Joined: March 1st, 2005, 3:53 pm

March 11th, 2018, 2:13 am #21

As for having higher speeds listed on the throttles - the Iowa class was designed for a "normal" max power of 212,000shp - which would produce 32.5 knots at their WW2 trial displacement of 53,900 tons*. They also had a designed "safe" overload rating of +20% - 254,000shp. This would theoretically get 35.4 knots at 51,000 tons (a very light displacement, considering that 45,155 tons of this is basic structure). Perhaps another 10% could be forced at the risk of damage - achieving ~280,000shp.

However, it was also expected that prop cavitation would begin at just over 34 knots at 53,900 tons - thus limiting their speed in a normal seaway to that regardless of what shp was developed. The two times that an Iowa actually achieved 35 knots were both just after refit, at a light displacement (machinery trials), and in shallow water (the bottom prevents water from being displaced downward, thus lifting the hull a bit and delaying prop cavitation by keeping air bubbles compressed).


* The USS New Jersey's weight table of 1943 showed a Full Load Displacement of 58,132 tons.


Now apply this to a CVN - what speed would the props cavitate at, regardless of shp applied?
There it is... the District of Columbia! You will never find a more wretched hive of scum and villainy. We must be cautious.
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Joined: May 5th, 2006, 5:38 am

March 11th, 2018, 4:06 am #22

I seem to recall something about the Action off Truk, where the CO of one of the US Battleships asked her chief engineer if he could get more speed out of her and he replied that he had steam but no place to put it ... the throttles were wide open.  The CVNs are probably capable of producing more steam than the propulsion turbines can use.
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Joined: May 5th, 2006, 5:38 am

March 11th, 2018, 4:13 am #23

One thing I don't understand is why cavitation would limit maximum speed.  Don't surface ships cavitate most of the time?  I know that on SSNs we tried not to cavitate, but if necessary we could increase speed well beyond the point of cavitation, depending on the depth we were at ... we would just be a lot noisier if we did.  Of course, the deeper you go, the higher the shaft rpm where cavitation begins.
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Joined: May 29th, 2010, 5:10 pm

March 12th, 2018, 3:40 pm #24

Cavitation builds up as the propeller speed increases. The power the propeller can put into the water decreases as it does. At some point, the point will be reached that more propeller speed just creates more cavitation and no more power into the water. Heck, at some point (theoretically) I bet you could have a super-cavitating prop putting absolutely no power into the water in terms of usable thrust.
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Joined: April 18th, 2006, 6:51 pm

March 13th, 2018, 5:45 pm #25

SaintsWillWin wrote: A site I was on posed the question of the top speed of a USN nuclear carrier; below is a response.

I did hear a story once from a very old hand, many, many years ago, about the time USS Theodore Roosevelt got into a race with USS Pegasus. He was there, so this is no sh!t:

According to the guy who told me, the ship's captain at the time was a former RA-5C Vigilante pilot, and he still had his need for speed.

So the two ships were operating in the same formation, when the Captain of the Pegasus kicked in the gas turbine, put the ship on her foils, and sped away.

The Captain of the TR saw this, and ordered the ship's top speed, and was still unable to catch the hydrofoil.

He started calling down to the engineering plants, and after making a lot of incredibly unorthodox changes in plant lineup, the ship was able to not only catch and pass the Pegasus, but did so with such a massive bow wave generated that it damaged the foils on the smaller ship, and she had to put in for repairs.
"He was there, so this is no sh!t:"

 . . . and there's the disclaimer.
Andy

("Never trust anything you've got to sign for - look what happened to Chamberlain.")
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Joined: March 1st, 2005, 3:53 pm

March 15th, 2018, 4:09 am #26

Steve Crandell wrote: One thing I don't understand is why cavitation would limit maximum speed.  Don't surface ships cavitate most of the time?  I know that on SSNs we tried not to cavitate, but if necessary we could increase speed well beyond the point of cavitation, depending on the depth we were at ... we would just be a lot noisier if we did.  Of course, the deeper you go, the higher the shaft rpm where cavitation begins.
pilikia wrote: Cavitation builds up as the propeller speed increases. The power the propeller can put into the water decreases as it does. At some point, the point will be reached that more propeller speed just creates more cavitation and no more power into the water. Heck, at some point (theoretically) I bet you could have a super-cavitating prop putting absolutely no power into the water in terms of usable thrust.
The point being "where are the bubbles being generated, and can the water-flow keep them clear of the prop blades"?

Yes, props to create air bubbles in water at most RPMs - but the ship's forward speed keeps most of the prop blade surface in solid water, with the bubbles trailing behind the props.

If the prop rotation is too fast (as happens in overload power situations), then the bubbles aren't cleared away fast enough, and the bubbles begin to surround the blades - and there is less and less (eventually no) solid water for the blades to grab and pull/push against.
There it is... the District of Columbia! You will never find a more wretched hive of scum and villainy. We must be cautious.
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Joined: August 30th, 2005, 9:15 am

April 5th, 2018, 9:48 am #27

Tedious stories of how incredibly fast CVs are usually boil down to 'she was going so fast we could waterski behind her'. That'd be 12 knots then.

Anyway, Pegasus was good for 48 knots. Roosevelt was according to wiki rather longer than an Iowa, and rather more powerful. If we generously give iowa 35 knots, then hull speed for Teddy was about 37 knots, so you've just got 11 knots to go.

Believe it if you like.
"I'm all in favor of keeping dangerous weapons out of the hands of fools. Let's start with typewriters."
- Frank Lloyd Wright (1868-1959)
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Joined: August 30th, 2005, 9:15 am

April 5th, 2018, 9:55 am #28

here's a handy curve
"I'm all in favor of keeping dangerous weapons out of the hands of fools. Let's start with typewriters."
- Frank Lloyd Wright (1868-1959)
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Joined: March 24th, 2011, 11:36 pm

April 5th, 2018, 10:23 am #29

The nuclear carriers are designed for a fleet speed and it's just over 30 knots. There's no point in going to the trouble and expense of making them faster as what would be the point? They can't out-pace the safety of their escorts.
A Moron Is Governing America

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Joined: May 5th, 2006, 5:38 am

April 5th, 2018, 11:49 am #30

Rmor wrote: The nuclear carriers are designed for a fleet speed and it's just over 30 knots. There's no point in going to the trouble and expense of making them faster as what would be the point? They can't out-pace the safety of their escorts.
I agree that CVN speeds are probably grossly exaggerated,  but they do make high speed redeployments without escorts or with only one, which they slow to refuel daily.  To say they "can't out-pace the safety of their escorts" is simply not true.
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