Title says it all.
It is not just ships that carry oil but seemingly most cargo carrying ships.
Yet naval ships frequently have two, three or four propellers.
I get that the design considerations are different for these ships. I’m just curious what the trade-offs are and why they choose what they choose.
Navy ships efficiency is not the main issue. A destroyer can have 50,000 shaft HP. by splitting it between two shafts then it is only 25,000 Shp per prop. They also have smaller props. The props have to turn at a high RPM and efficiency is lost. Also in a battle if one shaft is destroyed the ship is not dead in the water. Also maneuverability is also gained.
Now a carrier has 4 props. But the Essex class carrier were 250,000 shaft Hp. Not sure what the HP of the new carriers. Divide up the Hp and use a smaller prop.
Now a merchant ship efficiency is high on the list of requirements. One engine room one shaft alley means smaller engineering cargo and more room for cargo. Also less maintenance costs. The props are bigger and turn at a lower RPM. Normally 100 to 120 RPM.
Nimitz Class carriers have 4 shafts/propellers, 100,000 HP each. This almost double the HP of the conventionally powered Midway Class ships.
Yes.
A warship needs both redundancy and more power than can conveniently be handled by a single shaft and prop.
But multiple engines, shafts & props add significantly to construction, manning, operational and maintenance costs, which merchant ships prefer to avoid.
Are merchant ships lost to accidents much at all? It seems like a single point of failure is such a terrifying thing when you’re in the middle of the ocean. Is the engineering so good these days that’s it’s not really a concern anymore? Running aground? Wildlife? Weather? Ice?
Short of piracy, are there any real dangers to merchant vessels anymore?
Here’s a report from an insurer. Its a bit old, referring to 2013 numbers. 2014 saw 75 ships lost, the lowest in a decade. Weather was often a factor, as would be expected. Another report indicated 22% of the 2014 losses involved a machinery failure and/or fire. Given the number of ships sailing (in 2005 there were approximately 46,000 merchant vessels) that’s a pretty low number.
https://www.allianz.com/en/press/news/studies/news_2014-03-13.html/
You save a lot of money on construction, operating and maintenance costs. That offsets by a very large margin the extra insurance costs, if any.
You buy insurance, ships gets lost, you collect. The small odds of losing the ship aren’t worth otherwise bothering about.
If an accident takes out your propeller, you can just radio for help and sit tight until rescue arrives. If your situation is such that sitting tight won’t help you, then extra shafts probably won’t matter, either.
The situation is different for a warship, where sitting tight might mean remaining in the engagement that caused the damage to begin with.
Only sailed for a short time, have classmates who sailed 30+ years. Normally never thought about loosing main propulsion. If the machinery is kept up there is seldom a complete failure. But I was on a T2 tanker that had the rule to never go from two running hotel generators to one when crossing the bar going out of port. Last time that was done the engineer took one generator off line. Then closed the root steam valve to the other. Lost all lights, electrical auxiliary equipment, And DC power to the main engine and motor fields. That also meant the fires in the boiler went out. He had to get the engine room relight off with flashlights before loosing steam pressure.
The great majority of ‘ordinary’ non-passenger merchant ships are single screw, but actually it’s not uncommon now for tankers to be twin screw. Most aren’t, but some are, sometimes because of local regulations with an eye to preventing oil spills. Or in some cases instead there’s one prop, but some other form of emergency propulsion.
And there have been cases where very large merchant ships are twin screw because they need so much power there isn’t a readily available single engine (almost all merchant ships now are diesel powered) big enough. There have been some twin screw very large container in recent years. Otherwise dry cargo (container, bulk) ships are virtually always single screw, and in at least fairly big ones it’s almost always one low speed direct connected diesel (ie no gears) and fixed pitch propeller. They don’t break down often enough for the extra cost and lower propulsive efficiency (generally, on a big ship) of multiple screws to make economic sense. And it’s not generally required by regs except a few places for tankers.
For warships as others have mentioned the basic reason for more than one screw is redundancy in case of combat damage, although the US Navy built many single screw frigate type ships (the steam turbine FF types in the 1960’s then the FFG 7 gas turbine ships in the 80’s) which no other navy did in that period (beside ones building or buying second hand those same USN designs). Also between twin screw and triple or quadruple the issue is also in part how big you want to make each of those plants, not necessarily that you gain a lot of advantage in redundancy practically speaking between 2 and 4. For example in the 1930’s the Italian Navy went with more power per screw than others were willing to and so some of their heavy cruisers were twin screw where most others were quad. The Germans had a particular preference for triple screw in large armored warships in the steam era, which few others imitated. The trade offs between 2-4 are more marginal than between 1 or more.
Tugs and river towboats nowadays are almost always at least twin screw though. Single screw harbor and coastal tugs used to be the rule, but almost none built nowadays and gradually disappearing despite the typically long lives of tug boats. In part that’s because unlike a bigger ship the draft limitation often means it’s more efficient to put the power through two propellers than squeeze it into one. Also the economics of buying and running the plant don’t favor single screw as much in that power range. But there’s also just a different perception about redundancy. Back to tankers, most oceangoing tankers you see near the US east coast (say) are single screw. Almost all tugs you see towing oil barges offshore are twin screw. Charterers basically won’t employ single screw tugs for oil barge work anymore on liability concerns about spills and sometimes local regulations. The tankers and barges are virtually all double hull now so that’s not the difference (except to the extent the barges are smaller so protective space not as deep, but OTOH they wouldn’t spill as much because smaller). It’s partly just a difference in convention, and regulations (if banning single screw tugs from oil work) which aren’t a 100% rational if they don’t bar single screw tankers, which they don’t along US coast.
Yes. Storms still sink ships.
Rouge Waves take their toll.
They only happen when there’s a Red Tide or in the Red Sea.
As a side note cruise ships usually have 2+ propellers.
What if it’s a rouge rogue though?
As we often say in computer science: There are only 3 quantities: 0, 1, and more-than-one.
Each of those are very distinct classes with very different attributes. Once you enter more-than-one territory it’s all pretty much mud.
I’m exaggerating for effect, but only a little.
If that accident happens to involve bad weather then they cannot just sit tight and wait. The ship needs enough power to maintain steerage way, so that they can point into the waves. A ship left dead in the water in a storm is in deep peril.
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.
More and more tugs seem to be built not with conventional fixed screws at all but with systems like Z-drive - Wikipedia or Cyclorotor - Wikipedia.
As you say, the goal in any case is increased maneuverability as well as total power absorption.
Actually pilots use “cut” - as they call it - to good effect, to move the stern in a particular direction.