why not NASCAR as they suggested on mythbusters.

Here's the article on the new fast warships:

Navy's newest warships top out at more than 50 mph

BATH, Maine – The Navy's need for speed is being answered by a pair of warships that have reached freeway speeds during testing at sea.

Independence, a 418-foot warship built in Alabama, boasts a top speed in excess of 45 knots, or about 52 mph, and sustained 44 knots for four hours during builder trials that wrapped up this month off the Gulf Coast. The 378-foot Freedom, a ship built in Wisconsin by a competing defense contractor, has put up similar numbers.

Both versions of the Littoral Combat Ship use powerful diesel engines, as well as gas turbines for extra speed. They use steerable waterjets instead of propellers and rudders and have shallower drafts than conventional warships, letting them zoom close to shore.

Story continued here

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Anonymous Poster
#1

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 1:54 PM

These vessels appear to have planing hulls. As such Stokes (dragging a sphere through a fluid) law does not apply.

Dimples won't work.

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The Engineer
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#2
In reply to #1

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 2:00 PM

Guest Wrote "Dimples won't work"

Why? Because they don't reduce air resistance? I just told you that mythbusters had success with dimpling a car (even provided the link). Even planing hulls have some bit in contact with the water, don't you think they might be useful there?

Always great to have a guest stop by.

Are any of the regulars around so we can have a real engineering discussion about this?

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Anonymous Poster
#5
In reply to #2

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 2:25 PM

Is this thing a planing hull or a non-planing hull? Start there before you go off with your insults.

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The Engineer
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#7
In reply to #5

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 2:37 PM

My point is it doesn't matter if it's a planing hull (I'm assuming it is if it is designed for shallow water and high speeds). There is still some portion of a ship under water, even with a planing hull. Also air resistance on the hull becomes a bigger issue for planing hulls, which dimpling could help with. Why don't you log in so we can have a real discussion.

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Anonymous Poster
#8
In reply to #7

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 2:54 PM

I can't log in. CR4 took away my credentials a year ago and the problem has yet to be resolved.

OK, so we have a hybrid vessel. The physics will be different on the water side than on the air-side (maybe the dimples will help on the air-side). On the water side, not so much (Stoke's Law).

Also, keep in mind that as the water-line length changes (non-planing hull) the speed increases, but since we have a hybrid, when the vessel begins to plane, the speed may increase dramatically (hydro-planing).

This thing out to be able to do 75 knots!

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The Engineer
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#9
In reply to #8

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 3:02 PM

I'm asking the following question honestly, I don't understand why you're saying stokes law prohibits dimpling from reducing drag in water. The swimsuits worked. Can you elaborate on that more so I can get a better idea what you mean. Why would dimpling not work on the parts exposed to water.

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Anonymous Poster
#11
In reply to #9

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 3:23 PM

By Stoke's Law max speed (non-planing hull) = 1.34 x sq root of the water-line length.

This means that a vessel having a 30 foot water line length is limited to 7.4 knots, providing that the vessel does not surf (in a 20 foot sea) this same vessel can go maybe 9.0 knots from the crest of the wave to the bottom (as it begins to plane).

I know because I've done it in my sailboat.

Of course in light-air the maximum speed cannot be achieved. Instead the boat that gets to the mark first is the one with the smoothest bottom. I never saw another sailor attempt to make the bottom rougher. It would always be wash the bottom before the race, never was the bottom dimpled.

Perhaps that is why Dennis Conner lost the America's Cup. Actually the Aussie's came up with the winged keel.

The other physics is Reynolds number when flows are not laminar.

Remember that water is an incompressible fluid while air is not.

Also watch out for cavitation.

What ever happened to Henry Hudson?

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Anonymous Poster
#12
In reply to #11

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 3:28 PM
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The Engineer
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#14
In reply to #11

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 3:49 PM

I guess the way I see it is that you're trying to produce a nice regular turbulent flow. By creating periodic dimples on the hull of the ship I'm thinking you can achieve this. I think there is already turbulent flow over the hull of a ship (is that true, or is it laminar?). I think they make hulls as smooth as possible to reduce turbulence as much as possible. I'm also thinking that periodic dimples would create turbulence that would interfere with itself, preventing large energy draining turbulence to occur (only micro turbulence instead).

As I've said before, it seems to work on the swimsuits, but some claim it's buoyancy rather than the dimples that produce the savings.

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#3

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 2:15 PM

First of all 45 knots is exactly 45 nautical miles/hour and that does not impress me at all. It does not impress me because I know how to get speed up tp 300 knots. Do you know that Soviets had developped a torpedo technology that allows speeds comparable to speeds of airplanes and I know how to develop a large ship to do the same. If you want to know more just contact me.

As you know (perhaps you do not! do you?) resistanc to move ship in water is mainly caused by two type of drags: one is drag caused by friction and another by so calld wave drag resultig from displacing water by ship movement. I know ho to reduc both by 95%, but this information is propriotary, so "if you pay you would know"

Retired sea farrer.

Kazik

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#4
In reply to #3

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 2:20 PM

There was also that hovercraft type thing capable of carrying loads of tanks at ridiculous speeds but could hardly turn, I think it was the soviets that did that? Does anyone know anything of that?

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Anonymous Poster
#17
In reply to #4

Re: New Warships Hit 45 knots (52 mph)

10/24/2009 12:23 PM

Actually there is a fleet of Soviet built passenger ships transporting people along the coast of Bulgaria between Michurin and Kaliakria with speed 75 knots. The ships have a long hull and underwater wings to lower drag.

