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The Triple Screws

This discussion on "The Triple Screws" is in the Technical Construction Design section; SCREWS! Official weight and pitch from anybody! My research has dictated that the reciprocating 3 ...

      
   
  1. #1
    Stephen Stanger
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    SCREWS! Official weight and pitch from anybody!
    My research has dictated that the reciprocating 3 bladers were around 27.5 tons in weight with a pitch of 34'6" and the turbine screw weighing in at 17.5 tons (can't find pitch)
    I ask because many of my associates say that the bigguns were 38 tons (!) and the littleun at 28 tons.
    Personally I find their weights to be a bit over the top considering the the Queen Mary's screws weighed in at over 35 tons (biggest ever made at the time I read). Wouldn't the Mary's have been bigger considering it was twice the tonnage and over 150ft longer.
    Validation Please!

  2. #2
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    I can only offer confusion I am afraid.

    According to a contemporary press report, not the best source but one that was accurate in other respects, the central four-bladed 16 foot 6 inch bronze propeller weighed 22 tons and the two outer wing propellers (with a different hub material from memory?)of 23 feet 6 inches weighed 38 tons.

    The pitch of 34' 6" is accurate for the wing propellers as designed but in 1913 Olympic was given a modification to make the pitch 36', and the central pitch 17' (I believe from about 14' 6", but don't quote me). There is a rumour that Titanic's propellers had been changed to this before the maiden voyage, which is plausible considering they were given to the Olympic, but I have found no hard evidence.

    There was a further source I had which I will check, that I think also gave the above figures. I did however find it strange that the Queen Mary's propellers were apparently less, but even for her I've heard of reports varying between thirty and thirty-five tons, but I am unaware of propeler changes she went through during her life. (Olympic underwent at least three.)
    Mark Chirnside
    Webmaster: Mark Chirnside's Reception Room, www.markchirnside.co.uk

  3. #3
    Stephen Stanger
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    Hmmm, I'd just like to know if the 38 tons weight for the reciprocators is documented in multiple sources or if that is the most commonly agreed upon weight.
    I use the Queen Mary as a comparison as she was MUCH bigger (albeit she was a quad screw if I'm not mistaken) and in that, I would presume that her screws would have to be at least half if not twice as big as the Titanic's due to a larger engine propelling a ship twice as heavy.
    Do ya see where I'm coming from? (aargh!)

  4. #4
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    Yeah, I see where you're coming from. Most sources I've seen for the propeller wings state 38 tons, although I have seen one saying 26 tons.
    Thirty-eight tons appears in at least one book.

    However, although Queen Mary's quad screws were 30 to 35 tons, if I recall correctly they were solid casts and about eighteen feet in diameter (just from memory, can someone confirm this?).

    Bremen had 16-foot 9 inch propellers if I recall, and she had some 100,000 horsepower or even more, making 28.93 knots with her quad screws. Therefore the propellers may not increase in size so dramatically for the larger more powerful ships like Queen Mary... Oh, I do not know!!!

    Actually, I have made exactly the same comparison in a written work about the ships, and is something I've been wondering about.
    Mark Chirnside
    Webmaster: Mark Chirnside's Reception Room, www.markchirnside.co.uk

  5. #5
    Stephen Stanger
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    Thank You so much. Tis' reassuring to know that I'm not the only one.
    On a side note. As part of my spiel at my exhibition I play an engineer and it is natural to overwhelm guests with info (just for the looks on their faces!)
    When explaining the reciprocators I know: Two, four cylinder, triple expansion, inverted vertical, direct-acting, surface condensing, cylindrical multi-boilers operating on the Yarrow, Schlick and Tweedy balancing system. Am I missing anything?

  6. #6
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    Propellers are an arcane science practiced in the dark of night by naval architects by candlelight. The sun is up and I'm no prop expert, but here are some generalities:

    The diameter of a propeller is only one measurement of its size. Equally important is pitch--easiest visualized as the theoretical distance the propeller would move the ship forward in one revolution if there were no slip. A 34-foot pitch would move the ship 34 feet, not accounting for slip.

    Slip is usually expressed as a percentage of the theoretical pitch distance. At 22+ knots, Titanic probably had 12% to 15% slip. Surprisingly, slip goes down as speed increases. But, it never reaches zero.

    Another critical dimension is surface area of the blades. The more area, the more power the prop can impart to the water. Tugboats typically have blades with extremely large surface area that rotate extremely slowly. Race boats must keep engine revolutions high, so may have blades only an inch or two wide.

    Increasing pitch has the theoretical benefit of increasing speed because the ship will go farther with each revolution of the prop. There is a negative to this, however. The engine must work all that much harder. At some point, the increase of pitch overcomes the engine's ability to keep the prop rotating at the desired revolutions.

    One way to overcome this problem is to reduce the diameter of the propeller. That has the effect of reducing the surface area of the blades. Another way to accomplish the same thing is to make the blades narrower. The disadvantage in both cases is that a prop with less blade surface area can impart less thrust into the water.

