The Triple Screws

S

Stephen Stanger

Guest
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!
 
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.)
 
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!)
 
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.
 
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?
 
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
 
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
 
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
 
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.
 
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.?
 
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!)
 

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