Any ocean-going vessel with engines creates some amount of vibration, and whether or not
you can feel it enough to bother you is based entirely on its power and frequency.
So why did some ocean liners vibrate more than others?
Well, there are several factors to sift through here, so let's start first by isolating the
conversation to steam-powered ocean liners, and not the ones with combustion engines or
rigged sails.
When it comes to steam-powered ocean liners, there were two different kinds of engines
that drove them, either reciprocating engines which consisted of cylinders that filled with
pressurized steam which forced the crankshaft to turn, or steam turbine engines, which operated
similar to a pinwheel where rows of blades had pressurized steam blown through them,
causing the gears and propeller shaft to spin.
Well before there were steam turbines on ocean liners, there were reciprocating engines,
and as you might have already guessed, the motion of all those cylinders and linkages
moving, would have caused some vibration, but generally speaking, this isn’t bothersome,
if anything its the same kind of motion we are used to feeling inside a car.
However, where the vibration could get worse was if something was wrong with the alignment
of the crankshaft.
Everything had to be carefully calculated and balanced, or else if the engine were to
get moving at high speed, the unbalanced crankshaft could cause a destabilized motion.
Although this kind of situation was rare and could be fixed with an extensive repair.
More commonly though, the type of vibration that could bother passengers would come from
the propellers themselves.
The propellers, also referred to as screws, had to be designed perfectly balanced as well.
Depending on the ship, they could spin several times per second and with propellers as large
as 5 meters in diameter, that kind of imbalance could cause the screw to wabble as it spun,
shaking the aft end of the ship, sometimes unbearably.
You didn’t see this happen too often with the slower ocean liners, but rather with the
ones that competed for the blue riband, a race across the North Atlantic to claim the
title of world’s fastest passenger vessel to cross the ocean.
Another way the propeller could cause problems is if it threw a blade.
You see in the earlier days of ocean liners, collisions, improper casting, or even the
incredible hydrodynamic forces of the water, could sometimes sheer a blade right off of
the screw, causing an imbalance of weight and resulting in a dangerous amount of shaking
and shuddering.
If this happened the propeller would have to be out of service until the ship reached
the port and the damage could be assessed and fixed.
In days of single screw liners, they would then have to rely on the masts and sails to
propel them, but once the first dual screw liners came around, they no longer needed
sails as backup, and therefore the second propeller could get the ship back to port,
albeit at a very reduced speed.
With the introduction of dual screw liners, came another form of vibration.
You see, the propellers were located on either side of the extended part of the hull that
supported the rudder.
The motion of the screws constantly pushed and pulled at the water surrounding this specific
area of the hull, causing intense vibration, the engineers learned that by putting an opening
in the hull between the two screws, this permitted water to move more freely and all but eliminated
that form of vibration.
So that is how uncomfortable vibrations can be caused on a ship with reciprocating engines,
but you’ll find that a steam turbine ship had different issues.
First and foremost, a lot more care had to be taken with the fabrication of this type
of system because all the parts move at speeds much higher than the reciprocating engines
could.
For instance, if the shaft or the blades inside the turbine itself weren’t perfectly balanced
and centered, or if even one of the blades were bent out of shape it could result in
inconsistent motion that could cause vibration.
So the fabrication of the turbine systems was meticulously scrutinized, as a result,
the engineers were able to advertise that they invented a propulsion system without
vibration, but they would soon learn there were external forces to contend with.
Lusitania and Mauretania would become infamous in their early years for the violent shaking
they produced.
It wasn’t the engines, so what could it be?
It turns out it had to do with how the propellers interacted with the water around it.
When the ships moved at high speed through the water, it disturbed the water immediately
surrounding the hull, and that layer of disturbed water followed the contour of the hull, getting
sucked into the propellers.
However, outside that layer of disturbed water in motion, was the calmer water that was unaffected
and untouched by the hull.
As the outer screws turned, their blades struck this motionless water 9 times per second at
full speed, which resulted in the jarring shudder that rocked the ship.
Mauretania would later receive more specialized propellers to replace her outward screws,
helping to reduce the impact against the calm water by slicing through it more cleanly.
Interestingly, it was the inside screws were also contributing to the vibration in a different
way, the water would chew away at the edges of the blades, destabilizing them.
You see, it was Charles Parsons that had invented the modern steam turbine back in the 1880s
and he began constructing the first ship to utilize this technology, he named the vessel
after the engine itself, calling it Turbinia.
The steamship Turbinia had failed on its initial trial run, because the vibrations the propeller
produced had nearly shook the vessel to pieces.
Parsons didn’t understand, the turbine had been fabricated perfectly, so he couldn’t
find the source of the problem.
He discovered there was some kind of odd pitting that occurred on the edges of the propeller
blades, after multiple experiments, he discovered the phenomenon called cavitation.
Cavitation happens when the propeller moves so quickly, that vacuum bubbles form on the
edges of the blades, these bubbles quickly collapse in on themselves, and the mass of
the surrounding water hammers at the metal, causing the pitting he saw.
The pitting, in turn, worsened the effects of the cavitation, hastening the process in
which the water effectively chewed away at the blades.
The cavitation phenomenon also destabilized the motion of the water around the propeller
causing vibrations that interfered with the resonant frequency of the boat.
Cavitation wasn’t an issue with ships that had reciprocating engines because their screws
didn’t usually spin fast enough to cause the phenomenon to occur.
Parsons found that by designing better hydro-dynamic propellers, the effects of cavitation could
be lessened.
RMS Aquitania would receive four identical propellers similar to the outward screws that
were installed to correct Mauretania’s vibration because their sharp edges reduced the cavitation
that plagued the inside screws.
So as you can tell, even though vibrations could be caused by various factors, the engineers
were able to find ways to reduce the effects of it in the future.
But sometimes it would be months or even years before those issues could be resolved, so
how did they deal with an ocean liner that shook violently?
Well, as evidenced by Cunard ships such as Mauretania and Queen Mary, one of the key
ways they reduced the effects of vibration was by stiffening the structure in the aft
end of the ship.
A ship is not exactly a solid object, it's a honeycomb of steel framework, so by making
the structure more rigid, you can prevent the vibration from having such a big effect
on the vessel.
On Queen Mary, for instance, many of the lower aft decks received extra support pillars and
additional structural bulkheads.
The strategic placement of these reinforcing features, reduced the motion of the decks.
Queen Mary’s issue with vibration stemmed from propellers that produced too much cavitation,
this was resolved two years later when new screws were fitted to the ship.
And now you all know what caused these vibration issues and how they were generally resolved,
or at least reduced.
Compared to many ocean liner fanatics, my knowledge of the engineering of these ships
is still quite limited, but as I research these vessels and learn more about them, it's
fascinating details like these that I uncover and am able to share with all of you.
Thank you for watching, be sure to like and subscribe for more information about the age of steam.
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