# How fast an ocean liner could we build today? **URL:** \ **Category:** Factual Questions\ **Created:** [February 7, 2010, 5:11pm UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039 "2010-02-07T17:11:52Z")\ **Posts on this page:** 11\ **Page:** 3
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [February 10, 2010, 4:48am UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039/41 "2010-02-10T04:48:09Z")
> [@Philster](#): > > I disagree. > > Who can build it… now? Please advise. > > Some entity can launch into a feasibility study, design, engineering, etc. Henry Bessemer had a pretty good shot at it in 1875. [Játékok Rix - Ingyenes Online Játékok - Játssz Most!](http://scienceray.com/technology/engineering/henry-bessemers-bizarre-cure-for-seasickness/) Think we could do better now? ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [February 10, 2010, 5:34am UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039/42 "2010-02-10T05:34:05Z")
> [@Sailboat](#): > > Looks like my understanding is (roughly) correct for “subcavitating” propellers. Which part of the wiki article do you say supports this? It seems to me simply to be saying cavitation can be a problem. It is not saying that cavitation is by any means necessarily a problem that is inherent to high speed and cannot be avoided by appropriate design. ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [February 10, 2010, 9:07am UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039/43 "2010-02-10T09:07:08Z")
> [@Princhester](#): > > Which part of the wiki article do you say supports this? It seems to me simply to be saying cavitation can be a problem. It is not saying that cavitation is by any means necessarily a problem that is inherent to high speed and cannot be avoided by appropriate design. [QUOTE=from the Wikipedia link] Cavitation can occur if an attempt is made to transmit too much power through the screw. At high rotating speeds or under heavy load (high blade lift coefficient), the pressure on the inlet side of the blade can drop below the vapour pressure of the water, resulting in the formation of a pocket of vapour, which can no longer effectively transfer force to the water (stretching the analogy to a screw, you might say the water thread ‘strips’). This effect wastes energy, makes the propeller “noisy” as the vapour bubbles collapse, and most seriously, erodes the screw’s surface due to localized shock waves against the blade surface. [/QUOTE] Two things: one, “too much power through the screw/at high rotating speeds” probably varies by propeller design, but it seems likely that there exists a speed for any propeller that would be high enough to cause problems, just that it’s much higher in some designs; and two, if cavitation problems can be eliminated by designing special “supercavitating” propellers, the fact that many (most?) propellers in use today are NOT supercavitating indicates that there must some tradeoff inherent in the design choice that many designers are choosing not to make. Otherwise supercavitating propellers would have universally replaced other designs. Hence, cavitation is a problem, as speed increases, _unless_ you have a special supercavitating propeller. . ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [February 10, 2010, 9:28am UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039/44 "2010-02-10T09:28:18Z")
It seems to me you are just surmising, and without too much upon which to do so. The simple fact is that many fast vessels go much faster than ocean liners without problems and I think it is monumentally unlikely that cavitation is the limiting factor on the latter. > [@Sailboat](#): > > Two things: one, “too much power through the screw/at high rotating speeds” probably varies by propeller design, but it seems likely that there exists a speed for any propeller that would be high enough to cause problems, just that it’s much higher in some designs Which just adds up to: you’d use a prop designed for the speed and power and then you wouldn’t have problems. > [@](#): > > Hence, cavitation is a problem, as speed increases, _unless_ you have a special supercavitating propeller. No, your own cite says they are “employed to gain increased speed by reduced friction” and says nothing of any \*necessity \*to use them to avoid cavitation. ---
**Author:** ![pancakes3](https://avatars.discourse-cdn.com/v4/letter/p/ba9def/32.png) [@pancakes3](https://boards.straightdope.com/u/pancakes3)\ **Post date:** [February 10, 2010, 4:10pm UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039/45 "2010-02-10T16:10:41Z")
> [@Whack-a-Mole](#): > > To be fair are any warships seriously armored anymore? The days of the insanely armored battlewagons are long gone. Thought in the modern era with over the horizon targeting and accurate missiles and all that they opt for killing the other guy first rather than surviving getting hit. > > To be sure, by their nature and size and metal composition they have some toughness but nothing like WWII standards I think. your question jogged my memory. aluminum is also more COMBUSTIBLE than steel. that’s why the result was so catastrophic. ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [February 10, 2010, 5:01pm UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039/46 "2010-02-10T17:01:20Z")
