C Of G of ships

Force down through CofG. Force up through CofBouyancy. CofB moves when vessel rolls. These forces are vertical. The lower the CofG the greater will be the righting moment for any CofB. The greater the righting moment, the greater will be the speed at which the vessel rolls, ie the shorter the roll period. To short a roll period the motion may be "whippy" back and forth. Vessel stiff. The higher the CofG the shorter the righting moment the slower the roll and the longer the roll period. Motion described as tender. May also be uncomfortable (and disconcerting). Metacentric height is a measure of initial static stablity. At a glance this site illustrates graphically. Initial Metacentric Height - an overview | ScienceDirect Topics
 
MJWB said:
Force down through CofG. Force up through CofBouyancy. CofB moves when vessel rolls. These forces are vertical. The lower the CofG the greater will be the righting moment for any CofB. The greater the righting moment, the greater will be the speed at which the vessel rolls, ie the shorter the roll period. To short a roll period the motion may be "whippy" back and forth. Vessel stiff. The higher the CofG the shorter the righting moment the slower the roll and the longer the roll period. Motion described as tender. May also be uncomfortable (and disconcerting). Metacentric height is a measure of initial static stablity. At a glance this site illustrates graphically. Initial Metacentric Height - an overview | ScienceDirect Topics
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I'm confused by this, the paper refers to conventional ships, where the centre of mass is above the centre of buoyancy. This is in effect an inverted pendulum and the vessel is more stable upside down, it has an angle of heel where it has vanishing stability, the same is true for a cat or tri'. The vessel only stay the rightway up because of form stability.

A yacht is different, the centre of gravity is below the centre of buoyancy, and provides a passive righting moment, in addition to the form stability of the hull, which is much more dynamic. A yacht is much more like a conventional pendulum, and in a simplistic sense the roll frequency is near to the classic pendulum, roll period= 2 x Pi x square root of length of the pendulum divided g (9.81 N/kg). This tells us that the roll frequency is fundamentally lower for longer pendulums.
But yachts are much more complicated than this, there are more static and dynamic factors at play.
Form stability, which changes as the boat pitches and rolls, both as a results of the forces the sails apply and the waves.
The angular inertia is important, the higher the angular inertia the more energy the vessel stores as it pitches and rolls, which makes it slower to accelerate and decelerate.
The damping, from the sails, keel and hull form also play a major roll.
On a yacht, you need to keep the weight as low as possible, and have rig which is a light as possible. That will lower the fundamental roll frequency and improve the sailing performance. It also increase the moment of inertia, so it slows things down.
If you want a comfortable boat you need to increase the roll damping, here a large keel area really helps, but at the expense of drag, if you want a fast boat you need to keep the keel area as low as possible.
A comfortable boat will tend to have good form stability, but that increases the wetted area which adds more drag in a displacement mode, and makes the boat more sensitive to wave action.
This is a complex set of compromises, there's no single solution.
When a boat loses it's mast, the righting moment increase and the fundamental frequency goes down, but it also loses all of the damping effect of the mast rigging and sails and some of its angular inertia, so the boat is lot stiffer, and reacts faster to correct the roll and pitch caused by waves.
 
Buck Turgidson said:
From my understanding, the larger the metacentric height the shorter the rolling period and hence the motion being less comfortable. So whist it gives greater initial stiffness it isn't always a good thing. Only really a static issue so once you are shifting the game changes.
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I do not think that GM alone is the single determining factor but rather the radius of gyration. Of two (very) similar vessels, one with greater beam, hence a greater GM, yes, the beamier one would have a quicker roll. But, give it a higher/heavier mast while increasing it's ballast, this would not change the boat's GM, however it would increase it's radius of gyration and slow the roll period.

You could also confer weight to the beam ends to increase the radius of gyration, as is indeed frequently done on fishing vessels and for this very purpose. This would have very little to no effect on GM.
 
Ian_Edwards said:
On a yacht, you need to keep the weight as low as possible, and have rig which is a light as possible. That will lower the fundamental roll frequency and improve the sailing performance. It also increase the moment of inertia, so it slows things down.
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A light rig and a low CoG does indeed improve sailing performance. It does not , however increase the radius of gyration and hence lower the roll frequency. Reducing the weight of the rig does precisely not increase the moment of inertia and "slow things down".

This is indeed, as you have intimated, the great dilemma of designing a fast, safe and, to be sure, comfortable sailing boat.

