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Positioning Ballast for Stability


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2 minutes ago, Machpoint005 said:

^^ 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.)

 

 

...is the only relevant bit.

Indeed - but one should actually qualify which 'categorisation' you are using :

 

The RCD categories are :

A ~ Ocean;

B ~ Offshore;

C ~ Inshore;

D ~ Sheltered Waters.

 

watercraft

 

But the UK uses  their own (why not ? you may ask) categories :

 

Inland waters and how they are categorised

 

Category A - narrow rivers and canals where the depth of water is generally less than 1.5 metres

Category B - wider rivers and canals where the depth of water is generally 1.5 metres or more and where the significant wave height could not be expected to exceed 0.6 metres at any time

Category C - tidal rivers, estuaries and large, deep lakes and lochs where the significant wave height could not be expected to exceed 1.2 metres at any time

Category D - tidal rivers and estuaries where the significant wave height could not be expected to exceed 2 metres at any time

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2 hours ago, Murflynn said:

not to me it ain't     :banghead:

It seems that you wish to be argumentative simply for the sake of it. As it’s not obvious to you try asking yourself why Alan’s boat feels less stable since it’s been done. 

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52 minutes ago, john6767 said:

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.

It is said that if you can formulate the question correctly then the answer is obvious ?

 

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

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1 hour ago, WotEver said:

It seems that you wish to be argumentative simply for the sake of it. As it’s not obvious to you try asking yourself why Alan’s boat feels less stable since it’s been done. 

it is evident to me that you are guessing.  perhaps you have never been required to produce rigorous proofs of your hypotheses.

 

please refer me to the post where Alan had something done to his boat.

 

perhaps you are referring to someone called Allan?

Edited by Murflynn
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1 hour ago, Murflynn said:

it is evident to me that you are guessing.  perhaps you have never been required to produce rigorous proofs of your hypotheses.

 

please refer me to the post where Alan had something done to his boat.

 

perhaps you are referring to someone called Allan?

Of course I'm guessing.  I'm not a naval architect and wouldn't know where to start with the calculations. And yes, of course I mean Allan - who else has been in this thread talking about his overplated boat?

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2 hours ago, WotEver said:

Of course I'm guessing.  I'm not a naval architect and wouldn't know where to start with the calculations. And yes, of course I mean Allan - who else has been in this thread talking about his overplated boat?

I only know one Alan on this thread .............................. oh!   I see you've corrected your spelling.

 

 

 

............    see what I mean about rigour?     :P

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Thank you all for your enthusiasm in answering my question. Tippiness is probably a good synonym for my understanding of "tenderness"

When we first launched our boat we needed about a tonne of ballast to get her sitting correctly in the water. All of the ballast was required at the front.

We are in the happy position of having an 8 foot front cockpit with easy access to the baseplate over the full width.

The original ballast used was of a low density and currently covers the whole of the base plate also taking up a lot of space in the side lockers. I intend to replace the low density ballast with pig iron ingots and retrieve our much needed locker space.  

Having looked at all of your arguments I intend to try ballasting at the sides, leaving the centre of the base plate unballasted. It is not a big job to move it if it makes the tenderness worse.

We appreciate that narrow boats are tender compared to our previous boat (10 foot widwbeam) I am just hoping to make the best compromise I can.

Thanks again.

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On 02/06/2020 at 13:15, WotEver said:

However, as Bengo pointed out, the moment of inertia would be greater if the ballast was all to the sides as opposed to down the centre line.  Not that you could achieve that.  So no, it wouldn't affect the righting component, but it would make it less tender. 

 

Think of a dumbell.

 

 

Your response does indeed make me think of a dumbell

What you call the 'moment of inertia' is for a floating vessel, the Righting Moment, and is expressed in tonne metres. 

So to claim that "it wouldn't affect the righting moment, but would make it less tender" is a contradiction and an impossibility. 'Tenderness' is the manifestation of the Righting Moment and nothing else.

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50 minutes ago, yabasayo said:

Your response does indeed make me think of a dumbell

What you call the 'moment of inertia' is for a floating vessel, the Righting Moment, and is expressed in tonne metres. 

So to claim that "it wouldn't affect the righting moment, but would make it less tender" is a contradiction and an impossibility. 'Tenderness' is the manifestation of the Righting Moment and nothing else.

