Propeller Blades

[Senan Molony]

Here is work by Biles (one of the Assessors at the Titanic Inquiry) from 1911, showing that "blade theory" was being addressed at this time.

A chap called Froude seems to have done significant work...

 

[David G. Brown]

The probability of a significant difference between Olympic and Titanic is supported by the actions and admissions of J. Bruce Ismay. He admitted there was to be some sort of test on the Monday that never existed for Titanic. And, he seems to have been interested in comparing Titanic's speed with that of Olympic.

WHY?

Sister ships are never quite alike, but what advantage would there have been in proving one of the sisters was faster than the other? It makes no sense because such a claim would reduce sales on the other ship. This would have been the public relations version of shooting yourself in the foot.

But, if Titanic's screws were significantly improved...now, that's a different story. It would have been a simple matter to "improve" and give Olympic the same advantages by installing new props.

My guess is that Ismay was not interested in the public's reaction to any improvement in Titanic. Rather, he would have used such information to drive other shipping lines nut$. Not only would the two O-class ships fast enough, but they would also have been more efficient. Of course, "efficient" in a commercial ship is spelled with dollar signs and pound marks. It always boils down to money.

Several years ago I helped design the engine configuration for a twin-screw ferry. By careful choice of engines and props we reduced fuel consumption by almost half while increasing top speed by two knots. That meant about $125-thousand to the owner's bottom line every season. Big deal? You bet, because that improvement came with better passenger service.

A little thinning of the ogive of Titanic's props could have been a big deal in terms of White Star's economic position. Imagine the savings of 50 tons per day times 20 or 25 years of service.

I brought up this "techie" business because of the need to explain how unobtrusive changes (props same diameter, pitch, etc.) can achieve very big results.

-- Dave

 

[Michael H. Standart]

>>being a non-techie, it seems hard to believe that an adjustment in propeller blade curvature would make a significant speed difference, but perhaps it would and does.<<

Senan, you would be amazed at what a difference even some fairly trivial changes would make. This was one of the ongoing issues with the Lusitania and Mauritania, both of which suffered from severe vibration problems which made some of the after portions of the ships uninhabitable at high speed and even threatened to cause structural damage.

The after sections had to be beefed up but part and parcel to this were changes which were made with the propellors which served to reduce to level of the problems from threatening to the merely annoying.

Even in the absence of that, small changes can make some very big differences not only in the handiness of a ship, but their efficiency as well. All else aside, the more efficient the propellor is, the more useful work you get out of it, and the less fuel the ship consumes. In either instance, this is a very big deal.

 

[Senan Molony]

From The Times April 19, 1911, p. 24 of the Engineering Supplement -

 

[David G. Brown]

Has anybody seen that bucket o' prop wash? It was here a minute ago...


Re-examining the concept of a change in the ogival thickness of Titanic's blades --

I think we have ample evidence from the testimony of Fifth Officer Lowe that something was greatly different in Titanic compared to the established data from Olympic's performance. If the two ships were exact sisters, then the so-called "slip table" for Olympic would have been virtually identical for Titanic. It should only have been necessary to cross-check Titanic's actual performance with the Olympic table to fine tune it for the newer vessel. That does not seem to have been what they were doing in Titanic. According to Lowe it was necessary to build an entirely new slip table for Titanic. Consider Lowe's American testimony on the subject.

"We were working out a slip table, and we had not quite finished when she went down. ...how many turns of the engine it would require to do so many knots... We were working at our slip table, and that is a table based upon so many revolutions of engines and so much percent slip; and you work that out, and that gives you so many miles per hour. This table extended from the rate of 30 revolutions a minute to the rate of 85 and from a percentage of 10 to 40 percent slip; that is, minus. We were working it all out and, of course, it was not finished," Lowe testified.

I would suggest that all the work involved in the new slip table was to serve more than just navigational purposes. The data derived would also have helped engineers at Harland & Wolff improve both the "old" screws on Olympic and the as-yet unbuilt propellers for abuilding Britannic. It is my opinion that something was quite different about Titanic's props and that both H&W and White Star (in the form of Bruce Ismay) were hoping that difference was a marked improvement.

Senan's "Stresses" post (above) mentions "fast-running turbine machinery." That rang a bell with me regarding the museum ship SS Boyer in Toledo, Ohio. Several members of this forum have been aboard that ship. Originally, it had a pieced-up steel propeller. But, when it was changed from a triple-expansion reciprocating engine to a turbine in the 1950s the propeller was changed to a solid bronze one-piece casting. Hmmm....I wonder...

The "Stresses" article discusses reducing the "root thickness." This refers to the thickness of the blade where it attaches to the hub.

