Question about WWII German U-boat bow planes

Discussion in 'Boat Design' started by Gannet, Oct 22, 2009.

  1. Gannet
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    Gannet Junior Member

    Yesterday I was watching Stealth Submarine on NatGeoTV. It was about the U-480 which was a Type VIIC U-boat. There was a portion of the show that showed a close-up of the bow plane operating which caught my attention because forward of the bow plane was a fixed vane, which I have never notice before on any other submarine. I do not understand the reason for this fixed vane.

    I search the intenet and found that this design was used on subsequent German U-boats, see attachments.

    The German U-boats bow planes were not retractable like US Submarines. I am guessing this fixed vane serves some hydrodynamic function such as for surface running.
     

    Attached Files:

  2. Knut Sand
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    Knut Sand Senior Member

    or just protection? (for the rudder).

    (If I understand your question correctly)
     
  3. tom28571
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    tom28571 Senior Member

    Are you sure that it is fixed?
    Looks like it might be movable to me but I'm no expert on UBoats. That arc of holes around the forward vane is interesting. Wonder what that is.
     
  4. liki
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    liki Senior Member

    My understanding is that they were primarily to prevent tangling in mine cables or submarine nets.
     
  5. Gannet
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    Gannet Junior Member

    It appearred fixed on the show when they were showing the operation of the bow planes.

    I did noticed the arc of holes on the Type IXC/40 U-boat and not sure of their function.

    I think cable protection is a secondary function, because it so close to leading edge of the bow plane. Another fact I found out during my internet search was that the German U-boat's bow planes were electrically actuated instead of hydraulically actuated like the US. Maybe it is a hydrodynamic aid to reduce torque required to actuate the bow plane?

    Maybe it is a fixed slat instead of movable ones like those used on fixed wing aircraft to improve lift during takeoff?

    Maybe it directs the flow to be horizontal when near the wavy surface?

    If it primary function is hydrodynamic, I would appreciate if someone could enlighting me on theory.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    I'm not a sub designer, but a quick review of what the hydroplanes does help to explain, but it is also a function of the hull shape too.

    Older UBoats by today's standards are slow. Fwd fins are required for slow speed only. Since the subs were predominantly slow, i am assuming that it was sensible to fix the fins attitude to save weight. Not much point adding lots of weight and more systems to go wrong and take up more internal space, when it wasn't 100% necessary.

    Modern sub's for example though, have a common long.t axis, hence their frames (and hull shape) are circular. This makes the "zero fluid force" perpendicular to the direction of travel. Also, since the hull acts as an inefficient lifting surface, it is desirable to reduce the inefficiencies as much as possible. The older Uboats didn't have such luxuries, owing to the presence of the larger "bridge fin" and a not so symmetrical hull shape (by modern designs) - hence their MO was to troll about at slow speed, most efficient way is to have the fwd fins fixed.
     
  7. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Some American subs had fixed guards for rear planes

    as the rear planes weren't retractable. I heard they were for docking, but that seems like a lot of drag to haul around.

    You would think once in port they could deal with it, or have a Stand-Off section of dock. Notice the guards are out the water when surfaced.


    [​IMG]
     
  8. Gannet
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    Gannet Junior Member

    Thanks Ad Hoc for your comments; The WWII German U-boats did have moveable bow planes aft of this fixed fin. However, I found the following statement about WWI German U-boats at http://www.dreadnoughtproject.org/models/ships/SMS_UB_2/

    "Rather than having movable bow diving planes, the half-spade shaped fins were fixed at a downward angle and the deck casing limber holes were more closely spaced forward of the conning tower to create a natural bow-down angle and submerge the planes immediately on opening the ballast tank vents. Forward momentum of the boat would further force the bow underwater as the propulsion was shifted from diesel to electric. The movable stern diving planes (sometime referred to as horizontal rudders by the British) were proportionally oversized as compared with the planes on larger U-boats, thereby giving sufficient control authority to adjust the attitude of the boat. The boat could be operated with a slightly positive buoyancy, and as long as the motors provided forward propulsion the fixed bow planes would keep the bow down, but if speed was decreased, the bow would naturally rise even without putting a rise on the stern planes."

