Ship engines: 3-shaft?

[BrentD15]

I've noticed that the three-shaft propulsion for warships is seemingly uncommon for most navies. Except Germany.

In the game, most of the German battleships have 3-shaft engines, and I have to wonder why this design type didn't appear in most nations' navies (exception being the Russian Leningrad-class).

I've also read that the Bismarck's 3-shaft engine design gave her a weakness to her keel, particularly where the shafts exited the hull.

 

Was this design flaw typical among ships that chose 3-shaft engines?

[Murotsu]

The weak stern is a design flaw that really has little to do with the engine layout, and the choice of how the stern was laid out in general.  The KGV class British BB had a similar flaw that left the props exposed to torpedo hits.  The US with the North Carolina class an on went to the outboard shafts having those solid keels on them.  Those screened the inter shafts from damage and gave better support to the outer ones, in part to avoid the sort of damage that PoW took causing major flooding along the shaft alley.

The reason most countries went with 4 rather than 3 shafts is an engineering issue.  For a set amount of horsepower, four shafts allow it to be lower per shaft than three.  There is a real and practical limit to how much power you can put into a ship's shaft and prop.  The longer the shaft, the more torque is applied to it at any amount of output.  You can conceivably twist the shaft sufficiently to cause it to break.  The longer the shaft, the more vibration you get too.

For a 3 shaft layout you end up with an inconvenient machinery layout.  One engine room has to be on the centerline.  For the Bismarck, this wasn't a major problem as the boilers were grouped ahead of the engine rooms.  On a split-plant, unit machinery layout like the US used, having the engine rooms asymmetric would have caused issues with trying to lay them out in some regular pattern of alternating boiler and machinery rooms.

Designers have to make choices.  For the Germans, their capital ships could have lots of beam but had to have a relatively shallow draft to fit through the Kiel Canal.  The US had the opposite:  Beam limited, but could have more draft because of the Panama Canal.  The British were beam and length limited due to their mostly ancient (by comparison) and relatively small dry docks.

Sorry, rambling... It comes down to a design choice mostly.

[BrentD15]

14 minutes ago, Murotsu said:

The weak stern is a design flaw that really has little to do with the engine layout, and the choice of how the stern was laid out in general.  The KGV class British BB had a similar flaw that left the props exposed to torpedo hits.  The US with the North Carolina class an on went to the outboard shafts having those solid keels on them.  Those screened the inter shafts from damage and gave better support to the outer ones, in part to avoid the sort of damage that PoW took causing major flooding along the shaft alley.

The reason most countries went with 4 rather than 3 shafts is an engineering issue.  For a set amount of horsepower, four shafts allow it to be lower per shaft than three.  There is a real and practical limit to how much power you can put into a ship's shaft and prop.  The longer the shaft, the more torque is applied to it at any amount of output.  You can conceivably twist the shaft sufficiently to cause it to break.  The longer the shaft, the more vibration you get too.

For a 3 shaft layout you end up with an inconvenient machinery layout.  One engine room has to be on the centerline.  For the Bismarck, this wasn't a major problem as the boilers were grouped ahead of the engine rooms.  On a split-plant, unit machinery layout like the US used, having the engine rooms asymmetric would have caused issues with trying to lay them out in some regular pattern of alternating boiler and machinery rooms.

Designers have to make choices.  For the Germans, their capital ships could have lots of beam but had to have a relatively shallow draft to fit through the Kiel Canal.  The US had the opposite:  Beam limited, but could have more draft because of the Panama Canal.  The British were beam and length limited due to their mostly ancient (by comparison) and relatively small dry docks.

Sorry, rambling... It comes down to a design choice mostly.

Thanks.

[Stauffenberg44]

38 minutes ago, Murotsu said:

Sorry, rambling... It comes down to a design choice mostly.

Rambling like that is well received, thanks for that.

[dseehafer]

2 hours ago, BrentD15 said:

I've noticed that the three-shaft propulsion for warships is seemingly uncommon for most navies. Except Germany.

