# 2 and 4 stroke/low and medium speed



## blueprint2002 (Sep 7, 2011)

Senior members will probably remember that at one time there were 4-stroke low-speed diesels competing with the 2-strokes, though they disappeared from the market maybe around the early fifties. And the 2-strokes varied considerably, as far as scavenging methods were concerned: loop-scavenging and cross-scavenging were not uncommon, while scavenge blowers could be driven by electric motors, steam turbines, or off the engine crankshaft. 
And at the same time, many 2-stroke medium-speed diesels competed with the 4-strokes; some of them with opposed pistons, but mostly conventional. These disappeared around the seventies, if memory serves. Come to think of it, there were also a number of high-speed 2-strokes, such as the Napier Deltic and Detroit Diesel.
Today, however, all low-speed engines are 2-strokes, invariably with crossheads and uniflow-scavenged, while all medium-speed and high-speed engines are 4-strokes, always with trunk pistons and four valves per cylinder. Turbocharging is of course universal. (To my knowledge, the upper end of the “low-speed” range lies at 150-200 RPM, while the lower end of the “medium-speed” regime is about 400 RPM).
What I have never understood are the reasons why this came about. Could it be that the theoretical advantages of the 2-stroke (up to twice the power from the same swept volume) are only realised when
a)	There is sufficient time to achieve complete combustion, as much as a whole second for each complete cycle (60 RPM)
b)	Uniflow scavenging together with a really long stroke (twice to thrice the bore) is essential to achieve thorough removal of the burnt gases without too much excess air.
And conversely, do scavenge and combustion efficiencies drop for the 2-stroke, when RPM increases and strokes correspondingly shorten? With the result that only the 4-stroke cycle is then practicable, from the fuel efficiency point of view?


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## Tim Gibbs (Apr 4, 2012)

blueprint2002 said:


> Senior members will probably remember that at one time there were 4-stroke low-speed diesels competing with the 2-strokes, though they disappeared from the market maybe around the early fifties. And the 2-strokes varied considerably, as far as scavenging methods were concerned: loop-scavenging and cross-scavenging were not uncommon, while scavenge blowers could be driven by electric motors, steam turbines, or off the engine crankshaft.
> And at the same time, many 2-stroke medium-speed diesels competed with the 4-strokes; some of them with opposed pistons, but mostly conventional. These disappeared around the seventies, if memory serves. Come to think of it, there were also a number of high-speed 2-strokes, such as the Napier Deltic and Detroit Diesel.
> Today, however, all low-speed engines are 2-strokes, invariably with crossheads and uniflow-scavenged, while all medium-speed and high-speed engines are 4-strokes, always with trunk pistons and four valves per cylinder. Turbocharging is of course universal. (To my knowledge, the upper end of the “low-speed” range lies at 150-200 RPM, while the lower end of the “medium-speed” regime is about 400 RPM).
> What I have never understood are the reasons why this came about. Could it be that the theoretical advantages of the 2-stroke (up to twice the power from the same swept volume) are only realised when
> ...


Ring lubrication and piston speed are a greater problem on 2-strokes.
The efficiency of current turbo chargers allow high air pressures and flow to complete scavenging efficiently so I'm not sure that speed is a limiting factor. Doxfords eventually got the Seahorse (580 mm bore 2-stroke)to work ok at 300 rpm!


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## Varley (Oct 1, 2006)

Is it not as simple as weight. Naval Architects weight the equation in favour of medium speed crap heaps because they are lighter and allow for more cargo in a given hull providing the fuel cost do not tripover whatever the chartering rate is for the trade while those that must tend to the tiresome task of making thjeir design work weigh in in favour of the heavier but fewer moving parts of low speed two stroke. Perhaps they too would have liked the low speed four stroke but everything is compromise.

The always quotable Dick Swindall always reckoned that engines should be blown electrically so that the delivery could be exactly matched to optimum requirements regardless of engine output. Using the exhaust no ,longer used for turbo charger turning to generate electricity.


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## Andrew147 (Mar 23, 2009)

I seem to remember being told (Taylor, EK lecturer @ Hull) that the limiting factor for virtually any engine is the point at which the piston ring looses its lubrication and makes metal to metal contact with the cylinder wall. Again I think he mentioned 90m/sec as roughly the speed between the two.
So, long strokes get slower but get better combustion and last, with some care, forever.
Conversley, the F1 engine revs to 15000 revs but only 25/30mm stroke and it only last a couple of races.
Everything else is somewhere in the middle.