Yes, a bad choice of terms. It would make it slower to tilt, making it ‘feel’ less tender. 

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13 hours ago, yabasayo said:

Your response does indeed make me think of a dumbell

What you call the 'moment of inertia' is for a floating vessel, the Righting Moment, and is expressed in tonne metres. 

So to claim that "it wouldn't affect the righting moment, but would make it less tender" is a contradiction and an impossibility. 'Tenderness' is the manifestation of the Righting Moment and nothing else.

dimensional analysis proves that you are quite wrong.  

 

Righting Moment = mass x length.

 

Moment of Inertia = mass x length squared.

 

one relates to static mathematics, the other is about dynamics.  The difference, in practice, is subtle, but there IS a difference.

 

 

Edited by Murflynn
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5 minutes ago, Murflynn said:

dimensional analysis proves that you are quite wrong.  

 

Righting Moment = mass x length.

 

Moment of Inertia = mass x length squared.

 

 

Quoting basic applied mechanics is not dimensional analysis. Can you explain how your incomplete statement proves anything.

You apparently do not understand that in the case under discussion,  mass refers to the centre of mass of the whole boat and whatever it contains.

To make it simple, if you have a 10 metre pole with a 1 kg weight at each end, the centre of mass is half way along the pole ie 5 m from each end. If your pole is 20m long with a weight a 1kg weight each end, the centre of mass is still half way along the pole. So if the equal weights at each end of the imaginary pole represent the ballast, you can see that both weights combined can be said to act through a point half way along the pole. (if you can't accept that then I give up)

The boat rotates around the centre of buoyancy (which itself moves as the boat lists). Length here is the horizontal distance between the combined centre of mass and the centre of bouyancy. 

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3 hours ago, yabasayo said:

The only way to "make it slower to tilt" is to increase the Righting Moment.

No it isn't.

2 hours ago, yabasayo said:

if you have a 10 metre pole with a 1 kg weight at each end, the centre of mass is half way along the pole ie 5 m from each end. If your pole is 20m long with a weight a 1kg weight each end, the centre of mass is still half way along the pole.

Yes, and your 20 metre pole would be harder to tilt than your 10 metre pole, thanks simply to inertia.  Think tightrope walker.

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On the dumb bell analogy, it is not quite the same, as you you need to consider the point where the boat will be rotating around, which is not a point mid way between the mass that you add, assuming the mass is added at baseplate level.

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5 minutes ago, john6767 said:

On the dumb bell analogy, it is not quite the same, as you you need to consider the point where the boat will be rotating around, which is not a point mid way between the mass that you add, assuming the mass is added at baseplate level.

True, but the effect would still be measurable, even if it was slight  

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5 hours ago, yabasayo said:

Quoting basic applied mechanics is not dimensional analysis. Can you explain how your incomplete statement proves anything.

You apparently do not understand that in the case under discussion,  mass refers to the centre of mass of the whole boat and whatever it contains.

To make it simple, if you have a 10 metre pole with a 1 kg weight at each end, the centre of mass is half way along the pole ie 5 m from each end. If your pole is 20m long with a weight a 1kg weight each end, the centre of mass is still half way along the pole. So if the equal weights at each end of the imaginary pole represent the ballast, you can see that both weights combined can be said to act through a point half way along the pole. (if you can't accept that then I give up)

The boat rotates around the centre of buoyancy (which itself moves as the boat lists). Length here is the horizontal distance between the combined centre of mass and the centre of bouyancy. 

:banghead:  :banghead:  :banghead:

 

clearly you don't have a clue about the niceties of this discussion, which includes a discussion about dynamics, introduced by me and commented on by various others.

 

do you still maintain that Righting Moment is the same thing as Moment of Inertia?   Come on - be honest.

 

repeating a concept related to the centre of gravity of various different systems is so simplistic and obvious that it betrays your lack of understanding.  nobody has denied or refused to "accept" what you are saying but we have moved on long ago. 

 

you'd do better to open your eyes and ears and learn something.

 

if you don't understand my "incomplete statement" then I suggest you go back and read all my posts in this thread  .........................  I can't be bothered to repeat the facts just for your benefit.

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On 08/06/2020 at 16:54, Murflynn said:

:banghead:  :banghead:  :banghead:

 

clearly you don't have a clue about the niceties of this discussion, which includes a discussion about dynamics, introduced by me and commented on by various others.

