SES SWATH hybrid

Discussion in 'Boat Design' started by arn0, Nov 24, 2014.

  1. arn0
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    arn0 Junior Member

    Hi,

    Would you know a static air cushion and SWATH hybrid concept ? (just to be clear: not hydrofoils but SES)

    the SWATH would offset about 90% of the lift, the air cushion managing the other 10%.

    Advantages I see:
    - the SWATH would minimise the lift power of the SES (small cushion pressure, 10x less lift power)
    - the SES would bring stability to the SWATH, and smother the ride (air compression) while allowing controllable draft (smaller at low speed, higher at sea if low waves....)

    Disadvantage: shorter wave clearance than SWATH, bigger draft than SES

    Anyone ?

    just curious :)

    Thanks

    Arnaud
     
  2. BMcF
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    BMcF Senior Member

    The added drag from seals and cushion wave-making, even with a lift fraction of only 10%, would seem a pretty big "penalty" to pay for a little extra payload capacity. Then there is all the weight of the lift and seal systems..

    Then there is the issue of seal design...SES bow seals are not designed to operate at varying immersions; they are designed to operate at bascially only one design lift condition. So a new "type" of variable-height seal would be required.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    What lift?

    There is no 'lift' from a Swath hull. Only appendages provide that, and these being for motion control.
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I think he is means lift = buoyancy.
    The idea sounds about as sensible as a submarine/SES hybrid ;)
     
  5. BMcF
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    BMcF Senior Member

    sayyy..now there's an idea.
     
  6. cmckesson
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    cmckesson Naval Architect

    When DCN built AGNES in 1990 she was an attempt to be a SWATH/SES hybrid, but in her case she was a conventional 80/20 SES, with a bit of a wasp-waisted body plan shape. This was intended to provide improved ride quality during off-cushion loiter operations, without compromising the sidehull hydrodynamics at high on-cushion speed.

    You may also recall that Tom Lang built a planing SWATH. Again, she was a planing catamaran at high speed, and became a SWATH when at rest, at a waterline similar to an SES' off-cushion attitude.

    My bottom line: I'm open to a SWATH/SES hybrid, but I think it is more probable to be a much higher cushion fraction than the OP.
     
  7. cmckesson
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    cmckesson Naval Architect

  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    So that's two vessels that were built about 20 years ago, and neither are really SWATH/SES hybrids.
    How many have been built subsequently?
    My bottom line: the idea is not worth pursuing or it would have been given
    far more attention many years ago.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    It's hard enough to convince clients to use a Swath, a normal Swath hull, let alone any hybrid type of vessel.
     
  10. BMcF
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    BMcF Senior Member

  11. tspeer
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    tspeer Senior Member

    AFAIK, all SES ships work this way, with buoyant side walls in the water. It's just a matter of how much support is provided by the air cushion and how much by the water. That's the difference between a SES and a hovercraft, with the hovercraft being 100% supported by air.

    I doubt there's much point in having such a low cushion lift fraction as you propose. You'd be better off to just make it a catamaran and not have the weight of the air cushion machinery, bow and stern seals, and fuel.

    I think a cushion lift fraction more on the order of 90% would be more appropriate. After all, the whole point of a SES is to be able to go fast in a seaway, and having 90% of the hulls in the water would severely limit the ability to do that.
     
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  12. kerosene
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    kerosene Senior Member

    right Tom, but that would rule SWATH out of the equation. After all SWATH is based on the boyancy being in the pods.
     
  13. arn0
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    arn0 Junior Member

    some more considerations

    thank you all for your feedbacks.

    I will add a bit of context to my initial post:

    I understand those design, because of their complexity and cost, were mainly military research programs looking for high loads at high speeds.

    I am not looking for top speed, but for economy, comfort and safety, at speed around critical hull speed (as defined for a displacement hull of the same size).

    This is quite theoretical as I am not considering for now some essential parameters such as design complexity, cost to build and to maintain.

