Positioning Ballast for Stability

[WotEver]

13 minutes ago, Keeping Up said:

Our narrow boat has no ballast because the base plate is thick & heavy enough that we do not need any. When we had to have the sides overplated, from base to waterline, it added about a ton and a half to the weight, so the boat now floats about an inch and a half lower in the water. I wonder what were the effects on the levels of the centre of gravity and the roll centre? I am certain that since the work was done, the boat rolls more when the crew moves her (substantial) weight from centre to side and back as she walks down the boat. I would love to understand this further.

The CofG has been raised as the weight is no longer focused on the base plate, hence what you experience is to be expected. 

[Keeping Up]

19 minutes ago, WotEver said:

The CofG has been raised as the weight is no longer focused on the base plate, hence what you experience is to be expected. 

I can't get my brain around it properly. When weight is added below the CoG but above the base, does it raise or lower the CoG? My instinct says it lowers it. Or does it?

 

Also, if the boat is floating lower in the water, doesn't that raise the roll centre relative to the boat? Or does it?

 

Which of these is true? And as to the combined effect of the two ... my brain keeps crashing and when I press my mental ctrl-alt-delete I have to restart.

[WotEver]

57 minutes ago, Keeping Up said:

I can't get my brain around it properly. When weight is added below the CoG but above the base, does it raise or lower the CoG? My instinct says it lowers it. Or does it?

 

Also, if the boat is floating lower in the water, doesn't that raise the roll centre relative to the boat? Or does it?

 

Which of these is true? And as to the combined effect of the two ... my brain keeps crashing and when I press my mental ctrl-alt-delete I have to restart.

I'd have thought that although the centre of bouyancy has lowered, the centre of gravity is now higher than it was (in relation to the CofB),  and the higher the CoG is, the more 'tippy' the boat.  Just like putting bags of coal on the roof.  But maybe I'm wrong, I'm no naval architect.

1 hour ago, Keeping Up said:

When weight is added below the CoG but above the base, does it raise or lower the CoG? My instinct says it lowers it. Or does it?

I agree with your instinct (we could both be wrong), but it's the relationship between the CofG and the CofB that's relevant here. 

 

I think...

[Awayonmyboat]

The centre of buoyancy of a box shaped vessel (which to all intents describes a narrow boat hull) will be exactly half way between the waterline and the underside of the bottom plate. The centre of gravity of the boat before modification would have been somewhat lower than this - or you would have capsized. When the overplating was done on the sides it would have added new material with a centre of gravity roughly the same height as the original centre of buoyancy, that is above the original centre of gravity. So when the new plating was added centre of buoyancy went down a little (due to sinking deeper in the water) and centre of gravity went up slightly (new steel CoG above original CoG). Net result the two are now closer and the boat indeed has become slightly less stable. 

[Murflynn]

9 hours ago, Awayonmyboat said:

The centre of buoyancy of a box shaped vessel (which to all intents describes a narrow boat hull) will be exactly half way between the waterline and the underside of the bottom plate. The centre of gravity of the boat before modification would have been somewhat lower than this - or you would have capsized.

wrong.  In many boats the CoG may be higher than the CoB.  The critical issue is the position of the metacentre.  This link explains it better than I ever could:

 

https://www.marineinsight.com/naval-architecture/intact-stability-of-surface-ships/

[john6767]

I would say that the stability of any boat, even a narrowboat, is down to the relationship between the centre of gravity, the centre of buoyancy, and the resulting  metacentric height.  It is perhaps worth a search on here as this has been discussed before.  There are plenty of resources on the net though, for example https://en.m.wikipedia.org/wiki/Metacentric_height  explains it simply with some clear diagrams.

 

So what you need to do is think about what the adding of mass to a boat at various locations does to those parameters, and therefore what the impact is on the righting moment generated when the boat heels over.

Edited June 4, 2020 by john6767

[nicknorman]

On 02/06/2020 at 14:32, yabasayo said:

Sorry old chap but there is some unnecessary confusion here. 

