Propeller Vibrations in the collision

[Joshua Andersen]

The orders from the bridge were to run the engines full astern thats what most believe. I do not know if this has been mentioned before but wouldn't the vibration the outboard propellers running at full speed in reverse have caused a very bad vibration simply because of the direction they were traveling. The props in reverse and the ship still going forward would or should have caused considerable vibartion. But few survivors seem to mention this in the time of the collision. I remember Lawrence Beesley saying he heard the engines slow and stop but does not mention a vibration that would indicate the engines were in reverse. So maybe there was no time to reverse the engines and the order was not acted upon. The ships engines may only have stopped but not continued after the order went down from the bridge due to the lack of time between sighting the berg and the bridge ordering full astern. I realize this is only a theory put forward by a 18 year old kid but one that may have had consequences on that night. Any posts are appreciated. Thank you.

 

[Brian R Peterson]

Hi Joshua!

You have a very interesting question. However I do not think there would be vibrations if the engine was turning the propellers full speed in either direction, as there was no mention or passenger accounts of vibrations made when the ship was running full speed forward though I could be wrong.

Addressing your idea that this had consequences that night, I believe it is proven that the engines were reversed, the surviving officers seem to account for this at the inquiries. This had a major consequence the night of April 14th as reversing the engines first stopped the propellers, then reversed them - this cost the Titanic precious forward momentum and also slowed the amount of water passing the rudder thus making the turn to avoid the iceberg slower..... we all know what happens from here.

This is only my interpretation of the events leading up to the collision, any additional information is welcomed.

Best Regards

Brian

 

[David G. Brown]

Joshua -- I have long doubted that Titanic's engines were "crashed back" in a violent effort to stop the ship. The vibrations you speak of would have been far more noticeable than the sliding impact of the iceberg. In fact, there are stories of men being thrown out of bunks on military ships that performed unexpected crash stops to avoid collisions. I covered this subject in my book, "Last Log."

Today, I am of the opinion that a crash stop was ordered by Murdoch through an ASTERN FULL command on one of the telegraphs. It occurs to me, however, that he may not have been issuing so much an engine order as a warning to the engineers that something really bad was about to happen. During both World Wars crews of ships arranged special telegraphic signals to so the bridge could warn engineers if a torpedo was heading their way. Murdoch may have invented this idea on the spur of the moment.

Given the short duration between the ASTERN FULL engine order and impact, it is doubtful that the engines could have been reversed. The engineers needed were keeping a "sea watch" and not standing at the controls as they would have been in harbor when standing a "maneuvering watch." The only engine order that we know was obeyed was the STOP which came down on both sets of telegraphs immediately after inpact.

And, while there was no large vibration to be associated with a crash stop, there was definitely vibration in the stern. Few passengers noticed. However, reall the group of stewards in the first class dining area who thought the ship lost a propeller blade. For a short while they thought the problem was at the back of the ship and not the front.

To me, this indicates that the rudder was hard over so that the blade was in the direct flow of a propeller driven by a piston steam engine. Whether hard to port or starboard cannot be determined from vibrations, just that the rudder was hard over.

-- David G. Brown

 

[Erik Wood]

There is no physical or testimonial evidence to support that FULL ASTERN order was actually carried out by the engineering staff. Fourth Officer Boxhall had testified that it was ordered, but then Quartermaster Olliver speaks of ALL STOP being ordered directly after. The engine room testimony points to ALL STOP, Barretts testimony that he gave the order to shut the dampers does not agree with an attempt to fully reverse the engines. You need steam to back backwards and removing that steam from the system would indicate that ALL STOP was ordered.

Had the ship gone from 22 knots forward to 22 knots astern the vibration would have been enough to throw people out of bed. Remeber that Titanic's props where moving in one direction the water around them was being pushed in one direction, in order to reverse the engines you have to change prop direction which would then cause the momentum of the ship to fight the props hence cavitation.