In the game, most of the German battleships have 3-shaft engines, and I have to wonder why this design type didn't appear in most nations' navies (exception being the Russian Leningrad-class).

I've also read that the Bismarck's 3-shaft engine design gave her a weakness to her keel, particularly where the shafts exited the hull.

 

Was this design flaw typical among ships that chose 3-shaft engines?

 

Among other reasons, ze Germans opted for 3 shafts in order incorporate a VPP which would cause too much vibration if there was more than one VPP. Therefore by having 3 props the centerline can be VPP and you don't have to worry about vibration issues or the direction of thrust being offset. 

 

Though, I don't know that the ships were designed with only 3 shafts specifically so a VPP could be incorperated, or if it was because there were already 3 shafts that incorporating a VPP was a logical next step.

[DeliciousFart]

I'll copy and paste something I wrote earlier.

 

The elliptical symmetry of the German BBs is actually not optimal design and contributed to why sterns of German ships have a reputation for being weak. German sterns had a habit of snapping off because the elliptical tapering that made the stern very fine, and the keel structure in the rear raises quite abruptly to accommodate the shafts and rudder, reducing girder depth. This has a double whammy of both reducing stern volume (and thus buoyancy) and decreasing the area moment of inertia in the horizontal traverse axis (remember Euler-Bernoulli beam theory, or really just bending beam theory in general). Furthermore, the design of the longitudinal framing meant that the longitudinal bulkheads had some jarring discontinuities in the stern area, which tends to have an adverse impact on structural stresses. These combined means that the stern experiences much more structural stresses since the stern depends more on being held by the structure rather than buoyancy compared to other designs, in addition to the problem of longitudinal structural discontinuities. It also doesn't help that welding wasn't nearly as advanced or developed back then, and German sterns had a largely welded structure.

 

In contrast this is where THICC American sterns and skegs really comes stands out in terms structural strength. These skegs essentially act as double keels. In fact, updated model basin tests done for the Montana design show that skegs actually increase drag, but American BB designers still kept them because of their substantial structural benefits that were too good to pass up. That said, skegs did have some initial vibration issues in the North Carolina, which was mitigated, but not completely solved, by changing propeller blade combinations. Fourier analysis and modern FEA probably would've made this a smaller issue.

 

Another thing to consider is that drag can be minimized by both minimizing wetted area and making the transition of cross section areas in the longitudinal direction as smooth as possible, in other words, avoid very sharp dA/dx where x is the longitudinal axis. For example, with the South Dakota, the skegs allows for a tunnel in between, with the skeg area smoothing the decrease in hull area, and also allowing greater beam at the stern to provide adequate torpedo protection for the #3 turret, at the cost of some additional wetted area.

 

Interestingly, the British introduced the transom stern as a way to gain virtual length. This allowed the Lion and eventually the Vanguard to increase its top speed by 0.33 knots. In some ways this allowed the Vanguard to maintain a fairly conventional and flared bow, and avoided some of the Iowa's problems, where her very long and fine bow (her way of gaining virtual length) meant very little bow buoyancy; this means that the Iowa's bow tends to dig into waves rather than ride over them, which ships quite a bit of water over the decks and allegedly makes the forward turret difficult to use in heavy seas. The Vanguard is allegedly quite dry in comparison.

 

Funny enough someone on Reddit gilded me for this post.

[VonLippe]

A further disadvantage to the 3-shaft design is that when designing a big-gun capital ship, various ships systems tend to compete for the same spaces.  Specifically, gun turret systems involve a large barbette that extends down from the turret all the way to the keel.  This acts to support the weight of the gun house & training mechanism as well as house and protect the ammunition hoists and magazines.  A centerline propeller shaft competes for space with stern turrets and also tends to transmit vibration directly to the guns mounted above them, making long-distance accuracy (slightly) more difficult

[BrentD15]

I noticed that the Russian Podvoisky and Minsk weren't the only ships they have in-game with the 3-shaft design.

They also have it on the Kiev and the "battlecruiser" Moskva as well.

