Glosters UK

Can somebody answer why the top speed for modern warships, be it aircraft carriers of all sizes to Destroyers or Frigates seem to be 30 knots??

It seems to me that older warships, (Hood/Bismark etc) were capable of the same 30 knots.
If it is a restriction of engines then why can a 50,000+ ton aircraft carrier be the same speed as a type 42 destroyer or an invincible class 20,000 ton smaller carrier??

Does anyone know as it does not make sense to me that a modern destroyer is the same speed as a huge US carrier and they are both hovering around the 30 knots mark which seems to have been around since the war!?

Dreadnought

It has greatly to do with the design of the ship. Length,Beam, Tumblehome,Draft. Its truelly a science. Its no wonder they use wave pools for alot of design work before one sets out to build a ship.She has to be sleek enough to cut the water like a sword and yet long enough to sustain the speed and stability the powerplant offers and still remain responsive steerage and roll wise and have enough draft up front. There are so many mitigating factors in ship design it really baffels you to say the least.


As far as speed goes towards design characteristics think about the WWII era PT boats as an example. Very small, very light and extremely fast aprox 40 knots plus all out of two diesel engines. (Weight,Length,Propulsion and design all account for this and we know how good they were at their job). These were great lil boats with big teeth as far as threat wise to opposing navies.

The U.S. wont tell you the true speed of their carriers albeit above 30 knots.
for the nuke classes.

The newer U.S. DDG's are unbelievable as compared to the old. Gas Turbine propulsion is a big difference as compared to the old steamers. Instead of an few hours its now down to a matter of seconds and your underway. Technology is great.

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Glosters UK

Thanks,

So although quoted as 30 knots many destroyers etc are actually above that figure?

I was mainly thinking of the bigger end of warships, carriers,destroyers,frigates etc ... they all claim the same approx 30 knot mark! Given the size difference this does not make sense to me!?

Dreadnought

IMO and others may agree/disagree but almost any military will not post the true speed of their warships open to the public.

Weight/Size/Design for a class of ship = Speed and manuverability in so many words. Much more factors into this but Im no expert.

SnowLeopard

I seem to recall a mention somewhere that in order to get above 40 knots or so, the hull needs to be out of the water. I'll see if I can find it, find the equations for speed but while that seems to be the topish figures, someone can always come up with a new theory on how to do it better.

Ie, there was once a speed limitation for aircraft but Kelly Johnson found a way to bypass it but in order to do it, one had to get rid of the propeller.
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("Can't we go any faster?"--Federal Agent Mahoney
"We're too heavy for that ...... unless you would like to get out (of the boat) and swim along side for a while."--Dr. Jake Terrell, (w,stte), "Day of the Dolphin")

dundonrl

we don't even start our main engines till about 20 minutes prior to leaving the pier when were getting underway, and it literally could be done in less than a minute, from starting them to moving..

Dreadnought

From what I was told by an officer onboard a newer DDG 90 seconds is all you need to get underway. No sure in what kind of state though. But 90 seconds as compared to hours on burners? Luxury!

ArmchairGeneral

30 knots is about the speed that carriers need to be able to launch aircraft in a calm. Thus, all her escorts have to be capable of at least 30 knots, or at least fast enough to catch up. The actual maximum speed depends on the ship. Italians like to make fast ships, 35 or 40 knots sometimes, some others are fast too. A lot of modern frigates are a little slower than 30 knots, upper 20s typically, especially ships that aren't meant to escort carriers.

Most USN ships' speeds are given as "above 30 knots" which is a nice way of saying "none of your business." However, as I understand it, the rumors of US carriers making incredible speeds of 45 or 50 knots are just that, rumors. Max speed is maybe 35 knots.

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ArmchairGeneral

Although size does matter, the relationship between size and speed is rather counterintuitive. It is actually easier to make a large ship fast than a small ship. I believe it has to do with a better volume/surface area ratio. So if anything, large ships would be faster, except that you don't want to outstrip your escorts. Carriers don't need to be any faster than 30 knots, so none of the other ships need to be faster, although they sometimes are.