A good example is when I was relief captain aboard the S/S Herbet C. Jackson the Jackson is a one prop ship, but when traveling at only 14 knots and slamming the engines back, the cavitation was enough to pop the shaft seal and cause belongings in the after boiler house to fall to the deck, it also jossled loose the boom from it's cradle. This was only 14 knots, on a ship going 8 less knots, about 100 feet shorter and over 10,000 tons lighter.

Loosing forward momentum in any manner would have hindered Titanic's ability to move around possible danger. First Officer Murdoch being the competent officer that he had proven himself to be throughout his career would have known this, and it is my opinion that he ordered full astern, realized she would hit anyway and recinded it with ALL STOP.

 

[Parks Stephenson]

It has never been proven that Titanic's engines were crashed back. The eyewitness most quoted as having asserted as much (Boxhall) only talked about what the engine-order telegraph pointers indicated. Boxhall's testimony included no secondary indications that the engineers responded to or carried out the FULL ASTERN order that he recalled. Greaser Scott, from his position near the receiving end of the telegraphs, did not see the FULL ASTERN reported by Boxhall, but rather STOP.

Also, there is no account of Titanic having lost noticeable headway prior to the collision, so a loss in rudder effectiveness is unfounded speculation.

Parks

 

[Brian R Peterson]

Parks,

No account for loss of headway is needed, if the propellers were stopped, even if they were not reversed with the loss of the forward thrust they would provide, the flow of the water around the rudder would be reduced and in turn the rudder effectiveness would be compromised and the turning response time would be greatly hindered - the same effect could be demonstrated by pushing a car and attempting to turn with no engine or power steering.

Best Regards,

Brian

 

[Parks Stephenson]

Brian,

First of all, the outboard propellors would not stop, they would windmill while the engines are either stopped or had their direction changed. The centre prop would stop, but at the speed we're talking about here, the drag from that screw wouldn't slow the hull down enough to cause an appeiable loss of efficiency. I ran the numbers some time back on what speed the hull had to be at to see an appreciable loss in efficiency of the rudder, but I'm at work now and don't have those available to me. If you can't wait for me to dig those out, you should recalculate for yourself to see what I mean (the formulae I used were taken from White's manual, but more contemporary texts would give you workable forumulae to use if you don't have access to a White's). I seem to remember that the hull had to slow down to something like 15 knots before a loss in rudder efficiency was encountered, but I would have to double-check my numbers to state that for sure. I'm not aware of any testimony that would indicate that Titanic slowed that dramatically before the collision.

I don't see your analogy using a car as a good one...rudder efficiency peaks at a certain speed (which equates to hydrodynamic flow past the surface) and stays at that value for higher speeds. A car's tires have a different relationship with the hard surface of a road.

Parks

 

[Joshua Andersen]

So maybe Murdock only ordered full stop knowing he had only enough time to stop or the engine room did not have the time required to throw the reversing gears into use before the collision. I first developed this theory when I was stearing my dads 12 foot bass boat. I was running at top speed maybe 6 knots when dad told me to stop so I threw it into full reverse. The boat vibrated terribly dad thought I broke it. I then realized I had done the same the thing that supposedly happened on the Titanic. I realize the size difference but blow that one prop into two and 23ft wide it would certainly translate into a heck of a vibration. I'm just surprised that survivors only noticed the collision and not the vibrations which means that the engines weren't reversed.

 

[Michael H. Standart]

>>I realize the size difference but blow that one prop into two and 23ft wide it would certainly translate into a heck of a vibration. <<

You don't know the half of just how right you are. Multiply this times two props (The centreline prop had no reversing gears) trying to stop a mass of around 49,000 to 50,000 long tonnes of steel trucking along at 21.5 knots using the wheels available in 1912...none of which were as refined or as efficient as those in use today...and you get a dramatic introduction into the Wonderful World Of Cavitation.

It all translates into a rude awakening which nobody noticed happening. That despite the fact that the noise levels and vibration would wake the dead.