 

I can't help but wonder why the Russians would choose this layout, as they were also considering this for their Khronstadt-class and Stalingrad-class battlecruisers.

[Lord_Slayer]

On 9/2/2017 at 6:14 PM, Murotsu said:

 The US with the North Carolina class an on went to the outboard shafts having those solid keels on them.  Those screened the inter shafts from damage and gave better support to the outer ones, in part to avoid the sort of damage that PoW took causing major flooding along the shaft alley.

 

Just a quick correction.

The North Carolina and Iowa Classes did not have a solid keel or skegs on the outboard shafts. They were your typical 'free' shafts. The inboard shafts had the skegs on them.

 

The South Dakota Class were the US BBs that had the outboard shafts in skegs. The inboard were then shafts that were aimed down the 'tunnel' towards the rudders. The design had alot to do with how they laid out the armor and torpedo defense on the South Dakota class.

 

They were thinking of also putting skegs on the outboard shafts of the Iowas, but the hull design made this impossible.

 

On 9/2/2017 at 6:14 PM, Murotsu said:

The reason most countries went with 4 rather than 3 shafts is an engineering issue.  For a set amount of horsepower, four shafts allow it to be lower per shaft than three.  There is a real and practical limit to how much power you can put into a ship's shaft and prop.  The longer the shaft, the more torque is applied to it at any amount of output.  You can conceivably twist the shaft sufficiently to cause it to break.  The longer the shaft, the more vibration you get too.

For a 3 shaft layout you end up with an inconvenient machinery layout.  One engine room has to be on the centerline.  For the Bismarck, this wasn't a major problem as the boilers were grouped ahead of the engine rooms.  On a split-plant, unit machinery layout like the US used, having the engine rooms asymmetric would have caused issues with trying to lay them out in some regular pattern of alternating boiler and machinery rooms.

Designers have to make choices.  For the Germans, their capital ships could have lots of beam but had to have a relatively shallow draft to fit through the Kiel Canal.  The US had the opposite:  Beam limited, but could have more draft because of the Panama Canal.  The British were beam and length limited due to their mostly ancient (by comparison) and relatively small dry docks.

 

Another point towards the 4 prop design was control. In the event the rudder was lost, you could 'steer' the ship by throttling the props.

[MzFortune2001]

Murotsu-san (Renshi). Amazing knowledge. Should have paid more attention in the few engineering classes have taken.Were these props counter rotating? Just wondering if the flow over hull and props caused harmful shaft harmonic. as can happen with aircraft prop-engine combinations. This is fascinating. Thank you much 

[Drakenred]

 

Counter rotating props on dual shaft ships became standard early on from what I understand on Passenger liners simply because they reduced torque on the ship, and because early on 2 Screws and linked sets of power plants were simply more mechanically reliable than single screw designs. Apparently this was figured out by 1908. Apparently a couple of earlier shallow draft rivers boat builders in Louisiana and Missouri  had already built some multiple screw designs for  boats as they could be  shallow draft and flat bottomed, and had already ran into the problem of torque.

 

 

A bit of extra rivia, the boats they built were for use in the delta or in some of the tributary rivers, given the shallow draft of the boats a single screw just would not work at all, but the dual screw design did. Being basically flat bottom keel-less barges the torque issue turned up quickly. Apparently one of the early ones was built in Louisiana using a derailed and rebuilt/refurbished 2-2-0 Vulcan locomotive works locomotive for its steam plant, and stayed in use for about a decade.

[Murotsu]

3 hours ago, MzFortune said:

Murotsu-san (Renshi). Amazing knowledge. Should have paid more attention in the few engineering classes have taken.Were these props counter rotating? Just wondering if the flow over hull and props caused harmful shaft harmonic. as can happen with aircraft prop-engine combinations. This is fascinating. Thank you much 

The usual problem isn't the flow of water over the hull but that the props themselves are fixed in pitch.  This results in cavitation and vibration at some speeds on most ships.  Cavitation reduces the efficiency of the prop some and shaft and hull vibration are real problems that have to be considered in design.  

