Talk:Metacentric height

Too technical

I've added the technical tag as this article doesn't really make any sense. The terms are not defined - the worst case is in the formula, which doesn't even say what M is, never mind explain any of the other terms. The explanations are also very poor and in particular there is no definition of metacenter (other than the incomprehensible formula) in the section called metacenter, which is just weird. Metacentric height is clearly hard to explain since as far as I have been able to discover nobody in internetland has been able to do it yet. Somebody please rewrite the whole thing. Macguba (talk) 10:41, 16 March 2009 (UTC)

This concept of metacentric height doesn't make much sense to me... It says in the article that the metacentre is a FIXED point, however, as the boat keels and the centre of bouyancy shifts, the metacentre obviously moves up and down the centreline. I've done sample calculations on a square hull to verify. So all told, this seems like a useless indicator to me. Anyone care to shed some light? 128.100.53.193 19:44, 7 June 2006 (UTC)

The metacentre is assumed to be constant at angles of heel up to approximately 10 degrees, as the difference at that scale is inconsequential. //when the angle of heel is greater than 10 degrees, different calculations are used which do not involve the metacentre. 194.66.93.27 (talk) 12:24, 9 February 2015 (UTC)

You're probably miscalculating; the metacenter should stay at a fixed vertical distance from the keel, on the centerline, for all reasonable angles of heel. We had to derive the calculations on it as part of the Junior year naval architecture ship stability class; the assumptions that go into the "fixed position" statement are that the ship's sides are vertical, and that the angle of heel is small. In practice, the sides can be pretty non-vertical, and the heel angle can be high enough that you just barely put the deck under water, and the fixed position still doesn't shift significantly.

I can pull the info out of the textbook tonight. Georgewilliamherbert 20:29, 7 June 2006 (UTC)

As an indicator for the ships stability the GM is accurate enough and vital. I work as chief officer on a offhore construction vessel laying flexible pipe. Having heavy weights on high altitudes on the ship makes us squeeze the limits of the vessel at times. We keep a close eye on the GM at all conditions. It can be measured also to verify ones calculations using a normal stopwatch. One measures the time the ship uses to roll one period, and with the beam and a factor of 0.8 if you use meters, one uses the formula:$GM=((B*0,8)/T)2$ Where B is Beam and T is time in seconds. According to international rules and regulations one should have a GM higher than 0.15 metres. This give a roll period of approx 49 seconds on a vessel 24m wide.

The GM is not a fixed distance. GM₀ is, but GM_φ not. See nl:Stabiliteit (schip)#Dwarsmetacentrum for drawings. BoH 18:03, 20 August 2006 (UTC)

Those diagrams are exaggerating the effect; while it's true that "M is fixed" is a small-angle approximation (also depending on ships with largely vertical sides), in practice it's pretty accurate for real ships until the point that the deck edge goes underwater. I could paraphrase the whole chapter in "Principles of Naval Architecture" but i would likely go right past most readers and be point less verbage. We're not a textbook... Georgewilliamherbert 00:11, 21 August 2006 (UTC)

Those diagrams are not exaggerated, but are those of a not so typical ship, the DCV Balder. In practice it is valid to use GM₀ until 5º, depending on the type of ship. BoH 00:40, 21 August 2006 (UTC)

---

"exaserbating" is misspelled in the Free Surface Effect section, first paragraph.

The location of M will stay relatively fixed until angles of heel greater than 7° for a box shaped vessel, i.e bulk carrier at which point it does start to become harder to determine.

