Nautical know-it-alls: Are the hulls of big, modern metal ships utterly watertight?

I assume that back in the days of yore, the hulls of large oceangoing wooden ships and even riveted steel ships were not 100% watertight. They probably had small, harmless leaks that were just the nature of the beast, right?

Well, what about today’s welded-seam naval and cruise ships? My first guess would be that, of course, they are utterly watertight. But that’s an aweful lot of underwater acreage down there, and all it takes is a tiny pinhole from a bad weld or a rusty cavity to let in some water.

I’m not a nautical expert but … the shafts for the screws and the rudder control mechanism have to come through packing glands out of the hull and it’s awfully hard to keep such gland seals from leaking forever. Also, many modern ships have little maneuvering engines and screws so they can move around in docking, and there are stabilizing surfaces that keep the ship from rolling on rough weather, etc. The hull has quite a few holes in it.

Water is going to get into a steel hulled vessel just like it would get into a wooden hulled vessel or any other vessel. There are different vectors for this, but the result is the same. So, all ships have bildge pumps.

1/ Deliberate openings.

I’ve never heard of the rudder stock leaking. It is often above the water line.

The seals on the main shaft(s) are in an oil bath maintained at a positive pressure. If the seals leak, you get water in the oil, but I’ve never heard of the seals being in such bad shape that you get water back into the E/R. In fact, before that occurs, oil will be leaking out of the seals into the sea, which will result in the vessel being prosecuted and fined in any first world nation. I acted for a vessel involved in a significant brouhaha because of such a problem, recently.

David Simmons mentions bow thrusters. I don’t know if the seals on them leak. I suspect the same issues arise as for the main shaft(s).

Seawater lines with valves and joints may leak somewhat, which is not quite the same as the hull leaking but amounts to the same thing I suppose.

Overall, leaks through deliberate hull openings would be rare, but I certainly wouldn’t say that no ship is in poor enough shape for leakage to occur.

2/ Bad welds and corrosion: these do occur, but rarely. Any noticed would raise eyebrows and be fixed pretty promptly, particularly if into a dry space. It’s not so much that they would endanger the ship in themselves as that corrosion or defects that are that bad are going to cause very serious questions to be asked.

I suppose a small leak into a water ballast tank (and the areas directly adjacent to the waterline are often water ballast tanks) could go undetected for a fair while, particularly if the leak is slow enough to not make any appreciable difference to the levels in the tank.

Overall, I would say that a modern well maintained steel hulled ship would have almost no leaks. Older badly maintained ones may have some. The level of leakage would be orders of magnitude (proportionally) better than wooden hulled ships.

I’ve been told that if I were to be on even a modern submarine when it went underwater, I’d be shocked at how much it leaked. Is that true?

I don’t know if you’d be shocked, necessarily, but yes, submarines do leak. So do most steel hulled ships, and in fact, it’s an accepted truism that any large hull will leak somewhat. Back when steel hulls were of riveted construction, leaks were commonplace as the hull flexed with physical and thermal stresses. Modern metal hulls are almost always steam welded (to prevent cell corrosion and flexural stresses on the weld) and the joints are typically lapped or doubled, so leakage through joints is minimal, but there will eventually be leakage through hull fittings (thrusters, shaft seals) and even, on large hulls, small cracks which are not a structural threat.

Submarines, by their nature, are under greater pressure, see high thermal stresses and operating loads, and have a significant amount of through-hull fittings and aperatures (periscopes and antennae, loading and torpedo hatches, sensors, et cetera). Wherever there is the slightest flaw, water will leak in, one drop at a time. Since leaks large enough to pose an integrity threat are immediately evident (spraying it at hundreds of psi), it’s not really an issue, and Naval subs are routinely inspected and refit between cruises. All modern submarines are made of high tensile strength steel save for a few Soviet attack boats with titanium hulls permitting them to dive more deeply than similar size subs with steel hulls.