    There is quite an art and a science to picking the correct diameter, pitch and blade area for a given ship. And, the right prop for one day may be wrong on another. For instance, Titanic was lightly loaded on its maiden voyage. Its engines could have driven rather deep pitch props. On a fully-laden trip, however, the right prop would have to have been of lighter pitch.

    -- David G. Brown

  7. #7
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    David,

    Was the Titanic able to adjust the pitch of its props? When I look at pictures of the props, I see that the blades are secured to the hub of the prop by a circular connection. This type of connection is indicative of a variable pitch propeller on airplanes. So maybe Titanic?

    Yuri

  8. #8
    Parks Stephenson
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    The pitch on Titanic props was adjustable, but only in drydock. Basically, the blades had to be unbolted from one pitch setting, aligned in a new setting and bolts secured. There was no control from inside the ship.

    Parks

  9. #9
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    So they were adjustable. Thanks for the answer Parks! (I had only suspected they might be.)
    I wonder if all three were adjustable?

    Yuri

  10. #10
    Stephen Stanger
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    I would guess the reciprocators were easily adjustible as the blades were simply bolted to the housing. But the turbine was one solid mold. I figure they would have to replace the entire screw to adjust that one.

  11. #11
    Parks Stephenson
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    Stephen,

    Could you please explain what you are saying? You are using a lot of familiar words that, in the context you are putting them in, make no sense to me.

    Parks

  12. #12
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    I would guess the reciprocators were easily adjustible as the blades were simply bolted to the housing. But the turbine was one solid mold. I figure they would have to replace the entire screw to adjust that one.

    <FONT COLOR="ff0000">Stephen,

    Could you please explain what you are saying? You are using a lot of familiar words that, in the context you are putting them in, make no sense to me.

    That the wing propellers were adjustable with bolted blades, but the central propeller was a solid mould and so the whole screw needed to be changed, not the blade/bolts, etc.?
    Mark Chirnside
    Webmaster: Mark Chirnside's Reception Room, www.markchirnside.co.uk

  13. #13
    Stephen Stanger
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    Yeah, thats what I mean.

  14. #14
    Stephen Stanger
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    ALSO, in relation to the screw size of the Queen Mary to the Titanic, I have read that the QM (at her time) had the largest screws ever built, at 35 tons. (double Hmmmmmm!)

  15. #15
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    I had believed that her 35-ton screws were the largest 'solid' moulds ever made, not the largest screws.
    Mark Chirnside
    Webmaster: Mark Chirnside's Reception Room, www.markchirnside.co.uk

  16. #16
    Stephen Stanger
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    I was just thinking though, most propellers would not have been solid moulds in the 30's especially on the Queen Mary.
    I doubt Cunard/White Star would have crafted solid moulds for her considering what they thought her screw revolution/speed was going to be. Also any damage to a screw from any type of flotsam would require complete replacement of the whole thing which is why they started attach blades to the central key instead of solid molding them, and that is as far back as Titanic.
    SOD THAAT for replacing 35 tons of screw, it don't make sense that they would be solid.

    Anybody?
    Stephen.

  17. #17
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    Stephen! I've been trying to contact you by phone at the Titanic exhibition. Jim Trembowski told me I should get in touch with you. I tried your e-mail shown in your profile, but it didn't work.
    Kyrila Scully
    scullytunes@hotmail.com

  18. #18
    Daryl Dobbs
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    I have just been to a very entertaining talk at the Gloucester and District Model Boat Club on a recent tourist dive by the Russians on the stern section.

    One picture we were showed was very interesting, it depicted one of the screws projecting above the sediment. It must have been forced up by the impact on the sea bed. One aspect of the picture interested me is that the conical cover on the end of the keyway the blades are bolted on to is missing.

    Is is possible that;

    A/ It sheared off when it hit the bottom.
    B/ Someone salvaged it.
    C/ They were never fitted when the Titanic had to leave dry dock to make room for the Olympic.

    If C what happened to them? If they were fitted to one of the other two Olympic class ships what happened to the ones destined for them or did the White Star line carry spares?

    Many Thanks
    Daryl

  19. #19
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    I'm pretty sure they were installed before the ship left Belfast. As for whether they were sheared off or salvaged, I would assume they came off on impact. I haven't heard a word regarding that piece in any salvage, and the first photos of the props show them missing.


    Adam

  20. #20
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    Assume nothing regarding the propeller fairings. I can think of at least one reason why they would not have been installed for the maiden voyage. That reason's name is J. Bruce Ismay. There may be other reasons, such as lack of time or the failure of the shop to have completed the pieces. Until we have proof...photo or eyewitness...one way or the other, it is too dangerous to make an assumption.

    -- David G. Brown

  21. #21
    Daryl Dobbs
    Guest
    I have just received a very kind e-mail letting me know the correct name for them along with a good explanation. It appears that they were made of thin metal and hollow inside as the air pressure increased with the depth the assembly imploded.

