Steering astern!

GJO · 12-05-13, 09:59 AM

I am surprised by the preciseness of the steering with SH3's boats when running astern.

AFAIK, in RL, running astern is rarely used for navigation and is really only used for slowing or stopping. On my own boat in RL, running astern is sometimes necessary due to the narrow nature of the canal but it can be a steering nightmare especially with the slightest breath of crosswind. By contrast the boats in SH3 seem to handle perfectly and precisely with the props running astern (very useful when attacking with the stern torp!) but is this an accurate reflection of the way RL subs would handle?

Leandros · 12-05-13, 10:15 AM

Quote:

Originally Posted by GJO

I am surprised by the preciseness of the steering with SH3's boats when running astern.

AFAIK, in RL, running astern is rarely used for navigation and is really only used for slowing or stopping. On my own boat in RL, running astern is sometimes necessary due to the narrow nature of the canal but it can be a steering nightmare especially with the slightest breath of crosswind. By contrast the boats in SH3 seem to handle perfectly and precisely with the props running astern (very useful when attacking with the stern torp!) but is this an accurate reflection of the way RL subs would handle?

I believe it is fairly correct the reason being that the boats have contra-rotating propellers and are quite heavy but not showing much of itself above the surface for the wind to act upon.

The rotation direction of the (single) propeller, as you certainly know, is particularly influential while maneuvering at low speed, the rudder loosing its importance. This is particularly important when approaching the pier. When doing it from the right direction the reverse gear shall pull the aft nicely onto it.

Fred

Sailor Steve · 12-05-13, 02:05 PM

Quote:

Originally Posted by GJO

By contrast the boats in SH3 seem to handle perfectly and precisely with the props running astern (very useful when attacking with the stern torp!) but is this an accurate reflection of the way RL subs would handle?

No. Astern is most useful when backing into or away from a pier. In the early days of destroyers (1890s) experiments were performed to determine whether inward-or-outward-turning propellers were more effective. They discovered that outward-turning propellers tended to push against each other and keep the ship on a steady course. Inward-turning propellors tend to pull away from each other, causing the ship to wander and making it difficult to hold a steady course. What happens with two or more propellers is that the pivot point of the ship changes. When a ship or boat is sitting still the pivot point is near the center of gravity.

When the propellers are aft and pushing the ship forward the pivot point moves forward, which leaves the propellers pushing each side of the ship toward the pivot point. In reverse the pivot point moves aft until it is almost right on top of the propellers. This is compounded by the fact that in reverse the propellers are now turning inward, pulling away from each other almost around the pivot point. This means that a multi-screw ship running in reverse is extremely difficult to hold to a straight course. You also have to add in the helmsman is now standing near the "stern" of the ship, directionally speaking, and he's facing the wrong way.

No, it should be nearly impossible to hold a straight course in reverse.

Aktungbby · 12-05-13, 02:14 PM

What about the factor of each screw having it's own rudder as with a U-boat for more immediate wash bite and therefore greater control especially with a smaller craft such as a Uboat... as opposed to the Titanic, for example, which reversed engines and lost the 'bite' at a critical juncture and collided with the iceberg when probably it should have at least maintained speed and done a full port 90...

Sailor Steve · 12-05-13, 02:46 PM

Quote:

Originally Posted by Aktungbby

What about the factor of each screw having it's own rudder as with a U-boat for more immediate wash bite and therefore greater control especially with a smaller craft such as a Uboat...

Unfortunately going astern means that the rudders are now in front of propellers, and the wash never crosses the rudders.

Quote:

as opposed to the Titanic, for example, which reversed engines and lost the 'bite' at a critical juncture and collided with the iceberg when probably it should have at least maintained speed and done a full port 90...

Very true. The faster a ship is going the tighter it turns. Another possibility would have been to reverse engines and slow as much as possible, hitting the berg head on. This would have caused considerable damage to the bow, but would likely not have affected more that two of the watertight compartments, leaving the ship afloat.

But of course we never think with that kind of logic when panic strikes, and I probably would have done exactly as they did.

