Inclining Experiment - Back Against the Wall?

Discussion in 'Stability' started by Tad, Jul 30, 2015.

  1. DCockey
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    DCockey Senior Member

  2. Heimfried
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    Heimfried Senior Member

    The linked paper shows the movement of the (true) metacentre (page B-100). This leads me to an extension of my calculation sheet by a diagram showing the locus of the metacentre (0 deg. to 71 deg.).

    (Locus of metacentre: violet curve, WL: blue line, cross section of hull shape: red line, heel: 71 deg. )

    Not only a sqare prism shows this curve, but also a boat shaped hull.
     

    Attached Files:

  3. CDBarry
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    CDBarry Senior Member

    STANDARD GUIDE FOR CONDUCTING SMALL
    BOAT AIR-INCLINING STABILITY TEST:
    (Lightweight Survey and Air Inclining Test to Determine Lightweight and Centers Of Gravity)

    The Society of Naval Architects and Marine Engineers has published a new technical and Research Bulletin, Standard Guide for Conducting Small Boat Air-Inclining Stability Test: (Lightweight Survey and Air Inclining Test to Determine Lightweight and Centers of Gravity). This T&R Bulletin was developed under the Small Craft Committee of the Society in conjunction with the Ship Design Committee’s Ship Stability Panel.

    The bulletin covers the determination of a small boat’s lightcraft weight and center of gravity characteristics expanding on the guidance given in ASTM F3052 (2014). The procedure can be considered to consist of two separate tasks: 1) a scale weighing/deadweight survey that determines the weight and longitudinal center of the craft and 2) an air-inclining experiment that determines the vertical center of gravity. Although not required, the inclining test is strongly recommended for small craft upon their construction completion and/or after major conversions, especially for a first of class or type that will be subject to stability analysis and/or whose weights will be monitored for stability compliance.

    The complete experiment comprises weighing the craft from two points at different longitudinal locations then moving a known weight a known distance transversely across the craft to induce a heel angle, which is measured. It is typically conducted indoors and an enclosed facility to protect the vessel from any outside environmental disturbances.

    The new publication is identified as Technical and Research Bulletin 9-1. It is a 29 page report issued electronically and may be ordered for $40 ($20 for SNAME members) through the SNAME web site:

    (http://www.sname.org/pubs/viewtechnicalpaper?DocumentKey=a3f68694-dad2-4a4c-a7cc-d3968658c44e)

    or by contacting Kristin Walker at kwalker@sname.org or (703) 997-6710.

    Many thanks to the people on this forum who particpated in the review and/or offered comments.
     
  4. DCockey
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    DCockey Senior Member

    Good to hear that there is a SNAME TR describing the process.

    Is the air-inclining stability test method in T&R 9-1 Standard Guide for Conducting Small Boat Air-Inclining Stability Test compatible with the requirements of ISO 12217?
     
  5. CDBarry
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    CDBarry Senior Member

    One of the main reasons this was developed was because boats were being procured by various government agencies to ISO 12217 but there was no good reference covering air inclines, which ISO 12217 seems to prefer.

    There is no ISO reference for air inclines and the results of this procedure produce data that is compatible with 12217, so I see no reason why it wouldn't be unless ISO comes out with their own standard (and I hereby volunteer Todd to represent SNAME if ISO does decide to come up with their own standard).
     
  6. Heimfried
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    Heimfried Senior Member

    Last weekend I worked through the paper of R J Dunworth, "Back against the wall", you linked at first post. This new method needs a geometrically well known shape of the boat hull, because it is necessary to calculate KN (= eta(B)) for any used heel angle (phi). I calculate eta(B) with numerical integration of a array of numbers, which are modelling the boat hull.

    At page B-102 of the paper is the following remark:
    "The sign of the last term in Equation 9, TCGI cos phi, is dependent on the sign convention being used. If, for example, distances to starboard and heel to starboard are both positive or both negative, then the sign of the term will be negative as shown. If distances and heel angle are of opposite signs, then the term will be positive."

    That is an error, because the sign of the cosine is symmetrically to zero: cos(10) = cos(-10) .

    Instead of this that remark is applicable to the penultimate term in Equation 9, KGI sin phi.

    For a hull, which in its shape is not well documented, the inclining experiment in air will be the best procedure to find VCG and TCG.
     
  7. Heimfried
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    Heimfried Senior Member

    Deriving from own sketches of a boat hull I found a similar, but slightly different fomula to calculate VCG and TCG.

    VCG = z(G0) = [eta(B1) cos(phi0) - HZ1 cos(phi0) - eta(B0) cos(phi1)]/[sin(phi0) cos(phi1) - sin(phi1) cos(phi0)]

    TCG = y(G0) = [eta(B0) + z(G0) sin(phi0)] / cos(phi0)

    At a test it works well with an angle phi0 = -12.3 deg. and phi1 = -26.0 deg.
    Result VCG less then 1% off the correct value, TCG 2.2% off).
     
  8. Diego San
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    Diego San Junior Member

    Dear all,

    I have been reading this T&R Bulletin and I appreciate this wonderful job because it is very well explained. I have a question though.

    I understand the fore and aft pick/lift points must be at the same height in order to measure the B height from the baseline to the roll axis, but I still can't get why the keel line must be parallel to the baseline. The thing is that I find very hard to achieve this particular point depending upon the hull form and It represents a considerable amount of preparation time.

    I use the COG as an input for Orca plug-in (Rhino) or Maxsurf directly to stablish the hydrostatic properties of the vessel, so I wonder if one can just record the relative height between two points and their respective distance from the stern reference point (so one can estimate the trim from drawings or so) and then use it on the software rather than struggle with laser levels or any other leveling tool.

    Thanks for reading.
     

  9. Heimfried
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    Heimfried Senior Member

    Hi,
    I don't know that T&R Bulletin. Because there is no answer to your question until now, a general remark:

    Boats often show no physical straight part of the hull which represents a keel line on the contrary to large ships. So the meaning of keel line is an appropriate line parallel to the waterline, the former is the longitudinal axis of the boat fixed system of co-ordinates. If the boat in an air inclining experiment is not suspended in a position with horizontally waterline, you will get no correct measurements.
     
    Diego San likes this.
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