sail in reverse

[Angus MacDuff]

1 minute ago, disc said:

In regards to the viability of crash stops, aren't they not uncommon when avoiding ship collision? 

As I mentioned earlier, my old destroyer (HMCS Iroquois) could stop in her own length (gas turbine, variable pitch props)  Her immediate predecessors (the Annapolis Class) had steam turbines (designed in the 50's) and took a good thousand yards to stop if they had any speed on (+20knots).  Crash stop is just not a thing in our sim world.

[disc]

It's important to say that a crash stop is defined only as reversing the engines at speed.

The question is not so much "will the ship stop suddenly," which I think has been well-said to be "no," but rather whether applying a reverse to the engine will slow the ship demonstrably compared to the same vessel just stopping the engine. For the Annapolis example, was 1000 yards when stopping the engine, or when reversing, a crash stop? Or do we have reason to think this was not possible?

[RAMJB]

Maybe you're not getting me right or I'm expressing myself wrong.

First - crash stops are a thing of course and were tested. I didn't say "no you can't reverse propellers" - of course you can. But it'll take a pretty long time, far longer than what'd be needed for a sudden, urgent, emergency maneouver.

What I explained are the mechanisms that existed to do so, and that prevented to do it in a split second. If what you want is to test how long it'd take for a ship to completely stop from going full speed to 0 knots, of course part of the procedure will be stopping the machinery, engage the reverse, and then give power in reverse to stop it ASAP. The problem is that in the almost universal kind of machinery used at the time in warships since the dreadnought era (steam turbines) the whole process takes minutes, not seconds (and triple expansion engines weren't much better). 

And the corollary to that problem when we're talking "emergency maneouvers" is that you have seconds, not minutes, to react to whatever threat it is that so urgent to respond to. So you don't materially have the time needed for the process of reversing the engines in time to avoid whatever the emergency cause was.

Turboelectric drives were the exception, but again, they weren't common. Only the US made use of them, and then only in three classes (WV, Colorado and the single New Mexico that was built with it).


Your following comment:
"Are not the torsional loads imposed basically similar between a full-power start and a crash stop?"

No, because when you're talking "crash stop" you're talking about changing from all ahead to full astern. So you're talking about a shaft rotating at hundreds of rpm, with the associated gearing that converts the thousands of rpm from the turbines into the hundreds of rpm for the shafts, and you're talking about not just stopping that COLD, but imparting a strong force on the opposite direction. It's not nearly the same. I'll give you an instance, a Ferrari 588 is a sports car with an engine and gearbox specifically designed to stand the extreme forces involved in instant acceleration, fast gear changes, etc.

So now, go grab a 588, put it at mid revs on just 4th gear (just cruising, you know), engage the rear gear and see what happens with that gearbox.

And BTW, full-power starts, as far I can recall, were never done in ships of the time. To accelerate a ship you would raise steam in the boilers, when pressure was enough you'd gradually open the valves to the turbines and you'd gradually increase the power. You wouldn't build up steam and then rip open the valves.

You may be skeptical about it but geared drives were very very very susceptible to any kind of load that they weren't designed to take. There are plenty of reports of shaft gearings being shaken out of place by concussive forces of torpedoes that detonated quite a ways from the gearing itself, that misaligned them and rendered the shaft either unusable, or only operable at very low speeds and RPMs. Just to name an instance.


As  you correctly mention, gear drives (not gear boxes, there was no "changing gears" in a warship, the gearing was fixed ;)), were uncommon until after WW1. The first generation of turbine-driven ships were direct drives. On those probably you could be harsher on the procedure and engage the reverse turbine before you could on a geared drive warship - but in order to do so you STILL had to cut off the steam into the normal turbine, you STILL had to unclutch the shaft from that turbine, you STILL had to clutch it to the reverse turbine, and you STILL had to redirect steam to the reverse turbine. The process wasn't something you'd get done in seconds, and in emergency stuff such as emergency torpedo avoidances or collision warnings, you only have seconds to react. So while a bit better in that respect, direct drive turbines weren't still good enough, and not by far to be used on sudden emergency maneouvers.