Is it actually the case that "The distance between the center of gravity and the metacentre ... is usually between one and two metres. "? Regardless of the size of the boat? I find that remarkable. Very remarkable. Really extremely remarkable. Wow.KSONeill 04:45, 15 August 2007 (UTC)

an excessively low GM increases the risk of a ship capsizing in rough weather (see HMS Captain or HMS Vasa. - Is this right? Isn't it possible that the Captain, Vasa and Cougar Ace all had negative GM ? - If there are no sources should we be saying this? Also the article confuses a low gm which means close to the legal minimum allowable, taking into account damage stability and wind heel with a gm the is below the minimum. Ken E. Beck (talk) 16:53, 21 November 2007 (UTC)

The first diagram is backwards, the boat should tilt towards the center of gravity, not away from it. —Preceding unsigned comment added by 150.156.213.104 (talk) 16:19, 23 February 2011 (UTC)

Not sure what you're saying about backwards; the tilt can be imposed by waves, wind, etc., which have nothing to do with the CoG. Mcswell (talk) 15:49, 4 October 2018 (UTC)

Clarification & Comments

General Comments : This article attempts too much. I would suggest that it should focus on Metacentric height (GM) - what it is and how it is derived and how it is used. It should refer to Righting arm (Gz), Rolling Period, Damage Stability, etc. as these are not directly related to GM but GM affects them.

Usually between 1 and 2 metres : I would request that some reference be quoted because I have personally worked on vessels with GMs ranging from 500mm to 11m.

Good GM is the minimum necessary stability to maintain safety, so of course that will vary based on vessel and required safety. It doesn't seem unreasonable to provide a general figure as to what a reasonable GM might be as long as it is also acknowledged what factors go into determining a safe GM.KubalaC (talk) 03:08, 14 May 2008 (UTC)

Remains fixed with respect to the ship : This is misleading. According to "Principles of Naval Architecture" Vol. I, Section 3, p. 71 which states that "The location of this point will vary with displacement and trim, but, for any given drafts, it will be in the same place."

Righting Arm : I suggest that this be moved to an article of its own.

Stability, Rolling Period, et al : This has little to do with GM and should be referenced by this article instead of included in it. Free surface actually has its own article although it has little to do with stability. The problem is that someone searching for Free Surface will find the wrong article if they require an explanation of the stability term.

Inclining Test : KG is determined through the Inclining Experiment. KM is determined by adding BM & KB. Both BM & KB are terms derived by the hull shape. GM is calculated by subtracting KG from KM.

Stabilizer Solutions : This stub does not belong in this article at all but should be moved to a more relevant one.

Overall the article requires a significant re-write. I would suggest the structure should be as follows:

1. Definition of Metacentric Height with 'MetacentricHeight.png'
2. Transverse and Longitudinal Metacenters with graph of KMl & KMt vs draft
3. Calculating GMt
4. Calculating GMl
5. Using GMt to determine small angle stability
6. Using GMl to determine MCT and trim
7. Inclining Experiment reference
8. References

Please consider these recommendations and comment on the suitability of the proposed changes Jmvolc (talk) 01:39, 31 December 2007 (UTC)

I would consider these to be valid observations and excellent suggestions.LizardJr8 (talk) 15:59, 21 January 2008 (UTC)

I went ahead and removed stabilizer info. It is pretty much redundant with article Stabilizer (ship). LizardJr8 (talk) 02:05, 10 September 2009 (UTC)

Measuring Metacentric Height?

This is wrong!! The inclining experiment is used to find the centre of gravity of a ship. The metacentre is calculated. The formula is $BM={\frac {I}{V}}\$ where I is the moment of inertia of the waterplane and V is volume of displacement.

At the moment I am working on a passenger ferry with an average GM of 5 metres which is a bit stiff but good if we ever get damaged. There is so much variation between different ships that you really cannot generalise here.

I agree that it needs to be rewritten in a clear and logical way so that it can be easily understood.

{Welkinridge (talk) 17:55, 3 February 2008 (UTC)}

Yes, but the inclining actually measures GM via GZ*Tan(theta) and hence G from KM-KG. KM itself is calculated from the vessel's hull configuration

Subsea (talk) 14:14, 1 February 2010 (UTC)

Split "center of buoyancy" into new article

Searching for "center of buoyancy" redirects to this article, but isn't defined until after it has been used. Both it and "metacenter" need to be defined before metacentric height can be defined. I suggest doing this in a separate article for "center of buoyancy". ★NealMcB★ (talk) 15:51, 26 June 2011 (UTC)

Better figure?

