Titanic's propellers


Mar 22, 2003
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Hi A. Gabriel,
Just wanted to fill you in a bit further on my thinking regarding speed and the change in pitch.

There is an angle of attack where the propeller efficiency reaches a maximum, producing the greatest thrust for given brake power supplied to the prop. For a given pitch and rotational speed, this will occur at a certain forward speed. Unlike a vessel that has adjustable pitch props, these vessels had fixed pitch props. The propellers would be most efficient at one ahead speed at certain number of revolutions. The changes in pitch that you see through the years shows that they were trying to find the best setting of pitch for a certain forward speed. Basically, it was trial and error. The pitch of the wing props could be changed only when the vessel was in drydock.

Typical curves of propeller efficiency Vs. forward speed to rotational speed ratios look like the diagram below. It looks like a roughly linear change in pitch tends to move the peak efficiency point of the curves approximately linearly to the right. That was the basis for the assumption of speed for Titanic over speed for Olympic at a given rpm such as 75. More thrust would have resulted because of the higher resultant water flow speed over the blade area despite the angle of attack remaining about the same if they were working close to the max efficiency point of curve (not to be confused with the stall point of the prop).
I hope this all makes sense.
upload_2018-9-2_14-41-23.png

It's been a while that I was into this sort of stuff.
Cheers,
 
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A. Gabriel

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Jun 13, 2018
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Questions have come up from me -- it is known from Mr Chirnside's research that Olympic's damaged prop shaft was replaced with one such from Titanic (q.v. The Olympic Class Ships). But was this purloined shaft ever later replaced with a scratch-built one specifically for Olympic, i.e. a part that did not have a 401 on it?

And when Olympic threw a blade in March 1912, were parts once again diverted from Titanic to Olympic? It seems unlikely, considering that by then Titanic's propellers all would have been fitted on, but I've seen statements to that effect.
 

Mike Spooner

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Sep 21, 2017
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I don't know were the centre propeller was made, but if made away from a seaport. There was a railway limit of 16 tons to be consider.

Just a reminder this weekend November 17-18 is the last steam up for the world's largest working triple expansion engine at Kempton Steam Museum for 2018. Information can be found on website.

Mike.
 

Mike Spooner

Member
Sep 21, 2017
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138
Questions have come up from me -- it is known from Mr Chirnside's research that Olympic's damaged prop shaft was replaced with one such from Titanic (q.v. The Olympic Class Ships). But was this purloined shaft ever later replaced with a scratch-built one specifically for Olympic, i.e. a part that did not have a 401 on it?

And when Olympic threw a blade in March 1912, were parts once again diverted from Titanic to Olympic? It seems unlikely, considering that by then Titanic's propellers all would have been fitted on, but I've seen statements to that effect.
I think the date when she threw a blade was 24th February 1912. First week in March 1912 Olympic was in Thompson dry dock Belfast for repairs.
 

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