# Why do ships that carry oil use one propeller while other ships use more than one? **URL:** \ **Category:** Factual Questions\ **Created:** [July 30, 2017, 4:34am UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424 "2017-07-30T04:34:19Z")\ **Posts on this page:** 15\ **Page:** 2
**Author:** ![Corry\_El](https://avatars.discourse-cdn.com/v4/letter/c/ecae2f/32.png) [@Corry\_El](https://boards.straightdope.com/u/Corry_El)\ **Post date:** [August 1, 2017, 5:39pm UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/21 "2017-08-01T17:39:45Z")
> [@CannyDan](#): > > In addition to the other reasons given, there is the maneuverability issue. Single screw has some “yaw” built in, because of the screw turning in one direction. This is compensated for by “way” on the boat, as an action of the rudder and possibly the keel, and when going forward is pretty negligible. The effect is much more pronounced when reversing and can make backing into a tight area or directly to a specific target (dock, slip, other ship) somewhat difficult. Twin screws make for much greater control of directionality, both forward and reverse. This is especially valuable in tugs, given their constant interactions with other, often much less maneuverable, ships. Yes sure. I was thinking mainly in terms of the original ‘why do ships that carry oil’ and the reason for single screw tugs being banned (basically) from oil barge towing is redundancy. Articulated Tug Barge tugs rarely leave the ‘notch’ at the aft end of their particular barge, but they’re virtually all twin screw whereas similar size ‘one piece’ tankers much less often are. Although there are some tugs now which just do barge work which nonetheless have twin z drives (steerable props), not only twin screw. Stern z-drive was at one time a Far East thing mainly, the equivalent in Europe was cycloidal propeller (Voith Schneider by trade name), and such fancy gadgets were frowned upon by US tug operators. But now stern z-drives are very popular worldwide for shiphandling tugs (which tend to be somewhat specialized, jack of all trades work for tugs isn’t as common at it once was), and to some degree for offshore oil tug work, barge handling and river towboats. But relying on one engine is pretty much out of the picture for any newly built tug, whereas it worked well enough for many decades and is perceived to work reliability-wise for the great majority of big ships. ---
**Author:** ![CannyDan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cannydan/32/548_2.png) [@CannyDan](https://boards.straightdope.com/u/CannyDan)\ **Post date:** [August 1, 2017, 7:41pm UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/22 "2017-08-01T19:41:42Z")
> [@Princhester](#): > > Actually pilots use “cut” - as they call it - to good effect, to move the stern in a particular direction. Yes, I understand that and I’ve done it myself (if only on boats up to 30-ish feet). But the ‘particular direction’ is always _the same_ direction. If you need to sideslip the other way, you’re just out of luck. ---
**Author:** ![CannyDan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cannydan/32/548_2.png) [@CannyDan](https://boards.straightdope.com/u/CannyDan)\ **Post date:** [August 1, 2017, 7:45pm UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/23 "2017-08-01T19:45:12Z")
> [@Corry\_El](#): > > Yes sure. I was thinking mainly in terms of the original ‘why do ships that carry oil’ and the reason for single screw tugs being banned (basically) from oil barge towing is redundancy. Articulated Tug Barge tugs rarely leave the ‘notch’ at the aft end of their particular barge, but they’re virtually all twin screw whereas similar size ‘one piece’ tankers much less often are. > > Although there are some tugs now which just do barge work which nonetheless have twin z drives (steerable props), not only twin screw. Stern z-drive was at one time a Far East thing mainly, the equivalent in Europe was cycloidal propeller (Voith Schneider by trade name), and such fancy gadgets were frowned upon by US tug operators. But now stern z-drives are very popular worldwide for shiphandling tugs (which tend to be somewhat specialized, jack of all trades work for tugs isn’t as common at it once was), and to some degree for offshore oil tug work, barge handling and river towboats. But relying on one engine is pretty much out of the picture for any newly built tug, whereas it worked well enough for many decades and is perceived to work reliability-wise for the great majority of big ships. Thank you for this information! Although I did know about steerable drives/props, I wasn’t aware of all these variations. ---