Jimbuna · 12-05-13, 02:49 PM

Quote:

Originally Posted by Sailor Steve

No. Astern is most useful when backing into or away from a pier. In the early days of destroyers (1890s) experiments were performed to determine whether inward-or-outward-turning propellers were more effective. They discovered that outward-turning propellers tended to push against each other and keep the ship on a steady course. Inward-turning propellors tend to pull away from each other, causing the ship to wander and making it difficult to hold a steady course. What happens with two or more propellers is that the pivot point of the ship changes. When a ship or boat is sitting still the pivot point is near the center of gravity.

When the propellers are aft and pushing the ship forward the pivot point moves forward, which leaves the propellers pushing each side of the ship toward the pivot point. In reverse the pivot point moves aft until it is almost right on top of the propellers. This is compounded by the fact that in reverse the propellers are now turning inward, pulling away from each other almost around the pivot point. This means that a multi-screw ship running in reverse is extremely difficult to hold to a straight course. You also have to add in the helmsman is now standing near the "stern" of the ship, directionally speaking, and he's facing the wrong way.

No, it should be nearly impossible to hold a straight course in reverse.

Textbook fashion

Bumped my fathers former boat on a few occasions in learning the above the hard way many years ago

Would have been so much easier if they had the Azimuth pods of so many ships today...young Buna manoeuvres 90,000 tonnes in every direction, even on the ships own axis without the need of a tug.

Aktungbby · 12-05-13, 03:03 PM

Quote:

Originally Posted by Jimbuna

Bumped my fathers former boat on a few occasions in learning the above the hard way many years ago

As do we on my neighbor's 27 ft. Catalina in a narrow marina; Had the owner (his midlife crises toy) get two fending poles just to be safe as a 9.5 hp Nissan is a little unwieldy in tight quarters; The other marina residents are quick with theirs when we pull out! Still scraping a little creosote off the starboard gunwale...one of the myriad duties of a jibman/grinder...

What kind of fishing did you and your dad do? Any pics?

CaptBones · 12-05-13, 05:58 PM

Well, I don't really want to "torpedo" Sailor Steve's logic or contradict Jimbuna's experience...but...steering with astern way on is extremely easy and very accurate in twin-screw destroyers and "Fleet" type submarines (probably just as good for U-Boats). I once backed the USS Laffey (DD 724) through the inner harbor of Grand Harbor, Valleta, Malta, to "Med Moor" at Fort St. Angelo. The pivot point when going ahead is about one-third of the ship's length back from the bow (for surface ships) and a little further aft for "Fleet" type submarines; that's why their bridges are located where they are...just forward of the "ahead" pivot point. One action you'll see that differentiates an experienced conning officer from a "nugget" is that the "nugget" gives a rudder order and watches the bow move, the old salt gives a rudder order, steps out to the opposite bridge wing to check the swing of the stern, then comes back to the centerline pelorus and watches the bow.

The pivot point in the mentioned ship types moves forward (toward the bow) when going in the astern direction with way on. When starting to move astern, and having little way on, it's different; the location of the pivot point isn't as significant as the effect of the prop wash, or lack of wash, against the rudder(s). The trick to maneuvering a twin-screw ship, without good way on, either standing without moving ahead or astern, or starting to move in either direction, is to "twist" the ship by operating the engines in opposite directions. You the turn the rudder away from the direction you want the stern to go, otherwise it's just a big "barn door" keeping the stern from moving where you want it to move.

As alluded to, handling a single-screw ship (or boat) is quite different. I had the same kind of problems with my Watkins 26 (8hp Yanmar one-lunger) getting into and out of the slips at the Washington Sailing Marina. Usually, the screw turns clockwise (looking from stern-to-bow) when going forward, which gives the stern a push to starboard when starting to move ahead and a definite push to port when backing down. I won the Pacific Fleet Shiphandling Award in 1974 by backing a single-screw "Knox" Class Frigate to starboard while making a "two bell" landing maneuver at berth B-26 in Pearl Harbor. I taught that maneuver to the students at Surface Warfare Officer School in San Diego for several years after that..."The Book" says it can't be done.

Rammstein0991 · 12-06-13, 02:10 AM

Quote:

Originally Posted by Aktungbby

What about the factor of each screw having it's own rudder as with a U-boat for more immediate wash bite and therefore greater control especially with a smaller craft such as a Uboat... as opposed to the Titanic, for example, which reversed engines and lost the 'bite' at a critical juncture and collided with the iceberg when probably it should have at least maintained speed and done a full port 90...

