That center Prop question

Mark Chirnside

Mark Chirnside

Member
So where has the idea of the three bladed centre propeller come from?

The question is really 'where has the idea of a four bladed centre propeller come from?'

What does surprise me when looking at Stephen Pigott proposed notebook for Olympic/Titanic has apply slip for a three bladed centre propeller at -14% and a four bladed centre propeller at -11.7%. So if that is the case why an earth they wanted to use a less efficient three bladed propeller?

This was already addressed in my article two years ago, Mike.

The calculation in the notebook from the Stephen Pigott papers was based on two propeller designs with a different blade pitch. The pitch of both the 4 and 3-bladed propeller designs was subsequently standardised. Therefore we don't know what their calculation of efficiency was for the propeller specification as completed.

It must be remembered that they sought to maximise efficiency and minimise vibration.
 
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Rose F.

Rose F.

Member
However, Harland & Wolff kept a record of the propeller configuration of each ship they completed. That record confirms Titanic's port and starboard propellers had a pitch of 35 feet (greater than Olympic)

I must have missed this. I knew about the Centre prop being different between the sisters, but not the wing props having a greater pitch. Luckily this one's easier to prove to doubters, seeing as one of them's not buried and seems to be (mostly) intact.
 
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Mark Chirnside

Mark Chirnside

Member
I must have missed this. I knew about the Centre prop being different between the sisters, but not the wing props having a greater pitch.

Yes, the focus for many people has only been on the centre propeller. There's been little or no comment about the wing propellers!
 
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Samuel Halpern

Samuel Halpern

Member
Propeller efficiency is not measured by slip. Propeller efficiency is measured by the ratio of power produced to power supplied. For a fixed pitch prop (i.e., one whereby the pitch cannot be varied) there will be one point of max efficiency which will occur for a certain number of revolutions at a particular speed.
 
Mike Spooner

Mike Spooner

Member
The question is really 'where has the idea of a four bladed centre propeller come from?'
That has come from want was used on Lusitania and Mauretania turbines. Propellers on a new class of ship are always tricky to get right the first time around, where a bit of trail error is required. As proved with Lusitania & Mauretania stared life with three bladed propellers but change to four bladed propellers within about one year and proved to be an improvement. So I can see why Olympic turbine was fitted with a four bladed propeller. Further prove the three bladed centre propeller was tried on Olympic and wasn't as good as the four bladed. Who idea was to try the three bladed as would appear H&W have gone against the expertise in this field.
However giving H&W the benefit of the doubt built about twenty ships with this combination, where they three or four bladed centre propellers?
 
Samuel Halpern

Samuel Halpern

Member
Propeller design is very tricky. There are other factors that affect performance including the exact shape of the blades, the level of cavitation that is produced, etc. Generally speaking, the fewer number of blades, the better the efficiency but at a cost of greater vibration. There is and was always a tradeoff that needs to be considered. But the point is that the documentation from H&W shows that Titanic was fitted differently than Olympic, and there are people who simply refuse to accept that.
 
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Mark Chirnside

Mark Chirnside

Member
However giving H&W the benefit of the doubt built about twenty ships with this combination, where they three or four bladed centre propellers?

Mike, I've already covered this in my article over two years ago. I provided the data for all these ships.

Propeller design is very tricky. There are other factors that affect performance including the exact shape of the blades, the level of cavitation that is produced, etc. Generally speaking, the fewer number of blades, the better the efficiency but at a cost of greater vibration. There is and was always a tradeoff that needs to be considered.

All in all, it's a bit of a nightmare. By the way, one topic few people discuss is the erosion experienced on Lusitania and Mauretania's propellers in their early years of service. It was quite marked in a short period of time and so that was another factor to deal with in terms of material used, on top of all the performance considerations.

But the point is that the documentation from H&W shows that Titanic was fitted differently than Olympic, and there are people who simply refuse to accept that.

If they chose not to learn, that's their loss.

The simple reality is there is no debate as far as the primary source evidence is concerned.

I'm reminded of a Twitter post some time back. It was something along the lines of: 'We are not disagreeing - you are rejecting evidence. They are not the same, and if you are debating with me I will not allow you to conflate the two'.
 
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Samuel Halpern

Samuel Halpern

Member
The simple reality is there is no debate as far as the primary source evidence is concerned.
And that's the important point. It's not a case of conflicting sources of primary evidence, its a case of some people rejecting the primary source evidence without cause.
 