Because the air humidity in a modern nuclear sub is tightly controlled (to prevent corrosion and protect electronics) most leakage will evaporate on its own, and just a small amount will make its way into a bilge. Older, WWII-era subs used to leak constantly, though, but that was the least of the worries and discomfort of the captain and crew who were plowing mostly blindly through the water (without detailed charts or modern navigational aids) in a filthy, diesel-fume and rotting-potato filled clastrophobic tube not much bigger than a tour bus. See Wolfgang Petersen’s Das Boot for an accurate portrayal of what it was like to live on a WWII-era sub.

Aluminum is also commonly used for small-to-midsized boats, though its high thermal expansion and relatively low shear modulus and tensile strength (in marine-suitable corrosion resistant alloys) make it unsuited for large vessels or those that operate in arctic environments. Corrosion resistance (for steel hulls) is provided by sacrifical anodes (called “zincs”, as they’re made of zinc or zinc-copper alloy) which are more electronegative than the steel, and by copper plating or copper-based bottom paint. A properly built and maintained steel or aluminum hull will outlive then engine, the superstructure, and probably the owner, as well as survive many groundings and collisions.

Fiberglass is most typically used for sailing yachts, despite the fact that it’s not really an ideal material for boats. It does have numerous advantages–it does not suffer from chemical corrosion, it’s cheap to build and (fairly) easy to patch, the surface can be polished and waxed for smoothness or roughened to take paint, you can make just about any shape you desire, et cetera. On the other hand, fiberglass requires regular maintainence to keep it waxed and sealed; it suffers from continuous fatigue due to microfractures and if not layed properly can debond and come apart; it has the lowest toughness/damage resistance of any standard hull material (one solid grounding and a hull requires major repair) and low flexural modulus forcing hulls to be thick and surprisingly heavy, and will eventually take on water no matter how well cared for. Some older matrix formulations were also very sensitive to UV light, though modern epoxies are far more resistant. Oh, and it’s nasty, itchy stuff to work with. However, it’s cheap, it’s light, and for most recreational hulls, which will be used only occasionally and in conditions that aren’t regularly extreme, it’s fine. Most recreational boats seem to end up in permenant storage or washed up in the shoals at some point anyway.

Wood hulls, and particularly those made of teak and/or ironwood, are, if properly maintained, surprisingly watertight. If the hull is built correctly (accounting for the swelling of the wood when it is wetted) the hull will be self-sealing, and indeed, the majority of leakage in a well built completely wood vessel is through the deck, not the hull. A poorly built hull, in which the dry seams are too tight, will tend to warp, allowing leaks; so will plywood hulls when the (inevitably) start to delaminate. One expects wood to rot in a wet environment, but the salt water actually tends to preserve the wood, and if properly cared for (i.e. sealed or painted, not allowed to dry and crack in the sun, damage repaired promptly) a wood hull may be more watertight and last longer than metal. It also provides a fair amount of insulation and (generally) positive flotation, something metal hulls do not do. Wood does require a lot of maintainence, which is prohibitive for recreational owners, and it isn’t great at resisting contusions (though still better than fiberglass), but its biggest problem is that it requires skilled craftsmen to work a hull from wood, and the hull shapes are somewhat limited by the size of stock material and mechanical limitations of wood.

One other material people aren’t generally aware of as a hull material is ferro-concrete. That’s right; some people sail cement boats. :cool: In his amusing travellogue/journey of self-discovery The Water In Between, Kevin Patterson purchased and sailed (if inexpertly and haphazardly) a ferro-concrete yacht, Sea Mouse, from Washington to (eventually) Tahiti and back. He doesn’t seem to have had any leakage problems despite the very questionable maintainence of the boat, and indeed, ferro-concrete is, or at least was, more often used in large shipping vessels; the Liberty Ships that were operated by the Merchant Marine corps during WWII were made of ferro-concrete, and despite the low fracture toughness and tensile strength of the material they were surprisingly resistant to damage, owing to the thick hull which tends to spall when struck by a torpedo rather than through-fracture. It was also quite cheap to cast a hull and required little skill to fabricate. Most of these ships were deliberately sunk after WWII as target practice, but a couple that remain still demonstrate watertight integrity.

