# Could an oil tanker rescue someone at sea? **URL:** \ **Category:** Factual Questions\ **Created:** [April 11, 2002, 5:25pm UTC](https://boards.straightdope.com/t/could-an-oil-tanker-rescue-someone-at-sea/103059 "2002-04-11T17:25:09Z")\ **Posts on this page:** 7\ **Page:** 2
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [April 14, 2002, 1:48am UTC](https://boards.straightdope.com/t/could-an-oil-tanker-rescue-someone-at-sea/103059/21 "2002-04-14T01:48:16Z")
[USS Kidd Class DDG](http://www.fas.org/man/dod-101/sys/ship/ddg-993.htm): Length 563, Speed 33 kts, Hull speed 32.03 kts [USS Charles F Adams Class DDG](http://www.fas.org/man/dod-101/sys/ship/ddg-2.htm): Length 437, Speed 30 kts, Hull speed 28.22 kts (This Class hass been retired) [USS Farragut Clsss DDG](http://www.fas.org/man/dod-101/sys/ship/ddg-37.htm) : Length 512, Speed 33 kts, Hull speed 30.54 kts [USS Arleigh Burke Class DDG](http://www.fas.org/man/dod-101/sys/ship/ddg-51.htm) : Length 466, Speed 31 kts, Hull speed 29.24 kts (Hull speed in knots = 1.34 \* Sqrt(length in feet)) ---
**Author:** ![donkeyoatey](https://avatars.discourse-cdn.com/v4/letter/d/e47c2d/32.png) [@donkeyoatey](https://boards.straightdope.com/u/donkeyoatey)\ **Post date:** [April 14, 2002, 2:20am UTC](https://boards.straightdope.com/t/could-an-oil-tanker-rescue-someone-at-sea/103059/22 "2002-04-14T02:20:07Z")
I guess thread has been boarded by pirates and hijacked already, so > [@](#): > > (Hull speed in knots = 1.34 \* Sqrt(length in feet)) This can’t be a rigid rule, surely. Would a ULCC 1500 ft. long therefore have a hull speed of almost 52 knots? And does the difference between displacement and semi-displacement hulls have any bearing on what **Tranquilis** is saying about destroyers exceeding hull speed? ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [April 14, 2002, 2:35am UTC](https://boards.straightdope.com/t/could-an-oil-tanker-rescue-someone-at-sea/103059/23 "2002-04-14T02:35:59Z")
> [@](#): > > Would a ULCC 1500 ft. long therefore have a hull speed of almost 52 knots? Sure. As long as someone wanted to invest the money in installing engines and screws large enough to drive it. Hull speed isn’t how fast it _will_ go, it is the speed above which enormous amounts of power are required to push it. According to [John F. Hughes](http://www.cs.brown.edu/people/jfh/personal_other/boats/FAQ/node19.html) > [@](#): > > The hull speed, by the way, can loosely be thought of as the speed at which the boat, in order to go faster, has to start ``climbing up’’ over its bow wave, which takes a lot more power. I picked the formula I did simply because it appears on every on-line calculator I found. Someone who actually understands marine architecture will have to address the possible variables. (For example, I am a bit surprised that the hull speed takes no acount of beam. A supertanker has very nearly twice the beam of a large carrier, and I (not knowing the physics of marine architecture) would have thought that that would have a serious effect, yet no reference I have seen addresses it at all.) ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [April 14, 2002, 2:42am UTC](https://boards.straightdope.com/t/could-an-oil-tanker-rescue-someone-at-sea/103059/24 "2002-04-14T02:42:46Z")
(Additionally, all the U.S. 1,000+ foot carriers have a “rated” speed of 33 knots, yet I have heard persistent rumors that when they open them up, they exceed 40 knots–right in the range of the 42 knots that their hull speed would indicate.) ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [April 14, 2002, 2:55am UTC](https://boards.straightdope.com/t/could-an-oil-tanker-rescue-someone-at-sea/103059/25 "2002-04-14T02:55:27Z")
That formula is a _very_ simplified “rule-of- thumb” applicable mainly to small boats with very standard hulls (and the coefficient ranges from 1.2 to 1.4 depending on the boat type etc). I know I have a more complete formula somewhere but I cannot remember where and it is too late for me to search the Net. It is easy to see the simplified formula is aplicable only to certain narrow ranges because it is not applicable to catamarans due to the hulls being very narrow for their length. The destroyers have a length/beam ratio of about 10 whereas my boat it’s closer to 3 and that makes a huge difference. We should also realise the concept of hull speed is in itself a simplification even when calculated with the more complex formula. If you plot power/speed it is not like there’s a precise point, rather the curve gets steeper. The way I understood “hull speed” was defined was that speed which when exceeded would cause the boat to plane (surf). I’m not saying it is not possible for a destroyer to exceed hull speed but I’d like to see this confirmed by some authority rather than by our sloppy calculations. ---
**Author:** ![donkeyoatey](https://avatars.discourse-cdn.com/v4/letter/d/e47c2d/32.png) [@donkeyoatey](https://boards.straightdope.com/u/donkeyoatey)\ **Post date:** [April 14, 2002, 5:01pm UTC](https://boards.straightdope.com/t/could-an-oil-tanker-rescue-someone-at-sea/103059/26 "2002-04-14T17:01:44Z")
http://www.sciam.com/1097issue/1097giles.html.]This article lifted the fog a little: > [@](#): > > Froude noted several reasons why a ship’s speed-length ratio is typically somewhat less than that of an ocean wave, which has a ratio of 1.25. Most important, he noted, was the wave-making resistance (now called wave or pressure drag) imposed by the captive wave. Although in theory a modern cargo ship of 700 feet should be able to make 34 knots, its optimum speed in calm water is in fact only about 23 knots, which represents a speed-length ratio of 0.87. ---
**Author:** ![Tranquilis](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tranquilis/32/3639_2.png) [@Tranquilis](https://boards.straightdope.com/u/Tranquilis)\ **Post date:** [April 14, 2002, 11:48pm UTC](https://boards.straightdope.com/t/could-an-oil-tanker-rescue-someone-at-sea/103059/27 "2002-04-14T23:48:36Z")
> [@](#): > > \*Originally posted by sailor \* > \*\*Well, I’ve never heard of it and I’d like to see some proof. The link I gave shows a destroyer which cannot exceed hull speed. can you show me one that _can_? I’d just like to see it as I have never seen it. Can you give me some cites? \*\* Go away for a day, and you find all kinds of people have answered questions aimed at you! One small quibble: the Adams-Class was _admitted_ to be capable of 31.5 knots, vice 30. [Previous page](https://boards.straightdope.com/t/could-an-oil-tanker-rescue-someone-at-sea/103059.md?page=1)