How is a ship watertight

J

Jim Trebowski

Guest
Hi! Question that a few guests at the Orlando exhibit have asked...How is water prevented from seeping through the spaces between the rivets holding together the overlapping steel plates that make up the skin of the ship?..thanks.Also, we have a replica of one of the screws, and nautical old timers will ask me what the pitch is. Does anybody know?
 
Dec 4, 2000
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Pitch is the theoretical distance a propeller will move forward in one revolution. In 1912 it was expressed in feet.

Do not confuse "pitch" with the angle at which the blade is attached to the hub. This angle is steepest at the root (near the hub) and becomes less near the tip of the blade. Pitch is constant.

"Slip" is the difference between the theoretical pitch and the actual distance the blade did move forward. Some slip is necessary for a propeller to work. The amount of slip, as a percentage of pitch, actually goes down as speed increases.

Propellers do not "screw" their way through the water as once thought (which is why they were called "screws"). Rather, thrust is created by pressure differential on the front and back sides of the blades, which are foil-shaped like airplane wings. If you want to become totally confused on this subject, I advise Dave Geer's book, "Propeller Handbook" (ISBN 0-87742-988-X) Gerr does an excellent job of explaining propls, but the subject is extremely technical and difficult even for engineers to understand.

Now for rivetted seams. Calulking (always pronounced "corking") is the answer. Early on, they tried putting tarred oakum in the joints. This worked until the stuff squeezed flat over time, resulting in a weakened joint. So, mechanical caulking was invented. This is a process of splaying the end of a plate so that it is smashed into a watertight fit against its mating plate. The work was initially done with a hammer and cold chisel. Later, pneumatic caulking machines were invented.

A properly-set rivet is naturaly watertight. The body of the rivet expands as the head is being "bashed," or hammered over. The expanded rivet body should fill the hole tightly enough to be watertight.

A primary reason for scrapping Olympic was the method used to create the rivet holes in the actual shell plating. H&W apparently punched the holes with a mechanical press, as was the custom at that time. Later, it was learned that punching rivet holes causes the edges to "case harden" and become brittle. Micro-cracks develop which eventually result in quite visible cracking of the plate. These cracks radiate outward from the rivet hole. One or two cracked plates could be replaced, but Olympic needed virtually a new skin. The solution to case hardened rivet holes was simple, but time-consuming. The holes were punches a few thousandths of an inch undersize and then drilled to full size. Drilling removed the case hardened metal and ended the cracking.

Many of you may have heard the phrase "to cork off," meaning to take a nap on the job. It is nautical in origin and goes back to the days of wooden ships where deck seams were caulked with pitch. Pitch "corking" would soften in the sun and become sticky. A man who fell asleep on the deck in tropical climes would often arise with tell-tale imprints of the deck "corking" on his tunic...sure proof he had "corked off."

--David G. Brown
 
Dec 30, 2018
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Hi! Question that a few guests at the Orlando exhibit have asked...How is water prevented from seeping through the spaces between the rivets holding together the overlapping steel plates that make up the skin of the ship?..thanks.Also, we have a replica of one of the screws, and nautical old timers will ask me what the pitch is. Does anybody know?
Actually, I asked my selves that very same questions for years...
The answer is: riveted ships used taperings when overlapping steel met. Both vertical/horisontally. Plates were designed/manufactured precisely with pressed out angles before installing - when riveted with glowing rivets (30-40mmØ) - the force from the retracting cooling rivets; were sufficient to make a very powerful connection between plates.
FYG: Even into the 1980'es, HIGH pressure boilers, were still riveted, not welded. It was not until the adding of special cooling gasses and welding apparatus, that riveted boilers disappeared.
PS: No ship.. then or now… is completely watertight… thats why bilge-pumps have to active 24/7
 
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