A designer once told me: you can have any two of the above qualities (in their absolute sense), but not all three.
 
Ian_Edwards said:
A yacht is different, the centre of gravity is below the centre of buoyancy, and provides a passive righting moment, in addition to the form stability of the hull, which is much more dynamic.
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I'm sorry, but it is a fallacy to assume that the CoG in a yacht is below the centre of buoyancy. That might have been the case way back in the day when yachts were very narrow and deep, and even then only in rare cases. However, in most modern yachts the CoG is not only considerably above the CoB, but also above the DWL.
 
Daydream believer said:
I was reading a forum question on another forum by a member who had aquery about how to determine the displacement of a model tug he wanted to build. Like all forum subjects the thread wandered a bit & the following statement was made
"Full size shipwrights have to go an extra step. It's undesirable for all the weight in a ship to be too low because it results in violent rolling and everyone gets sick! By lifting the weight the ship becomes stiffer and rolls less. But not too high, because it risks capsize. Difference between the centre of gravity and the centre of buoyancy. Look up metacentric height if it's of interest. Probably doesn't matter how much a model rolls, and maybe the original had all the weight low down. Tug boats weren't built for crew comfort!
I find it amazing to suggest that the weight of a ship should not be too low. Does the above statement have merit? Because my yacht has a keel with a bulb on the bottom & in a sea with no sails up it sits sideways on & rolls like a pig. Is this the reason? Do yachts with a higher C of G roll less when just left to drift?
Do ships really need a higher C Of G?
What do forumites think? any one know or have a theory
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Sort of. As written it’s a bit mixed up.

The term stiff refers to a high tendency to resist heel or roll initially and to recover quickly, a stiff vessel will typically roll quickly and be quite uncomfortable.
excessively stiff may cause damage so.

typicaly a stiff vessel has a large GM distance between G centre of gravity and M the meta centric height.

A ship loads with ore, typically heavy will the ship will be full by weight with low volume the weight of cargo low down . The ship will be relatively stiff. Ore cargoes are often loaded in alternate holds raise G and be less stiff.

A ship loaded possibly the same ship with grain which is relatively light and will fill the entire volume of the holds and the G will be relatively high and the ship relatively tender, so much so there are special rules for loading grain.

A tender ship has a relatively low resistance to being beveled or rolled and will typicaly recover slowly from a roll and will have a more comfortable motion.

Load the same ship with a timber cargo and put some on deck. Again special rules for loading timber. The centre of gravity may be high enough for the ship to be initially slightly unstable. ( gets complicated the center of buoyancy B moves when heeled) resulting in an angle of loll. ( When B drops back bellow G. The ship will be stable again 2 or 3 deg from upright)
And very tender,
Timber is buoyant and when lashed on deck you don’t want to roll quickly.

Same goes for containers, Stiff not so good tender better.

passenger ships are traditionally built tender. Therefore more comfortable.

Ferries often tend to be stiff. So resistant to being heeled by of center vehicle’s . But less comfortable.

A tug I don’t know, I would suspect, big engines low down will lead to large low center of gravity large GM relatively stiff.
probably desirable to resist being heeled by the pull from a off center tow line. That’s a guess. I don’t know much about tugs.

For yachts the same general principles apply, different properties desirable in different vessel’s.
A motor yacht comfort is probably more desirable.

Racing sailing yacht, low centre of gravity to resist force of wind heeling the vessel is probably desirable.

Cruising sailing vessel somewhere in between. The damping effect of sail improving comfort.

Take the sails down, go out on a rough day, you can expect it to be stiff and less comfortable than a motor yacht.

Horses for courses.
 
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Laminar Flow said:
I'm sorry, but it is a fallacy to assume that the CoG in a yacht is below the centre of buoyancy. That might have been the case way back in the day when yachts were very narrow and deep, and even then only in rare cases. However, in most modern yachts the CoG is not only considerably above the CoB, but also above the DWL.
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Well that surprises me!

But here it is explained:
Center of Gravity and Buoyancy

BUT Just looking at that drawing again and that is for a boat without a keel.
 
Laminar Flow said:
A light rig and a low CoG does indeed improve sailing performance. It does not , however increase the radius of gyration and hence lower the roll frequency. Reducing the weight of the rig does precisely not increase the moment of inertia and "slow things down".

This is indeed, as you have intimated, the great dilemma of designing a fast, safe and, to be sure, comfortable sailing boat.