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## makko (Jul 20, 2006)

I think that just one consideration - spares. 2S, less parts needing replacement, downside, weight/size. M/H speed 4S, lots more spares and adjustment needed, upside, more compact/lighter. I once serviced Allen 4S 18 cylinder gennys - What a nightmare to adjust the timing correctly (TDC/BDC/stroke).
Rgds.
Dave


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## spongebob (Dec 11, 2007)

It is a long time since I had anything to do with marine engines be they reciprocation steam or four or two stroke diesels but I wonder how far the designs and durability have been improved since say the 1950's 
Many of us are familiar with engines of those times and I hark back to the car engines of the period .
As a still keen young man I used do due maintenance on three cars in the early 1960's , my wife's 1953 Vauxhall Wyvern, my mother in law's 1962 Morris 1000 and the firm's 1958 Mark 2 Ford Zephyr.
Each required a grease every 500 or so miles, an oil change every 1000 miles plus frequent spark plug cleaning and gauging, points checking , and tappet adjustments etc.
Now my 2016 CX3 Mazda requires an oil and filter change every year or 15000 kilometres , whichever is the sooner and apart from checking the tyre pressures and dipping the never changing oil level once in a while.
Other advances include no leaking oil seals , rocker covers etc and no oil drips on the concrete drive way.
A huge advance brought about by design improvements, better steels and alloys, better machining via CNC machine tools .
Has much of this transferred into the modern marine Diesel engine in a large container ship?

Bob


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## Engine Serang (Oct 15, 2012)

When was the last time you passed a car broken down with steam coming out of the bonnet? Modern cars are mechanically reliable but sometimes the electronics can be problematical.


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## Rosels (Dec 30, 2013)

Engine Serang said:


> When was the last time you passed a car broken down with steam coming out of the bonnet? Modern cars are mechanically reliable but sometimes the electronics can be problematical.


Or even a flat tyre!!!!!


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## skilly57 (Mar 11, 2008)

One of the last anchor handlers I was on a couple of years back had two 6,000 kW R-R Bergen V12s installed. The manufacturer can still get it wrong sometimes - trying to remember the exact problem, but I seem to recall that the fuel pump components were all faulty. So, reliability is only as good as the sub-contractors who are supplying the OEM parts. 
They are actually very nice engines when the problems are sorted. (now, if this was an event 30 or 40 years ago, I would be able to recall every detail!).


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## makko (Jul 20, 2006)

spongebob said:


> It is a long time since I had anything to do with marine engines be they reciprocation steam or four or two stroke diesels but I wonder how far the designs and durability have been improved since say the 1950's
> Many of us are familiar with engines of those times and I hark back to the car engines of the period .
> As a still keen young man I used do due maintenance on three cars in the early 1960's , my wife's 1953 Vauxhall Wyvern, my mother in law's 1962 Morris 1000 and the firm's 1958 Mark 2 Ford Zephyr.
> Each required a grease every 500 or so miles, an oil change every 1000 miles plus frequent spark plug cleaning and gauging, points checking , and tappet adjustments etc.
> ...


Bob,
You would be amazed! MAN B&W have technical papers on their website. They make good reading and can bring you up to speed on engine developments. CC Pounder's latest edition is also available to download, another great source to clue you up.

Having said that, I had a loss involving a B&W 10K98, used as a generator. The engine ran at 140 rpm with a 30 metre diametre, 360 pole generator! The engine had inherent vibration problems, eventually linked to crankshaft harmonics due to the even number of cylinders. This caused cracking of the bed plate and frames and, in the loss, the generator rotor contacted the stator with inevitable results.

It was interesting to see that, however new the engine, the same fitting skills are required! In other words, give you any new engine and, after flipping through the manual, yo would feel right at home.

In the same plant, they had the same engine, only 11 cylinder. This ran like a sewing machine! The same engine had a turbo driven auxiliary generator too. A real beaut, as you say downunder!

The only thing "new" under the sun are the electronic data systems/controls and exhaust gas washing sytems, really.

