Why Are SWATH Tubes So Long?

Discussion in 'Boat Design' started by zstine, Sep 16, 2021.

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  1. zstine
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    zstine Junior Member

    Tubes on many/most SWATH boats are near LOA in length. After studying submarine design, I find an optimum slenderness ratio of near 8 (L/D). I assume the submerged Tubes of SWATH boats would have a similar optimum L/D. If we take the case study of the 134ft, 600T 'Silver Cloud' SWATH ( Silver Cloud SWATH Specifications http://www.yachtsilvercloud.com/SSC/specifications.htm ). Each of her LOA length tubes average about 9.5ft in diameter to displace 300T resulting in an L/D of over 14. If she had 95ft long, 11.2ft diameter tubes, she would displace the same volume but have an L/D of 8.5 and a reduced wetted surface of 570 sq.ft per tube.

    So why are SWATH tubes so long? You would think the designers would want to reduce wetted area, especially considering that SWATH designs suffer from increased surface area compared to conventional designs.
     
  2. jehardiman
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    jehardiman Senior Member

    For that particular vessel you need the length for the separation of the struts (but why they designed them like that? I have some ideas), but generally because you can't structurally build the bridge deck as wide as you would need if you had fatter demi-hulls. I did a bibliography on SWATH design a decade or so ago, search that out. A little reading and you'll find SWATHs have very specific size and structural needs.
     
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  3. Ad Hoc
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    Ad Hoc Naval Architect

    It is not as simple as that.
    The buoyancy of a Swath of course comes from those tubes. However, if you reduce the L/D ratio, you are reducing the LD ratio.
    The LD ratio being the length-displacement ratio.

    As this is lowered, the residuary resistance increases, and the pressure distribution aft lowers further. The result is that there will be more drag and more trim.
    Simply increasing the length - for the same buoyancy required - reduces this effect.

    It is the same as any hull.
    Reducing the length for the same buoyancy makes a very heavy boat for its length = more drag = more trim.
     
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  4. tlouth7
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    tlouth7 Senior Member

    In what circumstances?

    Don't forget that SWATH vessels have two tubes, so there are interactions between the two. There may be advantages to keeping them far apart or minimising frontal area that outweigh the increased skin friction.
     
  5. zstine
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    zstine Junior Member

    Thanks Ad Hoc for the reply. With regards to Length-Disp and the statement "as this is lowered...there will be more drag". Is this only applicable to surface vessels with wave/wake residuary resistance? In other words, assuming the struts don't exist and the 2 tubes are sufficiently below the surface so there are no surface effects (waves/wake) ie a submarine; In that case residuary resistance is negligible and you just have From drag and Friction Drag. So the statement "as LD is lowered...there will be more drag" would not hold true if your L/Diam is higher than optimal. Do you agree with that theoretical circumstance? Further, is it true that "displacement hull speed" is a limit on surface ships associated with wave making that does not apply to a submarine. Correct?
     
  6. zstine
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    zstine Junior Member

    The circumstances are a streamlined submarine deep enough to have no surface effects. I read these.. One place was from "Submarine Tom" who states "8 is optimal" in the thread titled "HYSWAS Anyone". Calculating the Seawolf's fast attack sub's actual L/Diam is 8.8 and This paper https://www.alexlascelles.com/src/drag_project.pdf provides the below figure and states "6" is the optimal L/Diam.
    upload_2021-9-16_17-22-52.png
    As for the interaction between tubes.... I know that catamaran designs take this into account because of surface wake/wave interaction. But would 2 tubes/submarines running at depth (no wake) have interaction with each other at the distances we are talking (typical Beam is what 40-50% length)? I believe pressure decreases proportional to the square of radius/distance from the tube, so dissipates quickly, no?
     
  7. Blueknarr
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    Blueknarr Senior Member

    Swath tubes need to be long enough to achieve sufficient fore-aft stability. Tubes significantly shorter than loa would have significant bow or stern overhangs and reduced stability.
     
  8. zstine
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    zstine Junior Member

    i thought stability came from a shift in center of buoyancy, at least conventionally. Assuming the tubes are fully submerged, there is no shift in buoyancy of the tubes due to pitch. I believe pitch control on a SWATH comes from the bow and stern planes (dynamic control) as well as the change in buoyancy of the struts. In some cases the bow planes and struts are often set back quite far from the tips of the tubes. So in that case the tubes could be shortened a fair amount before impacting pitch stability in any way. But yes, i understand that limitation. However, that doesn't seem to be the reason almost all SWATHs I have seen have tubes ~equal to LOA by observation of the strut and foil locations.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    In the case of a submarine, that’s correct there is no wave making or residuary resistance, when deeply submerged.

