Talk:Metacentric height

From Wikipedia, the free encyclopedia

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)

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.