    A good explanation which I did not think of as I thought they were a more solid casting. It has surprised me that no one has thought of salvageing them. I know it is going off track but has any of the bells been located. I heard that a French expidition tried to get the one in the crows nest but dropped it when the crows nest fell apart.

    Regards
    Daryl

  22. #22
    Daryl Dobbs
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    Do any of the work records exist at Harland and Wolf as to what work was carried out on the screws. Would it have been possible that it sailed without the fairwaters.

    How much would they weigh?

    Regards
    Daryl

  23. #23
    Roy Currie
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    I unfortunately am not privy to all the info you guy's have but regarding the "prop fairings". would they not be some type of retaining cone rather than a piece of hollow metal for streamlining? Roy Currie

  24. #24
    Scott R. Andrews
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    Roy,

    The cone, also called a "fairing" or "fairwater", placed over the end of the each propeller, was a relatively thin, lightweight casting. It's basic purpose was to provide a hydrodynamically "clean" surface, reducing turbulence as the water was forced aft of the screws. They had a secondary purpose in preventing or reducing corrosion of the shaft, keyway and key within the propeller hub. The cone was sealed to the hub of the propeller with one of a variety of substances in use at the time, creating a watertight enclosure around the lock nut holding the propeller hub onto the tailshaft. The opposite end of the propeller hub was also fitted with a sealing device. The gap between the shaft bossings on the hull and the leading edge of the propeller hub was then nearly closed up by a metal band which was fastened in place.

    The actual lock nut on each shaft is visible in some of the dry-dock photos of the Olympic, and is readily visible in the photo of Titanic's starboard tailshaft being fitted prior to launch. It is located on the very end of the tailshaft. The type of nut H&W used on these ships was not the more familiar-looking hex type. Look for the large barrel-like protrusion having a series of holes bored into the periphery perpendicular to the centerline of the shaft at the end of the shaft. I would guess that the nuts were installed on the shafts and left in place during the launch and, afterwards, during fitting out, to protect the threads on the end of the tailshafts from damage.

    This type of nut is both tightened and broken loose with a large forged crescent-shaped spanner having a tooth at the outer end which locks into one of the holes on the periphery of the nut. These spanner wrenches are often unique to the specific ship, and because of this are sometime carried aboard the ship so that they are available for use in dry-dock in locations away from the builder's yard. Titanic carried hers lashed to the after outboard bulkheads of the after well deck - one having a smaller radius for the lock nut on the center screw, and one having a larger radius to accommodate the lock nuts on the wing screws. Photos taken 1912 and later of the Olympic show these propeller spanners stowed aboard her in similar fashion and in the same location.

    Regards,

    Scott Andrews

  25. #25
    Daryl Dobbs
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    Which direction did the screws rotate. I have heard that the port and centre screws rotated clockwise and the starboard rotated anti-clockwise.

    How did the rudder compensate for two screws roatating one way and the third roatating the other, I would have thought that the ship would tend to want to steer to one side without constant corrections.

    I have read the answer somewere but I cannot find the thred.

    Thanks
    Daryl

  26. #26
    Daryl Dobbs
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    Any experts care to answer the question above?

  27. #27
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    Looking at the ship from the stern -- starboard prop rotates clockwise, or in nautical terms is "right handed." Port prop rotates anti-clockwise and is "left handed."

    I would love to say that the center prop rotated "the other way." Of course, there are only left or right handed propellers. So, the center prop was right handed.

    Titanic's rudder did not "compensate" for the right hand rotation of the center screw. There is a rudder design, used on Liberty ships in WWII, that does compensate for the rotation of the propeller, but this had not been invented in 1912.

    There is a side pressure generated by a rotating propeller. On small boats this can be quite a problem and is known as "propwalk." The two outboard props on Titanic cancelled themselves out. The center prop gave no more problem in forward than any single screw ship (like Californian) would have encountered.

    Reverse is when side pressure becomes more apparent. However, the turbine powering the center prop did not have reverse gear. Without reverse power, the center prop had no effect on backing.

    --David G. Brown

  28. #28
    Daryl Dobbs
    Guest
    Thanks for answering my question

    Regards
    Daryl

  29. #29
    Philippe Delaunoy
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    Dear all,

    Just one question from a Titanic novice. Sorry, if the question was already asked on another thread :

    In J. Cameron's movie, when the Titanic is leaving Southampton's harbor, you can see the 3 propellers starting simultaneously.

    Question 1 : knowing that the Parsons' turbine took the exhaust steam from the reciprocating engines, is it correct to show 3 propellers moving at the same moment?

    Question 2 : is (or was) it custom to leave port with the turbine started?

    Your enlightened comments are welcome!

    Warm regards.

    Phil

  30. #30
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    According to Olympic's Engineers, the turbine was only started when the main engines were at 'half ahead' (50 r.p.m.).

    'Slow ahead' was used when leaving port, 30 r.p.m. without the turbine. So it would be wrong for them to start simulataneously; 50 r.p.m. would be needed to start the turbine.
    Mark Chirnside
    Webmaster: Mark Chirnside's Reception Room, www.markchirnside.co.uk

 

 

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