As for Titanic, indeed, I already mentioned it in my post- the central shaft was turbine driven. In the movie (which I refer to show how a reciprocating naval triple expansion engine was changed from full ahead to full astern), you see the outter propellers stop and then begin reversing. You see also the central propeller stop - but it never reverses. Which is correct, as Titanic didn't have a reverse turbine for the central shaft (was judged unnecessary given that the other two shafts could be put on reverse)

Edited November 22, 2019 by RAMJB

[Angus MacDuff]

Just now, disc said:

It's important to say that a crash stop is defined only as reversing the engines at speed.

The question is not so much "will the ship stop suddenly," which I think has been well-said to be "no," but rather whether applying a reverse to the engine will slow the ship demonstrably compared to the same vessel just stopping the engine. For the Annapolis example, was 1000 yards when stopping the engine, or when reversing, a crash stop? Or do we have reason to think this was not possible?

Its a process.  In order to stop as quickly as possible, you would have had to bring your revolutions down to zero then reverse.  If you simply stop engines, you will sail on for ages.  The words crash stop were never spoken.  If the bridge put telegraphs to full astern together, the engine room would go through their timely process to make that happen.  Now on the more modern ships, it was truly impressive when you threw the throttles to full astern.  People stagger forward and the guys on the quarterdeck get wet. 

[DeRuyter]

On ‎11‎/‎19‎/‎2019 at 7:28 AM, Cairo1 said:

Something like this actually did happen during WWII this is called Kedging, The seapalne tender Akitsushima Kedged in combat to avoid several diver bombs and lived to see another day.

It was a semi common practice for maneuvering in the days of sail, in fact the USS Constitution kedged during the war of 1812 while fighting an English squadron.

I am not saying this is something i want to see in game, its just neat.

Maybe I am missing something because this thread is about stopping and reversing engines. Kedging is using your anchors and windlass to move the ship generally forward. Was it done if you still had engine power?  In the age of sail it was used to maneuver in a tight space like a harbor or river, or as in the case of the Constitution  move forward while there was no wind. Keeping in mind this was done whilst attempting to escape and not in the middle of battle.

[Angus MacDuff]

6 minutes ago, DeRuyter said:

not in the middle of battle.

Using your anchor to affect your course or speed at sea is just not something to consider.  First, you are rarely in shallow enough water, and second, if you are at any appreciable speed, you would destroy your windlass equipment, and probably kill some people.

[disc]

2 hours ago, RAMJB said:

And the corollary to that problem when we're talking "emergency maneouvers" is that you have seconds, not minutes, to react to whatever threat it is that so urgent to respond to. So you don't materially have the time needed for the process of reversing the engines in time to avoid whatever the emergency cause was.

Is that always true? Is an emergency one that takes seconds? What if you do have two minutes to respond? Takes a long time to move your battleship's course. If you're trying to avoid the flagship, would that not be enough time to start (not complete, obviously) stopping? Why does, for instance, Lewis' Principles of Naval Architecture devote a pretty substantial amount of time to this? When is it useful, then? I ask as a genuine question, because it's pretty standard in trials.

Also not quite true for the German Diesel-powered units, which used push-button controls.

Edit: Found the answer in Lewis, pages 257-258 in volume 1. It is generally held that at high speeds, turning is superior to crash stopping for avoiding a collision, if turning is possible. This is evident because the advance is less than the ahead reach of a crash astern, and because directional instability will occur with crash stops. The example Lewis uses are from trials of turbine-driven single-screw tanker ships of 190000dwt, trialed at 200,000 to 225000 tonnes. They begin crash stops at ahead speed of 16 knots, attain full astern RPM between 44 and 102 seconds, and stop within ~20 minutes at head reach of 4000-5000 meters. This is basically maximum possible momentum (about the most possible in game), at moderate power and forward speed. If a hard turn is attempted, a maximum head reach of 1000 meters is incurred instead, showing the value of turning instead. However, at lower speeds, 12 knots and below, crash stops become more and more useful, until about 6 knots, when turning has no advantage. Lewis also mentions rudder cycling as a way to maintain directional control for radical maneuvers, but I think that would be too complicated to realistically add.