Actually the popular theory among experts now is that Titanic should have taken the iceberg head on, which would have resulted in maybe 2 flooded compartments, which would have been well within Titanic's ability to sustain flotation, as it was Murdoch's orders to turn to port and then reverse engines basically was like rearing your hand back then slapping someone, only it was a ship slapping an iceberg.

GJO · 12-06-13, 09:04 AM

Fascinating responses - thanks guys! Clearly there is a lot of RL experience on this forum. I was thinking of this problem when making a half mile run going astern with my narrow boat yesterday - the purpose of the journey was to get to the wharf and take on fresh water whilst discharging that which was anything but fresh! The fresh water tank (700 Imperial gallons when full) is at the bow and was nearly empty so the cross wind made going astern more challenging than ever and we only have a single three blade screw that is big enough to 'grab' the silt in the shallow canal and make the boat's stern 'walk' sideways . . .

Still we made it - and got back to our mooring for another patrol of SHIII which seemed so much more realistic with the real boat rocking and the real wind howling. And I am sure I heard my wife say "Jawohl Herr Kaleun, Alle Maschinen stop!" after I asked her to cut the engine . . .

Sailor Steve · 12-06-13, 10:50 AM

Quote:

Originally Posted by CaptBones

Well, I don't really want to "torpedo" Sailor Steve's logic or contradict Jimbuna's experience

Why not? Other experiences and opinions are not only welcome, but vital.

Quote:

...but...steering with astern way on is extremely easy and very accurate in twin-screw destroyers and "Fleet" type submarines (probably just as good for U-Boats). I once backed the USS Laffey (DD 724) through the inner harbor of Grand Harbor, Valleta, Malta, to "Med Moor" at Fort St. Angelo.

How fast? It's my understanding that the big problem isn't slow maneuvering in harbor but maintaining a straight course at high speed. Then again, I was a humble radioman and never got to do fun stuff like steer the ship.

That said, I use sources which describe the real problems encountered, so it's not just theory.

Quote:

The pivot point in the mentioned ship types moves forward (toward the bow) when going in the astern direction with way on.

This source says the pivot point moves aft when travelling in reverse. I don't claim to know, but...

Quote:

I won the Pacific Fleet Shiphandling Award in 1974 by backing a single-screw "Knox" Class Frigate to starboard while making a "two bell" landing maneuver at berth B-26 in Pearl Harbor. I taught that maneuver to the students at Surface Warfare Officer School in San Diego for several years after that..."The Book" says it can't be done.

Them's some mighty fine credentials. I'll bow to your experience.

Leandros · 12-06-13, 11:30 AM

Quote:

Originally Posted by CaptBones

The trick to maneuvering a twin-screw ship, without good way on, either standing without moving ahead or astern, or starting to move in either direction, is to "twist" the ship by operating the engines in opposite directions. You the turn the rudder away from the direction you want the stern to go, otherwise it's just a big "barn door" keeping the stern from moving where you want it to move

Problem in SH is that you cannot maneuver the engines independently......

Fred

Aktungbby · 12-06-13, 11:40 AM

Aktungbby · 12-06-13, 11:47 AM

Quote:

Originally Posted by Sailor Steve

Them's some mighty fine credentials. I'll bow to your experience.

It doesn't count that I placed second at Omaha in the 5-axel Parallel parking event of a truck rodeo! 65' and 80,000 lbs., and there was NO power steering?!!!

Lordy, I'm datin' myself again!

CaptBones · 12-06-13, 09:16 PM

Hello again Sailor Steve...yeah, you might have gotten the short end of the stick in some ways by being an RM, but then you didn't have to experience all of the really unpleasant topside evolutions in bad weather that the BMs and non-rated seamen did...or did you have a line-handler billet during UNREP "events" (always a millions laughs there

)? Anyway, you're right, low to moderate speed was critical. The "Laffey" event was done at around 7-8kts max and I backed the "Grampus" (SS-523) up the channel to change berths at the D&S piers in Norfolk at about the same speed (or maybe it was the "Clamagore" (SS-343)...my memory sometimes confuses which ship goes with which event)...and just for "grins"...I'm amazed that two of those old sea duty assignments of mine are museum ships at Patriots Point in SC.

