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Author Topic: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser  (Read 22588 times)

Talisman

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #50 on: February 07, 2011, 01:38:17 pm »

You can see that happening when people who have hired rowing boats bring them into the bank to get off. They half stand up in the middle causing the boat to oscillate rapidly and they then make matters worse by trying to keep their balance instead of sitting down again. Unfortunately the instinctive reaction is not the right one, sometimes with unwanted consequences....

Colin

OK, Take that idea further...

If we stick a superstructure on our rowing boat does that not have a similar effect to the standing up scenario?

Does the superstructure on our boats not contribute to roll and the need to compensate for the roll ?

Also if we put the weight out to the sides do we have a quicker onset of vanishing stability as the immersed hull shape changes pattern, centre of buoyancy moves etc...?


Rgds
Kim





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Talisman

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #51 on: February 07, 2011, 02:19:41 pm »

should I align on the centre line to avoid ignominy and the scoffing of the locals,
in the event of trying to sail it in say a 10 knot wind? 

Any advice welcomed with a gratitude born of desperation!

El Neave.

Going back to the original question -
The advise sought was for ballasting to allow sailing in a 10knt wind (the upper end of F3 , the point when some sailors might start to think about reefing?)

Does that change the advise given ?
I'd really like to know the correct answer but with so many variables i suspect this isn't the last time that Ballasting and stability are discussed and i look forward to seeing the article - Hopefully that will answer some of my questions questions.
Rgds,
Kim
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Colin Bishop

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #52 on: February 07, 2011, 02:30:36 pm »

There still seems to be confusion here. If you stand up in a rowing boat or add superstructure then you will raise the centre of gravity and reduce stability accordingly. My experiment has nothing to do with that at all. It just shows that if you move the ballast out to the sides of the hull the boat will roll more slowly and thus more realistically. The basic stability will not be affected. I suppose it is possible that if the boat rolls more slowly it may be more susceptible to being clobbered by a wave on the beam which might tend to push it over a bit further before it can recover but that's something else again.

As long as the boat has a good margin of basic stability stays fully watertight and the ballast etc. is firmly fixed down then it shouldn't matter how rough the conditions are within reasonable limits. But extreme conditions could overcome that. Just imagine your model broadside on in moderate surf at the beach. It would literally be picked up and hurled over bodily and not many models are strong enough to cope with something like that. In practice the danger often comes from short sharp waves at one end of a pond which have been kicked up by the wind blowing down the length of the water. Under these conditions the model will tend to be swamped although it will still be OK if 100% watertight. However most models are not 100% watertight in these conditions and often the superstructure is not firmly latched down. Once the superstructure is knocked off that's it! I have seen this happen on a number of occasions - example below.

Colin
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Colin Bishop

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #53 on: February 07, 2011, 02:56:17 pm »

Kim,

Going back to your original question, there is no magic formula. As others have said, if you have a relatively top heavy model then make sure she is ballasted right down to her waterline with the ballast as low as possible in the hull. As my experiment demonstrates, the rolling characteristics will to some extend depend on how you spread the ballast over the bottom of the hull. To play safe spread it evenly over the hull bottom as much as you can as this will hep keep the centre of gravity down compared with piling it up in the middle or at the sides.

If the model is unhappy with the conditions this will become evident as it will roll about all over the place with the decks regularly going under - then you need to think about bringing her in! The tug in the photos above was caught out along with several other models as general conditions on the pond were OK but the wind was causing waves to pile up at one end and these were being reflected back off the concrete surround which produced a localised but very nasty chop. The tug was flung about so much that the superstructure came off and it duly sank. Fortunately it was recovered and proved to be OK when dried out.

Where was it? At Wicksteed, where the Mayhem gatherings are held..... %)

Colin
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jinks8

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #54 on: February 07, 2011, 03:20:04 pm »

You know there is a quick solution to this that is to send Dicky D , Coilin and me tickets to go and help Dickand Colin can help and i will stand guard and keep all the well meaning on lookers from standing on ore nicking bits of the ship when your back is turned and i know what your answer will be  {-) {-) {-) >>:-( >>:-(
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pugwash

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #55 on: February 07, 2011, 03:45:04 pm »

Colin, personally if I were you I wouldn't bother any more, You have explained it in several different ways, all are logical and backed up by
various modelmakers/ books on the subject, but you are still going to have doubting Thomas's.
Geoff
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Mankster

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #56 on: February 07, 2011, 04:34:59 pm »

Well done on carrying out an experiment! I was going to do something similar myself but couldn't find the energy. Could those lead weights at the side of the hull been a little higher due to the hull curvature? If the centre of gravity is raised a little because of this the boat will have a longer roll period as you noticed.  Setting the weight down the centre line is like that is very hard to get the centre of gravity dead through the centre like of the hull as you don't have any significant moment forces to help you with fine tuning by movingthe weigths around. I was thinking about placing lead shot spread evenly along the bottom of the hull, rather than just on the sides.
Yes battle ships are relatively top heavy with deck armour and heavy turret and have longer roll periods. I see if I can hijack my sons bath toy boat which is flat bottomed and repeat the experiment.