    I am particularly considering a niche market of canal transportation, providing some particularities:
    - Limited legal speed around 20 km/h
    - Constant 2m deep canal, but some area might be lower (+ variations over the seasons).
    - bridges with 4m air clearance
    - Waves are forbidden to preserve the canal banks
    - Navigation is subject to moderate waves of other cruising ships (lets say 30° 1m high waves every 10 minutes, plus constant 0m to 0,3m waves because of the wind)

    As an example, let's consider a small ship of size: 15m long x 4m large, 6 tons.

    Assuming the drag of a displacement hull at speed around critical hull speed is approximately composed of equal order of magnitude of the following: wetted form drag, wave drag, viscous drag (skin friction), and drag from turbulence, I am wondering if one configuration could compete over the displacement hull (and the planning or semi planning hull) in term of power and comfort.

    HOVERCRAFT lowers the wetted form drag, but still have to face its wave drag (pushed by the cushion on the water). It starts to be interesting (on deep enough water to compare with the others...) at speeds above critical hull speed (like a planning hull), but is noisy and needs a lot of power.

    SES adds catamaran walls so it lowers the air gap of the hovercraft to front and rear only instead of full perimeter. Cat hulls can add more or less buoyancy to the SES, together with more or less form drag and wave drag, viscous drag and turbulence. But compared to hovercraft it needs less power to lift (smaller air gap + added buoyancy) and less power and noise at low speed since it can use water propellers instead of air propellers.

    SWATH, using submarine hulls 'far bellow the surface' allow to lower wave drag, while still having form drag (quite optimized) as well as viscous drag and turbulence drag.

    In my considered canal application the use of dynamically controlled systems such as foils or wings is not possible as the boat needs to manage its horizontal speed independently of its lift (and draft), as for stopping at anytime in shallow water.

    That is how I came to a potential interest in an Hybrid SES/SWATH.

    The cat submarine hulls of the SWATH would join the 'upper hull' (basically an aluminium deck with safety buoyancy) by two lateral thin walls in order to lower wave drag and host the air cushion.
    A 90% buoyancy from the submarine SWATH hulls would leave only 10% to be lifted by the air cushion (10 times less lift power than hovercraft, and 10 times less wave drag too, low pressure, lower noise).
    (ratio could go up to 80% / 20% to accommodate extra weight or get extra air clearance, but lets now considerate 90/10 in order to have 1/10 ratio to help neglect some drags overs others)

    So basically, back to the drag forces at speed around critical hull speed, compared with an equivalent sized displacement boat:

    - Form drag would be a little lower (smaller surface and better coefficient than displacement) but viscosity drag would be higher (bigger wetted surface). Assuming the turbulences are the same let's say those differences almost compensate (big assumptions I agree but I feel it's not feasible to go further without calculations)

    - Wave drag of the SWATH, if deep enough (let's assume a 0.5m diameter centered at 1m bellow the surface at around 20 km/h produces good results... another big assumption), is neglectful compared to the one of the displacement hull at critical hull speed.

    So we have to compare the lift power of the hybrid SES/SWATH with the power used to face the Wave drag of the displacement boat around critical hull speed.

    With the example given (15m x 4m x 6 tons) it seems obvious that a low pressure air cushion (600kg=10% of the weight over 60 m2 = 100Pa) uses far less power than pushing the equivalent sized displacement hull against its wave at critical hull speed (probably a 1/10 ratio: few kW for a 0,2m air clearance compared to fews tens of kW for the displacement hull).

    note: there might be some power advantages of the SWATH over the SES/SWATH at that speed, but I believe the comfort of the low pressure air cushion plus the variable draft are potential great advantages in that scenario.

    So far I think there might be an ecological interest for a SES/SWATH hybrid in canal transportation (not taking into account the design complexity and harder maintenance)

    I am not a boat designer, just a curious guy, so I welcome your expertise wether I made stupid mistakes or if it could be interesting.

    many thanks,
    Arnaud
     

  14. tspeer
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    tspeer Senior Member

    Perhaps what you want is something more like this, the Interflight Hydrofoiler.
    [​IMG]
    [​IMG]
    [​IMG]
    It was electric powered and had to be efficient.
     
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