procedure for an inclining experiment in air

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

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DCockeySenior Member

Did you look at the diagrams and read the other posts? The boat is suspended in the air by slings to either side of the boat with no other support. The slings and boat are free to pivot where the slings meet at the top. One side of the slings will not go slack when the weight is moved transversely; the boat will swing and settle at a new equilibrium heel angle. The slings are not adjusted when the weight is moved.

The moment of the boat, weight and slings about the pivot point are zero and remain zero as the weight is move transversly. This results in the CG of the boat, weight and sling combination being directly below the pivot point when in static equilibrium. If this is not clear you should consider conducting a simple experiment.

The pivot point distance from the boat remains constant. See above.

Entirely different "swinging" test for a different purpose, determining mass moment of inertia. Not related to what I described for determining CG location.

No paranoia by me, just consistency by you.

I provided an example of the usage in a reputable reference.

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Hmmm…let me see:

Ok…consistent as ever then.

Confused, your picture shows it changing.. which is it then?

To summarise.

The method you are poorly describing, since inconsistent each time, has nothing to do with “effective metacentric height” nor the calculation of the KG.

The metacentric height is calculated from the 3D shape of the hull when floating in water. This 3D shape, the hull, is displacing a known amount of water and floats at a given waterline. From this simple at rest steady state we can determinate several characteristics of the hull.

When a weight is moved transversely, or an external force is applied that makes the hull list/heel accordingly there is a change in the location of the centre of buoyancy and the waterplane area. Why does this matter?

The change in centre of buoyancy occurs so that the hull remains in equilibrium, the CoG and CoB are in-plane vertically, with a weight being moved, and a restoring force with a heel moment. The change in waterplane inertia effects the calculation of metacentric height. If both the hull's volume has changed (owing to a weight being added) and the waterplane area, then both the I(T) has changed and the V, in the BM. With a changing BM, the KM is changing, which means to determine the KG becomes problematic. Which is why an inclining a range of 2degrees is used to minimise the effects of the changing I(T). Since beyond this the change in waterplane area “moves” the location of KM, into what is called M curve, amongst others. As shown below:

Suspending a boat from a pivot point, wherever the merits of this, the location of the pivot remains unchanged (your words). The metacentre is not a fixed location. Additionally, the hull shape plays a significant role in determining the location of the metacentre, all your doing is aligning masses to be coincident in the same plane which is in the transverse axis, not vertical, which is required.

Then all you are doing is demonstrating a lack of understanding in stability, metacentric height (and “effective”) and how it is calculated and used for a vessel.

Since the “effective metacentric height” you refer to, is simply the reaction force from a grounding, as if removing weights. This is one of the first “simple” calculations done for a NA in their 1st year of a degree. It creates a virtual rise in the KG. But you need to know where the KG is to begin with! You have not established that.

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Graham.gemininaval contemplator

"E.2.2 Vertical centre of gravity
The vertical position of the centre of gravity (VCG) can be found using any of the following methods:
a) an inclining experiment in water (see 3.5.7), the results being corrected to the appropriate loading condition;
b) an inclining experiment in air using a known length of suspension and moving weights transversely (as in
c) calculation based on the calculated mass and centres of gravity of all individual components, raised by an
addition of 5 % of (FM + TC).
Method a) shall not be used for boats with a metacentric height greater than 5,0 m (such as multihulls), since
inclining experiments in water for such boats are liable to significant inaccuracies.
Method c) shall not be used for boats with a metacentric height of less than 1,5 m, since significant inaccuracies
might result. It can, however, be used for preliminary assessment.
For the purposes of determining the curve of righting moments, the vertical disposition of the mass of the
crew shall be
— as required by Annex B for calculations to show compliance with 6.2, or
— as required by 6.3.1 for calculations to show compliance with 6.3."

As this is for a RIB in our factory and we have both overhead cranes and a single point lift on the boat this is by far the easiest option available to me, just interesting that this procedure is not documented. I'll set up the experiment using the same weight movements as the procedure for water.

I assume i'll need to use the entire length of the cable from the point it comes out of the winch as the "known length of suspension", and then use a pendulum to measure the angle

then figure out how to use the moments of the weights to come back to the VCG. the equation should be quiet easy to figure out. i'll post what i come up with

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DCockeySenior Member

The sling length does not change during the tests for the method I have described.

Post #9 from which you have quoted one sentence completely out of context was in response to post #8 by TANSL about what happens when the sling length is changed but the weight is not moved. TANSL's post #8 was explicitly referenced in my post #9. When both sentences of paragraph from post #9 are read (copied immediately below) and the illustion is viewed it is clear that the reference to changing sling length was with respect to the difference between the two boats in the illustration.
My description has been consistent. You have attempted to make it appear inconsistent by taking a sentence entirely out of context as discussed above, followed by an irrelevant discussion of metracentric height. The CG location, KG, can be determind using the method I described in this thread without any knowledge of tghe metacentric height of the boat.

No weights are removed in the method I described. The combined weight of the boat and weight remains constant. The weight is only moved transversely.

It appears that you are demonstrating your inablility to understand the principles of simple mechanics outside of the standard naval architecture proceedures you have learned.

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In my 2002 edition this is listed in D2.2, not E2.2.

This appears to be based on the similar “rolling period” test used on large vessels as outlines in “Code on Intact Stability Resolution A.749”. But this is in water not air.