2 hours ago, RAMJB said:

On those probably you could be harsher on the procedure and engage the reverse turbine before you could on a geared drive warship - but in order to do so you STILL had to cut off the steam into the normal turbine, you STILL had to unclutch the shaft from that turbine, you STILL had to clutch it to the reverse turbine, and you STILL had to redirect steam to the reverse turbine

Uh, my mistake, I think I wrote poorly when I said "box." "Case" is better. I was not trying to claim there was anything like gear changes, except maybe for those weird Vulcan devices on the German "pocket battleships," and maybe their interwar light cruisers. I think you make a good point regarding increased stress. Also a great point regarding low steam pressures at starting.

There were no clutches, most of the time, for reverse turbines. The reverse was most often a separate stage on the low pressure enclosure. That is, certainly for Japanese ships -- the astern turbines were always connected to the shafts on their cruisers. This is per Japanese Cruisers of the Pacific War.

I think most navies used the same arrangement, from those examples I am familiar with. Per Anatomy of the Ship: Dreadnought, the eponymous ship lacked any clutches, even for the cruising turbines. There is a notable difference here in that the reverse turbines had their own enclosure aft of the ahead turbines on the outer shafts, though on the inner shafts they were part of the low pressure enclosure.

Koop and Schmolke's book on Bismarck apparently indicates the astern units were integral to the intermediate and LP ahead turbines. Likewise, their book on German destroyers indicates the astern stages were part of destroyer LP turbines. The Anatomy of the Ship book on HMS Belfast says they were integrated in the LP; the one on Bartolomeo Colleoni says they were stages on the HP and LP.

Edited November 22, 2019 by disc

[RAMJB]

fast answer, not much time in this end so I'll try to be brief.

Yes of course "crash stops" as you call them are part of the trials of any ship: part of those trials purpose is to investigate the extremes of performance of the ship to know the maneouvering characteristics it has, to know the safety margins on operation, etc. To that purpose of course testing the time and space required for a ship to go from full ahead to full stop was of vital importance. Don't see exactly how it applies to what we're discussing here (putting engines on reverse to move backwards in the middle of a battle) though.

as for "it's an emergency one that takes seconds?" - Most of them are. An incoming torpedo dropped a few hundred yards away doesn't give minutes of warning, for instance. A sudden collision warning due to a mis-turn during sailing in close formation doesn't either. Those things are usually a matter resolved in seconds , be it with a good or bad result. 

A practical instance of that is the incident involving Maryland, New Mexico and Oklahoma during the fleet maneouvers. As mentioned in http://www.navweaps.com/index_tech/tech-038.php

"The same ship also escaped two collisions in a matter of minutes during a close order fleet maneuvering exercise between the wars. When USS Oklahoma (BB-37) sheered out of column to avoid running down an errant destroyer, she intruded on the next column of ships, crossing the Maryland's bow. The Maryland performed an immediate "crash back" to avoid the Oklahoma, decelerating and letting the other battleship pass ahead, only to be confronted with the direct drive turbine USS Arizona (BB-39) vainly trying to back down behind her. Maryland's electric motors were immediately thrown back to flank speed ahead and the turboelectric ship accelerated ahead of the less responsive Arizona."

We can see the limitations of a ship with turbine direct drive (geared drive wouldn't have fared any better) when about to collide. The warship with turboelectric drive was able to go on full reverse to avoid colliding with another warship, to then go full ahead to avoid being rear ended by yet another one coming from behind, faster than what the warship coming from behind could just slow down. The whole sequence happened very quickly.

Meaning: as it was it was a very close call only averted because of the turboelectric powerplant aboard USS Maryland, because the ships with conventional powerplants could neither slow down enough to let the ship in front pass by, in the first case case, nor accelerate down enough to avoid being rear ended, in the second one. Had she had a conventional powerplant too (as Oklahoma and Arizona did), USS Maryland would've probably collided once, or twice.