Geoff, I am out the correct answer for myself , not to proved anyone right or wrong. Until a few days ago I had done little reading on the subject. Everthing thing I have read on the matter, all the equations given suggest that the Centre of gravity, mass, and Metacentric height are what is important in determing roll stability and period ( assuming no change in hull shape) in a static vessel. Always better to be a "Doubting Thomas" than a Lemming. I have never been one to subscribe to the notion that "this is the way to do it, because this is the way it has always been done". I have yet to see anything in print that explain why placing mass laterally will make any difference (assuming no changing the CoG, yet I have come across plenty of explanations that suggest there is no difference.

Talisman

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #57 on: February 07, 2011, 07:12:19 pm »

I'm still not getting it yet ....

So can someone explain to me why Yachts have keels / weights concentrated in the centre. (i know some have bilge keels leave that bit of learning for another day )
rgds
Kim


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Colin Bishop

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #58 on: February 07, 2011, 07:41:58 pm »

Mankster, you are grasping at straws! The bottom of the QM2 is dead flat and the experiment was quite conclusive. I think you are mixing up two areas of physics here, stability and conservation of angular momentum! Both can result in varying roll periods for quite different reasons and are quite separate. What do you think bilge keels are for? They substitute a resistance for weight at the side of the ship to slow down the rolling period, just two sides of the same coin. I appreciate you have read up on metacentric heights et al and it's all true but it is not the only influence upon rolling periods, check out the science of levers as well.

Kim, I have already explained about yachts. The keel is designed to counter the pressure on the sails to stop the boat blowing flat. If a yacht is dismasted it becomes the most uncomfortable thing on this planet as the weight of the keel with no counterbalance jerks it upright and scrambles the brains of the crew!

Sorry guys but this is all pretty basic stuff really.

Colin
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Talisman

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #59 on: February 07, 2011, 11:35:14 pm »

Ok, Colin i can understand the counter balance idea, my only concern was that a superstructure might present itself as an ''over sheeted sail'' and anything that can be done to help right the boat might be a good thing.

Sorry if this is basic to you Colin but I am still learning,  should we just accept and not question? I'm just surprised that the same physics applied to real boats is equally acceptable to models given that many factors are not scaled to the same degree.
 
In conclusion are we saying that weight spread over the bottom of the hull is an ideal to all types of boat without sails?

Happy to be wrong if we get the right answer...
Regards,
Kim


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Perkasaman2

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #60 on: February 08, 2011, 12:10:53 am »

I remember reading somewhere that a destroyer in WW2  did avoid capsize and 'righted' in spite of a registered  35 degree roll during severe weather.
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RaaArtyGunner

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #61 on: February 08, 2011, 01:10:11 am »

Being thoroughly confused it is time to put in ten cents worth.

The thread became confusing with the seesaw example which is not applicable to boats, ie buoyancy.

Don't think I am a doubting "Thomas" just trying to make sense of opposing view points.

A vessel is not simply supported at one point, the pivot as per the seesaw, which is more appropriate as an example of lever and fulcrum example.

It seems to me that a hull is supported along the entire length of the plank of the seesaw (there's that seesaw again) and not at one point and can also be said is evenly supported along the plank length.

The thread asked about a specific hull, namely a cruiser, it seems to me that the placement of ballast in say the QM, would be different than in the warship, as I take it that a warship has a keel and being "pointy" requires less  load to get down to water line whereas the flat bottom needs more, but we are not talking about amount of ballast but where to place it.

It is evident by the comments that ballasting is not cut and dried and compromises may have to be made such as do you want a boat that steers well or goes fast or a combination.

So examples and commentary of how the hull (in this case the warship) and also say other hull shape eg tugs, would react with ballast in alternate positions is required and the the builder can choose their own weapon of destruction.


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Talisman

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #62 on: February 08, 2011, 01:49:19 am »

Here we go again,
In my basic understanding i would suspect that in most model boats that the actuall CoG is higher than than that dipicted in builders drawings.
Hence my  reason for advising  a more central and lower positioning of ballast to counter act the Cog.


Afterall,  is our primary concern not (given the investment in time and money) to keep the boat upright?
more questions .....
Regards,
Kim
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tigertiger

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #63 on: February 08, 2011, 04:52:47 am »

Observations on this thread.

There are suppositions that we can make, that are inherently wrong. Even if they feel right.
This is where designers and builders can get into trouble.

