An Interesting Procedure

[John Hinkley]

On John Hinkley Said:

[…]  Please tell me where I’ve been going wrong.

John

 

That’s easy, John … as I have stated several times on this forum.

The word ”Levelling” is used to describe the process … in the same way that Surveyors use it.

It does not mean getting the lathe bed horizontal.

… although you can do that if you wish, in an appropriate location.

MichaelG.

.

Edit: __ We don’t have enough specific words in the language to describe things without ambiguity … but this page is worth a look.

https://eee.poriyaan.in/topic/levelling-10403/

[Colin BishopModerator @colinbishop34627]

Ref Richard’s comments earlier about fabricated propellers, the photo below shows spare pod thruster blades carried on the foredeck of the Queen Mary so that in case of damage, replacements can be made away from her usual maintenance facility. Cunard made a feature of them and they are nicknamed ‘Commodore’s Cufflinks’.

Colin

[John HinkleyParticipant @johnhinkley26699]

Richard, Jason and Michael,

Thanks for taking the trouble to answer my facetious question on levelling.  If I knew how to insert emojis on the iPad, I would have added a naughty winky one, if such exists. I was really poking fun at those who actually do level to a fraction of a gnat’s whatsit with no reference to twist in the bed, as Jason has said. Sorry to have wasted your time.

John

 

[John Hinkley]

On John Hinkley Said:

Richard, Jason and Michael,

Thanks for taking the trouble to answer my facetious question on levelling.  If I knew how to insert emojis on the iPad, I would have added a naughty winky one, if such exists. I was really poking fun at those who actually do level to a fraction of a gnat’s whatsit with no reference to twist in the bed, as Jason has said. Sorry to have wasted your time.

John

 

Not wasted John, still gave me the chance to tell a story.  🙂

 

[alan ord 2Participant @alanord2]

Richard I like the photo of the guy squating on the lathe cross slide with a sign Safety First in the background.

I like the prop model, looks great and I enjoyed the vids.

Alan.

[Richard Simpson]

On Richard Simpson Said:

On duncan webster 1 Said:

I think the pitch is constant from hub to tip, but the helix angle changes.

I’m afraid I disagree with that one.  Pitch is the theoretical distance travelled axially for one complete revolution, assuming zero slip.  Both large marine and aircraft propellers decrease in pitch the further away from the root.  This is required to maintain a constant angle of attack.  While pitch is usually quoted as a single figure for any propeller it is generally taken at a point 2/3 the distance from the root to represent the overall pitch.  Basically the blade twists as you move away from the hub.  This is probably more noticeable in aircraft propellers when viewed from the tip inwards.

I got my info from MIT but I suspect that is not consistent with constant angle of attack, which is the difference between blade angle and relative angle of turning blade to water flow. I think this means that pitch does change with  radius, and so MIT is wrong, but not by a lot.

[Nigel Graham 2Participant @nigelgraham2]

A fascinating thread!

Not only in how to make that lovely model screw, but also an insight into Below Decks, and the skills necessary to keep the ship moving.

I had never realised how complex the propellors are until I viewed the photo of the two technicians inside the hub! Nor indeed realised the hub is large enough to accommodate anyone. I assume they must work to tables of blade, stud and jacking-screw numbers and torque settings, or something similar, to maintain consistency but it would still be a demanding task, needing unbounded patience, and in what must be a very uncomfortable working space.

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