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grahame r

Positioning Ballast for Stability

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Would a boat be more stable (less tender) with ballast postioned primarily around the perimeter or with the ballast positioned close to the centre? Assuming the boat is already level side to side (no list) and acceptably head up in the water.

Thanks in advance.

Grahame  

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31 minutes ago, grahame r said:

Would a boat be more stable (less tender) with ballast postioned primarily around the perimeter or with the ballast positioned close to the centre? Assuming the boat is already level side to side (no list) and acceptably head up in the water.

Thanks in advance.

Grahame  

I would say center line

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53 minutes ago, grahame r said:

Would a boat be more stable (less tender) with ballast postioned primarily around the perimeter or with the ballast positioned close to the centre? Assuming the boat is already level side to side (no list) and acceptably head up in the water.

Thanks in advance.

Grahame  

I would say along the outside edges.

No evidence but just thinking about a knife edge - a knife would easily topple over off its edge.

 

Putting all your ballast down the centre line and then (you) moving to one side would have a greater effect as there is no counterbalance weight at all on the other side.

Putting it on the outer edges and your 'weight' trying to tip the boat would need to lift the weight of the ballast as well

 

Spread the weight as wide as you can - that's why NB's are far, far more tender than a WB

 

Not well explained & maybe confused thinking.

 

Edit to add found this on a boat forum :

 

Simple test anyone can do: get a broomstick and a couple of 5 lb. weights.
1: Put both weights at the center of the broomstick. Grab it with your hands about shoulder width apart (outside the weights, obviously). Rotate it 20-30 degrees back and forth.
2: Put one weight at each end of the broomstick. Repeat the grab and rotate.
You'll find that it is much harder to rotate it back and forth in the second scenario, even though the total weight is exactly the same.

 

My initial take is that this seems to be a case of initial stability vs ultimate stability. A simplistic explanation of the difference between the two (again per my limited understanding) is that initial stability is the amount of force it takes to heel the boat; ultimate stability is how far the boat can heel before it capsizes. Flat bottom boats generally have more initial stability than round or V bottomed boats, meaning that it takes more force (eg wind) to heel them to begin with, but they have less ultimate stability, meaning the angle of heel they can bear before the balance shifts and they capsize is less. Which you want depends on the application of the boat. Flat bottomed boats are perfect for things like fishing where you have people standing and want a lot of initial stability, but are unlikely to experience 45 degrees of heel and so don't care about the ability to recover from that. A seagoing boat that expects to routinely roll through 90 degrees of heel wants a lot of ultimate stability so it can recover, but doesn't need to stay rock solid most of the time.

It seems to me that putting the ballast on the chines causes a flat bottomed boat to have even more initial stability than it otherwise would, at the cost of less ultimate stability. If that is what you want, that is fine. I think another effect is that not only would the boat capsize at an lower angle of heeling, but would also be harder to right if it turned turtle (completely upside down), because again it takes more force to heel the boat back up.

 

This reminds me of a tight rope walker. The longer his balancing pole the easier it is to balance. If he had a pole with all the weight in the center it would not balance as well. Let us know how it goes.

Edited by Alan de Enfield

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Define stable.

If you use the naval architects definition of stable (will it come the right way up again, and how far can I tip it over before it won't come the right way up) then it is all about the righting lever.  Generally the lower the ballast is, the better.  So in most boats, in the centre gets the ballast lower, but not in narrowboats, cos the bottom is usually flat.

If you use the laymans version of stability (how much does it rock about anx how soon does it stop) then ballast further away from the centre of buoyancy is more effective, because it increases the moment of inertia about the centre of buoyancy.  In a narrowboat that means ballast at the sides.

 

In most nb's there is so much ballast that it has to go where it will fit!

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3 minutes ago, BEngo said:

ballast further away from the centre of buoyancy is more effective, because it increases the moment of inertia about the centre of buoyancy

And this is the key bit :)

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1 minute ago, BEngo said:

Define stable.

If you use the naval architects definition of stable (will it come the right way up again, and how far can I tip it over before it won't come the right way up) then it is all about the righting lever.  Generally the lower the ballast is, the better.  So in most boats, in the centre gets the ballast lower, but not in narrowboats, cos the bottom is usually flat.

If you use the laymans version of stability (how much does it rock about anx how soon does it stop) then ballast further away from the centre of buoyancy is more effective, because it increases the moment of inertia about the centre of buoyancy.  In a narrowboat that means ballast at the sides.

 

In most nb's there is so much ballast that it has to go where it will fit!

Methinks the last point is the deciding factor!

In any steel boat you'll need a lot of ballast just to get the correct / reasonable depth in the water. If the boat is out of the water then spread it evenly AFTER making allowance for heavy items such as stoves on a concrete base, poo tanks / whatever. Also make allowance for the engine / fuel tanks / water tank.