**Author:** ![busterpickle](https://avatars.discourse-cdn.com/v4/letter/b/258eb7/32.png) [@busterpickle](https://boards.straightdope.com/u/busterpickle)\ **Post date:** [August 1, 2017, 8:22pm UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/24 "2017-08-01T20:22:51Z")
All boils down to risk I take it. If one oil tanker looses propulsion it won’t affect the market that much if at all. They have rescue boats for the crew if need be. And, I wouldn’t be surprised if in and around the shipping lanes there were other ships that make their business by lending assistance, like tow trucks parked on the side of the highway waiting for a car in need. So, if the cost of all that is less than cumulative cost of the second prop, fuel, equipment, manning, etc. Then that’s why. If the cost was high then they’d go with the second prop. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [August 1, 2017, 11:34pm UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/25 "2017-08-01T23:34:29Z")
> [@CannyDan](#): > > Thank you for this information! Although I did know about steerable drives/props, I wasn’t aware of all these variations. See post #19. 🙂 ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [August 1, 2017, 11:52pm UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/26 "2017-08-01T23:52:51Z")
One thing that I haven’t seen mentioned is that, in general, the larger you make something, the more efficient you can make it. A one inch deep pockmark in the side of a engine cylinder is a major issue in an engine that you’d have in your car. But in an engine the **size** of your car? Much less of a deal. On a supersized engine, the tolerances (as a comparison to the total size) are extremely tight, and so the whole thing runs much more efficiently. A single engine that is very large ends up being more efficient than two smaller engines that produce the same amount of power. (Granted, there’s probably a small loss due to the single screw trying to turn the ship, and being fought against.) ---
**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:** [August 2, 2017, 1:25am UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/27 "2017-08-02T01:25:45Z")
> [@electronbee](#): > > All boils down to risk I take it. If one oil tanker looses propulsion it won’t affect the market that much if at all. They have rescue boats for the crew if need be. And, I wouldn’t be surprised if in and around the shipping lanes there were other ships that make their business by lending assistance, like tow trucks parked on the side of the highway waiting for a car in need. > > So, if the cost of all that is less than cumulative cost of the second prop, fuel, equipment, manning, etc. Then that’s why. If the cost was high then they’d go with the second prop. No there are not rescue boats just waiting around for that one ship that might break down every year or so. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [August 2, 2017, 2:19am UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/28 "2017-08-02T02:19:04Z")
> [@Snnipe\_70E](#): > > No there are not rescue boats just waiting around for that one ship that might break down every year or so. Sure there are, they just do other work while waiting if at all possible. > **[Services](https://www.westerntowboat.com/services.html)** > > ​Western Towboat offers ocean towing, harbor towing, ship assist, container and transportation services. Our modern fleet of 1000-5000 HP Z-Drive and conventional tugs from 365 - 4000 HP are ready... Plus with these engines you can remove entire cylinders or multiples from the operation of the system and keep moving. And the engines are rated for continuous use for 175,000 hours, or about 25 years of normal use before major overhaul. While this is a rendering, note that the crankcase has man doors and ladders for access. [![](https://www.dieselarmy.com/wp-content/blogs.dir/1/files/2015/01/man-diesel-cutaway-thumb-2.jpg) ](https://www.dieselarmy.com/wp-content/blogs.dir/1/files/2015/01/man-diesel-cutaway-thumb-2.jpg) This does make the need for vessel assist rare, but there is a lot of traffic. > **[MarineTraffic: Global Ship Tracking Intelligence | AIS Marine Traffic](https://www.marinetraffic.com/en/ais/home/centerx:-157.8/centery:39.7/zoom:3)** > > MarineTraffic Live Ships Map. Discover information and vessel positions for vessels around the world. Search the MarineTraffic ships database of more than 550000 active and decommissioned vessels. Search for popular ships globally. Find locations of... ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 2, 2017, 8:28am UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/29 "2017-08-02T08:28:06Z")
> [@CannyDan](#): > > Yes, I understand that and I’ve done it myself (if only on boats up to 30-ish feet). But the ‘particular direction’ is always _the same_ direction. If you need to sideslip the other way, you’re just out of luck. Pilots plan around this, in terms of where they put the tugs, their approach to the berth etc. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 2, 2017, 8:32am UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/30 "2017-08-02T08:32:28Z")
