Propeller Blades

[Nuno Viriato R R Cruz]

If it was so common for ships to loose their propeller blades, I bet that the ocean floor is ''filled'' with such items!

An apparently, if they do not rust away we will have them for many many years!

I wonder, is it still common for modern propellers to loose their blades?

Drop your comments on this!

Nuno

 

[David G. Brown]

Nuno -- some propellers are "built up" from a central hub and individual blades. The blades are held on by nuts on threaded studs. It is not unknown for one of these blades to come loose, although this is not a "common" occurrence.

A late friend of mine had this happen on a ship halfway across the Pacific. Fortunately, it was twin-screw vessel and they continued to a shipyard at diminished speed.

It was not uncommon in Titanic's era for a ship to be fitted with built-up props and to carry at least one spare blade. Lugs fitted to the underside of the counter allowed the attaching of block & tackle so that the crew could (in an emergency) change a damaged blade. I've never met anyone who went through such an operation, but I have seen the equipment that was supposed to get the job done.

The other method of constructing a propeller is to mold it of solid bronze. There is nothing to come loose or fall off except the whole propeller from the shaft.

Blades of either type propeller can be knocked loose by impact with floating objects. Again, not a common occurrence, but one that happens from time to time. Bent or damaged props are more common than dislodged blades.

With regard to Titanic, the key element to the stories of some of the hotel staff is that they thought the iceberg event was taking place at the wrong end of the ship. The propellers are at the back, while the berg was attacking the front. Even a steward or cook should be expected to know bow from stern. There are several possible reasons for this anomoly. A "crash stop" with the engines in reverse is one, although the vibration they described does not seem sufficient for that cause. Another would be screw-induced vibration from a hard-over rudder in a violent maneuver, for which there is corroborating testimony.

--David G. Brown

 

[Nuno Viriato R R Cruz]

Thanks so much Dave!

Your explanations have really thought me many things!

Nuno

 

[trevor william sturdy]

David, You wrote previously-

Lugs fitted to the underside of the counter allowed the attaching of block & tackle so that the crew could (in an emergency) change a damaged blade.


This certainly would have been an entertaining sight. I have seen nuts of a similar size to the ones used to hold on these blades and I dont think you are going to put the wrench in your back pocket to swim down and swap these blades..

Ropes and men into the water over the stern, new blade being swung over and lowered down, aligning and attaching the blade - All this in the middle of the ocean..
And no scuba in 1912.
Regards.

 

[David G. Brown]

Trevor -- I've seen references to the intention of the crew changing a blade. So far, I've not found any proof it was ever done. The whole procedure sounds like something invented by a bean counter to avoid a drydock bill. My point in mentioning it was partially to amuse with the thought of trying to accomplish such a project. Still, 100 years ago they routinely moved huge weights with nothing more than muscle power and simple machines such as block-and-tackle. I wouldn't put changing a blade past some of those ol' timers.

By the way, I doubt anyone ever thought seriously of doing it on open waters. It would have been an in-port operation.

-- David G. Brown

 

[Noel F. Jones]

The lugs under the counter were for use in normal drydocking: the dockside cranes could not plumb into the vertical prismatic projection of the hull without resorting to complicated and hazardous yo-yo gear.

Ships are of course built for particular services wherein routine port servicing facilities can be taken as granted but it must be taken into consideration by the naval architect that any vessel can find herself in extreme operating conditions. Consider the Mauretania in the Dardenelles under combat conditions for instance.

I can't cite an example but it is entirely possible to rig jury rudders and change propellers/blades without drydocking by severely trimming ship (by dint of shifting the available weights on board) so that sufficient of the job emerges from the waterplane to get a working purchase. This would be easier with a cargo vessel. A passenger liner would have fewer shiftable weights and a more critical curve of loads.

It would indeed have to be an extreme operating exigency for a tow to a drydock not to be opted for.

Noel

 

[trevor william sturdy]

David,
I agree that this would have sounded feasible(and cheaper) to some bean counter, and no doubt that if situations had deemed it necessary that the old timers would have got there backs into it and given it a go. Certainly would have been labour intensive and challenging..


Noel,
I can see that in the case of a cargo vessel, the amount of shiftable weight and the number of holds in which to move it would have made it easier to severely alter the vessels trim. In the case of a liner, with the only real means of adjusting the trim being manipulating the levels of the peak tanks, I wonder just how much difference you could make. These tanks only really provided a fine adjustment, no more than a couple of feet I would think. Any idea of what the actual figures may have been on an olympic class vessel?.

 

[David G. Brown]

Trevor -- We are probably beating a "dead horse" here. The suggestion that a propeller blade could be repaired without drydocking does not imply that it was routine procedure. In fact, I rather suspect it was only done under duress such as in combat--if ever done.

As to changing the trim of a ship, that's often done using the ballast tanks that extend the full length of the hull. In emergency situations, however, the drastic approach of deliberately flooding compartments can be used. That is, a hold may be flooded at one end of the ship to raise damage at the other above the waterline. Again, a drastic measure done mostly in combat situations or when a ship has gone aground.