The equilibrium shall always be to act in one axis, but where along that axis? In your diagram…you’ll need to move the location of where the line from the crane is attached to the hull (show as a cross in the hull) to “somewhere” to get the boat to balance with the CoG inline with the crane. That won’t be easy.

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DCockeySenior Member

Do you still claim in order to determine KG of a boat in air that the boat has to be placed at "at 90degress, i.e. on its side"?

Do you still claim that "Once the weight is move transversely, the string on the opposite side shall go slack as the TCG is no longer symmetrical"? If so may I suggest a review of basic statics and free body diagrams.

Start with one of the drawings in post #9, draw a free body diagram including the force of gravity acting thought the CG and the vertical support force at the top of the slings. Then write the expressions for the sum of the forces and sum of the moments in terms of the geometry including the change in heel angle as the weight is moved. Solve for the location of the CG of the combined boat and weights. Then use the weight of the boat, the weight of the weight, the locations of the CG of the combined boat and weights, and the locations of the CG of the weight to determine the location of the boat CG.

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DCockeySenior Member

I assume that the single point lift hook in Graham.gemini's drawing is rigidly attached to the hull. If the cable is flexible then there is an error in his drawing. The boat will stabilize with the cable vertical because a flexible cable cannot support a moment. The CG will be directly under the cable and the boat will be heeled. The "pivot point" will be at the attachement of the rigid lift hook to the cable.

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DCockeySenior Member

Method b is consistent with what I described above.

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philSweetSenior Member

Graham, your sketch in post 18 is badly flawed. I hope you can see why. The line will hang vertically in both cases and the cg will be in line with it at all times because there are no torques being applied. If it is suspended by three or four (noncolinear) points, then you can get a position like you have drawn, but not with just two points.

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Graham.gemininaval contemplator

thanks guys,

so as I understand it the cable would hang straight and the CG would be below the attachment. it pretty obvious now especially when looking at a boat being lifted beam on, the cable hangs straight and the boat aligns itself with the hook above the center of gravity. same thing will happen transversely just could not visulalise it!

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Convenor 12217Convenor 12217

Procedure for inclining experiment in air

Hi everyone,

I chanced across this website yesterday and came across this string quite by accident. If I had come across it sooner I could have saved you all a lot of time.

I write as the convenor (chairman cum secretary) of the ISO working group that developed ISO 12217.

David Cockey basically got it right - so I won't repeat all his reasoning, which is correct. Maybe I should write up a procedure and publish it on this website? I understand that this procedure has been used for establishing the VCG of aeroplanes.

I was confused by the reference to E.2, as in MY copy of the standard, it should refer to D.2.2.

It is worth doing a sensitivity analysis to determine what length of lifting strops to use. If they are too long, then the resulting heel angle will be too small. You don't need to restrict the angle to only 2deg, as the geometry works whatever the angle.

Conventional inclining experiments (in water) are normally limited to 2deg so that the waterplane inertia doesn't change significantly, and is most accurate if the boat is wall-sided. But given modern computer software, you can deduce the VCG even if heeled to (say) 20 or 30deg, provided you model the actual LCG+TCG with the offset weights and vary the modelled VCG until the calculated heel angle matches the physical experiment.

For about \$100-150 one can buy a digital inclinometer which makes measuring angles quick and easy provided the angle is steady.

Thanks for an interesting discussion.

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This only works if everything is 2D, in-plane.

Whist mathematically on a 2D vector diagram it appears fine, in practice it is rather more difficult than just a simple planar diagram. To balance a 3D object perfectly so that it only rotates about one plane is not an easy proposition at all. That’s where the misunderstandings arise.

The inclinometer must also be calibrated regulatory and by a certified authority.

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TANSLSenior Member

Rotate an object around "one plane" is something I can not understand how this is achieved.
In the inclinig test in the water, as the boat heels a bit, also changes its trim. Therefore, make the boat turn around a fixed axis, is impossible. So the angle of heel must not exceed 3 degrees, not to change too much inertia of the waterline.
I have to say I left totally surprised that, after 11 years, the creators of the standard 12217 tell us that they had to change/correct it. ???????. One can be wrong and should be corrected, but 11 years later ....

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DCockeySenior Member

If fore and aft slings or lifting points are used then as long as the pivot points for both slings are at the same angle the boat will heel but not trim. If the pivot points are not at athe same angle the change in trim angle with heel can be recorded as the weight is moved. Correction for trim variation can then be made using geometry and trigonomety, and would be the same calculation proceedure as corrections for equivalent trim angle changes with heel for an in-water inclining test.

Last edited: Jan 18, 2013

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And that's the whole point.

It is easy enough to demonstrate on paper. I even recall doing this with bits of 2D cutout cardboard shapes in school, and then later in O-level Technical drawing by graphically demonstrated this, using Bow's Notation....but the simple practicality of keeping everything constant in more than one plane, as you note is required, is not as easy as one is making out. It introduces the potential of large errors.

Even Olga Korbut or Nadia Comăneci wouldn't be able to balance that one!

The theory of a standard inclining expt is straightforward. Yet in practice is not so easy to achieve and many additional aspect are introduced to minimise such errors to obtain useful meaning full results.

The objective here, is to obtain a easily recordable and repeatable KG value. And one that a surveyor is happy to sign off.

Last edited: Jan 18, 2013
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