Displacement Speed Question

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

  1. mydauphin
    Joined: Apr 2007
    Posts: 2,164
    Likes: 53, Points: 48, Legacy Rep: 575
    Location: Florida

    mydauphin Senior Member

    You lost me on this one. Explain Overforced displacement please.

    My guess, you mean that given enough HP any speed is possible...
     
  2. apex1

    apex1 Guest

    They hardly can be called semiplaning, but with a L/B ratio above 10/1 they are fast as if they where a semiplaning vessel. But need a hell of a lot of power to achieve their target speed. At least in the old German Literature I have they are described "überforcierte Verdränger" which I (correctly) translated as "overforced". My shoolbooks call them "hybrid" not really semiplaning, not just only displacement vessels.
    That was my ability to explain that, I hope it was sufficient.
    And to your question:
    No, there is a limit, I was told some hundred years ago, above that, additional power would sink the vessel. Let me guess thats just a theoretical one. But I cannot elaborate on that, I know nothing about it!

    Regards
    Richard
     
  3. jehardiman
    Joined: Aug 2004
    Posts: 3,218
    Likes: 672, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Depends on the particular ship class and hull form. Apex1 is sort of correct about overdriving a hull, and if you read my previous post in this thread you will see that if you have enough power you can start to climb the 3rd major powering hump. However, that slope is almost vertical (i.e. large hp increase for almost no speed gain) for many "normal" displacement hull forms. This leads to specalized hull form for the smaller (150-300 LwL) combatants. Large combatants don't need to plane to achieve significant speed and can do it while devoting much less volume and tonnage to machinery.

    Check out the US Navy Cyclone PC's: 157' LwL x 25', 315 t, 13K shp, 35 knts. Not really planning, nor really displacement. Compare this to a non planning Spruance class destroyer: 550 LWL x 55, 9100t, 80K shp, 33 knts. The Spruance uses 1/5 the power per ton. On the other hand compare a Osa class missle boat: 120' LwL x 24', 210 t, 12K shp, 38 knts, which is a planning boat and uses about 50% more hp per ton.
     
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 7,276
    Likes: 1,165, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Just checkout any FastFerry.....they are running at very high Froude numbers, they are not planning, nowhere near it.

    HUll form dictates whether it will plane or not.
     
  5. mydauphin
    Joined: Apr 2007
    Posts: 2,164
    Likes: 53, Points: 48, Legacy Rep: 575
    Location: Florida

    mydauphin Senior Member

    I am confused. I understand planning, I understand displacement. I understand fast ferry and cats that split water and ride over bow wake. I don't understand Spruance that uses 1/5 the power per ton, could it be that Spruance gets more power to water because of bigger prop turning.
     
  6. mydauphin
    Joined: Apr 2007
    Posts: 2,164
    Likes: 53, Points: 48, Legacy Rep: 575
    Location: Florida

    mydauphin Senior Member

    Amazing little boats... Big fancy props, lots of power. Hull long and skinny, still displacement?


    USS HURRICANE (PC 3)

    PATROL COASTAL

    UIC: 21932
    Class: PC 1 Fleet: Atlantic
    Status: Active, in commission Homeport: NORFOLK, VA (LITTLE CREEK)
    Date status changed: 10/15/1993 Berth:
    Maintenance Category:
    Force: Auxiliary MARAD Type:
    Builder: BOLLINGER MACHINE
    Delivery Date: 07/21/1993
    Award Date: 08/03/1990 Age (since delivery): 15.9 years
    Keel Date: 11/20/1991 Commission Date: 10/15/1993
    Launch Date: 06/06/1992 Inactivation Date:
    Age (since launch) 17.0 years Decommission Date:
    Years from Commission to Decommission:
    Stricken Date:
    Overall Length: 170 ft Waterline Length: 0 ft
    Extreme Beam: 25 ft Waterline Beam: 0 ft
    Maximum Navigational Draft: 8 ft Draft Limit: 0 ft
    Light Displacement: 288 tons Full Displacement: 334 tons
    Dead Weight: 46 tons
    Hull Material: Steel hull, steel superstructure.
    Number of Propellers: 4
    Propulsion Type: Diesel Engines
    Accommodations: Officers: 5 Enlisted: 23

    Custodian: US NAVY Ships Program Manager: 325
    Planning Yard: Puget Sound (DET) Boston, Boston, MA
     

    Attached Files:

  7. mydauphin
    Joined: Apr 2007
    Posts: 2,164
    Likes: 53, Points: 48, Legacy Rep: 575
    Location: Florida

    mydauphin Senior Member

    These destroyer have some strange ways to get that speed


    Stern Flaps

    A stern flap (Figure 3) is a relatively small plate that extends behind a ship's transom, lengthening the bottom surface of the hull. A stern flap alters the water flow at the stern in ways that reduce the ship's resistance and increase fuel efficiency by a few or several percent. A stern flap for a Navy surface combatant in 2000 cost about $170,000 to fabricate and install.11 Preliminary tests of stern flaps on DDG-51s showed an annual fuel reduction of 3,800 to 4,700 barrels, or about 6.0% to 7.5%,
    --5--
    per ship.12 As of November 2004, the Navy had installed stern flaps on 98 ships (primarily surface combatants) and planned to install them on an additional 85. The 98 ships equipped as of November 2004 had accumulated 403 ship-years of service and saved $44 million in fuel costs.13 The Department of Energy stated in 2003 that by 2005, stern flap installations on Navy ships would save 446,000 barrels of fuel, or $18 million, per year.14
     

    Attached Files:

  8. Ad Hoc
    Joined: Oct 2008
    Posts: 7,276
    Likes: 1,165, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    mydauphin

    Im not sure what you're getting at...can you be more specific.
     
