Displacement Speed Question

Discussion in 'Boat Design' started by Tug, Jun 14, 2009.

  1. Ilan Voyager
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    Ilan Voyager Senior Member

    You're right, but submarine yachts and ferries are not common so I got stuck on ordinary displacement boats...the big problem of boats is the interface air/water. More I get old more I think that the best is to leave the surface and you have 2 options; up and down.
     
  2. Guest625101138

    Guest625101138 Previous Member

    Maybe a combination of both. The accommodation in the air and the displacement deep in the water.

    Would possibly be a very good option for bulk carriers. Need some means of reducing draft in harbours but once at sea they go deep and into high speed economy mode.

    Rick W
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    "...There is an exception to this.."

    Not when you're referring to a submarine.

    The resistance of deeply submerged bodies/vessels is given by:

    Rt = Rpv + Rf

    Which becomes

    Ct = Cpv + Cf

    When Cpv is very much greater than Cf it is considered “bluff”, since boundary separation produces a large wake. For example, a sphere or a flat disc across the flow. Whereas a streamed line shape is shaped because it does not experience large pressure gradients, or separation. The boundary layer actually becomes thicker and separation occurs much further aft, in some cases even avoided. When this occurs, the wave making component is sufficiently small that skin friction CF predominates the total resistance.

    In submarine design this is termed ‘slenderness ratio’ and coupled with its diameter defines how stream lined the vessel or sub is. The USS Albacore was the first to adopt a tear shape, because it is a nuclear powered and operates for very long periods under water, and hence ignores the air-surface interactions. All to reduce the Cpv.

    However as the slenderness ratio increases, for a constant immersed volume, the surface area also increases, which increases the Cf. But, the adverse pressure gradients over the aft section decreases as does the wake, hence Cpv is less. Contrary to this, when the ratio is reduced both the wake and Cpv are increased. The optimum ratio has been defined as 6, by trials, by Arentzen & Mandel in 1960.

    Hence long slim still dominates, even at extreme depths.
     
  4. Village_Idiot
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    Village_Idiot Senior Member

    plausible deniability
     
  5. Village_Idiot
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    Village_Idiot Senior Member

    Pumped air/water chambers for light/heavy ballast.

    Would be quite interesting to have a large submarine, the size of a modern freighter, that would submerge for long voyages, with just a small helm protruding above the surface of the water, connected to the mainship via a small shaft with stairwell...
     
  6. FAST FRED
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    FAST FRED Senior Member

    "Would be quite interesting to have a large submarine, the size of a modern freighter"


    I think they do something similar by towing a submerged bladder to the rich gulf states with arctic ice water.

    FF
     
  7. Guest625101138

    Guest625101138 Previous Member

    Fred
    Is this actually happening? I cannot find any references to it.

    Rick W
     
  8. Guest625101138

    Guest625101138 Previous Member

    Actually I was thinking the turret would be large enough to take a high capacity bucket elevator for discharging. It would hopefully have a lift because the hull would be something like 20m deep and you need to get 3 or 4 diameters below the surface to achieve the low wave drag. So the turret would be maybe 70 to 100m high.

    Certainly presents interesting challenges.

    Thinking about it I have no idea how much wave drag a large bulk carrier actually experiences. I do not think it is a large proportion of the overall drag. So there may not be a benefit for this size vessel. May be better suited to smaller vessels intended for high speed operation.

    Rick W
     
  9. mydauphin
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    mydauphin Senior Member

    My boat has that. I can change draft and trim using water tanks. It can also alter timing of rocking at anchor by raising and lower CG.
     
  10. loups1
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    loups1 Junior Member

    halo, congratulations, it`s a very useful forum-site.sorry if my english is pure. i am from greece. i have a traditional displacement fishing boat "trehandiri" made from fiberglass.it`s 6,20m long and 2m wide with an inboard nissan diesel engine of about 60hp at 4000rpm [used from an old taxi car 4cyl,2000cc-about 800,000 km] with a hurth [now zf] reverse [ratio 1:2,15] and it weights with fuel and fishing gear about 1,8ton. the max speed is 6,2mph. the question is: what can i do to gain more speed f.e.9mph or more as some other manufactures [exacly same hull design] say that their boats do. i dont know if its the right place for my question but thnks anyway.
     
  11. Willallison
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    Willallison Senior Member

    Loups1 - can you post a picture of the boat? Preferably out of the water so we can see what the bottom shape is?
    The speed you're getting now is about 'hull speed' for a displacement hull of that size. If the boat is a full displacement hull, then achieving 9 knots will either come at the expense of enormous extra power input, or not at all - at least not without major hull shape modification.
     
  12. loups1
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    loups1 Junior Member

    thanks very much.i will try to put a photo .
     
  13. loups1
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    loups1 Junior Member

    here it is DSCN0453.JPG
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    loups

    You'll be spending a lot of money trying to make her go faster. She is a "traditional" displacement boat. The harder you push her the more draggy she will be. This hull form does not lend itself to high speed owing to its shape. So as a rough guide/guess, it will be like doubling the power for each knot increase (not to mention the increase in weight of the ever increasing engine package)...so is it worth it?
     

  15. Ilan Voyager
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    Ilan Voyager Senior Member

    This kind of boat, called "pointu" in France, are common in all Mediterranean sea; it descends from the small boats using latin sails of the past centuries. Very well adapted to the "short" stepped seas that you can find when winds like the Tramontane or Mistral are blowing. Aegean Sea can be pretty hard in November.
    Very good boats but definitely not made for speed; a 60 HP engine is already a "big" engine on a such boat and you won't get any improvement of speed with a bigger engine.

    Certainly you have remarked that the speed you have now is not proportional to the RPM and the throttle; certainly at half throttle the boat is almost as fast than full throttle. Full power is only needed when going against the waves and wind. 6 knots is already a good speed on a 6.2m displacement boat.

    To go faster you need a different boat; I would like to see a "Pointu" of 6.2m at 9 knots, it must be a curious spectacle to watch a semi planing heavy double ended boat. Those announcing 9 knots with a 6.2m "pointu" are abusing of the ouzo liquor, retsina and Samos Glyko wines, delicious drinks but of none help in naval architecture...Or worst they are drinking too much Turkish raki.
     
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