An Interesting Procedure

[Richard SimpsonParticipant @richardsimpson88330]

Something I thought you guys might find particularly interesting.

Over the years I have worked with some incredibly talented crewmembers.  Their machining and engineering skills frequently put most of the engineering officers to shame.  One guy in particular stands out in my memories, particularly with a project I discussed with him.  I was discussing with him the possibilities of making some sort of presentation to our company President, who was retiring.  He suggested a brass model of one of our propellers on a base, which I though was an excellent idea.  Unfortunately we were fitted with what are known as highly skewed propellers, which I thought would be something extremely difficult to model in brass.

He asked if he could have time to look into it which I was more than happy to do.  After a couple of test tries to prove his set up he eventually produced this item.  When you have enjoyed looking at it and thinking about how it could possibly have been made I will provide links to some video clips that will show you how he did it.

When I retired a couple of years later he made one for me as well:

 

And before anyone with a keen eye informs me that the real ship has six blades and the model only has five, the picture of the real ship is a previous one that had six blades and I haven’t got a good shot of the last ship’s props!

[bernard towersParticipant @bernardtowers37738]

Beautifully done, my son does ret gifts where he works things like 1/8 scale chassis for a well known sports car maker. all parts being laser cut identical (but for size) to the original.

[peter1972Participant @peter1972]

3D printed pattern cast in brass at Lunt’s ?

[Richard SimpsonParticipant @richardsimpson88330]

I think most people would assume cast but it was machined from solid.

[Richard SimpsonParticipant @richardsimpson88330]

OK gents, for those who may find this interesting, and being basically a model engineering forum I figured there would be some, I offer four video clips of the basic machining process.  I’m sure you can see what is going on from them but basically the dividing head is unlocked and so free to rotate.  It is connected to the table cross feed shaft with a gear train such that when the table is being driven automatically by the cross feed shaft, it is also rotating the dividing head.

What is also very interesting and not immediately apparent is the fact that the pitch of the propeller blades accurately decreases the further away from the boss it is.  i.e. at the blade root the pitch is highest and at the blade tip the pitch is the least.  Frequently on model propellers the pitch is uniform across the blade but, on real propellers certainly of ship size, you will see this quite clearly in the blade design. The reason for this is that while angular velocity remains a constant across the blade the linear velocity, which is relevant to the speed at which the blade is cutting through the water actually varies with the distance from the centre.

I asked the Chief Mechanic who made this how he worked out the gear ratios and he showed me his calculations.  I was stunned by his level of in depth engineering understanding.

Anyway enjoy the video clips.

[Michael GilliganParticipant @michaelgilligan61133]

I will view those when adequately caffeinated !

Many thanks for posting them.

MichaelG.

[duncan webster 1Participant @duncanwebster1]

Pedant alert: I think the pitch is constant from hub to tip, but the helix angle changes. Having said that, propellor design looks to be fiendishly complicated, the blades themselves are twisted aerofoils, or is that aquefoils. Working out the gearing for thd dividing head would need several cups of strong coffee and an ice pack

[JasonBModerator @jasonb]

I had assumed you would not have asked if he had access to CNC

I thought he may have made a wooden master blade and then used some form of pantograph copy arrangement to replicate the master indexing the workpiece around 5 times.

We do have a few members here who have shown similar gearing arrangements to rotate the work as it is fed, myself I have done similar to form a helix but used a series of increments based on a set angle and distance to advance the mill table. Plenty of handwork to finish the job.

I’m waiting for someone like Dave to make a comment about the setup in the videos but I’ll assume it is due to having to use what is to hand on the ship though it’s a bit of tooling I would have thought would have been available.😈 I can also see a reason why it may have been used rather than the usual method and it is not as though I have never resorted to it either.

[Michael GilliganParticipant @michaelgilligan61133]

Jason,

Whilst I remain in awe of your own work, I think the big thing here is that it’s analogue motion, not digital.

Any tiny ‘steps’ in the workpiece will originate from profile inaccuracies in the gears, rather than from the inherently digitised motion of a CNC machine.

 

Sitting-back now, to be educated by wiser beings !

MichaelG.

[duncan webster 1]

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.

[noel shelleyParticipant @noelshelley55608]

Amazing ! Was it brass or did you use bronze ? No doubt very efficient going ahead but how did it do going astern ? What was the vessel ?

Jason, was there any technical reason the for the spiral fin or could a flat fin have been used ?  Noel.