[Drakenred]

Cavitation and Vibration resulting in excessive damage to the propellers was a common enough problem in the early days that when the Titanic first hit the Iceberg, one of the crew simply thought the ship had lost a propeller, and would be in for an extended stay in New York.

[MzFortune2001]

Thank you very much for the responses. Drarkenred-san, I just thought with gearing and larger props would that not lower rotational vibrations? Also done not turbulent flow over props and hull increase the drag coefficient. 

this would cause a loss of efficiency? No only basic fluid dynamics.? Ask me about Orbital Mechanics or EM spacecraft propulsion. lol 

Murotsu-san naval props are not variable pitch?   Is there a reason? sorry again not verses in subject. 

 

Thank you both for your time you spent on me.

 

 

[Drakenred]

There are a number of things that cause vibration, 

 

the shape of the hull and its impact on water moving past the propellor can cause vibration, cavitation from the prop operating at a speed it was not designed to operate at can cause vibration, or switching from full ahead to full astern van cause massive vibrations.

 

Compounding this in early piston ships literaly the engine could induce enough vibration on its own just by running at the wrong speed or from a minor defect in construction or desig,

 

Excessive "Prop drop" (the natural settling of the prop in its bearings due to its weight and normal wear) can cause vibration. This is Seperate from droping a propeller or propeller blade.

 

A turbine can suffer from vibration from a design or construction error, or unexpected wear. This can becom dangerously destructive very quickly which is why many turbines are designed to trip and shut down when vibration becomes excessive. Even seemingly minor damage to a turbine blade can cause vibration. 

 

[Murotsu]

10 hours ago, MzFortune said:

Murotsu-san naval props are not variable pitch?   Is there a reason? sorry again not verses in subject. 

 

Thank you both for your time you spent on me.

 

 

The reason most naval props aren't variable pitch is this adds considerable weight and complexity to the propeller.  Given the size of ship's propellers and that they're immersed in salt water all the time, not to mention all but inaccessible outside of a port or dry dock, they are difficult to maintain.

That's a wrench for taking the nut off holding a ship's propeller in place.  Imagine trying to do it underwater...   

[MzFortune2001]

Must hire really large men. (humor attempt). reasonable, guess that's why I don't have real job just crunch? (if that is correct) the numbers. 

Thank you Murotsu-san. I am supposed to be excellent at logic, but I guess no common sense. Forever a major flaw. 

[MzFortune2001]

Drakenred-san I have studied harmonic vibrations and turbine stator construction. Never heard of prop drop. Reciprocating engines I have only limited knowledge. just enough to make improper assumptions, so will (bow)? submit to your superior knowledge. Would have thought barrings would have been constructed for this expected condition. I realize, rotational forces, can create damaging harmonics, just would not have thought at such low revolutions this problem would have developed.   

Thank you for your kind answers. 

[Lord_Slayer]

On 10/7/2017 at 0:26 PM, Maggie_Saito said:

Drakenred-san I have studied harmonic vibrations and turbine stator construction. Never heard of prop drop. Reciprocating engines I have only limited knowledge. just enough to make improper assumptions, so will (bow)? submit to your superior knowledge. Would have thought barrings would have been constructed for this expected condition. I realize, rotational forces, can create damaging harmonics, just would not have thought at such low revolutions this problem would have developed.   

Thank you for your kind answers. 

 

From what I am aware (and someone can correct me) while they do have bearings and special seals, there is some water that is allowed to leak in to keep the bearings cool. And bearings do wear out.

There is a picture (that I cannot find online at the moment, but is in a book on the Iowa-class) showing the damaged shaft of the Iowa. The bearing gave out and the shaft rub was violent enough to be felt all the way to the bridge. In the photo, you can clearly see just how far the shaft dropped from it's normal position.

 

I'll dig up the photo when I have a chance.

[Lord_Slayer]

[Murotsu]

On ‎10‎/‎7‎/‎2017 at 9:15 AM, Maggie_Saito said:

Must hire really large men. (humor attempt). reasonable, guess that's why I don't have real job just crunch? (if that is correct) the numbers. 