But, back to the subject. The reference material you provided a link to, is likely oriented to merchant ships, which for the most part have significantly different hull shapes than warships (and submarines especially). The merchant hulls are typically more "boxy" (have less dead rise in hull transverse cross-section and at the stern in longitudinal cross-section) and are almost all single-screw/single-rudder with a transom stern hull form.

The warship hull is more often multi-screw/multi-rudder with more dead rise and with skegs for each shaft-line. The physics of the action of the rudder(s) when going astern is also bass-ackwards, in that rudder angle going ahead produces a strong turning moment at the "trailing edge" of the hull, and rudder action going astern produces a weak turning moment at the "leading edge" of the hull. All that gives some advantage to the warship when going astern, but even with that it's seldom done at "high speed".

Another BUT...I also got my directions backwards

thanks to that memory issue; I was recalling the handling characteristics of a couple of types of ships with large bow-mounted sonar domes (those puppies really "anchor" the pivot point near the bow when going astern at any speed). You're right about the movement of the pivot point aft (towards the stern) when going astern, for older DDs and "conventional" type submarines. It doesn't move beyond a point roughly amidships though; it's that hull-form thingy again...

Another "yup" regarding speed is also due. Because you're dealing with that weak "leading edge" moment, in any sea state above 2 or 3, or going astern at more than 7-10 kts, a surface ship helmsman will have an unpleasant time trying to keep a steady course. The submarine can do much better; the "Fleet" boat because of the hull form and "uniformly" deep draft of the hull as well as that huge single rudder (at the very front of the hull when going astern) and I'd suspect the U-boat also...hull form and twin rudders.

In SH, I haven't used sternway at more than 4-5kts, but in RL the "conventional" boat does handle well at astern speeds over 10 kts. You don't want to do any more than that though; the forces exerted on the rudder/rudderpost and steering mechanism that way, can seriously damage them all, starting with wiping the rudder post bearings and seals. I suspect the game has only one method of calculating turn rate and it doesn't change with a change in direction, so it's likely to be too good for steering with sternway.

Leandros's second comment hit another key point WRT the game, but I have to add that the rudder control in the SH games is not at all correct. Every course change ordered, more than a few degrees, gets the same response - 40 degrees of rudder - I'd keel-haul the helmsman who did that (or rather the conning officer who allowed it to happen). In warships you have three standard rudder commands; "right/left standard rudder", "right/left full rudder" and "hard right/left rudder". You can also specify the rudder angle; "-- degrees right/left rudder". "Hard" rudder is only used in extreme circumstances (that's why it's said differently than the other commands...gets the helmsman's immediate attention); it runs the steering engine mechanism "into the stops" where an actual mechanical device prevents further movement of the yoke or crosshead that turns the rudder post. That's asking for a jammed rudder. "Standard" rudder is usually half the travel, in degrees, as "hard" rudder (that'd be 20 degrees for our SH U-boats) and "full" rudder is usually halfway between "standard" and "hard". "Standard" is "standard" because it's the best compromise between loss of speed and good turning rate and size of the turning circle. For best maneuverability in a tactical situation, you'll typically sacrifice a little speed to increase the turn rate and tighten the turning circle...using "full rudder", not "hard".

Lastly...Aktungbby is clearly in serious need of psychiatric help...or another six-pack of Hamm's...same difference. My hat's off to anyone who can handle a big rig without power steering, parallel parking at that (!), regardless of "only" placing second.

Well, enough...I've got a five year old grandson standing here asking to go to the SH3 Naval Academy to do some target practice...get 'em "hooked" while they're young!

12-05-13, 09:59 AM	#1
GJO Engineer Join Date: Nov 2013 Location: On the Oxford Canal in England Posts: 202 Downloads: 40 Uploads: 0	Steering astern! I am surprised by the preciseness of the steering with SH3's boats when running astern. AFAIK, in RL, running astern is rarely used for navigation and is really only used for slowing or stopping. On my own boat in RL, running astern is sometimes necessary due to the narrow nature of the canal but it can be a steering nightmare especially with the slightest breath of crosswind. By contrast the boats in SH3 seem to handle perfectly and precisely with the props running astern (very useful when attacking with the stern torp!) but is this an accurate reflection of the way RL subs would handle?