 

do you still maintain that Righting Moment is the same thing as Moment of Inertia?   Come on - be honest.

 

repeating a concept related to the centre of gravity of various different systems is so simplistic and obvious that it betrays your lack of understanding.  nobody has denied or refused to "accept" what you are saying but we have moved on long ago. 

 

you'd do better to open your eyes and ears and learn something.

 

if you don't understand my "incomplete statement" then I suggest you go back and read all my posts in this thread  .........................  I can't be bothered to repeat the facts just for your benefit.

As a retired naval architect and marine engineer I'll give it one last try.

In this case, the mass moment of inertia (which I presume is what you are now talking about) is a resistance of the boat to radial acceleration. In other words it is a component of the speed at which the boat rolls and recovers, but not the extent to which it rolls, for given force at a given vertical distance from the axis of rotation. This is complicated by the fact that a certain amount of water also moves with the boat effectively adding to the mass. 

The amount a boat heels when loaded to one side or other of the centre of buoyancy, effectively the boat's centre line, depends totally on the Righting Moment.

End of conversation.

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28 minutes ago, yabasayo said:

it is a component of the speed at which the boat rolls and recovers, but not the extent to which it rolls, for given force at a given vertical distance from the axis of rotation

Absolutely, and that is exactly what we've been saying. Thank you for confirming it.

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30 minutes ago, yabasayo said:

As a retired naval architect and marine engineer I'll give it one last try.

In this case, the mass moment of inertia (which I presume is what you are now talking about) is a resistance of the boat to radial acceleration. In other words it is a component of the speed at which the boat rolls and recovers, but not the extent to which it rolls, for given force at a given vertical distance from the axis of rotation. This is complicated by the fact that a certain amount of water also moves with the boat effectively adding to the mass. 

The amount a boat heels when loaded to one side or other of the centre of buoyancy, effectively the boat's centre line, depends totally on the Righting Moment.

End of conversation.

But which is best, ballast on the centre line or along the edges?.

 

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25 minutes ago, yabasayo said:

As a retired naval architect and marine engineer I'll give it one last try.

In this case, the mass moment of inertia (which I presume is what you are now talking about) is a resistance of the boat to radial acceleration. In other words it is a component of the speed at which the boat rolls and recovers, but not the extent to which it rolls, for given force at a given vertical distance from the axis of rotation. This is complicated by the fact that a certain amount of water also moves with the boat effectively adding to the mass. 

The amount a boat heels when loaded to one side or other of the centre of buoyancy, effectively the boat's centre line, depends totally on the Righting Moment.

End of conversation.

yes, we knew that; as I said in an earlier post, the Moment of Inertia is one of several factors influencing the period of roll.    Others were saying it was irrelevant to the tenderness of a narrowboat, not me.

 

why don't you bring something new to the discussion?

 

I am surprised that a naval architect (which is a profession requiring formal further education - I assume that, by using that description, you are AMRINA or MRINA or FRINA) and "marine engineer" (which could mean something or nothing) did not know yesterday that the dimensions of Righting Moment and of Moment of Inertia are different, but now acknowledges that same fact.  Did you have to refer to your revision notes?

 

before you ask ...............................................   BSc CEng MICE

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1 hour ago, ditchcrawler said:

But which is best, ballast on the centre line or along the edges?.

 

And when you have answered that one, does the addition of plating to the sides from base to waterline make the boat rock more?

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8 hours ago, Keeping Up said:

And when you have answered that one, does the addition of plating to the sides from base to waterline make the boat rock more?

it might .... it might not .............

 

the real world is often complicated because there are several different factors at work - which is why engineers were invented.

 

if you provide all the weights and dimensions involved then we can analyse it for you. 

 

one generalisation we can offer is that if all the ballast was concentrated immediately on top of the baseplate and as far as possible at the sides, not the centre, the boat will rock more slowly and therefore comfortably, at least on undisturbed water ..............  however even that is not necessarily the best outcome because if the period (frequency) of any waves causing the boat to rock actually coincides with the period of rocking, then the rocking will be exaggerated.

 

so you also need to tell us if the boat is being rocked by movements inside the boat, or by external forces, and if the latter, then these need to be defined.

 

 

 

 

my personal advice - just get on with overplating if you love your boat that much.

 

 

Edited by Murflynn
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