Given that everything else is constant (fixed), Inertia is directly proportional to mass (of the whole displacement, not just the ballast) Therefore, yes, by adding ballast you are increasing the inertia slightly.

However, as discussed earlier, the whole mass can be considered to act through a single point. So moving the ballast around while maintaining zero list and the required trim, ie moving it symmetrically around the current centre of gravity, alters nothing.

 

Static vs dynamic. You are correct for the static case. Eg the boat is level, you step on the gunnel, the boat settles at a certain angle regardless of where the ballast is located, so long as they centre of mass is in the same place. But the dynamic cases are different between all the ballast at the middle, vs all the ballast at the edges. In the former case, the boat will rock more quickly, in the latter case, more slowly.

[dmr]

43 minutes ago, john6767 said:

I would say that the stability of any boat, even a narrowboat, is down to the relationship between the centre of gravity, the centre of buoyancy, and the resulting  metacentric height.  It is perhaps worth a search on here as this has been discussed before.  There are plenty of resources on the net though, for example https://en.m.wikipedia.org/wiki/Metacentric_height  explains it simply with some clear diagrams.

 

So what you need to do is think about what the adding of mass to a boat at various locations does to those parameters, and therefore what the impact is on the righting moment generated when the boat heels over.

That isn't answering the question, that is just re-stating the question with a few bigger/more technical words thrown in.?

 

................Dave

[dmr]

44 minutes ago, nicknorman said:

Static vs dynamic. You are correct for the static case. Eg the boat is level, you step on the gunnel, the boat settles at a certain angle regardless of where the ballast is located, so long as they centre of mass is in the same place. But the dynamic cases are different between all the ballast at the middle, vs all the ballast at the edges. In the former case, the boat will rock more quickly, in the latter case, more slowly.

But static stability does not usually exist in isolation on a boat, because a boat is a dynamic thing (except on the K&A ? ). The boat moves and the people on board move. Static stability would be the more relevant thing if we were building a pontoon and planned to locate a big shed at one side of it.

 

...............Dave

[Keeping Up]

17 minutes ago, dmr said:

That isn't answering the question, that is just re-stating the question with a few bigger/more technical words thrown in.?

 

................Dave

Indeed. The referenced article is illuminating, but I do not know where my Centres of Gravity and Buoyancy are, nor where the metacentre is; and I do not know their positions relative either to the water line (hence to the added weight of the overplating) or to each other (except their order of occurrence as shown in the diagram). So I am still as unsure as I ever was.

[WotEver]

1 hour ago, nicknorman said:

But the dynamic cases are different between all the ballast at the middle, vs all the ballast at the edges. In the former case, the boat will rock more quickly, in the latter case, more slowly.

Hence the comments made by Alan De E and myself earlier - think of a dumbell or a tightrope walker's pole.

[Murflynn]

1 hour ago, nicknorman said:

Static vs dynamic. You are correct for the static case. Eg the boat is level, you step on the gunnel, the boat settles at a certain angle regardless of where the ballast is located, so long as they centre of mass is in the same place. But the dynamic cases are different between all the ballast at the middle, vs all the ballast at the edges. In the former case, the boat will rock more quickly, in the latter case, more slowly.

well at least someone understands 

[WotEver]

32 minutes ago, Keeping Up said:

Indeed. The referenced article is illuminating, but I do not know where my Centres of Gravity and Buoyancy are, nor where the metacentre is; and I do not know their positions relative either to the water line (hence to the added weight of the overplating) or to each other (except their order of occurrence as shown in the diagram). So I am still as unsure as I ever was.

I believe my clumsy explanation and the clearer one below answered your question, did it not?

11 hours ago, Awayonmyboat said:

... when the new plating was added centre of buoyancy went down a little (due to sinking deeper in the water) and centre of gravity went up slightly (new steel CoG above original CoG). Net result the two are now closer and the boat indeed has become slightly less stable. 