12-05-13, 02:14 PM	#4
Aktungbby Gefallen Engel U-666 Join Date: Jul 2013 Location: On a tilted, overheated, overpopulated spinning mudball on Collision course with Andromeda Galaxy Posts: 30,000 Downloads: 24 Uploads: 0	What about the factor of each screw having it's own rudder as with a U-boat for more immediate wash bite and therefore greater control especially with a smaller craft such as a Uboat... as opposed to the Titanic, for example, which reversed engines and lost the 'bite' at a critical juncture and collided with the iceberg when probably it should have at least maintained speed and done a full port 90... __________________ "Only two things are infinite; The Universe and human squirrelyness?!!

12-05-13, 05:58 PM	#8
CaptBones The Old Man Join Date: Nov 2010 Location: Rockton, IL Posts: 281 Downloads: 208 Uploads: 0	Not exactly correct... Well, I don't really want to "torpedo" Sailor Steve's logic or contradict Jimbuna's experience...but...steering with astern way on is extremely easy and very accurate in twin-screw destroyers and "Fleet" type submarines (probably just as good for U-Boats). I once backed the USS Laffey (DD 724) through the inner harbor of Grand Harbor, Valleta, Malta, to "Med Moor" at Fort St. Angelo. The pivot point when going ahead is about one-third of the ship's length back from the bow (for surface ships) and a little further aft for "Fleet" type submarines; that's why their bridges are located where they are...just forward of the "ahead" pivot point. One action you'll see that differentiates an experienced conning officer from a "nugget" is that the "nugget" gives a rudder order and watches the bow move, the old salt gives a rudder order, steps out to the opposite bridge wing to check the swing of the stern, then comes back to the centerline pelorus and watches the bow. The pivot point in the mentioned ship types moves forward (toward the bow) when going in the astern direction with way on. When starting to move astern, and having little way on, it's different; the location of the pivot point isn't as significant as the effect of the prop wash, or lack of wash, against the rudder(s). The trick to maneuvering a twin-screw ship, without good way on, either standing without moving ahead or astern, or starting to move in either direction, is to "twist" the ship by operating the engines in opposite directions. You the turn the rudder away from the direction you want the stern to go, otherwise it's just a big "barn door" keeping the stern from moving where you want it to move. As alluded to, handling a single-screw ship (or boat) is quite different. I had the same kind of problems with my Watkins 26 (8hp Yanmar one-lunger) getting into and out of the slips at the Washington Sailing Marina. Usually, the screw turns clockwise (looking from stern-to-bow) when going forward, which gives the stern a push to starboard when starting to move ahead and a definite push to port when backing down. I won the Pacific Fleet Shiphandling Award in 1974 by backing a single-screw "Knox" Class Frigate to starboard while making a "two bell" landing maneuver at berth B-26 in Pearl Harbor. I taught that maneuver to the students at Surface Warfare Officer School in San Diego for several years after that..."The Book" says it can't be done.

12-06-13, 11:40 AM	#13
Aktungbby Gefallen Engel U-666 Join Date: Jul 2013 Location: On a tilted, overheated, overpopulated spinning mudball on Collision course with Andromeda Galaxy Posts: 30,000 Downloads: 24 Uploads: 0	Aktung ASEA! __________________ "Only two things are infinite; The Universe and human squirrelyness?!! Last edited by Aktungbby; 12-28-13 at 01:35 AM.