A designer once told me: you can have any two of the above qualities (in their absolute sense), but not all three.
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There is a big “a but!” about heavier rigs on cruising yachts - which often had heavy furling main mandrels, radar etc etc onto a rig that may not have been designed for it originally.
These can raise the CoG considerably over what the designer intended. And a high CoG and high moment of inertia will slow the rate of roll - but will also slow the cessation of the roll. So if a boat with a heavy rig gets a serious force to start rolling, it will keep on rolling to a much larger angle of heel - which can be very disconcerting for crew in moderate conditions, and potentially leading to a knockdown in serious conditions. Plus it will reduce the Angle of Vanishing Stability, AVS, quite considerably.

So putting a heavy rig on a boat is not necessarily a good thing.
 
dunedin said:
There is a big “a but!” about heavier rigs on cruising yachts - which often had heavy furling main mandrels, radar etc etc onto a rig that may not have been designed for it originally.
These can raise the CoG considerably over what the designer intended. And a high CoG and high moment of inertia will slow the rate of roll - but will also slow the cessation of the roll. So if a boat with a heavy rig gets a serious force to start rolling, it will keep on rolling to a much larger angle of heel - which can be very disconcerting for crew in moderate conditions, and potentially leading to a knockdown in serious conditions. Plus it will reduce the Angle of Vanishing Stability, AVS, quite considerably.

So putting a heavy rig on a boat is not necessarily a good thing.
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There is a case of balance...
You are correct, simply adding weight aloft without further consideration is poor practice and in fact one of my pet peeves.
But, the weight of a rig is simply just one of the factors in regards to a boats propensity to roll, the other aspects concern roll attenuation.
Here, hull shape and keel area and aspect ratio, boat speed and sail area play significant rolls (pardon the pun).

For further reading I highly recommend Marchaj, "Seaworthiness, the Forgotten factor".
 
Even aside from rolling, you do not want a sea boat to remain perpendicular to the surface, because the sea surface ain't flat: so there's a limit to the value of form stability in particular.

I have a Canadian canoe which is essentially flat bottomed, and hence has very high 'initial stability' but is (aside from being hard work to paddle and to turn) most inappropriate for rough water - you don't want to be perpendicular to the front of a wave!

By contrast one of my sea-kayaks has negligible initial stability and effectively depends on the paddler to keep it upright. It's great in rough water (fast, and easily propelled and turned) but inappropriate for a beginner and hazardous if the paddler becomes incapacitated.

As Laminar Flow ands others have suggested, there's always a balance to be struck.
 
Buck Turgidson said:
no I'm not a troll and more than willing to learn so please explain what is wrong with my statement.
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Sorry, I was not implying you are one.. I just didn't want this to degenerate into a furious keyboard war of words.

Your semicentrical hull form with a CG lower than CB will have stability but not form stability.

As pointed out, many vessels have CG above CB and rely on form stability.

Have you read up on initial stability and metracentric height?
 
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davidej said:
Sorry, I was not implying you are one.. I just didn't want this to degenerate into a furious keyboard war of words.

Your semicentrical hull form with a CG lower than CB will have stability but not form stability.

As pointed out, many vessels have CG above CB and rely on form stability.

Have you read up on initial stability and metracentric height?
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OK, thanks.

So it very much does have form stability just not as much as it could have were it to have a shallower dead rise. Any time the CB is offset by heel you create a moment due to the "form" of the hull, hence form stability. For it to have "ZERO" form stability it would need to have GM=0 which is the point I was trying to make.
Sorry for the lack of clarity in my post.

EDIT: See #20 for metacentric height. I assume your "metRracentric" was a typo.
 
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Buck Turgidson said:
OK, thanks.

So it very much does have form stability just not as much as it could have were it to have a shallower dead rise. Any time the CB is offset by heel you create a moment due to the "form" of the hull, hence form stability. For it to have "ZERO" form stability it would need to have GM=0 which is the point I was trying to make.
Sorry for the lack of clarity in my post.

EDIT: See #20 for metacentric height. I assume your "metRracentric" was a typo.
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As I understand it, form stability is the name given to stability caused by the form (shape) of the hull as distinct to the stability caused by the position of the CG. When the vessel has the shallower deadrise you refer to, the CB moves as the vessel heels, This allows vessels with CG above the CB to be stable. This will not occur with a semicircular hull form .

We may just be differing on definitions so l shall let it rest to avoid the sort of bickering that has been complained, about elsewhere.
 
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