C Of G of ships

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Daydream believer

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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
 
I think this could be a bit like the effect a mast has. The mast creates inertia and of course sort of raises the c of g. I know when I motored my boat down Southampton water sans mast and went over some ferry wash it was VERY uncomfortable and disconcerting. That super stability actually meant she developed a pendulum motion. Others who have lost masts at sea will no doubt testify to the reduced stability
 
Yes, it's a well-known phenomenon that too low a CoG gives an uncomfortable motion. I've never really considered the physics behind it, but it's not controversial.

As Matt says, the rig on a yacht means the CoG is a bit higher than you might think by considering only the hull.

Pete
 
A mast carries weight up high which creates that stability, not in preventing capsize, but reducing the rate of rolling.
There has to be a compromise between the two.
 
. [QUOTE= 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 [/QUOTE said:
Your boat "rolls like a pig" when no sail is hoisted is more to do with the lack of damping due to air pressure on the sail as the boat rolls. 'Tis true that weight of the hoisted sails will also raise the CG which reduces metacentric height which in turn promotes a slower and more comfortable roll but not neccesarily a lesser angle. This is most probably a lesser effect on crew comfort though compared with the damping effect of the sails.
Click to expand...
 
So does a bilge keeler or shallow draft keel with bare poles drifting give a more comfortable motion than a same design with a deep fin keel in the same situation?
Does a shallow draft motor boat roll less than the yachts of similar size if left to drift in the same sea?
 
Apart from the wave period, having a higher mass such as a mast or even wooden pole slows the rate of acceleration due to the larger force needed to make it move, the moment of inertia mentioned above. This slows the acceleration of the roll and is likely to feel more comfortable rather than a quick sharp roll which might be tiring.
 
It has never happened to me, yet, but I have heard that the motion of a dismasted boat can be pretty unpleasant. I expect, though, that the move to carbon fibre rigs and light weight aloft has little effect compared to the damping affect of sails. Hull shape and keel area have a lot to do with roll behaviour. My 1.5m keel leads to more roll than our sister-ships with 1.8m, though performance is otherwise very similar.

I also believe that the metacentric height of many ships can be very small, though from the look of some one might expect it to be negative.
 
But lowering the centre of gravity (assuming all other things equal, which they rarely are!) further below the centre of buoyancy will presumably have a similar effect to raising a mast or other weight above it: i.e. slowing the rate of roll, while increasing the extent of roll.

In answers to the question raised by Daydream Believer, the lowering of the weight in the deep fin keeler will not be the only difference from the shallow bilge keeler - the area of the fin itself will significantly dampen rolling. The shallow bilge keeler will also probably have been given more form stability (a mixed blessing, like all the other factors) to make up for the (relative) lack of 'weight' stability.
 
Daydream believer said:
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.
Click to expand...
This is backwards. Raising the CofG makes the ship more tender, reducing the roll rate and so making the motion more comfortable (and less likely to break the lashings on the cargo). The amplitude of roll is increased, eventually to the point where capsize is a risk, so as ever there is a balance to be struck.
 
Large portions of Marchaj's treatise on seaworthiness concern itself precisely with this phenomena. Larger, heavier rigs have greater inertia and, consequently require a greater amounts of energy to incite rolling and or cause sudden capsize.
Countering the inertia build up of the rig/ballast pendulum, are hull shape, i.e firm versus slack bilges and areas of keel or fin appendages and sail, if set.

Tests have shown that vessels with little or no salient keel (area) are much more prone to capsize.

In a real life case study:
I used to own a large lifting keel and heavy displacement cutter. She was quite beamy as well.
Without sails and with the 5ton keel raised i.e with a considerable raise in CoG, she would roll quite alarmingly (AVS with the keel raised over 110 degr.) Once the keel was lowered, the rolling subsided to an acceptable (tolerable) level. The rolling was effectively stopped or brought under control by the dampening resistance of the keel.