There was a lot of knowledge among artisans that was based on common practice.
In answer to a question, 'Why do we do it this way?'. The answer would often be, 'We've always done it that way'.
And unless the method changed it was good enough for those who did not have the resources to experiment.

And so there are two distinct questions.
First, what is the best way to ...?
Second, why is ... the best way to ...?

The first will give us a solution to a problem.
The second may give us an answer to a question, or just more questions that we do not know how to answer.

This thread has turned into the latter.

Just my 2 cents.
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Talisman

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #64 on: February 08, 2011, 10:45:14 am »

OK, i have found an article that makes sense without using complicated techy formulas etc -
You can read it here -
http://casanovasadventures.com/catalog/watersports/p40104.htm

Seems to tie in with what has been suggested so far -

So my new understanding goes something like this -

- In model boats righting arm maybe a better theory to observe to improve overall stability?

- Where we place the ballast should be predicted by righting arm and hull shape?

Am i getting somewhere?

All this new theory should help me give new life to a rather tippy boat i have gathering dust lol

Regards,
Kim
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El Neave

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #65 on: February 08, 2011, 12:05:31 pm »

Ah well, said he, scratching his pate, and now to the Open University for a course in hydrodynamics!  Clearly, the penalties of NOT getting things right are grim, as your photos of the tugboat suggested, Colin. But supposing I do
by the sheer laws of probability, manage to get things right, I will think very seriously of issuing tickets to all and sundry since I have the idea of testing the ship out on July 14th with White Ensigns flying from fore and main;  This is likely to incense the St Germanois mightily. Protection will almost certainly be needed.  Support for this provocative act from Mayhemers will be greatly appreciated!

Before that, the building of a test tank is obvious.  Better than a bath any day and more so if one remembers the immortal words of the late O. Nash
"I test my bath before I sit
and it always moves me to wonderment.
That which chills the finger not a bit
Is so frigid to the fundament."

Test tanks do away with this form of prior verification!

And finally just as a gesture to all who have cudgelled their wits on the matter of ballasting and its location, here is a pict of the ship. There will be cries of horror at the White Ensign flying on an American hull.  But, curiously after
twenty years spent over here, one grows strangely patriotic.  Accordingly I have renamed this Ship.  In my flotilla it will be HMS Quiberon Bay - a strategically highly significant victory on November 20th 1759 - some say almost as
important as Trafalgar - at least for us and the Canadians, though the Québecois will not take so even handed a view. The same battle is known over here as the bataille des Cardinaux, they being the rocks off which it was
fought on the Britanny coast.  Not surprisingly, few if any over here remember it!

El Neave.

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derekwarner

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #66 on: February 08, 2011, 12:47:51 pm »

Thanks..... O0 about blu##y time we were invited to see .jpgs of the vessel..... >>:-( <*< why not post more?,,,,,,,,  O0 Derek
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Talisman

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #67 on: February 08, 2011, 01:17:10 pm »

The only real way to do this is to put the completed boat in water along with installed batteries and then ballast it down to the waterline.


Test tanks do away with this form of prior verification!



As i said earlier maybe that is the best piece of advise given - maybe that should be the stock answer the next time stability is questioned.
Do as we have allways done and not question...

Bon Chance! with her launch she looks good and i hope she performs on the water as you wish.

Regards,
Kim
 

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unbuiltnautilus

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #68 on: February 08, 2011, 01:17:48 pm »



And finally just as a gesture to all who have cudgelled their wits on the matter of ballasting and its location, here is a pict of the ship. There will be cries of horror at the White Ensign flying on an American hull.  But, curiously after
twenty years spent over here, one grows strangely patriotic.  Accordingly I have renamed this Ship.  In my flotilla it will be HMS Quiberon Bay - a strategically highly significant victory on November 20th 1759 - some say almost as
important as Trafalgar - at least for us and the Canadians, though the Québecois will not take so even handed a view. The same battle is known over here as the bataille des Cardinaux, they being the rocks off which it was
fought on the Britanny coast.  Not surprisingly, few if any over here remember it!

El Neave.




