Olympic vs. Titanic Screws definition of pitch?

[Stephen Walker]

I understand that the screws on these two ships differed in pitch. Being familiar with prop-driven aircraft, I had assumed that pitch on a marine screw refers to the angle of attack of the propeller blades. But I've just read Mark Chirnside's article "The Mystery of Titanic's Central Propeller" which contains the statement that "pitch refers to the distance the screw would move over the course of one revolution...not allowing for any slip."

So does this refer to the distance the screw (and the ship) would move in space over one revolution? If so, it seems like that would involve several variables, including blade angle of attack and the speed at which the propeller shaft is being turned. Can you guys enlighten me on this? Thank you!

 

[Frederik-Matthias Davids]

Propeller pitch

The distance that a propeller theoretically (i.e. without slip) advances during one revolution.

www.wartsila.com

Yes, it refers to the distance Made good in a Medium. E.g. a Propeller driven in wood or whatever makes this more clear.

Best regards Fred

 

[Stephen Walker]

Thanks, Fred, for the clarification—as well as the link to Wartsila’s marine encyclopedia. I’m looking forward to browsing this!

 

[Incony]

cavitation, determines the effectiveness of a propeller at a chosen speed of rotation... in effect if the propeller angle of attack is too severe, then one negates the action of turning it faster.. because vacuum bubbles form behind its rear faces - those vacuum bubbles - caused by the pitch of the prop and its rotational speed are - because they are a vacuum - the surrounding water has not had time to fill, are lighter than the water, since they contain nothing, so - they draw the water up toward the surface... because the water surrounds them.. in doing so. they negate the whole purpose of the prop, which is to push water out behind it in a horizontal plane, not a vertical or diffused one.. So the pitch of the prop blades and the speed they turn is important... there is an optimum based on the effort needed to turn the prop at a chosen speed, and its effect on cavitation at that speed.. i.e being able to turn at any speed does not guarantee efficiency... one wastes effort for no benefit.. and that is expensive. so the attack angles ( the pitch, the means of fuel supply and labour to make it possible, the load one wants to push along, and the speed one wants to achieve) , reach an optimum based on all those things.. its a mathematical computation and has a desired median based on input, and output.. for example, in a car, you can go very fast.. but your fuel economy goes down, and your cost goes up... in the Titanic the median really didnt have time to be known for certain.. even though some of the maths had been done.. they were still trying 3 and 4 bladed props.. they were still discovering how much fuel of the right type was needed, how many folks to use to deliver it... and what that all meant to cost and profit depending on load..

 

[Gokul Gopan]

Hi. I'm new to this community. I have a question regarding the propeller pitches.

How was the propeller pitch adjusted? I know it depends on the blade angles. But how was the blade adjusted to change it? Was it by repositioning it in the hub? Or was it a new set of newly casted blades? Hoping for an answer.

Regards,
Gokul Gopan

 

[Roger Southern]

As regards the wing propellors, the blades were bolted to a central boss so changing the pitch would involve making (casting) new blades with a different pitch to bolt on. The central propellor was a cast item all in one unit so would have required a whole new casting of boss and blades as one.

Although there was a very slight adjustment allowed at the hub when mounting the side blades it wasn't enough to seriously change the pitch.

 

[Gokul Gopan]

As regards the wing propellors, the blades were bolted to a central boss so changing the pitch would involve making (casting) new blades with a different pitch to bolt on. The central propellor was a cast item all in one unit so would have required a whole new casting of boss and blades as one.

Although there was a very slight adjustment allowed at the hub when mounting the side blades it wasn't enough to seriously change the pitch.

Thank you.. and I have also been trying to find articles on the whole process of casting. Any idea where to find them? Any detailed articles regarding Olympic class propellers or propellers at the time in general?

 

[Samuel Halpern]

Any detailed articles regarding Olympic class propellers or propellers at the time in general?

Mark Chirnside’s Reception Room

markchirnside.co.uk

 

[Gokul Gopan]

Mark Chirnside’s Reception Room

markchirnside.co.uk

Thank you for the reply. I've gone through that article. It's really a good one. I'm actually looking for anything related to the casting of the propeller blades and, like mentioned above, the adjustment of pitch. Anything regarding the whole process of making the blades. I can't seem to find it.

 

[Bill West]

A quick Google Image search for "propellor casting mold" brought up this link Making Giant Propellers - Wonders of World Engineering The forming of 2 bakeable halves around temporary "sand" propellor blades seems to be the key.

Bill