    That’s where the slenderness ratio is the dominant factor, since you’re only concerned with vicious pressure and skin friction.

    It is not the displacement that limits the speed of the vessel, but the LD ratio coupled to the shape of the hull to either stunt or promote water flow, that limits, or otherwise, the speed of any hull.
    A bit more on this is noted HERE.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Pitch.. how do you describe/calculate pitch?
    What are the governing factors of pitch and trim.... the waterplane area!.

    Thus the waterplane area, like any vessel, governs the amount of pitch stability.
    But with a Swath, it is also influenced by the Munk effect at higher speeds.
     
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  11. jehardiman
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    jehardiman Senior Member

    Zstine, there is not enough bandwidth to directly address your instruction on SWATHs and why certain factors are chosen over others. Suffice to say, there are design decisions based upon the SOR. There is no one "perfect" SWATH layout, but there are specific optimal sizes based upon practical environmental and structural considerations. Go read the stuff in my bibliography ... Published data for SWATHS https://www.boatdesign.net/threads/published-data-for-swaths.36420/#post-437857
     
  12. zstine
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    zstine Junior Member

    Haha, well yes and no. Yes, pitch on a SWATH at rest is governed by waterplane area, but the foils have some minor effect at speed.. And yes I asked originally about SWATH so you are right.. But I'm really interested in Hybrid foiling SWATH where the main hull is waterborne at rest and while in motion the waterplane only plays a small roll in pitch and trim. The hydrofoils are the main contributor for pitch and trim control... and of course a submarine's waterplane is not going to govern pitch and trim much at all. I will spill the beans on my SOR below
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Depending how 'sensitive' you make the Swath, the fins have a significant effect!

    Sounds like you're mixing up terminology, if not, the concept is illogical. As the raison d'etre of a Swath is to run on the thin struts!
     
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  14. zstine
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    zstine Junior Member

    Thanks for the list of references, It's quite the list! I'll have to hunt them down.
    So the SOR... Well, I spent 18 months cruising the Caribbean on a monohull with the family to find that seasickness (wife and 1 of the kids) kept us from sailing as much as we could/should. So I'm looking for an island hopper that ain't gonna get wifey sick. The SWATH is attractive, but I think the hybrid model is going to be more efficient. What I'm thinking ultimately is to have a
    - 45ft ish LOA,
    - electric drive,
    - 20,000lbs full load,
    -Hybrid-SWATH with very small/fine struts (no man access, tubes as close to optimized submarines as possible),
    - tubes displace 60-70% ships displacement (13,000lbs ish) and foils lift the rest as well as control pitch, heave, etc..
    - cruise 10-12knts
    - take-off under 4knt
    - 50 mi range on battery (maybe too far)
    This reduction in displacement exacerbates the slenderness ratio issue that I asked about. A full LOA tube would be L/Diam over 26! Too much drag from friction. Looks like my tubes are about 50% LOA to get under 10 for slenderness. I'm trying to get rough numbers on drag to figure out power to cruise at 10-12knts. I'm hoping to have 50miles range on batteries, which shall be extended by charging via roof covered in solar panels and/or a generator when needed. I'd like to make a minimal safe flight speed, 5-6knts, under generator power alone for extended passages. Sub 4knt lift-off. .. My initial hand calcs show that at 10 knts, a 22ft long, 1.2ft radius tube will each require about 700lbs force or 20hp into the water, or about 30kW total for both sides. 5 hours for 50mi range is 150kW-hr of battery (that's 4000+ lbs of battery!). And there's efficiency loss I've neglected. I'm just looking at rough feasibility now, maybe just academic.. oh, And the Cd I used of 0.06 is a guess. At 5 knts the power drops way off to just 3hp per side, 6hp total into the water. no losses. BUT even doubling that (50% system efficiency) is under 10kW generation for min flight.
     

  15. zstine
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    zstine Junior Member

    yes, I don't know what it is called... Like the HYSWAS "Quest", where the tube is less than full displacement and hydrofoils make up the difference in dynamic lift. But I'm interested in a twin tube, not a single like Quest.... I've not found a lot of info on such a boat.
     
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