[JasonBModerator @jasonb]

Just to confirm that model of mine was done on manual machines. But you could say the helix is digital due to using co-ordinates.

I’m not sure if there is any tapering in the thickness of those blades but if there is then any manual stepovers or angular adjustments to the dividing head would also leave facets or steps. The path the tool follows to any of those settings would be analogue.

[JasonBModerator @jasonb]

So if there were changes to the pitch then the will have been some faceting of the surface to be blended out with plenty of needle file and abrasive work before polishing.

 

Noel, I can’t think of a reason that the original engines used the spiral fins. I just replicated it as best I could and enjoyed the challenge.

[bernard towersParticipant @bernardtowers37738]

Have only done it once making spiral slots for internal throttle grips and got all my info from Machinery Handbook. a real headache to set up but very satisfying once there.

[Richard SimpsonParticipant @richardsimpson88330]

This is probably the best shot I have of a marine propeller, where you can get a better idea of the shape:

The model is made of brass, having a piece of bronze that size as stock would be a little over indulgent.

Noel touches on an interesting point, going astern.  Propellers tend to be designed for the itinerary they are expected to be on.  Long lengths of time at sea under fairly consistent conditions require a particular design that is comparatively efficient under those conditions but which is then less efficient when manoeuvring or going slow.  Ships that require extreme manoeuvring capabilities, such as tugs, ferries and anchor handlers use propeller designs that give the best torque and response but tend to be far less efficient when full speed running.  All propeller design is a compromise to best fit an infinitely variable set of conditions.

The most interesting, and frequently most misquoted are, what are commonly known as, variable pitch propellers.  However, as I’ve mentioned above, all propellers are variable pitch.  What is really being referred to are controllable pitch propellers.  These are usually found on ships that require very fast response and manoeuvrability but at the expense of efficiency.  These are nearly always found on tugs, anchor handlers etc. although when I first went to sea on small North Atlantic container ships they were fitted with Controllable Pitch Propellers, or CPPs, to enable fast ahead and astern movements to break the ice when going up the St Lawrence in winter.  That was in the days when fuel efficiency deep sea wasn’t as significant as it become a few years later.

As for the pictures, the first shot from astern above is the Disney Wonder, they are around 20 tons of single cast bronze each and the single propeller is from the Disney Dream.  This one is of interest because it is fabricated not a single casting.  The blades are all bolted to the hub with stretched studs from inside.  A man can comfortably sit inside the hub while he spends all day slowly stretching the studs.  While basically fixed there is an ability to adjust the pitch by a couple of degrees either way to enable the best possible deep sea efficiency.  That propeller is a bronze hub and stainless steel blades and end cap.  Big advantage of course is that a damaged blade can be replaced, yes, even with the ship in the water.

[Richard SimpsonParticipant @richardsimpson88330]

Just out of interest a picture of the process of tightening up the studs on a fabricated propeller.  The bare studs can be seen on one blade and the nuts have been fitted on the studs behind the technician.  All the stretching is done manually, no hydraulics are used.  The nut is fitted hand tight to the main stud  with the ring of hex headed jacking studs all backed off.  Then begins an incredibly lengthy process of tightening the jacking studs, in a specific sequence and in a number of stages, to stretch the main stud.

There are 12 jacking studs per nut and 18 nuts per blade.  There are five blades per prop and two props.  That is a total of 2160 jacking studs to be tightened up by hand, with a torque wrench, in a number of specific stages.

The hub is normally filled with oil in service.

[Richard SimpsonParticipant @richardsimpson88330]

Beautiful job Jason.

[parovozParticipant @parovoz]

Lovely job ! Helical milling at it’s most elegant…..  With multi axis CNC it’s an art far less practiced these days.

 

[JasonBModerator @jasonb]

Richard, do you have any more photos of what I assume is the ships workshop that the videos were take in? I’m sure members would be interested to know what facilities are available on board.

Now that could be a good seller – a model engineering cruise, better than all that Micky Mouse stuff🤣

[Richard SimpsonParticipant @richardsimpson88330]

Jason, despite the fact that I have in the thousands of pictures around the machinery spaces and many events such as dry docks and engine overhauls etc. I only have a few shots of specific items in the workshops and none at all of the workshops in general.  How could I have missed that over the years?