  9. mydauphin
    Joined: Apr 2007
    Posts: 2,164
    Likes: 53, Points: 48, Legacy Rep: 575
    Location: Florida

    mydauphin Senior Member

    I am trying to understand these high-speed non-planning boats. I think these boat are super skinny length to beam, have flat bottoms at rear quarter to half of ship. So the front acts like a knife, the back as a planning boat. The destroyer ship is using bow appendages to get lift and rear flap for lift. They may also be getting some lift from props.
     
  10. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    "The point is, to my knowledge, no such boat has ever been built.

    Are you aware of one?"
    __________________
    Tom Lathrop


    Would seem that most of the Atkin Box keel reverse deadrise are quite similar, however Atkin goes two steps further in an attempt to capture the velocity of the water dragged by the canoe body and hull above and feed it to the prop.

    And in using the reverse deadrise to lift the stern from the water , perhaps reducing drag even further.
    No stern flaps anti squat required.

    We all dream of a series of tank tests to see if this old design is as efficient as claimed 50 years ago , and weather a modern version would indeed make a super efficient SL 2.5 or 3 cruiser .

    With modern materials weight and maint could be minor , compared to planks.

    FF
     
  11. Ad Hoc
    Joined: Oct 2008
    Posts: 7,276
    Likes: 1,165, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    mydauphin

    It is more than just looking at hull shape and saying...hmm..flat bottom, with a Vee, must be a planning boat...or wow, boat is fast so why is it not called a planning boat.

    Any "body" moving through the water, ie at the air-sea interface, will create pressure variations around the 'body'...these pressures variations manifest themselves as waves. These waves are a measure of energy and hence drag.

    The shape of a hull can affect this pressure distribution considerably. In a nut shell to cut a long story short, the stern experiences suction pressure fields. Speed then also begins to play a part....ie trim/squat, the faster one goes.

    For a "normal" boat, the faster one tries to go, the more trim and the greater the power required for little gain. The back is sucked down and dragging creating a lot of wash/waves. Too much energy is being used in making waves. The reason is the hull shape and its length displacement ratio. This is seen in the resistance curve by humps, and the main prismatic hump. A hull must over come this main hump to go faster, ie make "lesser waves" or better still no waves. Long thin hulls, hydrodynamically, behave differently to short fatter ones.

    However, just making the aft section flat with/out a vee, doesn't mean that is all that is required just to go fast, or get over the "hump" in the resistance curve. That is just hull shape.

    Making the hull longer and thinner, reduces the 'near vertical' curve in resistance of a 'normal' hull form...ie its length displacement ratio. Not only does the resistance curve become less step, but also the main hump is much less pronounced. The longer and thinner one makes it (that is light for its total length), the curve slowly approaches a smooth curve and almost no discernable pristamtic 'hump'. This is why fast ferries, for example, go fast, have a high froude number, but are not planning. They are long and thin.

    The length displacement ratio (ie long and thin) allows the hull to be driven faster than would normally be the situation. The wave making reisatnce gets less and less, the longer and thinner the hull becomes.

    The down side is, one of these long thin hulls is generally far too unstable to be used effectively. Solution, put two side by side, ...a catamaran. Utilising the benefits of the long thin hydrancimc effects, but providing a stable platform for use in almost any application safely.
     
  12. mydauphin
    Joined: Apr 2007
    Posts: 2,164
    Likes: 53, Points: 48, Legacy Rep: 575
    Location: Florida

    mydauphin Senior Member

    Ran across this reverse transom, it work its way to the front were it became a traditional V hull. So the boat had a large reverse planning area in between chimes. Anyone seen this type of hull, what is it called and do you anything more about it? thanks
     

    Attached Files:

  13. Ad Hoc
    Joined: Oct 2008
    Posts: 7,276
    Likes: 1,165, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    there are 3 lines, showing er...what?...is this plan, profile, body plan...not very clear...which is centreline, or baseline etc....????
     
  14. mydauphin
    Joined: Apr 2007
    Posts: 2,164
    Likes: 53, Points: 48, Legacy Rep: 575
    Location: Florida

    mydauphin Senior Member

    Sorry, The wavy line is the bottom of hull at the transom. The vertical and horizontal line mean nothing...
     

  15. Ad Hoc
    Joined: Oct 2008
    Posts: 7,276
    Likes: 1,165, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    still confused, in what plane is the view?
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.