 

[Murflynn]

32 minutes ago, Keeping Up said:

Indeed. The referenced article is illuminating, but I do not know where my Centres of Gravity and Buoyancy are, nor where the metacentre is; and I do not know their positions relative either to the water line (hence to the added weight of the overplating) or to each other (except their order of occurrence as shown in the diagram). So I am still as unsure as I ever was.

all the data is available to you if you are prepared to do a few measurements and a few educated estimates of your boat's dimensions, component weights and live loads (humans, dogs, water, fuel, beer, etc.) - there is nothing secret about it. 

 

of course if you are not of a mathematical bent it may all seem a bit overwhelming 

 

 

 

 

[Keeping Up]

3 minutes ago, WotEver said:

I believe my clumsy explanation and the clearer one below answered your question, did it not?

 

I thought they did until the posts about metacentre revealed that the CoG is probably ABOVE the centre of buoyancy hence the new weight may actually be below the original CoG

2 minutes ago, Murflynn said:

 

of course if you are not of a mathematical bent it may all seem a bit overwhelming 

 

I am ... but it still does 

[Murflynn]

2 hours ago, Keeping Up said:

I thought they did until the posts about metacentre revealed that the CoG is probably ABOVE the centre of buoyancy hence the new weight may actually be below the original CoG

you're getting there!

 

perseverance pays  

 

 

 

it just occurred to me to describe an example.    imagine a block of balsa wood floating in the bath.   the CoG is well above the CoB yet it is as stable as a raft.

Edited June 4, 2020 by Murflynn

[WotEver]

4 minutes ago, Keeping Up said:

I thought they did until the posts about metacentre revealed that the CoG is probably ABOVE the centre of buoyancy hence the new weight may actually be below the original CoG

No, the new CoG will be above the old CoG  and that is why the boat is more tender.  Just like putting bags of coal on the roof.

[Murflynn]

2 minutes ago, WotEver said:

No, the new CoG will be above the old CoG  and that is why the boat is more tender.  Just like putting bags of coal on the roof.

really?

 

perhaps you know the height of the old CoG and the depth and therefore the CoG of the overplating - I don't - please let us know what those dimensions are.

[WotEver]

Just now, Murflynn said:

really?

 

perhaps you know the height of the old CoG and the depth and therefore the CoG of the overplating - I don't - please let us know what those dimensions are.

Why would I need to know that?  The extra weight has been put on the sides, so it doesn't take a mathematical genius to know that the CoG will therefore be higher, it's blindingly obvious.

[Keeping Up]

The plating is effectively from baseplate to waterline, hence its CoG will be exactly halfway between the two (ie just under a foot above the base). I would have expected the boat's COG to be higher than that. The baseplate weighs just over 5 tons and the overall loaded weight must be around 20 tons.

[Machpoint005]

2 hours ago, john6767 said:

I would say that the stability of any boat, even a narrowboat, is down to the relationship between the centre of gravity, the centre of buoyancy, and the resulting  metacentric height.  It is perhaps worth a search on here as this has been discussed before.  There are plenty of resources on the net though, for example https://en.m.wikipedia.org/wiki/Metacentric_height  explains it simply with some clear diagrams.

 

So what you need to do is think about what the adding of mass to a boat at various locations does to those parameters, and therefore what the impact is on the righting moment generated when the boat heels over.

 

Well posted; have a greenie!

 

I am amazed that it took two days before anyone mentioned metacentric height. That's the one bit of of naval architecture I remember from my student days (a mate was reading Nautical Studies, and he had a number of lectures in common with my Mech Eng course).

[Murflynn]

2 hours ago, WotEver said:

Why would I need to know that?  The extra weight has been put on the sides, so it doesn't take a mathematical genius to know that the CoG will therefore be higher, it's blindingly obvious.

not to me it ain't    

 

 

I believe year 6 have returned this week - perhaps you should join them.     

[john6767]

5 hours ago, dmr said:

That isn't answering the question, that is just re-stating the question with a few bigger/more technical words thrown in.?