And "german diesel warships" ammounts to a grand ammount of 3 ships in six decades. Namely Deutchland/Lutzow, Graf Spee and Graf Scheer. I know the H-39 design was intended to use diesels, but obviously they don't really count as the ships of that class were cancelled when the first one wasn't even 20% complete. I didn't really feel like adding a long paragraph about how diesel plants operate on an already kilometric post just for the sake of covering three heavy cruisers, because I guessed everyone is already familiar with how internal combustion engines work, and what's required to put those engines in reverse ;).

Edited November 22, 2019 by RAMJB

[disc]

Gotcha, makes sense. On the contrary, I don't think most people are familiar with the process of air-injection braking on Diesels, so I talked about that previously in this thread.

For that matter, I've never heard a good description of exactly how the Vulcan transmissions worked on the German ships. Could they actually change gears? I always imagined them as a bunch of planetary reduction differentials with clutches so each Diesel could be disconnected for maintenance.... 

A quibble, but there were seven Diesel-powered German cruisers, out of the twelve. The K light cruisers, Leipzig, and Nuernberg had combined turbine/Diesel plants, per Koop and Schmolke's book.

Plus, for Diesels, we have hundreds of the American escort destroyers (including a bunch of Diesel-electrics! I agree, electric transmission would be fun to see), or the eclectic Japanese Diesel warships,  or the planned Spaehkreuzers, or the Z-51, or yada yada yada. I would be curious to know more about their transmissions too.

On an unrelated note, did WWI or WWII German submarines have air injection braking? Relevant for diving, as at least on Japanese subs the engines had to be completely stopped for declutching and transfer to electric propulsion. Obviously the Americans dodged that problem by abandoning direct drive in favor of pure electric transmission.

 

I don't know if you saw my posts earlier in this thread, but I strongly agree that stopping mid battle is idiotic. I support adding a reverse only if ramming is introduced.

I support adding crash-astern maneuvers only because it would help avoid torpedoes and collisions. You don't have to come to a complete stop to avoid something bad, is my point, deceleration to a slower speed may be enough. If a destroyer launches a 40 knot torpedo 1000 yards away, that's a solid 45 seconds to impact, within the timespan Lewis gave for a merchant ship to achieve full astern RPM. You may be able to dodge it by slowing down a little. It would be nice to have as an option, is my argument. I think we can agree that actually travelling backwards is probably a bad idea.

 

Very cool information on those turbo electrics!

Edited November 23, 2019 by disc

[RAMJB]

Completely forgot about the german CLs, you're right on that one. Then again, they were mixed propulsion and I don't know a lot about them (they were pretty marginal units anyway) so I guess that little slip of mind has an explanation ;). The "paper ships" of Plan Z I just don't even factor in - I try to focus on ships that were built and used in service, not on drawing board projects that never were completed ;).

there were obvoiusly a lot of small sized combatants with machineries of different types during WW2, ranging from triple expansion in older small ships pressed into service as patrol craft, to turbines (Geared and not geared), diesels, diesel-electrics, etc. But I didn't even give a second thought about them during the whole thread,
I was talking about major combatants only (destroyers and avobe, the ones relevant for this game ;)). Though now I think of it, a couple US Destroyer Escort classes had turboelectric and others used (as you mention) diesel-electric drive...so I guess I should've been a bit more precise, yes :).


As for the german subs, I'm not really that knowledgeable about the particular mechanic details of the german diesel transmissions in the U-boats. I know in general how they worked, and the general differences between the geared diesel-electric drive of the german submarines vs the turboelectric model of the american submarines (where the diesels weren't geared to the propellers at all, while on the german machinery layouts, they were). As far as I recall from memory alone, german subs did have a clutch and gearbox, but (again, talking out of memory here so don't take this as gospel AT ALL) those were used to either engage the diesel to drive the propeller directly, or the electric motor when battery power was in use. Not to "change gears". Or at least I don't think they had that type of gearbox. I don't recall many specifics about that particular extreme, I'm afraid, been a while since I last read anything related with WWII subs and what I've read wasn't that in-detail either.