> [@rat\_avatar](#): > > Sure there are, they just do other work while waiting if at all possible. Yeah, sorta. What you are talking about is salvage tugs. There are fewer and fewer all the time as ships get more reliable. They are stretched pretty thin. It really just depends where you lose your main engine. If it’s near a port there will be options. If not, it can become a race between the nearest salvage tug and the ship’s drift towards a lee shore. I get woken up in the middle of the night from time to time by a master saying he’s lost his ME and he’s drifting towards danger at x knots, and it then becomes a matter of hasty back of the envelope calculation as to whether the tugs are going to make it. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [August 2, 2017, 8:51am UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/31 "2017-08-02T08:51:38Z")
> [@Princhester](#): > > and it then becomes a matter of hasty back of the envelope calculation as to whether the tugs are going to make it. And if they don’t? Sounds like a bad day. ---
**Author:** ![CannyDan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cannydan/32/548_2.png) [@CannyDan](https://boards.straightdope.com/u/CannyDan)\ **Post date:** [August 2, 2017, 6:59pm UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/32 "2017-08-02T18:59:44Z")
> [@Princhester](#): > > Pilots plan around this, in terms of where they put the tugs, their approach to the berth etc. Sure! Professionals act in a professional manner, solving problems as needed. I get it, and I wasn’t suggesting that single screw made maneuvering impossible, just that twin screw makes it significantly easier. (And pivoting turrets or other steerable drives make it easier still.) Thousands of years ago, back in college days, a couple of us decided to become commercial fishermen for our summer jobs. We bought a small single screw inboard cabin boat (actually a Chris Craft made in the 1950’s) as our platform. At the time, dock space could be free if you fished exclusively for the dock owner, who supplied fuel, bait, and ice at cost and purchased your catch at market price. One such operator had the ice house at the end of a long, narrow canal. Both the canal and the ice house dock were constricted by other boats tied alongside. To get fuel and load/unload we had to put our stern up to the dock. This involved turning 180 degrees then backing up to the dock. Twin screw boats came in, used the twins to pivot, then backed straight up to the dock. For them, it took little more time and effort to perform than it does to say. We on the other hand required thrown lines, considerable effort, and some spectacular cursing to achieve the same evolution. Needless to say, it made an impression on me. 😃 ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 3, 2017, 9:05am UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/33 "2017-08-03T09:05:46Z")
> [@Francis\_Vaughan](#): > > And if they don’t? Sounds like a bad day. Well. Awkward point. Very bad day in general. But given that I make a living out of marine casualties… (Hey, don’t look at me like that. I don’t _cause_ the casualties!) ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [August 3, 2017, 9:38am UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/34 "2017-08-03T09:38:20Z")
> [@Princhester](#): > > I get woken up in the middle of the night from time to time by a master saying he’s lost his ME and he’s drifting towards danger at x knots, and it then becomes a matter of hasty back of the envelope calculation as to whether the tugs are going to make it. Still married are we? :D;)😃 How much does the nature of the Cargo affect the decision tree? If most of what he is carrying mostly CIF or FOB? ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 3, 2017, 9:53am UTC](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424/35 "2017-08-03T09:53:09Z")
Yes, I am still married. We have a common joke amongst my colleagues in the marine casualty game. What do you do if you get a phone call at 11am on a Tuesday, when you aren’t otherwise very busy, about a marine casualty in a temperate, comfortable, location with good communications, and excellent hotels and restaurants? The answer? Wake up, you’re having a nice dream. We all know that in reality marine casualties only happen at 2am on a public holiday when you are already overworked, in mosquito infested mangrove swamps hundreds of km from the nearest mobile phone tower or hotel. As to cargo, not much. The decision tree in the early stages is the same either way ie try to save the ship. [Previous page](https://boards.straightdope.com/t/why-do-ships-that-carry-oil-use-one-propeller-while-other-ships-use-more-than-one/792424.md?page=1)