procedure for an inclining experiment in air

Discussion in 'Stability' started by Graham.gemini, Jan 16, 2013.

  1. Graham.gemini
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    Graham.gemini naval contemplator

    Hi Guys,
    does anyone have know the procedure for an inclining experiment in air to determine VCG as described in ISO 12217:1 annex E 2.2:

    " an inclining experiment in air using a known length of suspension and moving weights transversely (as in
    water), the results being corrected to the appropriate loading condition"

    there is a well established procedure for inclining in water but I can find anything on this suspension in air method.

    Regards
     
  2. gonzo
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    gonzo Senior Member

    The center of gravity is influenced by the displaced volume of water.. A submerged keel weighs less than one out in the air.
     
  3. DCockey
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    DCockey Senior Member

    The center of gravity as used in stability calculations remains the same whether the vessel is in salt water, fresh water, air or a vacuum. (Assuming the vessel and its contents are the same.)
     
  4. JSL
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    JSL Senior Member

    I don't know about the ISO method but:
    if it is possible to block the boat at about 30 deg. to 40 deg.heel in a cradle with some accurate scales set athwartships (port and stbd), it is possible to determine the VCG. And, if you had 4 scale you could also determine the LCG as well. Either set-up will give you the weight (displacement).
    Trouble is, such a set-up could be a challenge.
     
  5. DCockey
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    DCockey Senior Member

    Another possibility is to hang the boat in slings such that the boat is free to "heel". Then conduct a inclining test as would be done with the boat floating; move weights transversely and record the inclination/heel angles. The effective "metacenter" is the location that the boat and slings rotate about as the boat "heels". Given the effective metacenter height, and heel angle vs transverse weight location the VCG can be calculated the same as using floating incline test results.

    Update: The use of the term 'effective "metacenter"' is not meant to imply any connection or relation to the actual metacenter which arises from the buoyancy characteristics.
     

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  6. Petros
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    Petros Senior Member

    I think what this is referring to is a method where you set two scales when on the hard, one in front and one in back. with the boat level, take the measurements off both scales, than raise the front to a known measured angle, take the weight measurements again. You can repeat this for the rear as well, but it should not be necessary.

    With accurate measurements of weights, distance and the angle, you can determine the height of the CG as well as its location front to back relative to the scale location.
     
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  7. JSL
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    JSL Senior Member

    I have used the scales method on all my boats for the past 15 years but only for determining weight, LCG, TCG using a vertical 'lift' with scales fore and aft. Some years ago the people that 'weigh' the boat offered to heel and then determine the VCG. This was an 85' aluminum motoryacht (completed) and the builder was a bit nervous about the idea (as was I)so we decided not.
    Using a similar fore/aft method might work but you would either have to trim the boat a significant amount or have some incredibly accurate scales and measurements... plus absolutely no external forces such as wind*, etc. (*When weighing outside, the wind must be less than 10 knots or the scales never settle down for a good reading.)
     
  8. TANSL
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    TANSL Senior Member

    Maybe I am very wrong but I think that the pivot point is, always, the point of attachment of the slings to the vertical cable. That is the only point around which the assembly can rotate. The rotated angle, however, the same weight to heeling is always the same. It is not, therefore the false metacentre of the boat nor can be used to obtain the metacentric height. Or am I so wrong?
    On the other side, but not least, I have the ISO 12217-1, 2002, in Spanish and English versions, and none of them has Annex E.3.2
     

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  9. DCockey
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    DCockey Senior Member

    I agree the pivot point is the point of attachment of the slings to the vertical cable. The stable equlilibrium location of the center of gravity of the boat, weight and slings assembly is aligned directly below the pivot point of the sling attachment to the vertical cable. The rotated angle is not the same for different sling lengths.

    In the attached illustration the CG location of the combined boat and weight is fixed relative to the boat. The sling length is changed and the boat heel angle then adjusted so that the CG location is directly below the sling attachment to the vertical cable.

    The sling attachment to the vertical cable might be considered as the "effective metacenter" for calculation purposes though it is not related to the actual metacenter which arises from the buoyancy characteristics.
     