As for the way german u-boats went into reverse, those diesels had two sets of cams. A "normal" set for standard operation, and a second set to allow reversing the rotation of the driveshaft. It was a pretty simple system to maintain and operate, you only needed to switch the engine off, operate a lever at each end of the banks of cylinders to change the set of cams, and when switched back on the engine would be put on reverse.

this was on the Type VII btw. I'm not really sure about the Type IX but I'd wager it was done the same way. As for the Elektroboots and WW1 era german subs...I'm afraid I don't have a clue, I have barely read anything detailed about them ;).

Edited November 23, 2019 by RAMJB

[Steeltrap]

I note Titanic was mentioned, and indeed in one of my earlier posts I initially had a reference due to her triple expansion engines being the tried and tested technology for the period the game covers before the widespread adoption of turbines, but removed it as it was adding more to the post than was necessary for the points I was making.

In her sea trials, one test was to see how quickly she could stop. While we don't know the answer, sadly, we DO know the distance and the starting speed.

At 20 knots she stopped in ~780m. It would've taken approximately 75 seconds to cover that distance if maintaining 20 knots. If it takes a minute and a quarter without changing anything, it has to be true we're looking at a number of minutes at least before she came to a stop, presumably with her engines at full astern (but not the turbine running the smaller centre propeller it had no reversing ability, as RAMJB pointed out).

Titanic was the largest ship afloat at the time of her trials, and indeed a good deal larger than warships. Comparing her with Iron Duke, for example, we have 882ft/52,310t displacement v 622/30,000. Iron Duke had a top speed of 21 knots, which was what Titanic was supposedly at when the iceberg was sighted. Their relative engine power is quite different but, due to the 16,000HP of the turbine not being available for reverse on Titanic, their available power astern was pretty much the same at around 30,000HP.

All of which means it's pretty much certain Iron Duke could stop more rapidly than Titanic due to having the same power available but being only 60% the displacement. Not sure how different the time to reverse the turbines might be vs the triple expansion reciprocating engines, however.

As an aside, it's rather fascinating how much study has and continue to be done even now of the Titanic collision. Some of them present very different pictures from the one in popular awareness. Here, for example, is just one example, well worth reading as it suggests, rather startlingly, that not only was there no "hard to starboard" (they were still using 'tiller commands', so a starboard order in fact had the effect of turning the rudder to port) command ever given, but that Titanic was in fact turning back TO starboard as part of a "port around" attempt when she struck, and that technically she grounded as opposed to collided:  http://www.williammurdoch.net/articles_14_Revisionist_Theories.html 

I read another in an article ( https://www.telegraph.co.uk/news/uknews/8933109/The-30-seconds-that-sank-the-Titanic-fatal-delay-in-order-to-change-course-doomed-liner.html ) that suggested the entire thing could have been avoided as they had approximately a minute's warning. The 'facts' of the sighting of the iceberg, the orders given, and the nature and indeed questioning the inevitability of the strike, are all still very much contended.

Sorry to derail with the Titanic adventure, but it's still pretty fascinating even today.

Cheers

[Steeltrap]

To @disc and @RAMJB , you guys are having a great technical discussion. Very interesting.

@RAMJB I believe you've got the German Type VII set up pretty much correctly. Personally I like the system the USA fleet boats used as for some reason it strikes me as 'cleaner', but clearly both were very much fit for purpose. Obviously the USN boats were larger, too, which probably was a factor in being able to use their system.

Thought you might enjoy this more detailed description posted by Rainer Bruns on uboat.net:

Both propulsion systems (port and starboard) were set up identically. From forward to aft end machinery was set up in the following way:
Diesel engine==>Short Shaft==>Electric Motor==>Main Clutch==>Shaft with Main Thrust Bearing==>Shaft inside Stern Shaft Seal==>Shaft outside and screw support by strut bearing. In addition the shaft could be mechanically decoupled at the aft end of the Diesel.