The truth is that a multiple-screw ship like Titanic would not have done a prop repair on its own. It would have been returned to drydock, as happened with Olympic when it lost a blade. And, in the drydock those lugs would have served as Noel suggests as attachment points for the tackles used by the workers to handle the new blade or replacement prop.

-- David G. Brown

 

[Senan Molony]

I have never seen this suggestion (below) before - the suggestion that the *curvature* of the Titanic prop blades was different to Olympic and thought better.

Believe it is a new one for techie community - it is not about the pitch of the wing props, which was different from the sister ship.

It is specifically about the curvature of the blades.

Deplorable spelling of 'principle,' incidentally.

 

[David G. Brown]

Regarding Senan's propeller curvature citation:

This is extremely significant information. It represents a serious difference between Olympic and Titanic. If true, it means that comparisons between the speeds and distances achieved by each ship cannot be directly compared.

It is hard to tell what the word "curvature" means in the article. This word is not used in modern texts about propeller design. Most likely, it refers to the cross section of the individual blades. The two most common today are "ogival" and "airfoil."

Ogival means the face of the blade is flat while the back is symmetrically curved in a section of a circle or ellipse. The maximum thickness of an ogive is at the midpoint of the blade.

Airfoil blades resemble aircraft wings. The face is flat, while the back is curved with the maximum thickness at the leading edge. In practice, airfoil shapes produce too much negative/positive pressure and thus cavitate too early.

While I can't say for sure, it's most likely that Titanic's blades were ogival.

The thickness of the blade has greatly affects performance. As a rule, thinner blades are more efficient than thicker ones. Obviously, making a blade thinner changes the curve of the ogival section. And, that is probably what the paragraph Senan quotes speaks to -- a change in curvature by reducing the ogival thickness.

How much of this was known in 1912 is something we have to learn. However, we do know that metallurgy was improving apace in that period of time. Better metal alloys meant thinner propeller sections were possible.

The only other possible "curvature" would be called "twist" today. As a propeller rotates the inner portions of the blade move slower through the water than the tips. To account for this blades are given "twist" so that the pitch is less at the tip than the root. The result is to produce even thrust along the length of blade.

For many years I have suspected there was something different about the propellers of Titanic as compared to Olympic. Diameter and pitch do not seem to be the issues. But, an improvement in ogival section would explain J. Bruce Ismay's overriding concern with Titanic's speed compared to Olympic's performance. My hat's off to Senan because I think he's found the "bingo" fact.

If Titanic's propellers were more efficient it would follow that the ship would achieve either better speed, or reduced fuel burn, or both. The speed increase would have been exciting to watch, but lower fuel cost would have warmed Ismay's heart all the more.

-- David G. Brown

 

[Michael H. Standart]

Question is, where does the information come from? The paragraph looks like something from either a newspaper or a period technical journal or industry news publication. I don't recall seeing this in any of my copies of The Shipbuilder Specials but this isn't the sort of thing that the newsies tend to make up whole cloth either.

The smart money is on Harland & Wolff as the primary source.

 

[Steven Hall]

That is quite interesting Senan; nice find.
Dave, I've long suspected Olympic got Titanic's wing props in October 1911.
I've not looked at Olympic's speed or coal consumption figures after that date.

 

[Michael H. Standart]

>>I've not looked at Olympic's speed or coal consumption figures after that date.<<

I'll bet Mark Chirnside has. He's just about the only researcher I've seen who's made a habit of noting this sort of thing and publishing the information in his work. Perhaps somebody should get a hold of him to see if he's noticed anything of interest.

 

[Senan Molony]

Wow... such erudition on propeller blades! I've come over all ogival with pleasure.

This extract is from an American newspaper of early 1912. I am afraid I have lost the exact reference. I have looked at it a few times and wondered about it - 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.

The warning here is that this is an orphan article - yet it appears to cite a relatively widespread belief among "experts on navigation," so why has this belief not been expressed elsewhere?

We need Mark Chirnside on this thread as another propeller expert. It would be interesting to know if changes in propeller blades started to become general at this time or shortly afterwards, and whether greater speed was thereby conferred.

If so, we could perhaps backtrack an expectation to Titanic. It is interesting that David suspected a propeller difference for many years, presumably since the propulsion plant was the same as Olympic and yet there is all this evidence lying about that indicates they expected the new sister to make better speed. How?

If they had developed revolutionary new blades, then Steve's suspicion about fitting the former ones to Olympic in late 1911 would gain a lot of credence.

 

[Steven Hall]

New blades for Titanic could be considered a strong possibly. With T blades not fitted and sitting on the side of the dock when Olympic returned in Oct. they may have thought — ‘nothing to lose, bolt them on’. Of course that’s speculation on my behalf. At least you’d get to test them. Examination of images of Olympic's blades taken in March 1912 could provide an answer. You never know, could this have been the cause of the failure to one of her port blades which seen the hapless Olympic back to Belfast.
This only adds fuel to the thought Titanic (may have) sported a 4 blade centre setup; possibly used to offset any adverse issues from using the newer (wing) blades.
Get it wrong and you could end up with a ship that has 'a happy tail'.