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Mike Spooner

Mike Spooner

Member
This is not a case I don't believe a three centre propeller was used on Titanic. I am only looking at the evident be put forward and I see a there is lack of evident for the full story here. The evident is based on just a proposal hand written into a notebook. The original coming from Stephen Pigott of John Brown shipyard. Who is a top line designer in turbines and propellers has come to the conclusion with his calculation that the proposed three bladed propeller has -14% slip and four bladed is -11.7%. Therefore four blade propeller has better efficiency and will be the reason why was used on the Olympic.
What is missing here is the next stage after the proposal propeller, a blueprint drawing with an issue date is required for the manufacturing purposes. It would appear has gone missing. However one must question if Stephen Pigott a specialist in propeller design suited for turbine speeds and has calculated in favour for a four bladed propeller so why use a three bladed centre propeller?
 
Michael H. Standart

Michael H. Standart

Member
This is not a case I don't believe a three centre propeller was used on Titanic. I am only looking at the evident be put forward and I see a there is lack of evident for the full story here.

Objection Your Honour: Asked and answered! Mark has presented primary source material on this to back it up. It's as good as it's ever going to get.

All you're doing now is resorting to the moving the goal posts fallacy.
 
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Rose F.

Rose F.

Member
We also know having two otherwise identical ships be a test bed for future design goals was not unheard of: See Carmania and Caronia being used to test the efficacy for turbine engines while the Lusitania class ships were under construction, as well as Magentic and Laurentic doing the same for the Olympic Class ships' hybrid propulsion system.

And if it were set in stone that 4 bladed props were more efficient than three bladed props before the ships were made, why would the Olympic have had to temporarily used a three bladed centre prop design, as we knew she did? After all, there'd be no point in giving her a less efficient propeller design if it was already known to be less efficient, right?
 
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Mike Spooner

Mike Spooner

Member
Like I have said before I do not dispute Mark primary source material and is good find to. But when an expertise this field as Stephen Pigott was and has calculated four bladed propeller is better than three bladed centre propeller. As Stephen Pigott notebook is also primary source material so why use a three bladed propeller? Further more ships using turbines like Lusitania & Mauretania have turned to four bladed propellers. Then Aquitania a very similar size ship used four bladed propellers. Followed on by Germany super giant liners SS Imperator, SS Vaterland and SS Bismarch all have used four blade propellers. It would appear if H&W have used the three bladed centre propeller on Titanic is the odd out ball here.
Then the slight increase on speed of Titanic over Olympic. We do know there was a slight change of wing propellers design between the two ships. Was that the reason for increase speed?
 
Mark Chirnside

Mark Chirnside

Member
when an expertise this field as Stephen Pigott was and has calculated four bladed propeller is better than three bladed centre propeller.

That's a very sweeping statement. 'Better' in which way?

We know they made a slip calculation but there are lots of other factors involved - and that has already been explained to you, above.
 
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Michael H. Standart

Michael H. Standart

Member
But when an expertise this field as Stephen Pigott was and has calculated four bladed propeller is better than three bladed centre propeller. As Stephen Pigott notebook is also primary source material so why use a three bladed propeller?

Mike, you are seriously straining on a gnat with all of this.

The reasons for the different propellors have been explained. This was a time of a lot of experimentation to see if something better could be done with different arrangements. it still happens too. Sometimes it works out and sometimes it's back to the drawing board.

What matters in the context of this discussion is the nearly religious devotion people have to the silly idea that "The Titanic just had to have had a four bladed center prop because.....because....well....because she just, just, just HAD to."

Only she didn't. This has been shown. Time to let this go.
 
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Mike Spooner

Mike Spooner

Member
That's a very sweeping statement. 'Better' in which way?

We know they made a slip calculation but there are lots of other factors involved - and that has already been explained to you, above.
Mark I am only quoting from your article in ADB. summer 2020:
In case of a propeller with slip of 14% (three bladed as in Stephen Pigott proposed notebook) this means it is working at 86 per cent of it maximum efficiency whereas a propeller with a slip pf 11.7 per cent ( four bladed as in Stephen Pigott proposed notebook) is working at 88.3 of its maximum efficiency.
You also compared Aquitania propeller slip at 14.3 percent and when change over to new propellers reducing the slip to 6 percent with increase speed of 2 knots for the same fuel consumption.
Now correct me wrong as I read that the slip rating does make different in speed and fuel consumption. I would agree propeller design is far from straight forward as can be found in a book: Verbal Notes and Sketches for Marine Engineer Officers by J.W. M Sothern published in 1917 coving 27 pages for the correct design. Yet still land up as a bit of trail error.
 
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