Thus under normal operation the Diesel would be turning the E-motor (switched to and functioning as a generator charging batteries) and turning shaft and screw. Once batts are full, Diesel can be declutched, E-motors switched from generator charging mode to propulsion (drawing juice from batts) etc.

Since everything is duplicated on both P&S, you can see that the engineer could stop one Diesel, switch its E-motor to propulsion (drawing juice from the other E-motor in generator mode), and do routine maintenance or emergency repairs on the Diesel with both screws turning. So you can see, there are a couple of options how to get on.

In addition you could stop one side completely and proceed with Diesel or electrically with the other.

This happened on U175, when homebound off West Africa she got a pasting from aircraft DC causing an uncontrolled dive down to 310 meters and severe damage. Among other damage, the shaft strut and bearing just foreward of the screw broke off, leaving the tail end of the shaft assembly unsupported. They made it home to Lorient on one shaft using the other diesel for charging only.

You are correct that there is a sideways slewing under one shaft propulsion. This is easily corrected by applying a couple degrees of counter rudder. This has no ill effects on the rudder shaft and bearings as they are designed for full hard-over turns at max. speed. Obviously, it is a bitch to steer in any kind of sea; and manoeuvring at close quarters you might as well call a tug.

Edited November 23, 2019 by Steeltrap

[RAMJB]

3 hours ago, Steeltrap said:

I note Titanic...

Don't worry about the slight off-topic. That ship is a matter of interest for almost everyone who loves ships ;).

I'd like to comment about your estimations about the respective ability to stop in an emergency of Iron Duke vs Titanic. THey pass over some important considerations ;). I'm not trying to discuss your conclussion that Iron Duke would be able to stop much quicker than Titanic, just that you're not taking some important things in consideration in your calculations ;).

For one HMS Iron Duke power you listed I guess is when forcing the engines over 100%. I don't recall them having more than 30000 shp total. But even taking your number as valid, remember that's the power available on the normal operation turbines. But when put in reverse, the ship would have to redirect steam to the reverse turbines which, honestly, I don't have a clue of how powerful they were (And all my reference books are 500km away from me), but for sure they would've been able to deliver only a fraction of the power the high output turbine (the one delivering the top power you listed) could produce. So you wouldn't have the full rated power available for operations on the astern turbines. Meaning that RMS Titanic had more power available on reverse (triple expansion engines could deliver the same power in whichever direction they were operated) than Iron Duke, even while her midline turbine couldn't be put on reverse.

The other consideration however heavily favors Iron Duke. Titanic was a 800+ feet ship with a pretty large lenght/beam ratio, both of which favor lower drag, and a hull form designed (as far as confort and luxury weren't affected) for efficient high speed travels, meaning it's form was suited for low drag at high speeds. Of course she was bigger too - far more inertia involved.

Iron Duke was a bulky 620 footer with not a particularily large L/B ratio. Both are undesirable for high speed performances. Her hull wasn't designed with any special consideration for low drags and high speeds. Having both Titanic and Iron Duke launched at 21 knots besides each other, and immediately cutting engines on both would net Iron Duke slowing down much faster than Titanic. First because of sheer inertia. And second because Iron Duke's hull created more drag at high speeds than Titanic, so even leaving inertia aside, she'd have a much easier time stopping on her own.

Again, not trying to discuss your conclussion that Iron Duke would be able to stop in less time and space - I agree with it. It's just that there were some things you weren't taking in consideration ;).


That's some nice extra information on the submarines, btw. Glad my memory isn't totally crap ,lol. Thanks for bringing it :).

Edited November 23, 2019 by RAMJB

[Steeltrap]

Yes, I knew that when I wrote it. Didn't really want to go into the whole hydrodynamics and various engine specifics beyond basic, but of course you're correct and the straight power comparison in particular was pretty crude.

Edited November 23, 2019 by Steeltrap