> [@Princhester](#): > > It seems to me you are just surmising, and without too much upon which to do so. Admittedly most of my information comes from reading fiction and nonfiction about submarines, where increased speed and decreased depth definitely causes cavitation damage despite enormous expense and care in prop design. I have also read (nonfiction) accounts of it being a problem in large surface ships, but that was specific and might not generalize to all large surface ships. Perhaps for sub props silence is such an important design factor that it dominates prop design, and perhaps supercavitating designs are unsuitable for subs. Would cavitation noise be noticeable on a cruise ship, and thus a design consideration? If I am erring, it is in assuming that the physics work the same for a sub at the surface and a ship at the surface, and that engineers are not hopelessly stupid (by continuing to produce subcavitating props when they could just make them all supercavitating – I assume they have a _reason_ for continuing to make the subcavitaing props for so many vessels). ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [February 10, 2010, 5:12pm UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039/47 "2010-02-10T17:12:59Z")
You can see the damage cavitation causes [here](http://upload.wikimedia.org/wikipedia/commons/e/e6/Cavitation_Propeller_Damage.JPG) (and this is a small propeller). On small propellers the loss in efficiency is probably minimal and they are not all that expensive to replace. On a large ship the expense of a new propeller is considerable (not just the prop but removing and installing the new one) so they really want to avoid that damage. Cavitation is very bad for subs due to the noise it creates (which is directly counter to the sub remaining hidden). As mentioned above the US spent considerable effort in prop design for subs to minimize this (although they can still cavitate with enough power applied). IIRC Toshiba got in big trouble for selling computerized milling machines to the Soviets that, at the time (80s?) gave their subs a considerable jump in stealthiness that would have taken years for the Soviets to achieve on their own. For other surface ship prop design will be determined by what you want the boat to do. Speed, power or whatever. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [February 10, 2010, 5:21pm UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039/48 "2010-02-10T17:21:26Z")
These are the fastest medium large ships I know of. [US Navy littoral ships](http://www.naval-technology.com/projects/littoral/) > [@](#): > > Core capabilities of the littoral combat ship > A full load displacement draft of 10ft allows the ships to access very shallow waters. The ships will have a top speed of about 50kt and the range at sprint speed is 1,500nm. At an economical speed of 20kt, the range is 4,300nm. > > "Mission packages will be: mine warfare (MIW), anti-submarine warfare (ASW) and anti-surface warfare (SUW)."The ships are configured with a helicopter deck and hangar. The deck is capable of the launch and recovery of the MH-60R/S helicopter and a tactical unmanned air vehicle. The ships can carry out aircraft launch and recovery in conditions up to sea state 5, i.e. in winds up to 27kt and average wave heights between 6.4ft and 9.6ft. The ships will be capable of launching and recovering watercraft, for example 40ft high-speed boats, within 15 minutes in conditions of sea state 4, i.e. waves up to 5ft and winds up to 21kt. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [February 10, 2010, 8:32pm UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039/49 "2010-02-10T20:32:55Z")
I remember the rule that for an aircraft to go twice as fast, it needed 8 times the power. Likely it’s less for a ship, as it can get up and out of the water at speed. But the question, as with cruise ships, is “why?”. If you want to get there fast, nothing beats an aircraft. If you accept it’s three days across the pond, give or take, why spend the money on the engines and fuel? So yes, they can design hydrofoils, ground effects, etc. - but the need isn’t there usually. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [February 10, 2010, 9:00pm UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039/50 "2010-02-10T21:00:45Z")
> [@Whack-a-Mole](#): > > For other surface ship prop design will be determined by what you want the boat to do. Speed, power or whatever. You are correct in this statement. Look at the difference between a merchant ship’s screw and a Navy screw. Take a ship like a destroyer. 8,000 tons, 50,000 hp, flank speed excess of 30 knotts. Flank RPM I believe in the range of 400+RPM. Large freighter 20,000 tons, 10 to 20,000 hp, full speed maybe 20 knotts, full speed RPM some where under 120 RPM. the destroyer at flank speed is cavating like mad but it gets there. The merchant ship does not want cavation because of loss of efficiency. ---
**Author:** ![jdub22](https://avatars.discourse-cdn.com/v4/letter/j/4da419/32.png) [@jdub22](https://boards.straightdope.com/u/jdub22)\ **Post date:** [February 11, 2010, 5:54pm UTC](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039/51 "2010-02-11T17:54:44Z")
If you want to build a fast ship or sub, you have to use nuclear propulsion. not only will you be fast, you can stay at sea for a long period of time. There’s a site all about Navy nuclear propulsion and how you can get involved in working with this stuff. Check out [navy.com/nuclear](http://navy.com/nuclear). [Previous page](https://boards.straightdope.com/t/how-fast-an-ocean-liner-could-we-build-today/528039.md?page=2)