Rgds.
Dave


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## Tim Gibbs (Apr 4, 2012)

Sulzer has a medium speed 2-stroke, the ZH40. It had a clever rotating piston to improve ring/liner wear and control lub. oil consumption.It eventually morphed into a 4-stroke to improve fuel consumption the ability to burn low grad fuel.
But my favourite medium speed 2-stroke was the Wichmann WX28. A clever 600 rpm loop scavenge engine that gave remarkably good fuel consumption mainly due to a clever modulating auxiliary blower. After Wartsila took over Wichmann they offered an upgrade to 720 rpm resulting in a piston speed of 9.5 m/sec bit I don't know how well that turned out!


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## makko (Jul 20, 2006)

makko said:


> Bob,
> You would be amazed! MAN B&W have technical papers on their website. They make good reading and can bring you up to speed on engine developments. CC Pounder's latest edition is also available to download, another great source to clue you up.
> 
> Having said that, I had a loss involving a B&W 10K98, used as a generator. The engine ran at 140 rpm with a 30 metre diametre, 360 pole generator! The engine had inherent vibration problems, eventually linked to crankshaft harmonics due to the even number of cylinders. This caused cracking of the bed plate and frames and, in the loss, the generator rotor contacted the stator with inevitable results.
> ...


Fe de Errata - The vibrating 98 Cm bore engines were 11 cylinders, the "good 'un" a 12 cylinder! It was bothering me all day, so I went onto the VPN and checked my notes/reports. The plant is "Internal Combustion Plant Baja California".

Rgds.
Dave


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## Steamseadog (Apr 11, 2008)

Reading about lubrication problems reminded me of an engine that was built by the North British engine works. It was a double acting fitted with sliding cylinders.
This arrangement managed to reduce the relative movement of piston to cylinder by 30% thus decreasing wear
One was fitted to the "Swanley" in the early 20's -3 Cylinder 2000Hp. She lasted for about 15 years. I guess sealing of exhaust pipes and cooling water kept the engineers busy.


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## Chillytoes (Dec 9, 2006)

Referring to #11 where Tim says about the engine being uprated from 600 to 750 rpm, this is a not uncommon way manufacturers can offer a more powerful engine. Just increase the revs. 
The great advantage of multiple medium speed diesels is that they can be coupled to generators for both domestic needs and for electric propulsion. No need for expensive, complicated reversing gearboxes and the propulsion power can be varied as required. But still, there's a lot of units, valves, injectors, fuel pumps etc, plus the individual management systems to worry about.


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## skilly57 (Mar 11, 2008)

Seem to recall that 9 m/sec is the preferable max piston speed, rather than 90 m/sec. The latter speed would really keep us engineers busy looking over the side for blown piston crowns & cylinder heads.


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## Engine Serang (Oct 15, 2012)

Spent years at sea but never really got the hang of this 2 stroke and 4 stroke lark. I assumed it had something to do with the up and down movement of the piston thing in the engine. Some ships had both 2 and 4 strokes in the engine room, were they compatible? Did the crew ever mix them up?
Impulse and reaction turbines were also a complete mystery to me and any engineer I asked said it was all Parsons fault. Was there similar conundrums on the Bridge?


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## chris8527 (Jul 26, 2008)

Engine Serang said:


> When was the last time you passed a car broken down with steam coming out of the bonnet? Modern cars are mechanically reliable but sometimes the electronics can be problematical.


I must admit that it's a rare occasion to pass a car with steam coming out from under the hood but it's not unusual for steam to come out from under my wife's bonnet when I am driving and I do something that doesn't comply with an her concept of safe driving.


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## skilly57 (Mar 11, 2008)

Engine Sarang, you need 4 things to happen to make horsepower in an engine cylinder - you need to Suck, Squeeze, Bang, and Blow.
Suck in the fresh air (Induction)
Squeeze it to make it hot enough to ignite the fuel (Compression)
Bang - the fuel igniting to produce the power (Power stroke)
Blow - to vent the exhaust gases out of the cylinder (Exhaust).

The 2-stroke engine does all this in 1 complete revolution of the crankshaft i.e., 1 piston stroke down, and one piston stroke up.
The 4-stroke does it all in 2 complete revolutions of the crankshaft, i.e., 1 piston stroke down, 1 piston stroke up, another piston stroke down, and another piston stroke up.

