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| There is only one set of letters used to describe stability. G is center of gravity, there is no term called "center of mass." B is center of bouyancy. Metacentric height is M. Always was, always will be.
The working measure of stability is GM the distance between G and M, and as long as M is above G the ship is stable. The closer G and M the longer the roll until when G and M are at the some point the vessel will keep rolling.
Center of bouyancy "B" is, with the exception of submerged submarines, nearly always below G. The location of B changes with heel and draft. It is the location of the force which provides a righting moment when due to heel it moves sideways from G, creating a lever equivalent to the displacement and distance - GZ.
Back to the books old boy.
Yes the posting is getting weird. My quote of your post is all in gray and I cannot edit anything either, line spaces are screwed up as well. |
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Yes, Rick, as I suspected, we were both meaning the same thing. No need for books. The problem was the M's you see. Our B's are the same, but my M was equivalent to your G (in most cases centre of mass = centre of gravity. Your M clearly the vertical distance between the two. As to the lever arm GZ - no argument there. Hey, I wonder if this post will come out as it is supposed to...?
Hey, as an afterthought - the submarine thing is interesting, is that possible because the ballast tanks to submerge are above the centreline, but when completely submerged have an effectively 'neutral' effect on G, (? assisted by the vanes/stabilisers and forward motion), but when vented, returning the vessel to the surface, re-establishing the 'floating' status, B is again above G?
PB
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| Sorry, you must go back to the book. M is not the distance between G and B it is a calculated location defined by the point where a vertical line above B intersects a line drawn from K (keel) through G extending upward toward space.
Vanes and stabilizers do not change the location of B, G, or M. They only influence roll or trim dynamically. Go back to the book and it will be clear.
As far as the submarine goes, the only time B is above G is when it is submerged. In surfaced submarines and practically every other vessel (I can't think of an exception but there probably is one) B is below G. Remember that fact - B is below G. B moves around depending on heel. G does not move except by moving, loading, or offloading weight. |
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Ok Rick - you got me. I Googled the subject in Wikipedia, and yes, my memory of the stability /righting moment diagrams I recalled from my yachting days had somehow become a bit scrambled. The way a deep bulb-keeled yacht gains extra righting from all that weight low down and even moreso when canted to windward takes a bit of getting the head around in terms of this metacentric centre stuff, but finally worked out the effect is gained from increasing the length of that GZ thingie. This Google thing is good - couldn't find it in any book I still had. So pax, ok?
PB.... I wonder..... "..........................
Distance GZ is the righting arm: a notional lever through which the force of buoyancy acts.
Sailing vessels are designed to operate with a higher degree of heel
than motorized vessels and the righting torque at extreme angles is of
high importance. This is expressed as the righting arm (known also as GZ — see diagram): the horizontal distance between the center of buoyancy and the center of gravity.[2]
Monohulled sailing vessels are designed to have a positive righting arm (the limit of positive stability)
at anything up to 120º of heel, although as little as 90º (masts flat
to the surface) is acceptable. As the displacement of the hull at any
particular degree of list is not proportional............ " Yeah - it copied across ok. Now others might get the concept a bit clearer like me. Now why didn't you do this Rick?
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Here is another posting to their blog.
We encountered flat calms and seas/winds a bit bigger than desired. Seas to
25' (my crew a Hawaii veteran swears they were 30') with steady trade winds
of 30-35 kts gave a real test. Nothing in the boats structure
worked, no bulkheads, no drawers or doors that didn't open as always, etc. 2
days of dead on the nose westerlies out of Cabo San Lucas tested our water
tightness and also our teeth. About 50 hours of steep 8-12 ft head seas made
for slow going but again showed SS to be very solid boat. We carried our 1500 lb 15' tender up on the boat deck and she remained firmly attached for the entire passage.
Lots of folks have asked us how our anti roll tank, aka ART, performed. My
crew said it was absolutely amazing. He would not have believed it if
someone told him. An example is that we shut down once a day to check fluids
in the engine/transmission. As most trawlers do SS comes beam to the
wind and seas when she loses way. So each day when we stopped we would sit
parallel to the seas. ART just keeps on working and SS sits without rolling
as the waves pass under us. Works as well in the small seas in the middle of
the passage as well as the large ones at both ends of the passage.
Underway it keeps most of the rolling under 5 degrees with an occasional
roll towards 10 or so. The largest roll was a braking wave that hit us
broadside just about 10 hours from Hawaii. Perhaps a 30 degree roll but I
don't think any type of stabilization other than a Sikorsky heavy lift could
help with those types of waves. The largest following wave stood us on our
end with a pitch angle of perhaps 50-60 degrees. Looking almost straight
down to the bottom. One wave that slapped the stern hard as it broke on us.
No green water aboard at any time fore, beam or over the stern. However we
did have quite a bit of sea foam from some of the tops so we weren't dry out
on the decks for sure!
Oh, I given the shop a drawing of the tank to be built sometime between now and end of October with adjustable baffles to play with. I have a picture and drawing of ART, so its not like I am starting from scratch. Besides if 1500 lb is going to effect the ultra stability of the Eagle that wieghs 80,000+lbs there is a bigger stability problem. I mean filling the water tanks, 400 gallons is like 2,500 lbs or filling the fuel tanks is 1000 gallons or 7,000 lbs. Anyway, ART is an option?
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Greetings,
Um....P/F, your math is off....1 gallon (US) of water weighs 8.34 lbs. 400 gallons would weigh 3336 lbs. NOT 2500 lbs! Remind me to NOT go on the Eagle even if at the dock!
Peter.
ps: You have WAY too much time on your hands. |
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Peter, glad to hear your first leg went well. Thank you for the information. I knew it was 6 to 8 lbs, but too lazy to look it up. Well the heaver the better to counter ART. 
I do not understand why there is such a negative against ART? ART just sits there, requiring no maintenance, and dose its thing weather at the dock or crossing an ocean? I mean ART sounds great. I am also having the plant make the plates required to add stringers to strengthening mast/boom in preparation for installing fish stabilizers which I am not to sure about as I don’t think my wife is strong enough deploy and/or retrieve the fish? 
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Phil/Fill wrote
I do not understand why there is such a negative against ART? ART just sits there, requiring no maintenance, and dose its thing weather at the dock or crossing an ocean? I mean ART sounds great. I am also having the plant make the plates required to add stringers to strengthening mast/boom in preparation for installing fish stabilizers which I am not to sure about as I don’t think my wife is strong enough deploy and/or retrieve the fish?
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Phil, to answer why most are anxious about this method of stabilising a vessel, just go back a couple of posts and have a look at the diagram & explanations re boat stability I copied from Wikipedia. (Thanks to Rick's urging, I must admit). With these ARTs, as you call them, you are in effect just adding weigh high up, raising the C of G, and therefore decreasing the righting arm GZ (did I get that right Rick?), and thereby decreasing the roll period, yes, because there is a lessened righting moment (force) but at the expense of absolute stability. So although more comfortable, if stability was ever thrown into REAL question...... go figure, as they say....
PB
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