Hulls and speed potential

[ADeblois]

I don't know if you guys could help me out on this question, but here goes:

Say I wanted to design a fast ocean liner. Would it be better to use the minimum waterline length required for maximum speed or should I lengthen the hull for greater speed potential with greater efficiency? Essentially, I'm asking if pushing a minimum-length hull at the greatest speed possible is better than having a longer hull but operating at a much slower speed relative to its length. I think having a longer hull but operating much slower than optimum speed is costly and inefficient.

Basically I was thinking: wouldn't it be cheaper to build small and go at the fastest speed you can versus build bigger and not using all the speed you possibly can out of the hull. (ex: Titanic was designed for 23 knots max, but its 850' waterline length would allow her to betetr 34 knots...see the wasted potential in a longer hull?)

Tell me your thoughts on this...

 

[Anna Simpson]

Basically I was thinking: wouldn't it be cheaper to build small and go at the fastest speed you can versus build bigger and not using all the speed you possibly can out of the hull. (ex: Titanic was designed for 23 knots max, but its 850' waterline length would allow her to betetr 34 knots...see the wasted potential in a longer hull?)

Yes. But, from a business point of view, a smaller ship means fewer passengers. With a bigger ship, you can carry more passengers in less time, thus increasing your yearly profit. In 1912, the big ships got the media attention. And more media meant more passengers. And more passengers meant fewer trips. And fewer trips meant less time. And time is money.
Now, I'm no marine engineer, I'm no businessperson,but I just thought I'd throw my ideas out there. Any correction is appreciated.

 

[Michael H. Standart]

>>Tell me your thoughts on this... <<

Speed is not without some importance but it's not quite as important as you might think it is.

For the express liners it mattered because at the time, it literally was the only way to go, time was money, and keeping to the schedule was everything.

The catch: Speed is expensive...very expensive and this is one of the big reasons why very few of the express liners were even designed to break records, and why the ones which were needed a state subsidy to stay in the black.

An example of one of these "Ships Of State" was the S.S. United States, which remains the fastest passenger liner ever built. While there was a lot about the design which was secret for almost half a century, there wasn't really anything remarkable about it. What Gibbs and Cox did was take the propulsion plant of a Midway class aircraft carrier, re-arranged it a bit, and built a passenger ship around that plant instead of a warship. As long as the subsidy lasted, the ship did as well, but once it was withdrawn, her remaining in service became a non-starter.

There were other factors as well...a lot of them...but this was a big one. Mr. Boeing's 707 didn't help either.

If you take a look at passenger ships today, you'll notice that even the two Cunard vessels which are designed for the North Atlantic run are not designed for speed. They cater to a niche market and are designed to maximize passenger capacity.

 

[Rusty_S]

I don't know if you guys could help me out on this question, but here goes:

Say I wanted to design a fast ocean liner. Would it be better to use the minimum waterline length required for maximum speed or should I lengthen the hull for greater speed potential with greater efficiency? Essentially, I'm asking if pushing a minimum-length hull at the greatest speed possible is better than having a longer hull but operating at a much slower speed relative to its length. I think having a longer hull but operating much slower than optimum speed is costly and inefficient.

Basically I was thinking: wouldn't it be cheaper to build small and go at the fastest speed you can versus build bigger and not using all the speed you possibly can out of the hull. (ex: Titanic was designed for 23 knots max, but its 850' waterline length would allow her to betetr 34 knots...see the wasted potential in a longer hull?)

Tell me your thoughts on this...

The Olympic class liners were designed with a top speed of 23 knots or roughly around there. Titanic nearly hit 22 knots turning on 75 revolutions of her absolute maxium of 80 revolutions as stated by Bruce Ismay, so I have to say with ideal conditions Titanic like Olympic had a very good possibility of reaching maxium speed without running the engines at maxium speed. The Lusitania for instance was built for speed and she was powered by steam turbines which were not very fuel friendly. She and her sistership were mainly built for speed and not for the business of moving passengers. Reason why the government stepped in and helped cause they wanted to recapture the blue ribband from the germans. The Olympic class was built as a balance between speed and economy. Her Recprocating engine`s were more fuel efficient than turbines were, this means she could travel larger distances on less fuel. But to improve the economy instead of just wasting that low pressure steam after it exited the low pressure cylinders, they were instead fed to power a parson low pressure steam turbine which would further improve the speed but not hender fuel economy.

When the New York class of battleships were built, they were built around the largest triple expansion recprocating engines ever to be installed in a battleship while other classes of battleships being designed and built in the US were using turbines. The idea of this was fuel economy. Sure she wouldnt be fast but she could travel the world without refueling. I even did math on this comparing fuel consumption between the New York class of battleships and the Iowa class because they used two different types of powerplants. The Iowa class while it was faster top end her most economical speed was just 12 knots where as the New York class which had a top speed of 22 knots had a best economical speed of 15 knots. This could be done to compare the Mauritania class to the Olympic class. While the Mauritania class could run faster than the Olympic class her most economical speed was lower than that of the Olympic class. So when it comes down to it turbines are great for speed but you eat up a lot of fuel reguardless of hull design but recprocating engines are great for a balance of speed and economy. While they might not push a ship as fast as a turbine can they sure can run faster than a turbine can when trying to conserve fuel.

 

[William D. O'Neil]

The answer to all this can readily be found in the pages of Charles F. A. Fyfe, Steamship Coefficients, Speeds and Powers, 2nd ed. (London: E & FN Spon, 1920) (available on Archive.org) which tells you everything you need to know to choose ship coefficients and dimensions to meet a given set of requirements. Computers have made it easier today, and we have more data to work from, but the basic principles are the same.

The S.S. United States aside, no liner I know of ever had a service speed whose numerical value in knots significantly exceeded the square root of her LWL. That is, for liners V/SQRT[L] < 1.

Contrary to what ADeblois appears to assume, LWL does not inherently impose any limit on the speed of a ship. If it desired to design for V/SQRT[L] = 50, it's entirely possible. But there's a price, particularly in power and hence also in fuel consumption. Again leaving the special case of the S.S. United States aside, the tradeoffs always favored V/SQRT[L] < 1. For the Olympic class, V/SQRT[L] < 0.8, permitting a considerably fuller form than the Lusitanias. That brought all kinds of advantages from the standpoint of design and economy.