Because of the different way they achieve the same result, the 2-stroke engine is normally bigger than a 4-stroke, it runs slower, and is made up of fewer bits.

That's about as simple as I can put it.


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## Varley (Oct 1, 2006)

You have to understand, S...57, that E-S is a steam man.

With hot fog:

It is the fuel consumption that sucks.

Only partners or budgets get squeezed (until one gets to the age when Avocardo's hypothesis rules).

Bangs are only good if emanating from a connubial couch, with much the same applying to blowing.

Stroking is associated with manual de-watering equipment employed by certain of those on the Iberian peninsular.

The only things two top plumbing tickets have in common is ignition and exhaust. E-S would have become very exhausted indeed if he had to light a fire four times for every revolutions of the screw. Even twice would bring on a sweat.


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## Engine Serang (Oct 15, 2012)

The fog is lifting but why do Doxfords not need a spark plug?


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## Andrew147 (Mar 23, 2009)

skilly57 said:


> Seem to recall that 9 m/sec is the preferable max piston speed, rather than 90 m/sec. The latter speed would really keep us engineers busy looking over the side for blown piston crowns & cylinder heads.


True, where did I get the 0 from. Don't trust an old memory.


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## spongebob (Dec 11, 2007)

Trying to think through all this fog , both steam and time gone by , is it correct to say that a two-stroke piston bottom end bearing is always loaded on the top half whereas a four-stroke cycle the load goes on to the bottom half during the induction stroke.
A excess bearing clearance in a 4/ might knock but in a 2/ the bottom half is only a keeper.
Tell me to get back to my gardening if you wish

Bob


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## makko (Jul 20, 2006)

spongebob said:


> Trying to think through all this fog , both steam and time gone by , is it correct to say that a two-stroke piston bottom end bearing is always loaded on the top half whereas a four-stroke cycle the load goes on to the bottom half during the induction stroke.
> A excess bearing clearance in a 4/ might knock but in a 2/ the bottom half is only a keeper.
> Tell me to get back to my gardening if you wish
> 
> Bob


Far too simplistic, Bob! Think about it and draw a simple vector diagram by crank angle, number of cylinders and what the other cylinders are doing! I have, somewhere, the B&W report on the 11 cylinder 98 Cm. engine. Also, in that case, the effect of the alternator and the overhang effect. Rather a splendid paper, but "for internal use only"!

Another interesting claim was a cracked crankshaft in a Wartsila V18 cyl. medium speed 4/S. Someone asked for an analysis of propagation of the crack versus rotations of the engine and why there were no auxy alarms to alert operators to the pending disaster. These engines have also an auxiliary system (cannot remember how it works, precisely) to confirm splash lubrication.

I did the analysis and the maths. Less than 2.2 seconds from crack initiation to failure! On "yer mudders grave" it was not predictable, merely fortuitous! It took out a couple of cylinders and required a new crankshaft, but I was quite taken aback how the engine resited more damage!

Rgds.
Dave


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## skilly57 (Mar 11, 2008)

Bob, with boost pressures of up to 3 Bar in today's modern 4-strokes, the induction stroke hardly puts any load on the bottom end bearing bottom half when loaded at the top end of the rpm range. For a constant-speed 4-stroke (i.e., a lightly loaded generator), there is not much boost pressure acting on the piston crown, so there is the potential for more wear on the bottom half. 
On a variable-speed 4-stroke propulsion engine, the boost pressure rises as the rpm rise, so load on the bearing top half is much greater on the induction stroke than, for example, a naturally aspirated diesel engine. The boost pressure of up to 40 psi exerts a good downward force on the piston crown when passing over TDC, thus reducing the reciprocating load on the bottom half of the bottom end bearing as the piston reverses direction.


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## Varley (Oct 1, 2006)

Engine Serang said:


> The fog is lifting but why do Doxfords not need a spark plug?


They stopped putting plugs in early on after the first time a junior left one out and the bang leaked.


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## Varley (Oct 1, 2006)

#23 ,

Ah, Dave, a true disciple of the medium speed crap-heap. Who else would describe wrecking two units and a crankshaft as "fortuitous"? Only in the manner Capt. Smith might have regarded his last iceberg.