FWIW - I didn't make proper allowance for the stove and had to correct it by getting some church roof from a scrappy and putting it on one side...

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I believe it makes no difference. The hull shape is obviously fixed so all you can do with ballast is alter the centre of mass of the boat ie given that you don't want any angle of heel you can only move the centre of mass vertically. The centre of mass is a combination of the the weight of the structure and everything in it, plus the ballast you are adding and acts through a single point. If for example you add 1 kg of ballast at an equal distance from the centreline on each opposite side of the boat, the centre of mass of the combined 2 kg is on the centreline of the boat.

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4 hours ago, yabasayo said:

I believe it makes no difference. The hull shape is obviously fixed so all you can do with ballast is alter the centre of mass of the boat ie given that you don't want any angle of heel you can only move the centre of mass vertically. The centre of mass is a combination of the the weight of the structure and everything in it, plus the ballast you are adding and acts through a single point. If for example you add 1 kg of ballast at an equal distance from the centreline on each opposite side of the boat, the centre of mass of the combined 2 kg is on the centreline of the boat.

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.

 

 

Edited by WotEver
Removed erroneous reference to stability.

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4 minutes ago, WotEver said:

Think of a dumbell.

or (as I posted earlier)

 

This reminds me of a tight rope walker. The longer his balancing pole the easier it is to balance. If he had a pole with all the weight in the centre it would not balance as well. 

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15 minutes ago, 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 more stable port to starboard.

 

Think of a dumbell.

correct, up to point where you use the word 'stable'.  

 

the righting moment that relates to the crude idea of stability is unaffected by the position of the ballast with respect to the centreline - the righting moment depends only on the vertical distance between the centre of buoyancy and the centre of gravity (and other factors like hull shape that you cannot change).  The ballast should always be as low as possible which is why most boats have a thin layer of ballast covering most of the baseplate.

 

(the static list or righting moment is entirely unrelated to 'dumbells').

 

the moment of inertia will be greater if the ballast is at the sides, but that will only effect the 'tenderness' of the boat as describing the speed at which it will rock from side to side.  Different wave conditions may result in exaggerating or damping the rocking, and it is unlikely that there is one perfect ballast layout to suit all conditions. 

 

having the ballast at the sides will not influence the static list to one side when someone stands on the gunwale (after the initial dynamic rocking has damped out).

Edited by Murflynn

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24 minutes ago, Murflynn said:

….          the moment of inertia will be greater if the ballast is at the sides, but that will only effect the 'tenderness' of the boat

 

Which was the OPs question.

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30 minutes ago, Murflynn said:

correct, up to point where you use the word 'stable'.

Okay then, less tender. 
 

5 minutes ago, Alan de Enfield said:

 

Which was the OPs question.

indeed. 

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1 hour ago, Murflynn said:

correct, up to point where you use the word 'stable'.  

 

the righting moment that relates to the crude idea of stability is unaffected by the position of the ballast with respect to the centreline - the righting moment depends only on the vertical distance between the centre of buoyancy and the centre of gravity (and other factors like hull shape that you cannot change).  The ballast should always be as low as possible which is why most boats have a thin layer of ballast covering most of the baseplate.

 

(the static list or righting moment is entirely unrelated to 'dumbells').

 

the moment of inertia will be greater if the ballast is at the sides, but that will only effect the 'tenderness' of the boat as describing the speed at which it will rock from side to side.  Different wave conditions may result in exaggerating or damping the rocking, and it is unlikely that there is one perfect ballast layout to suit all conditions. 

 

having the ballast at the sides will not influence the static list to one side when someone stands on the gunwale (after the initial dynamic rocking has damped out).

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.

 

Edited by yabasayo

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The amount of stability needed in this instance needs to be spread over the whole base-plate as far as is possible. If its all on the center-line or the extreme outer edges would over time cause the base-plate to bend either concave or convex. The thickness of the base-plate would of course help to counteract the bend.

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26 minutes ago, John Hartley said:

The amount of stability needed in this instance needs to be spread over the whole base-plate as far as is possible. If its all on the center-line or the extreme outer edges would over time cause the base-plate to bend either concave or convex. The thickness of the base-plate would of course help to counteract the bend.

But that doesn’t answer the OP’s question, which was ‘which arrangement would make the boat less tender?’  
 

As already answered, if it were possible to only ballast at the sides that would result in a less tender boat. It wouldn’t however affect the righting moment. 

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3 hours ago, 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.

 

you are wrong.   you should review your maths.  

the subject matter is not 'inertia' - 'it is the moment of inertia'.

the moment of inertia refers to rotary motion - the further the mass is from the centre of gravity, the greater the moment of inertia - if I recall correctly, the moment of inertia is proportional to the cube of the distance from the centre.