    Squidly-Diddly - Thanks also for your comments I have that model
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Gannet

    Having an aft and fwd fin is the only way to maintain constant depth with an even keel, since the aft and fwd fins enable the sub to find its common long.t axis.

    These two fins create a vertical or rather a force perpendicular, to the long.t axis. The result force must be applied at the hydrodynamic centre of the hull, or neutral point. However for fwd motion at constant velocity the hydroplanes (fwd and aft fins) may be adjusted so that the resultant force is fwd of the neutral point.

    It then gets a bit messy, as there is a location called the "critical point", call it C. This varies with the speed. At low speeds this position is well aft, at high speed it is well fwd.

    At low speeds, there is a 'critical speed', below this speed C's position is abaft of the aft fin. Therefore the fwd fins are used for maximum rate of rise.

    At high speed, the location of C is far fwd, hence the aft fins provide the control.

    Hence having both fwd and aft fins, the hydroplane provide both trim and depth control at any speed.

    As i said before, i'm not a sub designer, i design surface boats/ships. However a quick review of my old notes suggests it is not so straight fwd and is rather complex, since there are many different 'forces' acting with or against the hull in its desired motion to maintain stability.
     
  10. apex1

    apex1 Guest

    The fins simply provided a lifting force at the bow. Due to the fact that the pressurized compartement was well behind the bow, there was no (or not enough) lifting force when the boat operated at the surface. But that was the standard operation mode for all submarines until the 50ies.
    So, Ad Hoc got it right!

    Regards
    Richard
     
  11. Gannet
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    Gannet Junior Member

    I guess I was not clear about my question

    Thanks Ad Hoc for the refresher on diving planes mechanics.

    My question is about the Fixed Fin directly forward of the moveable bow plane, see attachment.

    As I stated in previous post
     

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  12. Ad Hoc
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    Ad Hoc Naval Architect

    Im not sure if you are just making a statement or asking a Q?
     
  13. Gannet
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    Gannet Junior Member

    I am asking a question about the Fixed Fin just forward of the Moveable Bow Plane on WWII German U-boats. I have not see this arrangement on any other submarine.

    I was wondering if anyone would know why the Germans would use this configuration over the normal moveable bow plane only which have been around since the Hunley.

    Thanks again for your comments
     
  14. wardd
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    wardd Senior Member

    early war u-boats had net cutters on the bow, so i would assume they were to keep the bow planes clear of obstructions and as the bow planes were not retractable they would have protected them when docked

    the type XXI had retractable planes and didn't have the fixed protrusions
     

  15. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Ad Hoc and Apex....??????

    Ad Hoc: All the sub novels I've read include lots of references to trimming the ballast tanks(fore and aft) after all sorts of events(launching torpedoes, etc) and that even minor out of balance becomes noticeable (although it can be corrected with movable planes). Sort of like you CAN force a car to go in a straight line with the steering wheel even if the front-end(or rear end) has been smacked way out of whack in an accident.

    Any overall trim imbalance of even very small amounts would cause the sub to sink or rise. Almost all the submarining was done with very minor trim adjustments rather than the planes except for diving and surfacing or dodging depth charges.


    Apex: Are you saying that when a U-boat was under way on the surface it needed the bow planes to keep the bow up? Never heard that one. I'm sure you could remove the bow planes and the boat would cruise on the surface just fine, as their hull shape is very similar to American subs that cruised with bow planes retracted.

    I'm pretty sure you could remove either bow or stern planes, or both, and just use fore and aft trim tanks for everything and you would only lose a little quickness in diving or surfacing or otherwise changing depth.
     
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