12-06-13, 09:16 PM	#15
CaptBones The Old Man Join Date: Nov 2010 Location: Rockton, IL Posts: 281 Downloads: 208 Uploads: 0	Mo' betta' info... Hello again Sailor Steve...yeah, you might have gotten the short end of the stick in some ways by being an RM, but then you didn't have to experience all of the really unpleasant topside evolutions in bad weather that the BMs and non-rated seamen did...or did you have a line-handler billet during UNREP "events" (always a millions laughs there)? Anyway, you're right, low to moderate speed was critical. The "Laffey" event was done at around 7-8kts max and I backed the "Grampus" (SS-523) up the channel to change berths at the D&S piers in Norfolk at about the same speed (or maybe it was the "Clamagore" (SS-343)...my memory sometimes confuses which ship goes with which event)...and just for "grins"...I'm amazed that two of those old sea duty assignments of mine are museum ships at Patriots Point in SC. But, back to the subject. The reference material you provided a link to, is likely oriented to merchant ships, which for the most part have significantly different hull shapes than warships (and submarines especially). The merchant hulls are typically more "boxy" (have less dead rise in hull transverse cross-section and at the stern in longitudinal cross-section) and are almost all single-screw/single-rudder with a transom stern hull form. The warship hull is more often multi-screw/multi-rudder with more dead rise and with skegs for each shaft-line. The physics of the action of the rudder(s) when going astern is also bass-ackwards, in that rudder angle going ahead produces a strong turning moment at the "trailing edge" of the hull, and rudder action going astern produces a weak turning moment at the "leading edge" of the hull. All that gives some advantage to the warship when going astern, but even with that it's seldom done at "high speed". Another BUT...I also got my directions backwards thanks to that memory issue; I was recalling the handling characteristics of a couple of types of ships with large bow-mounted sonar domes (those puppies really "anchor" the pivot point near the bow when going astern at any speed). You're right about the movement of the pivot point aft (towards the stern) when going astern, for older DDs and "conventional" type submarines. It doesn't move beyond a point roughly amidships though; it's that hull-form thingy again... Another "yup" regarding speed is also due. Because you're dealing with that weak "leading edge" moment, in any sea state above 2 or 3, or going astern at more than 7-10 kts, a surface ship helmsman will have an unpleasant time trying to keep a steady course. The submarine can do much better; the "Fleet" boat because of the hull form and "uniformly" deep draft of the hull as well as that huge single rudder (at the very front of the hull when going astern) and I'd suspect the U-boat also...hull form and twin rudders. In SH, I haven't used sternway at more than 4-5kts, but in RL the "conventional" boat does handle well at astern speeds over 10 kts. You don't want to do any more than that though; the forces exerted on the rudder/rudderpost and steering mechanism that way, can seriously damage them all, starting with wiping the rudder post bearings and seals. I suspect the game has only one method of calculating turn rate and it doesn't change with a change in direction, so it's likely to be too good for steering with sternway. Leandros's second comment hit another key point WRT the game, but I have to add that the rudder control in the SH games is not at all correct. Every course change ordered, more than a few degrees, gets the same response - 40 degrees of rudder - I'd keel-haul the helmsman who did that (or rather the conning officer who allowed it to happen). In warships you have three standard rudder commands; "right/left standard rudder", "right/left full rudder" and "hard right/left rudder". You can also specify the rudder angle; "-- degrees right/left rudder". "Hard" rudder is only used in extreme circumstances (that's why it's said differently than the other commands...gets the helmsman's immediate attention); it runs the steering engine mechanism "into the stops" where an actual mechanical device prevents further movement of the yoke or crosshead that turns the rudder post. That's asking for a jammed rudder. "Standard" rudder is usually half the travel, in degrees, as "hard" rudder (that'd be 20 degrees for our SH U-boats) and "full" rudder is usually halfway between "standard" and "hard". "Standard" is "standard" because it's the best compromise between loss of speed and good turning rate and size of the turning circle. For best maneuverability in a tactical situation, you'll typically sacrifice a little speed to increase the turn rate and tighten the turning circle...using "full rudder", not "hard". Lastly...Aktungbby is clearly in serious need of psychiatric help...or another six-pack of Hamm's...same difference. My hat's off to anyone who can handle a big rig without power steering, parallel parking at that (!), regardless of "only" placing second. Well, enough...I've got a five year old grandson standing here asking to go to the SH3 Naval Academy to do some target practice...get 'em "hooked" while they're young!

12-06-13, 09:04 AM	#10
GJO Engineer Join Date: Nov 2013 Location: On the Oxford Canal in England Posts: 202 Downloads: 40 Uploads: 0	Fascinating responses - thanks guys! Clearly there is a lot of RL experience on this forum. I was thinking of this problem when making a half mile run going astern with my narrow boat yesterday - the purpose of the journey was to get to the wharf and take on fresh water whilst discharging that which was anything but fresh! The fresh water tank (700 Imperial gallons when full) is at the bow and was nearly empty so the cross wind made going astern more challenging than ever and we only have a single three blade screw that is big enough to 'grab' the silt in the shallow canal and make the boat's stern 'walk' sideways . . . Still we made it - and got back to our mooring for another patrol of SHIII which seemed so much more realistic with the real boat rocking and the real wind howling. And I am sure I heard my wife say "Jawohl Herr Kaleun, Alle Maschinen stop!" after I asked her to cut the engine . . .