In many modern boats keel areas have been declining, while rigs have been getting lighter. Ballast is carried in a bomb at the lowest possible point to create the greatest possible RM. This may create an unbalanced couple in the pendulum equation, particularly as very short fins with a high aspect ratio are much more prone to stalling than a low aspect fin or a long keel and thus present much less resistance to rolling.
 
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Good article.
However, it is primarily concerned with hydrostatics and not so much the dynamic relationships that this thread is concerned with.

Stall resistance of low aspect profiles and the roll dampening ability of a substantial underwater areas are, in this context, the single most beneficial aspect of longer and long keels. This is really only of consideration in (survival) conditions that the average yachtsman, even a long distance, ocean crossing sailor, is unlikely to encounter.
 
Laminar Flow said:
Good article.
However, it is primarily concerned with hydrostatics and not so much the dynamic relationships that this thread is concerned with.

Stall resistance of low aspect profiles and the roll dampening ability of a substantial underwater areas are, in this context, the single most beneficial aspect of longer and long keels. This is really only of consideration in (survival) conditions that the average yachtsman, even a long distance, ocean crossing sailor, is unlikely to encounter.
Click to expand...
.

This is something I've thought about often with the yacht I am building.

I have a heavy mast and long keel. To compensate for the heavy mast I have used lead for ballast instead of iron.
I suppose I won't know how it sails until I try it.....? :unsure:
 
Laminar Flow said:
Large portions of Marchaj's treatise on seaworthiness concern itself precisely with this phenomena. Larger, heavier rigs have greater inertia and, consequently require a greater amounts of energy to incite rolling and or cause sudden capsize.
Countering the inertia build up of the rig/ballast pendulum, are hull shape, i.e firm versus slack bilges and areas of keel or fin appendages and sail, if set.

Tests have shown that vessels with little or no salient keel (area) are much more prone to capsize.

In a real life case study:
I used to own a large lifting keel and heavy displacement cutter. She was quite beamy as well.
Without sails and with the 5ton keel raised i.e with a considerable raise in CoG, she would roll quite alarmingly (AVS with the keel raised over 110 degr.) Once the keel was lowered, the rolling subsided to an acceptable (tolerable) level. The rolling was effectively stopped or brought under control by the dampening resistance of the keel.

In many modern boats keel areas have been declining, while rigs have been getting lighter. Ballast is carried in a bomb at the lowest possible point to create the greatest possible RM. This may create an unbalanced couple in the pendulum equation, particularly as very short fins with a high aspect ratio are much more prone to stalling than a low aspect fin or a long keel and thus present much less resistance to rolling.
Click to expand...
There are a lot of factors to consider.
The damping of the keel is very significant, but so is the hull form.
Some dinghies are really quite tippy with the centreboard right up and the boat not moving. Get the boat moving at even a few knots and it changes.
Weight aloft is like a longer pendulum, it lowers the roll frequency, but for a given amount of damping that can increase the amplitude.
If you've ever sailed the same dinghy with both ali and carbon masts, or even a wooden mast, you'll probably prefer the lighter rig.

I think modern ocean racers have an element of this, they are safer when they keep moving under control. lying ahull is not a great option for them.

Boats like small offshore fishing vessels are the best example of 'designed to roll'. They could easily be made 'stiffer' but the crew would be thrown around too much. A long slow roll is less acceleration.
 
coopec said:
.

This is something I've thought about often with the yacht I am building.

I have a heavy mast and long keel. To compensate for the heavy mast I have used lead for ballast instead of iron.
I suppose I won't know how it sails until I try it.....? :unsure:
Click to expand...
Best of luck, Coopec. I hope that she turns out to be all you have dreamed of.
 
TernVI said:
I think modern ocean racers have an element of this, they are safer when they keep moving under control. lying ahull is not a great option for them.
Click to expand...
I would agree. This is precisely the situation in which and in turbulent water, a high aspect keel stalls and loses much of it's dampening effect.
 
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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