He was in the Navy a long time ago, so things may be different now.
Yeahuh. All that proves is that… you don’t know jack about ship design. Sure, you can shove enough power into the screw propellor to make it go a little faster, but on a practical level, the hull is the same bloody shape ships (sailing, not galleys) have been in since Carthage and Rome went at it, and there are some soft but tough limits that elementary physics puts on it. These principles have been known since Newton walked the earth, at least.
All our advances in shipbuilding knowledge have involved engineering the ship’s power systems and making them tougher. We changed How we build but not What we build. You can model fluid mechanics all you want, but it ain’t going to change the fact that a ship’s length and width fundamentally determine what kind of speeds it can attain at any given power output.
If a ship’s speed is limited by hull length, how on earth would a city-sized nuclear carrier outdistance a fast frigate or destroyer? I’m assuming the bows of all warships are more or less the same shape- is that correct?
This figure File:HullSpeed.PNG - Wikipedia shows power requirements versus speed and has a mark at the conventinal “hull speed”. It can be seen that power requirements grow exponentially with speed but that there is no clear cutoff point.
A clear sign that it is not as simple as performing a few calculations is that computers are used for creating models and that real models are tested in basins.
So if this bulbous bow is more… fluiddynamic than a raked bow, why don’t smaller ships use the design? Does the ship have to be a certain size to make it practical? IOW, is there a downside?
My WAG is that it gives the ship a much larger turning radius, which is fine for a carrier which isn’t going to be making any sharp turns anyway, but not good for a littoral ship or frigate.
Those are bow thrusters. They are propellers which push the bow to one side or the other and help the ship maneouver without the help of tugs.
A smaller boat finds more resistance in the waves while a larger ship finds more resistance in the resistance of the water flowing. They are basically very different.
In small sailboats a longer keel does indeed give the boat more directional stability and makes it harder to turn while a shorter keel makes it more maneouverable. I do not think this is a big concern for an airplane carrier.
Point: fluid dynamics ain’t that awesomely relevant to shipbuilding, because there’s only so many ways to shape a ship. Trial and error may not be perfect, and it doesn’t enable much in the form of radically different options, but it’s very good for finding the optimum under a set of reasonably known constraints. Over the last few centuries, constraints have loosened but in no way been eliminated. Fluid dyhamics may help you tweak a ship to get a slightly better performance, but without going to a very, very radical design (like a trimaran hull with a souped-up propulsion to skate the surface) you ain’t changing the basic math.
Sure, you can get some improvements, but not only were those not in use with our aircraft carriers (which were built before the use of computerized FD anyway), but we’re not talking vast changes here. Whether that’s significant or not to soldiers is another matter; you’d have to talk to them with the final product in hand. But if it can be done it will, since some computer modelling is pretty cheap relative to the output.
As far as I’m concerned, the only major changes in navies have been weapons technology, metal hulls, and modern powered drives. And only one fundamentally affects ship performance characteristics, and that by altering thrust capability and not fundamental forces. You’d be better off getting improved hull materials than trying to find some new super-design for ships.
Excuse me but after reading this I thought I would add to it.
Years ago as an engineering Midshipman I remember in one of the deck classe I had to take there was discussion of ship design speed. If I remember right there is a ratio between the lenght and with of a ship and the required HP to reach a definate speed. If I remember right it is a parabolic curve. The amount of power to increase the ships speed would require larger and larger amounts of power where it becomes impratical to exceed the max design speed.
As to the time required to bring a steam turbine up to speed. If a ship is at a stop bell and all boilers are line when a full ahead bell is rung up the throttle valve is opened until the screw is turning at the designated RPM, this takes 10 to 30 seconds.
Now the boiler may go from having one burnner with low oil pressure to three or more burners with a higher oil pressure. The boiler water level will have to be watch. The forced draft fan’s speed will have to be increased.
Just turnning the screw faster after a point will not increase the speed of a ship because of cavitation, the screw slipping in the water. I was taught that above 82 RPM the efficiency of a screw dropped of by the square of the increase.
The fastest designed merchant ship was the SS United Stated. Design speed was in the range of 40+ knotts. Required 250,000 HP in 4 screws.
You are talking here about “hull speed” and sailor already commented earlier in the thread.
On any given ship, ie for a given hull and screw, this may be true, but I think the number you quote will be specific to a particular vessel. There are vessels that turn the prop very much faster.
Look, this is just plain stupid. It is not true that calculating the resistance of a hull through the water is easy and can be found in 300 year old books. And if you say it is true you better provide some proof because that is an extraordinary claim.
My citations show that it is a very complex thing, that theory and equations have been developed much more recently than 300 years ago, that even today modeling is used because theoretical calculations do not provide the entire solution and that some equations still need to be developed and there is a prize for whoever does it.