 

................Dave

I was attempting to give the tools for the OP to work the answer out themselves.  If you can picture how the rotation works then you should be able to picture the effect.  I think Nick makes a good point about the dynamic situation, that may well be the bigger effect in determining how stable the boat feels, as to a posed to how stable it ultimately is.

[Alan de Enfield]

The RCD requirements for stability are covered in the EN ISO 12217-1 specification.

 

 

BS EN ISO 12217-1:2013 Non-sailing boats of hull length greater than or equal to 6m

This part of the stability standard covers “the stability and buoyancy of non-sailing boats of hull length greater
or equal to 6 m”. It offers one option for the assessment for Design Category A (Ocean) and B (Offshore) boats
built with full watertight deck, quick draining cockpits and small deck recesses, one option for Design Category
B (Offshore) boats with any form of deck and various options for Design Category C (Inshore) and D (Sheltered
water) boats.
These options mean that a vessel without a full watertight deck, quick draining cockpits and small deck
recesses can never be assigned with a Design Category A and will only ever be assigned Category B if it is able
to float with a required level of reserve buoyancy when fully swamped.
The criteria for assessment for each option is detailed in the BS EN ISO 12217-1 and depending on the type of
craft and design category requires the assessment of the some of the criteria detailed below.
Downflooding Height Measurement & Assessment
All options address the risk of downflooding which is the risk of water entering non-draining parts of a boat.
The size and position of openings in the hull and their height, or in an open boat, the gunwale height above
the water level must be greater than a given limit. Engine exhausts, discharges connected to watertight
systems (bilge outlet for example) and openings that are provided with a watertight means of closure are not
considered, although the companionway is always considered open however watertight it may be. The full
procedure is detailed in the standard and involves measuring the height of openings above the loaded
waterline with the boat at the Loaded Displacement Mass (mLDC).
BS EN ISO 12217-1 gives tables with the required limits for the downflooding height which depend on the
Design Category and length of the craft. If the craft does not meet these values given in the tables a more
detailed calculation provided in Annex A (Full method for required downflooding height) may be used to
demonstrate compliance with the standard.

Downflooding Angle
This requirement is to show that there is sufficient margin of heel angle before significant quantities of water
can enter the boat. A simple method of calculating this is provided in BS EN ISO 12217-1, Annex C (Methods
for calculating downflooding angle) and similarly to the downflooding height, if this method does not show
compliance with the requirements, practical testing or computer simulation may provide an alternative
method.
Offset Load Test
This test demonstrates sufficient stability for the boat at loaded displacement mass against offset loading by
the crew. BS EN ISO 12217-1, Annex B (Method for offset-load test) gives the procedure for conducting the
test with the maximum allowable heel angle that may be obtained which is a function of length given in Clause
6.2 of the standard.
Resistance to Waves & Wind (Categories A & B only)
These calculations require a righting moment curve. Righting moment curves are normally produced by the
designer or a naval architect from the lines plan, with an inclining experiment on the completed boat and
addresses the forces likely to be applied and the energies dissipated when a boat is rolling in beam waves and
wind.
Heel due to Wind Action (Category C and D)
This is only a requirement for boats where in the minimum operating condition ALV ≥ 0.55 LWLBH. The standard
gives limits for the maximum heel angle resulting from the application of a wind heeling moment. The purpose
of the test or calculation is to show that the actual angle of heel is less than the assumed maximum. If
performed by practical test, the maximum moment required for the calculation is the maximum offset load
moment. These calculations should be carried out for the craft in both conditions.
Floatation Requirements
Depending on the assessment schedule used from Table 2 of BS EN ISO 12217-1 a floatation test may be
required for open boats to demonstrate adequate swamped buoyancy and stability. These are detailed in
Annex F (Method for level floatation test) and Annex G (Floatation material and elements).

[Machpoint005]

^^ Doesn't all this just mean that a flat-bottomed narrowboat is never going to be properly seaworthy?

 

(It doesn't have to be, of course.)

 

 

12 minutes ago, Alan de Enfield said:

 >>>D (Sheltered water) boats <<<

...is the only relevant bit.

Edited June 4, 2020 by Machpoint005