The letters M, G, and B in the first image are practically illegible, even when fully zoomed in. Can this be fixed by someone with decent image editing software?

roll period

"Greater mass and/or arm length means a slower swing; and less mass and/or shorter arm length means a faster swing." Source ? Since when mass has any effect at all on roll period ? Seems like total bogus to me. 87.95.47.187 (talk) 13:50, 3 December 2012 (UTC)

You are correct for pendulums. See: period of oscillation. Seems to me other factors related fluid fiction are involved, but not mass. Dger (talk) 18:57, 3 December 2012 (UTC)

comments

in the section metacenter, the phrase starting by "G is" seems the result of a misended editing

in the section roll period GM is called stability index; if stability index is a synonymous of metacentric height, this should be stated at the start

I feel kind to point out that the metacentric height appears in the roll period in the opposite way that does a pendulum length.

About the mass-question immediately above, the mass does not appear but its distribution does (through the barycenter position and the gyration radius); perhaps this may be stated more clearly (the compromise between the needs of expert and non-expert readers is hard) May help something like "Insofar gravity is the only force (buoyancy arises from the gravity on the fluid), the total mass cannot appear in the kinematic equations and formulae".

I am not native english and I find the style "too little international"

pietro

188.14.97.4 (talk) 16:44, 27 September 2016 (UTC)

ambiguous and contradictory article

1) Metacenter as a point might be well defined, but apparently means both M0 (the original position when ship is not heeled) and instantaneous position as a function of heeling angle. Metacentric height is not. Is it: a) GM b) KM c) BM If the term has a clear meaning it should be defined, not left to be guessed.

2a) "Ignoring the ballast, wide and shallow or narrow and deep means that the ship is very quick to roll and very hard to overturn and is stiff" Really? Narrow and deep means that B is well below surface of the water but as K is even deeper, KB has a large value. If there would be a lot of ballast placing G close to K, it would mean a stiff hull. But when ballast is ignored, G might be at the waterline. In that case GM might even become negative, implying just the opposite that is claimed. In that case if the hull remains watertight at 90 degrees heel, it will be stiff at that position. If not it might sink or totally overturn.

2b) "If a ship floods, the loss of stability is caused by the increase in KB, the centre of buoyancy, and the loss of waterplane area - thus a loss of the waterplane moment of inertia - which decreases the metacentric height" Interesting. in 2a) it was claimed that increase in KB improves stability and makes the hull stiff, now it's just the opposite.

In correct text, it should point out that if for some reason there would be no free surface due to flooding, if (as is common) added mass of water due to flooding is located below added center of buoyancy (which is located above where waterline used to be before flooding took place using ship fixed co-ordinates), it is certain that righting moment will increase (although righting arm might not), and in any other case righting moment will decrease.

3) Under header free surface effect: "This results in a displacement of the centre of gravity of the tank or space relative to the overall centre of gravity." Correct, but a very important issue is ignored. In which direction does the center of gravity move, and does it improve the righting moment or just the opposite? It turns out that in some sailing boats with very high angle of heel (but still less that angle of vanishing stability =AofVS) the righting moment will indeed improve. If this is the case the typical angle of heel is around of 135 degrees and the effect can even improve AofVS. And even better, for practically all sailing monohulls the effect of free surface will help to right the boat from 180 degrees angle of heel. 5.61.93.234 (talk) 18:40, 7 March 2019 (UTC)

Material Copied to Wikiversity

I am working on a robotics course and am copying content to the following site: https://en.wikiversity.org/wiki/Robotic_Mechanics_and_Modeling/Vehicles Admazzeo (talk) 18:03, 18 February 2020 (UTC)