So please stop spouting this bullshit that " he showed them how to calc it, and that the information was in 300 year-old books". It is not in 300 year old books and it is not any simple calculation then or today. And that incident never happened as you tell it. It is ridiculous in its face. If you want to contradict this please feel free to use some cites but please stop posting your own ignorance as if it were fact. Anyone who knows anything about the topic is laughing at you.
The advances in fluidics has been phenomenal and computers allow calculations which were impossible 300 years ago.
There has been more advancement and change in hull designs in the last 75 years than in all earlier history together. Bulbous bows and bulb keels, to name just two obvious ones, did not exist before then.
It is extremely ignorant to say that in the last 300 years nothing has advanced in the knowledge of how hulss behave in the water. It is just not true. A lot has been learnt and a lot is still to be learnt and my cites prove it.
I am going to go out on a limb here and say you are not an engineer and you are not into sailing because if you ask any engineer or any sailor this is something they would know. Anyone who has followed the America’s cup knows this.
Cavitation is not “the screw slipping in the water”.
Cavitation hurts prop performance and damages the prop but with or without cavitation props have an optimal design speed and their performance will suffer above that.
Some props have blades which can be rotated to change the attack angle. These are caled “feathering” props. These can perform better in a wider range of RPM but will not perform as well at any one given point.
Prop design is another field which is very complicated and theoretical calculations do not provide the full answers so that extensive testing is needed.
Both hull and prop have a speed at which they perform best.
Bad analogy. These figures are given regularly by manufacturers of aircraft.
The aircraft I fly can go to 25,000 feet, it can be loaded to 43200 lbs on the ground and 43000 lbs for take-off, and it can go 2200 nautical miles. Your point is kind of valid in that most answers given should come with a caveat. It’ll do 2200 miles flown at long range cruise speed but not at max cruise speed for example, but that doesn’t stop the manufacturer from giving precise figures.
sailor, if there is a tiny grain of truth in what smiling bandit is saying it is that it has been known for a very long time how to calculate a vessels’ very approximate “hull speed” which (as you said earlier) does give you a rule of thumb indication as to how fast a conventional hull can be made to go easily. But as you say refinement of these sorts of calculations has come ahead in leaps and bounds in recent decades.
And furthermore, the idea that you could use 300 year old knowledge to panic anyone in the navy who had even the most basic knowledge of hull design is laughable. That knowledge is what they would have learned in the first hour of the first day they started receiving training. If smiling bandit’s friend panicked anyone with this stuff, its because he was speaking to the wrong guys.
You are making my point. The answer is “it depends”. All those things interact with each other so that you can fly heavier but closer or you an fly farther but lighter. They depend on altitude of the airport. They depend on air temperature and humidity etc.
The fact that a car manufacturer gives you a MPG figure in the brochure or the airplane manufacturer gives you some figures in the brochure is only an indication and does not mean those are guaranteed figures nor that they are the maximum figures. They are just to give you an idea.
The equivalent answer for the Nimitz is to be found in the brochure and it says 30+ knots. But the OP specifically said he was looking for an actual performance figure not for the brochure answer which we already had.
I am sure you can find somewhere the max speed for a Honda Civic. That does not mean it is an absolute barrier and that no one has gone beyond it or even that they would all reach it. Much depends on the circumstances.
Well, my point is that if the Nimitz brochure said 34.5 knots, the OP would’ve been happy, even though the figure still comes with a big “depends.” So does the Navy give a figure of 30+ knots because the figure varies (it doesn’t stop other entities giving exact figures), or do they just not want to put an actual figure for security reasons? Is it possible to get more exact speeds for other naval vessels? The Australian Navy seem happy enough to claim their Armidale Patrol boats are capable of 25 knots, not “20+”, or “24+”, or “it depends.”
Even the concept of “hull speed” is much more recent than 300 years. The concept of Hull Speed is a rough simplification of the Froude Number which dates to the second half of the 19th century.
Hulls 300 years ago were extremely complicated to build out of timber but as to their proportions the books at the time just gave some general proportions for length/beam/height and left it at that. Designing hulls was an art and they were very often built as small half models and then those proportions were enlarged to build the actual frames.
I have read several old books about sailing ships and they are generally very primitive. All the knowledge was gained from experience and there was little or no scientific calculations (except in astronomical navigation and even that was very primitive and did not mature until the late 19th century with the development of the intercept method, later improved in the 20th century).
I do not think anyone 300 years ago, even ship designers and builders, worked with numerical concepts like resistance of hull to advance through water or actual power delivered by sails. It was all just practice and experience.
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