Yuri Singleton
Member
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?
Welome To The Titanic Tech Thread
- Thread starter Michael H. Standart
- Start date
Samuel Halpern
Member
Who said cavitation caused shaking or pitching of the stern?
Yuri Singleton
Member
Of any stern on any ship. How does running engines full astern cause any ship to shake? That is the question.
David G. Brown
RIP
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
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