I do agree that there cannot be alarms for everything although I have often thought that crank case hot spots could be detected at a much earlier stage with a low vapour pressure (and low hazard) luboil additive and a more sophisticated detector targeting its vapour.


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## Tim Gibbs (Apr 4, 2012)

skilly57 said:


> Bob, with boost pressures of up to 3 Bar in today's modern 4-strokes, the induction stroke hardly puts any load on the bottom end bearing bottom half when loaded at the top end of the rpm range. For a constant-speed 4-stroke (i.e., a lightly loaded generator), there is not much boost pressure acting on the piston crown, so there is the potential for more wear on the bottom half.
> On a variable-speed 4-stroke propulsion engine, the boost pressure rises as the rpm rise, so load on the bearing top half is much greater on the induction stroke than, for example, a naturally aspirated diesel engine. The boost pressure of up to 40 psi exerts a good downward force on the piston crown when passing over TDC, thus reducing the reciprocating load on the bottom half of the bottom end bearing as the piston reverses direction.


About 30 years ago I had the misfortune to be involved with some Mirrlees MB275s ([email protected]) driving a CCP and a front end 1500KVA alternator and after about 15k hours (2 years) we started to get BE bearing failures. Mirrless said it looked a bit like light load overspeed but we were convinced that that could not be the case as, although the engines had huge load swings whilst they were driving the discharge system, we never saw anything above 1020rpm on our analogue instruments. Eventually we fitted a digital rpm recorder and to our horror we saw that in fact we had in excess of 1090 rpm for a few seconds every couple of minutes when the load came off. Working it out it seems that we were subjecting the engines to about 25 hours of very light load overspeed every year. Oops!


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## makko (Jul 20, 2006)

Varley said:


> #23 ,
> 
> Ah, Dave, a true disciple of the medium speed crap-heap. Who else would describe wrecking two units and a crankshaft as "fortuitous"? Only in the manner Capt. Smith might have regarded his last iceberg.
> 
> I do agree that there cannot be alarms for everything although I have often thought that crank case hot spots could be detected at a much earlier stage with a low vapour pressure (and low hazard) luboil additive and a more sophisticated detector targeting its vapour.


The original fault was an inclusion in the crankshaft forging, below 2 microns, which is the limit of resolution detection. This caused the initiation of a crack which propagated over time with the crankshaft cyclic loading, until it ran completely out of the journal and through the web.

I have seen other faults on Vee engines where the piston rods share a journal, usually due to seizing but without damage to the crankshaft which, normally, only needs in situ machining, honing and the fitting of oversize bearings (They even get a very nice brass plate on the entablature of the particular crancase, below the access cover).
Rgds.
Dave


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## Duncan112 (Dec 28, 2006)

makko said:


> Far too simplistic, Bob! Think about it and draw a simple vector diagram by crank angle, number of cylinders and what the other cylinders are doing! I have, somewhere, the B&W report on the 11 cylinder 98 Cm. engine. Also, in that case, the effect of the alternator and the overhang effect. Rather a splendid paper, but "for internal use only"!
> 
> Another interesting claim was a cracked crankshaft in a Wartsila V18 cyl. medium speed 4/S. Someone asked for an analysis of propagation of the crack versus rotations of the engine and why there were no auxy alarms to alert operators to the pending disaster. These engines have also an auxiliary system (cannot remember how it works, precisely) to confirm splash lubrication.
> 
> ...


Crack failure analysis is interesting, for my dissertation I did a project on hull life estimation for a bulk carrier. The really interesting thing from my point of view was that critical crack length is not dependent on structure size, and once it has been achieved propogation takes place at the speed of sound in the material under stress. Messers Miner and Basquin have theories and rules!! (But you knew that anyway.)

The finest engine I ever sailed with was the B&W 9K98FF, unusually it had second moment compensators at front and back ends and No 9 Exhaust Valve had shims under the springs to reduce vibration in the camshaft. That well built and engineered that all the liners were still original (although not in the same holes) and some of the piston rings were still original (or at least 20 years old, some of the early records had been lost) none of the crowns had been welded when she went on her final voyage at 27 years old.