RustyBattleship

Regardless of how powerful engines you put in, or how efficient the propellers are, it's the hydrodyamic shape of the hull that's the restricting factor.

Many of our latest DD's can go over 35 knots thanks to two things, variable pitch propellers and bulbous bows. The bulbous bow is actually the housing for the SONAR transducer.

Many years ago, David Taylor did hydrodynamic studies of scale model hulls and foung that a sharp edged bow is merely cutting through the water but water resistance was still constant along the sides of the ship. But a bulbous bow CARRIES the bow wave of water with it, thus it's the bow wave that pushes aside the rest of the water. (Also it's the bow wave that Dolphins and Porpoises swim in giving the appearance that they are as fast as the ship when in reality they are only hitch hiking).

We do have some 50 and 60 knot ships, but they're not very big and when up to speed there is very little of the hydrofoils UNDER the water. They're sort of like a water skier with his own inboard-ouboard motor.

Oh dear! I shouldn't have said that. Now we're going to have some nut out their riding twin surfboards with a Mercury outboard on it.

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dundonrl

Pegasus class comes to mind, the USN's hydrofoils of the 70's and 80's.. I don't think they carried a big enough weapons loadout to justify their cost, but they were definitely fast..

SnowLeopard

IMHO, they probably justified thier cost in the Cold War; they were used for choke point tactics.

Considering of the Israeli destroyer Eliath was attacked, looking at the traffic of Gilbralter and the straits of Messina, and just imagining that it would be similar in other places, consider:

One has a boat with a 60 mile missile that one can launch while in the background clutter of coast, ports, lots of boats. Eight missiles, either against a single ship or a fleet, could make a heck of a mess. Even if none of the missiles hit, there still could be damage from evasive action collisions with the boats in the area. Even if the fleet is further out than 60 miles, the launcher can jet out to a launch point, engage, and jet back in to its hiding point.

Now, were those the tactics they had in mind? Don't know, saw a Pegasus only once and was never involved with it. But maybe.

One interesting little sidepoint, though. When it comes to chaff, most ships have SRBOC, Super Rapid Blooming Offboard Chaff. Pegasus class, however, didn't. They only had RBOC. At 60 knots, I guess they didn't need the super capability.
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("Well, we can still out maneuveur them!" and Han puts the Em Falcon into a spinning dive between two star destroyers, causing them to take screaming action to avoid immenient collision, (w,stte), "The Empire Strikes Back")

Garry

There is ships which has speed well beyond that... WIGs. Lun is armed with six mosquit missiles and is being now in Caspiysk.....

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ArmchairGeneral

Might be useful in the littorals where tactical speed seems to be of comparatively greater importance. Or so say the LCS people. Yay for 40 knot frigates.

SnowLeopard

We-ll, yes, now we are getting into an interesting area of things ...... or perhaps the original question should have been restricted to just hull borne.

Hydrofoils, hovercraft, ground effect machines: are they ships?

Oh, that could be argued back and forth but the basic point is that the more one gets the hull out of the water, the faster they can go.

Personally, I've always seen ground effect machines as very low flying aircraft, using the built up air between the surface and their wings to ride on. It's a nice way to go but there are limitations.

Consider the overweighted airplane taking off. It gets off the ground and it is flying, right? Well, as any number of accident reports might show, it will be in the air until it leaves the runway, runs out of it. Then there isn't a "smooth" surface, the air cushion breaks up on rough land, and the airplane crashes.

Similar thing here: as seas get rough, it is harder for hovercraft and gound effect machines to move on that cushion of air.
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(The tanker Captain watched the Iranian (of the Shah's time) hovercraft be bounced around on the seas. Hovercraft were great for the calm waters of the Persian Gulf but out in the open ocean, they were at the mercy of the elements.--(w,stte), Book "The Ship Killer" by Justin Scott)