So, it’s generally taken for granted that all large hulls will leak, whether through seams, cracks, through-hull fittings, or at least, though the deck; hence, all boats have bilge pumps.

It’s not the definitive reference on hull-building, but Dave Gerr’s The Nature of Boats provides a pretty comprehensive introduction to monohull design and outfitting of sail and motor yachts. Introduction to Naval Architecture is a general survey and a standard reference of large shipbuilding.


[ nitpick ]

Liberty Ships were steel.

During WWI there were about a dozen freighters built for the war effort that were of ferro-concrete. (They were built with the typical (for the time) island and engines amidship layout that the Liberty Ships used 25 years later, and perhaps this has led to some confusion on that issue.)

Ferro-concrete has also been a fairly popular experimental material in the construction of small oilers and tankers, with several being built during WWI and another couple dozen or so being built during WWII.

However, the Liberty Ships were built using proved technology in an effort to produce as many as possible as quickly as possible. The slightly larger Victory Ships were also built of steel.

Here is a site dedicated to ferro-concrete ships that includes a list of the ones commissioned during WWI and WWII. (It may not be complete; I know the Navy had some small oilers that were only numbered, not named, and I did not find them on the site–which has some dead pages.)
Note that the photo of the WWII era ship is an engines-and-island aft with a small navigating island forward design (typical of tankers and coasters of the era, although none of the WWII ships on that site appear to have been tankers).

[ /nitpick ]

Grr…Jacques Cousteau does it to me again. I stand corrected.


How does the water get through the pressurised oil in the seals?

What does the vessel’s class society have to say about this? I’d be stunned to find out they’d allow it to remain.

This is an interesting one! I’m familiar with the types of seal you’re describing, having recently investigated a failed one from a different application. I didn’t know they were used on prop shafts.

As you describe, the oil in the seal is at slight positive pressure w.r.t. the water outside. All things being equal, this should result in oil passing along the prop shaft into the water. In the seal I was looking at, this was prevented by a slinger arrangement on the shaft itself, which would throw the oil outwards after it had migrated along the seal labyrinth and act as a centrifugal seperator. The slung oil was repressurised and pumped back into the seal. In effect, the seal oil was constantly circulating to hold the water at bay.

There’s a bunch of ways this arrangement could fail, and for a fact I’m surprised it’s used on a propshaft. The active mechanisms required - oil recirculation, oil pressurisation, feedback to maintain the small positive pressure as the prop speed changes - seems more complicated than just letting the shaft leak water through packing or a labyrinth seal and pumping it out.

Some ships can use water lubrication for a time, but at least in my experience, they tend to overheat the bearings.

So would I. My office is notified anytime an inbound merchant vessel discovers any type of crack or leak in the hull or cargo tanks. Typically for minor cracks, the vessel will be allowed to conduct cargo operations only after the crack has been temporarily repaired to the satisfaction of an attending class surveyor and CG inspector. Then the class society will require permanent repairs made prior to loading next cargo, and the US will require permanent repairs prior to next call in a US port.

If it’s a big enough crack in the hull, the vessel will not even be allowed in US waters.

[Sort of a hijack]It is surprising that there aren’t a lot of hull cracks. When I was on the liner Ile de France I noticed finger joints in the decks to allow the hull to sag or hog without warping the deck planking. And those joints were working all of the time as could be seen by their opening and closing. So it’s clear that the metal in the hull was subjected to constant stress reversals and considering the metallurgy of the 1920’s when she was built the endurance of the hull was rather surprising.[/sort of hijack]

Fortunately for me, 99% of the merchant traffic in my port are ships less than 10 years old. The US began its port state control initiative in 1994, and as a result the quality of merchant ships calling in the US has steadily improved. While cracked hulls in older ships may be common in other world ports, it aint here.

We’re starting to see ships outfitted with stress monitors on the decks, which enable the ship to adjust their course and speed to minimize the stress and flexing in a seaway that lead to cracking.

[/continued sort of hijack]