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## makko (Jul 20, 2006)

Duncan112 said:


> Crack failure analysis is interesting, for my dissertation I did a project on hull life estimation for a bulk carrier. The really interesting thing from my point of view was that critical crack length is not dependent on structure size, and once it has been achieved propogation takes place at the speed of sound in the material under stress. Messers Miner and Basquin have theories and rules!! (But you knew that anyway.)
> 
> The finest engine I ever sailed with was the B&W 9K98FF, unusually it had second moment compensators at front and back ends and No 9 Exhaust Valve had shims under the springs to reduce vibration in the camshaft. That well built and engineered that all the liners were still original (although not in the same holes) and some of the piston rings were still original (or at least 20 years old, some of the early records had been lost) none of the crowns had been welded when she went on her final voyage at 27 years old.


Fascinating stuff, Duncan. I learned a lot with a metallurgist and in a specialized lab in Dallas. 

A US company had bought an independent production plant in Quintana Roo, just outside the arqueological remains at Chichen Itza.

About two months after the sale, the "single shaft" generator shaft cracked in the location of the field bar bushings. Now, a single shaft generator is comprised of a gas turbine coupled through a reduction gearbox to an alternator and, on the other end of the alternator, is coupled a direct drive steam turbine (Alsthom design, cogeneration).

The electron microscope detected a red residue on the crack surface. It turned out that the original bushing had been fitted with crossed threads that had initiated a crack in the thread profiles. The final layer of dielectric paint had penetrated the crack, giving rise to the evidence of the smoking gun barrel!

The other telltale of failures are the tide marks, like an arrow pointing to the culprit.

I got my first lesson on crack propagation from my Dad, when we visited a Blue Funnel Liberty (Memnon or other "M") when I was about four years old. Visiting with the Sparks to his shack, my Dad pointed out the only riveted joint in the hull, abaft the wheelhouse, to ease the tensions and avoid "notch cracking". It has stayed with me all my life!

I must admit that my Dad, although dedicated to lecturing in Marine Engineering, probably harboured a desire to be a metallurgist. When he did his time with BF in all departments, there was a dedicated metallurgy lab in Odyssey Works. 

My Dad was involved in the solution to cracking in cast cylinder liners due to the struvture of the spherical cast iron (crystal boundary tension). The usual fix were tensioning bands but the metallurgist devised a method to reform the crystalline structure. I cannot remember the details, but my Dad always refers to him as a very clever man! 

I have the metallurgy book published by the Metallurgist, dated 46 or 47 and signed by the metallurgist and gifted to me by my Dad - And it is still a mine of information.

Rgds.
Dave


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## Duncan112 (Dec 28, 2006)

Liberty Ship cracking, another fascinating subject, solved by Constance Tipper, who lived about 5 miles as the crow flies from where I'm sitting now, always wondered if she was buried in the local churchyard, one of my failings is visiting the graves of the unrecognised!!

One thing I was told by one of the Chief Engineers on the B&W engined ship was that if you need a new stainless steel pump shaft machining ashore specify that the nut and shaft should come from different heat numbers of steel otherwise they will gall and seize - naturally pump manufacturers know this and will manufacture OEM spares accordingly.

Cylinder liners - centrifugal casting, ensured that the fine grains were on the wearing surface.


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## Tim Gibbs (Apr 4, 2012)

makko said:


> The original fault was an inclusion in the crankshaft forging, below 2 microns, which is the limit of resolution detection. This caused the initiation of a crack which propagated over time with the crankshaft cyclic loading, until it ran completely out of the journal and through the web.......
> Dave


2 microns? How sensitive! We has a 75LB6 Doxford crank fracture across No 4 aft side crankweb, It had initiated in the internal fillet between web and crankpin and when we broke it open we found a real inclusion - the remains of a half-round file(K)


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## makko (Jul 20, 2006)

Tim Gibbs said:


> 2 microns? How sensitive! We has a 75LB6 Doxford crank fracture across No 4 aft side crankweb, It had initiated in the internal fillet between web and crankpin and when we broke it open we found a real inclusion - the remains of a half-round file(K)


Ha ha ha! Like the old joke about the 2/E and the torch - and it was still on!

That would have been on the British bulkers with the "weld filling"!

Yes, MAN B&W have a limit of 2 microns.

Rgds,
Dave


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