Interestingly, the Royal Navy was offered the first four of the class, when the delays set in on the Type 45 project, so you can legitimately fly the White Ensign as a ' What If' project. Also, its good to see some practical application of the theorys being discussed here.
I had a small destroyer which had to be fitted with a ballast weight on a removable rod, about 6" below the hull ( the model rolled over and sank on its first outing, a result of all that is being discussed here!). A gentleman watching the model commented on the models tendency to heel in 'all the wrong ways' on a turn. When I pulled the model out of the water to reveal the 'yacht like' keel hanging under the model, his statement was "that would explain it..." However wrong this solution was, it did result in a 39" x 4" model that could be run in a near Force 8 gale. Great Fun :-))
We staged a Battle Of Quiberon Bay event at Westerham in Kent two years ago, on the 200th Anniversary of the battle. We were the fleet out at sea, while 300 re-enactors were shooting muskets at each other on the shore, it was like Last of the Mohicans.....
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Colin Bishop

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #69 on: February 08, 2011, 02:04:00 pm »

Quote
as I take it that a warship has a keel and being "pointy" requires less  load to get down to water line whereas the flat bottom needs more

Most warships have flat bottoms, they just tend to have more slender hulls than merchant vessels although there are exceptions in both cases. Certainly high superstructure will increase windage and act as a sail which will tend to push the model over when the wind blows.

But having built the boat, the options, are strictly limited. You can only add ballast sufficient to bring the model down to her scale waterline although in practice you can often get away with just a bit more as an extra half inch or so may not be noticeable except in still water. In fact you could even cheat a bit and paint the waterline a little bit further up the hull, no one is likely to notice and it may give you just that little bit extra you need to keep the model stable.

Having determined how much ballast the model will take then this needs to be placed as low as possible in the hull. Where outright stability is needed then lead sheet across the bottom of the hull is the best bet as it will keep the centre of gravity as low as possible plus you will probably get a decent roll period too as the weight isn't concentrated in the middle of the hull.

The only further measure you can take is to 'add lightness'. As has already been suggested, is there any opportunity for trimming surplus material from within the superstructure to reduce weight? Any such gains can be doubled by placing an equivalent weight to that removed in the bottom of the hull as extra ballast.

If none of this is sufficient then the only alternative may be an external keel.

Colin
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Perkasaman2

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #70 on: February 08, 2011, 02:24:13 pm »

My compliments to El Neave. You are a gentleman............... and a scholar.  :-)
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RaaArtyGunner

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #71 on: February 08, 2011, 08:26:56 pm »

Most warships have flat bottoms, they just tend to have more slender hulls than merchant vessels although there are exceptions in both cases. Certainly high superstructure will increase windage and act as a sail which will tend to push the model over when the wind blows.

But having built the boat, the options, are strictly limited. You can only add ballast sufficient to bring the model down to her scale waterline although in practice you can often get away with just a bit more as an extra half inch or so may not be noticeable except in still water. In fact you could even cheat a bit and paint the waterline a little bit further up the hull, no one is likely to notice and it may give you just that little bit extra you need to keep the model stable.

Having determined how much ballast the model will take then this needs to be placed as low as possible in the hull. Where outright stability is needed then lead sheet across the bottom of the hull is the best bet as it will keep the centre of gravity as low as possible plus you will probably get a decent roll period too as the weight isn't concentrated in the middle of the hull.

The only further measure you can take is to 'add lightness'. As has already been suggested, is there any opportunity for trimming surplus material from within the superstructure to reduce weight? Any such gains can be doubled by placing an equivalent weight to that removed in the bottom of the hull as extra ballast.

If none of this is sufficient then the only alternative may be an external keel.

Colin

:-)) :-)) :-)) O0 O0 O0
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El Neave

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #72 on: February 09, 2011, 10:09:47 am »

Your every command, Derekwarner, is my whim. I will have a shot  sending a few more picts of the Quiberon Bay.  Tried yesterday, but the limitations on Mayhem' server meant that, like Mr Jagger, "I don't get no satisfaction" and
also like Mr Jagger, "I have tried, I have tried" (latter to be read in a high stragulated falsetto!) javascript:void(0);  Unbuiltnautilus,  so you fired off muskets in celebration of the 250 anniversary pf Quiberon Bay.  My heartiest congratulations on your surviving the event.  By the way, how many models (scrub that, I mean ships, unless you were also marking some definitely pagan festival) did you sacrifice? 

And now for a further display of incompetence!

Best,
El Neave;

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Talisman

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #73 on: February 09, 2011, 10:17:44 am »

Sorry to keep quoting from you Colin It's not personal, just you seem the most willing to debate the matter.



If none of this is sufficient then the only alternative may be an external keel.

Colin



Could we not think about pinching a bit of technology from one of the other R/C disciplines and maybe start thinking about Active Ballasting?
Now that could help fill a page or two in your Magazine?
Regards,
Kim

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Perkasaman2

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Re: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser
« Reply #74 on: February 09, 2011, 12:03:01 pm »

The self-righting  pendulum analogy may be better understood if it is considered as  double ended - the superstucture or mast/sail above and the machinery mass or keel as the bottom weight. The pivot point of this pendulum is the hull  at it's waterline. Viewed in cross section the pendulum is a vertical axis perpendicular to the horizontal water line. The roll rate of the model is dependant on the relative values of these opposing  weights and their distance from the pivot.  :-)
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