Also ships had been develop with wings to fly over water of Caspian Sea. CIA had spotted and given these code name "Caspian Sea Monster" in sixties. To lunch these a catapult was used on land and then those flew with speed 540 knots utilizing WIG (wing in ground) effect that improves lift many times.

Purpose of these was to cross Atlantic fast with invading forces against USA. The plan has never materialized because Afghan war had crippled economy of the Soviet Union.

I think that 52 knots is not impressive at all. Nothing special special. My boat can do better (70knots)

Stan

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The Engineer
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#6
In reply to #3

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 2:32 PM

Ok, well, good luck with your super fast ship invention.

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Anonymous Poster
#21
In reply to #3

Re: New Warships Hit 45 knots (52 mph)

10/25/2010 3:12 PM

see:

www.vortexosc.com

http://www.vortexosc.com/images/pdf/sorokodum24.pdf

=Cd=0 or MINUS!

henryk

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#10

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 3:11 PM

I am impressed by something that big going that fast.

I remember Dr. Olson in Fluid Mechanics (many tears ago) explaining it this way, I think. The dimples trip the flow from laminar to turbulent. This will cause less drag because the low pressure (trailing) zone of the golf ball becomes smaller (or a not as low, low pressure) when turbulent as compared to a larger zone (or lowest, low pressure) when laminar. I'm sure the prof did a better job and he had a chalk board.

The method of causing turbulent flow on a trailing edge (to decrease the affect of the low pressure zone) has been in use for other applications for a long time. Dimples are not the only way to do it although they work for something that is rotating.

I think I also saw a reference to dimples causing a thicker boundary layer. This making a less abrupt velocity difference and so reduces drag. This point I am not so sure about. I bet this has been studied very much in certain industries.

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Anonymous Poster
#19
In reply to #10

Re: New Warships Hit 45 knots (52 mph)

11/06/2009 3:12 PM

The idea of the dimples is not so much to maintain laminar flow, or even to "trip" it into turbulent flow. Laminar flow is very delicate, and maintaining it is tricky. Eventually, it will transition to turbulent flow somewhere along the surface (whether that surface be a ship's hull or an airfoil).

What the dimples really do is delay the onset of a DETACHED flow. Detached flow is when the big vortices occur and drag really increases. Not that turbulent flow is draggy, too. It is. Everything is draggier than laminar flow. But detached flow is the worst, and anything you can do to avoid it or delay it will help reduce drag. Even if that means trying to keep the flow turbulent after it has already transitioned from laminar.

Detached flow can have other negative affects too, besides causing drag. Large-scale detachments can cause control surfaces (ailerons, elevators, rudders, etc) to lose effectiveness, causing control problems. This is one reason why a wing is designed to stall (massive flow detachment) at the root first, so that the outer ends of the wing (where the ailerons are) stay flying and roll control is kept as long as possible.

Bottom line, keep flow laminar as long as you can, then make sure it transitions to turbulent flow and does NOT detach.

There was another comment in this thread about water being incompressible and air not. While that's technically true, it's not relevant in most discussions of subsonic aerodynamics. Under a Mach number of about 0.7, air can safely be assumed to be an incompressible fluid, too.

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#13

Re: New Warships Hit 45 knots (52 mph)

10/22/2009 3:31 PM

The navy tried something similar to what you suggest. It was a retrofit to add buoyancy for the new equipment installed. They welded protruding dimples on the outside. The problem with it was it caused the ship to whip back and forth instead of a steady roll.

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#15

Re: New Warships Hit 45 knots (52 mph)

10/23/2009 12:04 PM

Back in the 80's I rubbed down the bottom of my windsurfer with wet and dry to get a matt finish, I thought it was to encourage the water to stick to the surface encouraging a laminar flow. I thought if I didn't do this the water would "trip up" and form little vortices causing resistance. I have no data but I got the idea from a TV programme about boat hulls. The same programme showed a new shape for ribs on the bottom of a speed boat inspired by looking at the streaks made by the water in marker paint on the bottom of a smooth hull as it moved through the water. In the way of these programmes there was a significant improvement. In many cases (Colani teapot recently came up) consumer resistance is a significant barrier.

In the yahoo article which I also read, the dimples filled up with pollen which increased the drag again, so the car had to be washed regularly. In the case of a ship, not being a marine engineer, I imagine getting profiles smooth enough to matter at that scale might be difficult and then there is the problem of antifouling.

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Anonymous Poster
#16
In reply to #15

Re: New Warships Hit 45 knots (52 mph)

10/23/2009 3:14 PM

I too recall surfers roughing up the gel coat hoping somehow for a performance boost.

If you look at a Darcy-Weisbach nomograph you will see that the laminar zone is very narrow (f = 64/Reynold Number) followed by a likewise narrow critical and transition zone between laminar and turbulent zones.

Such efforts seem to be aimed at trying to extend or maninpulate the benefits of not having complete turbulence at boundary conditions in the velocity profile between the fluid and the boundary interface.

The bottom line is it is very difficult if not impossible to extend the laminar zone over a wider range with any measurable velocity since the Reynolds number increases dramatically with fluid motion (as well as the boundary friction).

Very difficult but interesting problem. The subject of much research at places like Webb Institute and MIT.

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#18

Re: New Warships Hit 45 knots (52 mph)

11/01/2009 11:25 PM

Impressive but can't out run a missile!

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The Engineer
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#20

Re: New Warships Hit 45 knots (52 mph)

05/20/2010 4:03 PM
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