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  10. TANSL
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    TANSL Senior Member

    Agree that the angle of rotation may be not the same. I made a silly mistake.
    Disagree, however, considering the pivot point as the metacenter. It has nothing to do with the boat metacentre that, for each list, is defined by the position of the center of buoyancy to the angle of heel. Don´t call it metacentre and we both agree.
     
  11. DCockey
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    DCockey Senior Member

    The combined CG of the boat, weight and slings must be directly below the pivot point. Assumption is that any stiffness or friction in the pivot point is negligable.

    In an inclining test afloat the combined CG of the boat and weight must be directly below the metacenter. Given the weight of the boat, and the heel angles and position of the weight's CG as the weight is moved transversely the position of the boat CG can be found if the metacenter location is known at each heel angle.

    Same proceedure can be used to determine boat's CG location based on an "inclining test" with the boat hung in slings and free to heel using the location pivot point of the slings in where the afloat inclining proceedure uses the metacenter location.

    The pivot point location was labeled "effective metacenter" in previous posts given it's role in the calculations. Not meant to imply any relationship to the boat's actual metacenter location which arises from buoyancy.
     
    Last edited: Jan 16, 2013
  12. Ad Hoc
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    Ad Hoc Naval Architect

    In my 12217-1:2002 Annex E, E.2 it states:
    "During the test, the boat shall be in calm water in the light craft condition and then the equipped as follows:..."

    Has this now changed from the 2002 version?
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    What you are describing here is not related to finding the KG. The methods you are describing above are related to the mass moment of inertia, totally different.

    You cannot find the KG of a boat in air, unless you place the boat at 90degress, i.e. on its side, and place 2 scales, one around the keel, and the other around the mast. Just as for the LCG, but rotated to the correct axis. Not that anyone would do that of course. Even in model testing we incline the model and perform a simple inclining expt.

    You shouldn’t make reference to metacentre, even when you amend and add a qualifier later. It is misleading. Since it bears no relationship to the metacentre. Simply describing it as you have will confuse those unaccustomed to the correct terminology and usages, such as used in an inclining experiment, and its references to stability.
     
  14. DCockey
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    DCockey Senior Member

    Sorry Ad Hoc but you are either wrong or not able to understand the method as described in posts #5, 9 and 11. No need for placing the boat at 90 degrees. Suspending the boat by slings such that it is free to swing, and then heeling it by moving a weight transversely on the boat and measuring the position of the weight relative to the boat and corresponding the angle of heel is sufficient. Two positions of the weight and corresponding heel angles are sufficient though accuracy may be increased by using additional weight positions.

    The equations for determing KG, with the exception that KM is replaced in the equations by the distance from K to the pivot point, are exactly the same as used in determing KG from an in the water inclining test when KM is known.

    I'm surprised it took you this long to criticize my terminology. You are consistent in that practice though.

    Since use of terminology was confusing I ammended it and added the qualifier. The relationship to the metacenter is how it is used in the equations. The term "effective metacentric height" is used in a similar manner in the discussion of grounded vessel stability by Moore on p 74 of Intact Stability, The Principal of Naval Architecture Series SNAME 2010. But as has been pointed out "effective metacenter" can be confusing and perhaps is best avoided in the future.
     
    Last edited: Jan 16, 2013

  15. Ad Hoc
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    Ad Hoc Naval Architect

    All you are doing is moving weights transversely onboard, effecting a transverse change in the CoG. Once the weight is move transversely, the string on the opposite side shall go slack as the TCG is no longer symmetrical. Readjusting this opposite sling just realigns the TCG with the pivot point it is being suspected from, nothing else.

    With the datum (pivot point distance from boat) constantly changing, the location of the KG is still an unknown.

    In terms of its “swinging” this is the method, as noted above, that we use for determining the mass moment of inertia in the plane required. Not the KG. The location of the CoG (in the plane under investigation) is required for such “swinging” methods in determining the mass moment of inertia.

    Why??...I do not sit at my desktop waiting for you to post and see if there are any mistakes or errors. I pop on, when not overloaded with work and comment upon posts what I see and when I have time too. Not sure what it is you are suggesting, other than paranoia?

    I have never come across the term “effective metacentric height”…?
     
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