In particular my last ship, the Disney Dream, had a superbly fitted out workshop with the usual array of large lathe, pedestal drill and milling machine as well as a separate welding bay with dedicated ventilation and even a separate area for the electrical team to have their own testing bench.  One of the first things I did was to arrange the purchase of a 7 x 14 mini lathe with accessories, justified of course by the need to make those small one off parts that the big lathe was too cumbersome to make.  Needless to say it was for our use in the evenings!  Having said that the crew were pretty adept at making the most of whatever machinery was available, as this chap demonstrates making a small bush on the main lathe:

 

This is a shot of the Disney Wonder workshop during an engine overhaul:

And this is another job being done on the milling machine.  This time I asked the lads to make a display piece for the Chief Engineer’s Office that demonstrated as many different types of gearing as possible.  This was all mounted into a perspex box so when a handle was turned all the different gears could be seen rotating.  I was always very conscious of the fact that the engine room crew tended to be out of sight and out of mind so I was always looking for ways in which the skills of the lads could be shown off to the rest of the ship:

[Richard SimpsonParticipant @richardsimpson88330]

Just another shot of the propeller model.  The machined blank is now in the process of being carved out to the basic shape, mainly with angle grinders and files.  As Jason points out the ‘steps’ the machining process leaves can still be seen at the periphery of the blade:

[John HinkleyParticipant @johnhinkley26699]

An awful lot of people feel that it is necessary to “level” a lathe using an engineer’s spirit level during initial installation – something I’ve never attempted nor felt the need to do.  Richard would appear to be the perfect person to explain how it’s done on board ship while bobbing about on the briny.  Please tell me where I’ve been going wrong.

John

 

[peter1972Participant @peter1972]

That propeller is a bronze hub and stainless steel blades and end cap.

With there being a quite a large electrochemical protential difference between bronze and stainless steel, how is galvanic corrosion prevented?

[JasonBModerator @jasonb]

John, I’m also not one for “leveling” lathes but the process is more to do with comparing readings at each end of the bed to see if there is any twist rather than the need to be horizontal. As for doing it on a ship – just wait for a calm day or better still turn a test piece.

Thanks for the additional photos.

[Richard SimpsonParticipant @richardsimpson88330]

John, bearing in mind that, even on the most stable of ships such as a large cruise ship, the trim of the ship still varies, even when in port.  When at sea there is movement in all three planes so the idea of something being level simply doesn’t exist.  An interesting one was many years ago on a large bulk carrier we had a major engine failure in the middle of the Pacific.  There is nothing worse than the movement of a dead ship at sea as it simply drifts around until it is beam on to the weather and then it starts to roll heavily.  We were adrift for over fifty hours completing repairs, without sleep.  During this time we had to machine a couple of new parts so the second engineer was operating the lathe while the ship was rolling quite heavily.  No-one asked whether the lathe was level!  It might also interest you to know that, on my last ship, there was a pool table in the crew recreation room.  I believe they are normally supposed to be level as well!

Peter, my apologies but I’m not sure why you are asking about the electrochemical potential difference here.  What is relevant is the position of the two metals in what I have always known as the Galvanic Table, or the Galvanic Series. As you suggest the larger the difference between two metals on this table then the bigger is the potential for a galvanic cell to be created in the presence of an electrolyte.  The page below demonstrates perfectly the positions of metals in a Galvanic Series.  As you can see Manganese Bronze, the most common propeller material, is at position 17 and Stainless Steel is at position 19, 23, 30 or 32 depending on the composition.  I would expect the propeller material to be closer to 410 or 416. There is therefore very little potential.  This is also further shown in the lower table where the risk between these two metals is classed as low.

https://structx.com/Material_Properties_001.html

Jason, normally critical machinery, which could well include a large lathe, will be sat on poured resin chocks.  As you say this is more to ensure they remain in shape rather than level.  Main and auxiliary engines are always sat on poured chocks.  Interestingly the very large slow speed, long stroke diesel engines are fabricated such that each unit is made vertically separate from its neighbour and then they are bolted together.  This is to allow the engine to flex with movement of the ship.  Likewise the main engine crankshaft is fabricated and all held together with interference fits again to allow flex.  I remember the first time I saw a large slow speed, long stroke diesel engine started.  The second engineer told me to stand at the end of the engine and pick a common point on all the cylinder heads and line them up by eye.  You could see the engine ‘flowing’ as the different cylinders fired and each unit stretched a couple of millimetres.

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