Thank you Murotsu-san. I am supposed to be excellent at logic, but I guess no common sense. Forever a major flaw. 

Actually, they usually use an explosive charge initially to loosen the nut, sort of like a giant impact wrench.

[MzFortune2001]

Is this true how do I obtain the employment. Enjoy exploding things lol

[DeliciousFart]

The Burkes (DDG-51) have variable pitch propellers. In fact, those particular 5-bladed propellers are nearly the diameter of those found on the Iowas.

 

More recent propellers have a sweep to their blades to improve efficiency, like the ones you see on the Ford class CVN or on the Nimitz during their overhauls.

 

[wtfovr]

On 10/7/2017 at 10:50 AM, Murotsu said:

The reason most naval props aren't variable pitch is this adds considerable weight and complexity to the propeller.  Given the size of ship's propellers and that they're immersed in salt water all the time, not to mention all but inaccessible outside of a port or dry dock, they are difficult to maintain.

That's a wrench for taking the nut off holding a ship's propeller in place.  Imagine trying to do it underwater...   

I remember we had to change the prop on my ship and the shipyard asked the chief engineer where the ships propeller nut wrench was?   He had a puzzled look which meant the ship lost its wrench and he had no idea where it was.   So a new one was fabricated by the shipyard at considerable cost to the taxpayers.   After we left dry dock, the chief engineer found a place where the bulkhead was relatively clear and had the propeller nut wrench welded to the bulkhead in the event another prop ever needed to be changed.

 

 

The reason why modern warships have variable pitch propellers is due to the source of propulsion.   On steam ships when there is no engine order, the prop can be stopped for short periods of time.  And the direction of rotation can be changed from ahead to astern.   The prop cannot be stopped indefinitely, you do have to maintain some rotation during idle periods to prevent bow on the prop shaft., i.e. when the propulsion plant is shut down, the prop is put on a jacking gear to rotate the shaft at 6 rph.  

 

On a gas turbine ship, the prop can only turn in one direction, so to go astern the prop pitch is changed from ahead to astern to change the direction without changing the rotation of the prop.  Also at all stop the propeller is still turning at a minimum speed typically 35 to 55 rpm depending on the class of ship. So to go slower than 5 or 12 knots, you set the prop blade angle to zero so that even though the prop is turning when the gas turbine engines are running, the prop doesn’t impart any ahead or astern forces when tied up next to the pier.

[DeliciousFart]

5 hours ago, wtfovr said:

I remember we had to change the prop on my ship and the shipyard asked the chief engineer where the ships propeller nut wrench was?   He had a puzzled look which meant the ship lost its wrench and he had no idea where it was.   So a new one was fabricated by the shipyard at considerable cost to the taxpayers.   After we left dry dock, the chief engineer found a place where the bulkhead was relatively clear and had the propeller nut wrench welded to the bulkhead in the event another prop ever needed to be changed.

 

 

The reason why modern warships have variable pitch propellers is due to the source of propulsion.   On steam ships when there is no engine order, the prop can be stopped for short periods of time.  And the direction of rotation can be changed from ahead to astern.   The prop cannot be stopped indefinitely, you do have to maintain some rotation during idle periods to prevent bow on the prop shaft., i.e. when the propulsion plant is shut down, the prop is put on a jacking gear to rotate the shaft at 6 rph.  

 

On a gas turbine ship, the prop can only turn in one direction, so to go astern the prop pitch is changed from ahead to astern to change the direction without changing the rotation of the prop.  Also at all stop the propeller is still turning at a minimum speed typically 35 to 55 rpm depending on the class of ship. So to go slower than 5 or 12 knots, you set the prop blade angle to zero so that even though the prop is turning when the gas turbine engines are running, the prop doesn’t impart any ahead or astern forces when tied up next to the pier.

I was going to ask about that actually. Gas turbines only run in one direction, but is there not a clutch that allows the shaft to go in reverse? Also, as far as I know gas turbines usually like to run within a certain RPM range, which would further compel the use of variable pitch props.