The term "throwing the tip" is a bit of a mystery. It could refer to either "rake" or "skew." The leading and trailing edges of a blade with skew are shaped differently, usually the leading edge more rounded and the trailing more straight. In a blade with rake the tip is generally farther aft than the root. Skew is generally reserved for high-speed propellers. Rake is one way designers can increase the effective diameter of a prop without having the tips too close to the hull. The few photos I have of Olympic's propellers appear to show neither rake no skew -- or so little as to be undiscernible.

And, a modern propeller text states that for most "normal" applications vertical blades are still considered "optimum." Of course, any modern opinions are based in large part on the experimentation into propeller design that was ongoing at the time of Olympic/Titanic. Modern opinions can't really be applied backwards in time.

To me, the underlying importance of the "Stresses" article is concern over strength versus blade loading. The "load" on a blade can be huge. Remember, those relatively tiny propellers were pushing 50,000+ tons of deadweight ship plus the water resistance. Even divided among the total number of blades (those of each of the three propellers) the stress was enormous.

Looking at efficiency again, the most efficient conventional propeller would have a single blade. That single blade would always be turning in water undisturbed by any other blades. Two blades are better in this respect than three, and three better than four. Or, at least that's so in theory.

Three, four, and five-bladed props have other advantages, the biggest being more blade area over which to spread the stress. Also, adding blades tends to reduce vibration caused by the blade tips passing close to the hull. A "pulse" of water off the end of each tip hits the hull and creates a "thump." Increasing the number of "thumps" by increasing the number of blades changes annoying vibration into a more pleasant "hum."

Vibration would most likely have been the reason for changing the center shaft prop from three to four blades. The tips of the center prop passed close to the stern of the hull, so passengers would have appreciated less "thumping." The two wing propellers had more hull clearance, so the better efficiency of a 3-bladed prop most likely would have overruled vibration considerations.

In looking up slip and efficiency I came across one of those factoids that are curious and pretty much meaningless. As a ship moves forward, it drags water along due to "skin friction" and other factors. As a result, the propellers are always moving through the water at a slower rate than the ship and its load of attached water is moving forward. Put that in your paradox pipe and smoke it.

If you are a bit confused by this "prop talk," so are the experts. Buried somewhere deep in every text on the subject is a sentence that in effect says, "when all the math is done, you still have to put the new prop on the ship and give it a test." Even the experts recognize there is no substitute for good old "cut and try" in this arcane science.

-- David G. Brown

 

[Senan Molony]

Very interesting.

The problem seems to have been that the turbine gave them lots more pent-up power than the props could handle.
They initially resolved it the old way, through gearing. Which still gave a degree of economy, but my sense of it is that it was walking a tiger on a strict leash.
So the big prize was in upgrading propeller efficiency, such that it broke through an apparent band of 'diminishing returns' into the huge profit territory of enhanced performance.

The reason I posted the "Stresses" article from 1911 is that it appears to go to the heart of David's observation -

quote:

As a rule, thinner blades are more efficient than thicker ones.

And I think this is the correct interpretation of what is meant by "curvature" in the newspaper article - in other words, that the rearward bulking out of the blade (its curvature) would be thinner (or less curvy).

Hmmmm.

Maybe reassessing Lowe's slip table work in this light would also draw in Ismay's conversation with Chief Engineer Bell at Queenstown. What was all that about?
Speed is the general suspicion, but equally it ?might? have involved appraising him of expected fuel savings from the experimental improvements that we are now supposing, inviting his monitoring of matters from his end, etc.
A possibility arising from a possibility, perhaps. But I am way out of my depth in these matters. I have skin friction itching.​

 

[Michael H. Standart]

>>The problem seems to have been that the turbine gave them lots more pent-up power than the props could handle.<<

That's a large part of it. The problem with turbines...steam or gas...is that they operate most efficiently in the range of several thousand RPM whereas a ships propellor operates most efficiently down in the very low hundreds. With a reciprocating engine, this wasn't much of an issue, but with turbines, this was a real headache.

Reduction gears were the response to that but the problem with some of the early sets was that they weren't all that reliable, and they were very expensive. That's one of the reasons why some of the early turbines used a direct drive. This wasted quite a bit of energy by simply chewing up the water or running the turbines at less then their optimum speeds, but with no gears to worry about, nobody needed to wonder if they were going to strip themselves.

Dave may be more up to speed on this then I am...I'll have to dig up one of my steam engineering histories to check it out...but I seem to recall that the problems with building reliable reduction gears for steam turbines weren't really solved until the time of the Great War.

 

[Senan Molony]

From The Times, Thursday April 11, 1912 [The day she called at Queenstown on her maiden voyage], page 13 -