The 'tenderness' and pendulum frequency depend very much on the moment of inertia.  

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4 hours ago, Alan de Enfield said:

 

Which was the OPs question.

not necessarily - the term 'tenderness' needs to be properly defined before we can all agree what was meant.

 

static stability (as measured for example by the RCD test for stability) is not quite the same as the initial tippiness as the boat rocks after a load is suddenly imposed at the side.   I chose 'tenderness' to mean this initial tippiness, but OP has equated it to 'stability' which is not necessarily the same thing.

 

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4 minutes ago, Murflynn said:

I chose 'tenderness' to mean this initial tippiness, but OP has equated it to 'stability' which is not necessarily the same thing.

I think by the way he worded the OP that he was defining what he meant by ‘stability’. Namely ‘tenderness’. And I think we would all agree that tenderness means tippiness. 


I visited a Barry Hawkins open day many years ago and as we stepped onto an unballasted brand new tug the ‘I’m only here for the day’ salesman stated that he had no idea why the boat rocked so much and possibly it was down to the design...

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2 hours ago, WotEver said:

I think by the way he worded the OP that he was defining what he meant by ‘stability’. Namely ‘tenderness’. And I think we would all agree that tenderness means tippiness. 

 

would we really ?

 

are you suggesting that there is no difference between static stability and dynamic stability?  are we all agreeing on that?

 

I'm not.   ........................... but I suppose the average boat-owner would not notice much difference between the ballast being all on the centreline and all at the edges.  Among other things it depends on the ballast ratio.

 

for a 12 tonne boat with 2 tonnes of added ballast there won't be a tremendous difference.  For a 10 tonne boat with 4 tonnes of added ballast there would be a noticeable difference, most obviously felt by the difference in the pendulum period.

Edited by Murflynn

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4 minutes ago, Murflynn said:

are you suggesting that there is no difference between static stability and dynamic stability? 

Nope.  Why would I?  The subject is the tenderness of a narrowboat.

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2 hours ago, Murflynn said:

you are wrong.   you should review your maths.  

the subject matter is not 'inertia' - 'it is the moment of inertia'.

the moment of inertia refers to rotary motion - the further the mass is from the centre of gravity, the greater the moment of inertia - if I recall correctly, the moment of inertia is proportional to the cube of the distance from the centre.

The 'tenderness' and pendulum frequency depend very much on the moment of inertia.  

I think its the square, but otherwise nicely put.

 

Now I read somewhere that it's good to have enough stability but actually bad to have too much, if the righting force is too great the boat sort of snatches back into the upright position which is less pleasant than more gently rolling back.  I know that if I put too many logs on the roof the boat takes on a rather pleasant rolling motion, but I would probably not want to go out onto a big bad river in that condition.

 

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

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14 minutes ago, dmr said:

I think its the square, but otherwise nicely put.

 

Now I read somewhere that it's good to have enough stability but actually bad to have too much, if the righting force is too great the boat sort of snatches back into the upright position which is less pleasant than more gently rolling back.  I know that if I put too many logs on the roof the boat takes on a rather pleasant rolling motion, but I would probably not want to go out onto a big bad river in that condition.

 

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

just checked - you are right - it is squared.

 

 

having too much stability can be bad in the case of salty boats. 

my dad had a tubby little yacht and it was most uncomfortable in a chop.  It was 'stiff'.

a comfortable yacht moves with the swell and is quite relaxing.

52 minutes ago, WotEver said:

Nope.  Why would I?  The subject is the tenderness of a narrowboat.

is that the static version or the other one?   :rolleyes:

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It is no coincidence that racing yachts have heavy keels in the middle. Weight low down obviously gives stability, weight at the ends makes the boat pitch (rocking up and down at the ends) more than weight in the centre.  I don't know but I would expect the same principle would apply to weight at the sides which would making rolling worse. In the case of a yacht pitching shakes the wind out of the sails thus losing power, side ways rolling doesn't usually occur because the sails act as a huge damper fan.

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1 minute ago, Detling said:

It is no coincidence that racing yachts have heavy keels in the middle. Weight low down obviously gives stability, weight at the ends makes the boat pitch (rocking up and down at the ends) more than weight in the centre.  I don't know but I would expect the same principle would apply to weight at the sides which would making rolling worse. In the case of a yacht pitching shakes the wind out of the sails thus losing power, side ways rolling doesn't usually occur because the sails act as a huge damper fan.

That's a different issue though.  Yachts usually have a deep keel, which itself resists rolling, and that is often coupled with a heavy counterweight at the bottom of the keel which will result in a very strong righting moment. Necessary for a boat which is likely to exceed 45 degrees of list. 

 

By contrast, narrowboats already have the ballast as low as it will go (the base plate) and are not expected to roll more than a few degrees..

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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.

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