>>I'm just surprised that survivors only noticed the collision and not the vibrations which means that the engines weren't reversed. <<

Note that most of the people actually slept right through the event which would kill 1500 of them and the ship they were riding on.

 

[David G. Brown]

On a bass boat, reversing is a matter of moving a hand lever. Not so a 1912-era steam engine. It took a small crew of men per engine. These men were stationed at their respective controls while maneuvering in port. At sea, however, the ship was not expected to suddenly have need to reverse. So, the "maneuvering watch" was secured and replaced with a "sea watch" of one or two men.

Murdoch's ASTERN FULL command could not have been obyed in time simply because of the need to rouse out a maneuvering watch. Sure, anyone handy might have been pressed into service, but On-the-job training isn't the best way to conduct an emergency maneuver.

Proof that the "crash stop" was not accomplished lies in the lack of vibration expected from throwing the reciprocating engines into reverse. However, this does not negate the testimony which supports that an ASTERN FULL order was telegraphed to the engine room.

Murdoch must have known there was too little time for the engines to be reversed. And, if he did, the question arises, "Why did he send down ASTERN FULL on one of the telegraphs?"

My suggestion is that this order was not meant to stop the ship or to avoid an iceberg. Rather, it was a quick-witted attempt to warn the men working far below the ship's waterline that an accident was about to happen. I believe that the ASTERN FULL engine order was meant as "heads up" to the engineers and stokers.

-- David G. Brown

 

[Jamie Bryant]

In Ghosts of the Titanic there is an image of a telegraph, but it is set in the position "full ahead"

Jamie Bryant

 

[robert s hauser]

Dear Sparks,
What is cavitation exactly?

Thanks, Rob

 

[Brian R Peterson]

Hi Robert,

Cavitation which is often confused with ventilation, is a phenomena of water vaporizing or "boiling" due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation can result in metal erosion or "cavitation burn" to the prop's blade surface. There are numerous causes of cavitation such as incorrect matching of propeller style to application, incorrect pitch, physical damage to the blade edges, or "crash stopping" the engines

Best Regards,

Brian

 

[Brian R Peterson]

Here is an illustration of propeller cavitation:

Best Regards,

Brian

 

[Noel F. Jones]

CAVITATION:

This phenomenon was first knowingly encountered during the trials of the destroyer Daring and was consequently described in the Proceedings of the Institute of Civil Engineers Part IV, 1894-95 by J.L.Thorneycroft and S.W.Barnaby arising out of their experience with the Daring.

She underperformed at 24 knots whereas her intended speed was 27 knots. Remedial work on the propellers resulted in a performance of over 29 knots. Thorneycroft and Barnaby concluded that there was a critical thrust pressure of 11.25 lbf per square inch of blade area which should not be exceeded.

The term 'cavitation' was suggested by W.E.Froude, the son of William Froude, devisor inter alia of the Froude Number. There is in fact a derivative 'cavitation number' used in propeller performance analysis.

In addition to enervating performance, cavitation gives rise to damage consisting of pitting of the blade back surface and this is referred to as 'cavitation corrosion'.

Essentially, cavitation consists of the generation of pockets of a Torricellian vacuum on the blade back and this is attributable to insufficient pressure gradient in the fluid. The resultant cavitation corrosion arises from the physical impact of the implosion of the capricious cavities. There may however be a chemical contribution due to precipitation of gases along with water vapour and the disparate rates of their re-absorption may generate and contribute acidic corrosion.

Remedies for both corrosion and performance enervation, consist, variously, of increasing blade area, reducing speed of rotation and increasing propeller immersion.

Most towing tank facilities include a 'cavitation tunnel' specifically for testing the efficiency of propellers. The first cavitation tunnel was devised by Parsons. He was experiencing the same problem with his Turbinia and cured it by using nine propellers on three shafts.

A cavitating propeller can generate a sound known as 'singing'.

Paradoxically, there can be an intentionally-designed 'supercavitating propeller' whereby the total blade back surface runs dry at service speed and thereby escapes corrosion. It is applicable to fast small craft.

Noel