Welome To The Titanic Tech Thread

Dec 2, 2000
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>>didn't have a crystal ball, he didn't know for 'certain' that Titanic would sink, only that Andrews said it would.<<

If he got it from Andrews...who knew an Olympic class liner better then anyone on the planet, and who also know what numbers to crunch and what they meant...then Smith knew the Titanic would sink with certainty. The engineers could do little more then fight a holding action to buy time and he knew that too. Whatever else Smith may have been, he was a realist.

It helps to know that the crew he kept for any damage control effort were the engineers, not the seamen. The seamen were needed to both man and launch the boats and they were in short supply. Short enough that scratch crews for the boats had to be cobbled together from hotel staff, redundant engineers and even some of the passengers.

It also helps to know that Smith didn't just order the boats to be made ready. He ultimately gave an explicit order to fill them up with people and get them away. Since the best chance for survival at sea lies in staying with the ship, such an order would be given only if the master understood that there would be no ship to stay with.

What his officers believed is certainly open to debate, but I have to wonder if they were as in denial of reality as they might have been inclined to express in the inquiries. Admitting to knowing that would tend to open up avenues for questioning that they had good reason to want to avoid...like pressing even harder about why they didn't load the boats to full capacity. When you're answering to lawyers who have no idea about what the realities of your vocation are, you tend to be very very very careful about what you admit to. Especially when you know that total candor can come back to haunt you.

I regret that the boatdeck on the Willis B. Boyer is no longer going to be accessable for a demonstration of the davits. I've had the opportunity to work with them and it was quite an education.
 
May 9, 2001
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You mean we can't even go onto the boat deck of Boyer? Or just that the lifeboat davits aren't going to be used? Either way - very disappointing indeed.
 
Dec 2, 2000
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>>You mean we can't even go onto the boat deck of Boyer?<<

I don't know if the boat deck is off limits, but I understand there are some corrosion issues which make going up there a tad more risky then the museum is willing to chance. This sort of thing is an on going problem with a lot of museum ships. The Boyer just held up longer because she's in fresh water instead of salt.

The davits are similar in general configuration to the Titanic's. Though not exactly the same, they're close enough so one can get a sense of just how manual and demanding they really were. Since they were a very real improvement over davits used on such vessels as the Lusitania, you can well imagine that dealing with what came before was no sweet picnic.

Perhaps when Captain's Dave or Erik drop in they can speak more to the matter of the Boyer's condition since they know more about it. I hope it's something that can be fixed.
 
May 9, 2001
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I feel it would be very educational if someone could offer a good explanation of just what is 'cavitation', what causes it, how it affects a ship like Titanic, and what forces are at work when it is occurring.

Thanks,
Yuri
 
Mar 22, 2003
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In the simplest terms, cavitation is water boiling around the propeller blades. Most of us know that water boils at 212°F at sea-level. But it can also boil at room temperature if we can lower the atmospheric pressure well below sea level. As a propeller cuts through water at relatively higher and higher speeds, the water pressure on the back of the blades is lowered. If it gets low enough, boiling will begin. This occurs most often near the back of the leading edge of the blades. The vapor bubbles that are produced then move downstream into a region of higher pressure and collapse back into liquid water with the release of energy that actually can cause damage to the blades under prolonged conditions. When a prop is cavitating energy is waisted. Submarines can follow a target ship by the signature sound of a cavitating prop caused by minor imperfections on one or more propeller blades.
 
May 9, 2001
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Ok, thanks Sam. Now how does the occurrance of cavitation, or boiling of water due to low pressure areas at the prop blade, result in a shaking or pitching of the stern of a ship?
 
Dec 4, 2000
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Even though it is destructive to the metal, cavitation is not necessarily accompanied by any vibration or noise inside the vessel. It is not the culprit for the sort of noise and vibration at the focus of Yuri's questions.

Yuri's questions point out the confusion in the public mind (and a lot of sailors, too) between cavitation and all of the other "things" that happen when a propeller rotates.

He wants to know why a ship vibrates and its stern jumps around when the engines are "crashed back" to full reverse from full speed forward.

In simple terms, a propeller blade creates a spiral discharge current. Photos of these spiral patterns can be found in most boating books and propeller catalogs. As each blade rotates so that it's tip approaches the ship's hull, some of the discharge current gets "pinched" between the two. At cruising speed, this produces a noticable "thump" of water against the hull. The hull in turn acts like the sounding chamber of a guitar body. At 70 rpm, a 3-bladed prop like those of Titanic's outer engines produces 210 of these "thumps" per minute. That's well within the range of human perception as sound, and can create enough physical vibration to jiggle objects off tables, etc.

There are other engineering factors in the choice of four or five-bladed propellers, but on passenger vessels the overriding factor can be vibration. A five-bladed prop produces more "pulses" per minute, so there is less time between each "thump." The result more closely approximates a steady flow of water instead of a pulsating flow--and passengers complain less.

Look at Titanic's stern. The shape is designed to ease the flow of water aft of the ship. So, the discharge current of the propellers has "someplace to go" as the ship moves forward. But, look at what happens when you apply reverse thrust. Instead of that smooth run aft, the discharge current is sort of bunched up under the hull.

Thump, Thump, Thump, each spiral slams into the hull. With "nowhere to go," the flowing water begins to pile up underneath the stern. I've seen water well up in this fashion to a height of at least 10 feet when a freighter on the Maumee River had to make a quick stop after the King Bridge blew a fuse and stopped working in the half-open position.

A dearly departed friend and outboard motor wizard was aboard a U.S. Navy aircraft carrier that nearly T-boned an Italian ferry back in the middle of the last century. (Not that far back in nautical time, however. Ships are pretty much the same.) All four shafts were "crashed back" and disaster averted. His berthing area was aft over the screws. He told me that men were literally tossed out of their bunks and more than 35 in his section required first aid, many for broken bones. That illustrates the kind of vibration and jumping involved.

Titanic did not have as efficient of propeller blades, four shafts, or the horsepower of an aircraft carrier. In a "crash back" (the term is modern U.S. Navy slang and would not have been used in 1912 aboard a British ship) Titanic would have vibrated and jumped, but with the decorum of a dowager on the dance floor. Passengers would have noticed, probably remembering that event above the actual iceberg impact. But, I doubt that many would have been tossed out of their bunks as were those navy sailors.

There are other noises and vibrations which have to be considered in the Titanic story. If any attempt was made to reverse engines, the center turbine had to be taken off line. But, stopping steam flow would not have caused the center prop to stop spinning. It would have "windmilled" from the force of moving water on its blades. A dragged prop never quite gets enough speed to match the water flow, so turbulence is produced that translates into both noise and vibration.

It is interesting that experiments have found that a spinning propeller produces almost as much drag as a solid disk of the same diameter. This is why many sailboats have "propeller locks" to stop windmilling and reduce drag.

The same sort of turbulence and vibration is produced when a driven propeller (such as the piston-powered wing props) are slowed while the hull is still moving at high speed.

-- David G. Brown