Ballast

[claudia]

This might be a stupid question but here goes, why do they ballast the boat is it to put so much under water to make it more stable or some other reason. If I am right how deep should it be?

[Max Sinclair]

For canal cruising it should be the minimum depth that is stable and the propellor grips the water without cavitating, ie sucking in air.This can only be acheived by trial and error. For ballast find a local concrete products company that makes fence posts, they often have cracked wasters they can't sell.

[john6767]

I would say the purpose is basically to make the boat more stable, but in a narrowboat I would suggest also partly to put more of the boat under water, ie make it sit lower in the water so you can get the prop under water for example.

 

For boat stability there are 2 things to consider the "centre of buoyancy" and the "centre of gravity". Basically if the centre of gravity is lower than the center of buoyancy the boat is intrinsically stable. The centre of buoyancy is the centre of gravity of the displaced water, so with a narrowboat that you could as a first approximation consider to have vertical sides, that would put the centre of buoyancy halfway between the surface of the water and the bottom of the boat. It you put mass in the bottom of the boat you can lower the location of the centre of gravity of the boat towards the centre of buoyancy. The closer it gets the more stable it gets.

 

I think that is correct, my physics may be a bit rusty though!

 

What I don't know is with a typical narrowboat, is the centre of gravity actually lower than the centre of buoyancy, ie intrinsically stable, or is it the more general case where it is above until the boat rolls?

Edited October 27, 2011 by john6767

[Theo]

I would say the purpose is basically to make the boat more stable, but in a narrowboat I would suggest also partly to put more of the boat under water, ie make it sit lower in the water so you can get the prop under water for example.

 

For boat stability there are 2 things to consider the "centre of buoyancy" and the "centre of gravity". Basically if the centre of gravity is lower than the center of buoyancy the boat is intrinsically stable. The centre of buoyancy is the centre of gravity of the displaced water, so with a narrowboat that you could as a first approximation consider to have vertical sides, that would put the centre of buoyancy halfway between the surface of the water and the bottom of the boat. It you put mass in the bottom of the boat you can lower the location of the centre of gravity of the boat towards the centre of buoyancy. The closer it gets the more stable it gets.

 

I think that is correct, my physics may be a bit rusty though!

 

What I don't know is with a typical narrowboat, is the centre of gravity actually lower than the centre of buoyancy, ie intrinsically stable, or is it the more general case where it is above until the boat rolls?

 

I fear that you have got the boat stability a bit wrong. I see where you are coming from. what you are saying is that if the point of support (centre buoyancy) is lowere than the c of g then you have an unstable situation. In the case of a float ing boat that is not the case because the centre of buoyance moves as the boat heels. It moves towards the low side. To understand this you need to look at the shape of the volume of displaced water. A wedge of displaced water moves from the low side to the high. The position that you are looking for is known as the metacentric height which is the point where the vertical through the new centre of buoyancy runs through the original vertical through the C of G of the craft.

 

Nick

[claudia]

I bet your physics aren't as rusty as my boat but I think I get your drift, I was on the right lines.

[john6767]

I fear that you have got the boat stability a bit wrong. I see where you are coming from. what you are saying is that if the point of support (centre buoyancy) is lowere than the c of g then you have an unstable situation. In the case of a float ing boat that is not the case because the centre of buoyance moves as the boat heels. It moves towards the low side. To understand this you need to look at the shape of the volume of displaced water. A wedge of displaced water moves from the low side to the high. The position that you are looking for is known as the metacentric height which is the point where the vertical through the new centre of buoyancy runs through the original vertical through the C of G of the craft.

 

Nick

 

That is what meant by my closing comment. I fully accept that the general case for boats is that the centre of buoyancy it below the centre of gravity ,and that the boat is unstable, until if rolls at which point the centre of buoyancy shits and it generates a righting moment, unless it rolls to far and woops....

 

I was just wondering, in the case of a narrowboat can you get the case where the c of g is below the centre of buoyancy in the steady state condition. A ex-working boat ballasted with slabs would seem to be a good candidate?

 

 

 

[bizzard]

I bet your physics aren't as rusty as my boat but I think I get your drift, I was on the right lines.

With that SR Lister weight in the stern i doubt whether it would need much in that area,mine certainly doesn't Lister ST2,but of course it depends on the shape of the stern and swim area (buoyancy).

[claudia]

The reason I ask is I have removed the very large poo tank from the front which made it stand very high at the front so I removed several large pieces of steel from the engine room, this leveled it nearly but I thought it now seems to rock from side to side much more.

[Theo]

That is what meant by my closing comment. I fully accept that the general case for boats is that the centre of buoyancy it below the centre of gravity ,and that the boat is unstable, until if rolls at which point the centre of buoyancy shits and it generates a righting moment, unless it rolls to far and woops....

 

I was just wondering, in the case of a narrowboat can you get the case where the c of g is below the centre of buoyancy in the steady state condition. A ex-working boat ballasted with slabs would seem to be a good candidate?

 

 

No. The boat is not unstable even if the Cof B is below the C of G. It is stable because a disturbance from the equilibrium position will give rise to a righting moment which acts to restore the original position.

 

N

[Ray T]

Also a boat may need ballasting more on one side than t'other so she sits level in the water.

 

CF has all her furniture along the port side so she need additional ballast on the starboard side to bring her level.

Edited October 27, 2011 by Ray T

[WotEver]

The reason I ask is I have removed the very large poo tank from the front which made it stand very high at the front so I removed several large pieces of steel from the engine room, this leveled it nearly but I thought it now seems to rock from side to side much more.

Oh yes. Loads of ballast removed = rocky boat.

 

Tony

[Neil2]

I was just wondering, in the case of a narrowboat can you get the case where the c of g is below the centre of buoyancy in the steady state condition. A ex-working boat ballasted with slabs would seem to be a good candidate?

 

 

 

The way to think about these issues is that the c of buoyancy is an upward force, the c of gravity a downward force. These forces must be equal otherwise the boat would sink or float into the air.

 

So in a floating boat, the c of g must be above the c of b. What happens when a boat heels is that the c of b shifts (depending on how much more hull is in the water, or how much less) but the c of g stays put. It's the relationship between these two forces that determine whether the boat stays upright or not, imagine a lever with the forces at either end. Once the c of b is above the c of g the boat wants to tip over. With a c of g very low down when you think about it even when the boat heels the leverage caused by the opposing forces isn't as strong as it would be with a high c of g.

 

In reality it's much more complicated. If you think about an extreme example of a boat with a wheelhouse, (albeit it would have to be watertight), heeling over to the point at which the house is in the water, the boat might go from being in danger of capsizing to becoming stable again. I believe some sea rescue vessels use this design principle. A narrowboat could be in a similar position. There's been a lot of debate on this and other forums about narrowboats at sea and there is an issue about stability because narrowboats have a very high c of g compared to say a blue water yacht. But if you could make the superstructure lighter and make it watertight, you could have a very stable construction. At least as regards side to side movement. Fore and aft is a different story...

 

Boats that are designed for inland use tend to have internal ballast as it isn't as crucial to have a low c of g in the form of a heavy keel, it's more useful to be able to shift ballast around depending on loading requirements. One of the drawbacks of modern NB contstruction is that it is usually very difficult, to make changes to the ballast once the boat is fitted out, which sort of defeats the object. That being the case you might as well have the ballast where it can do more good, on the outside ie the base plate. Which is why there's been a move towards heavier and heavier bottom plating. Apart form the cost, it makes a lot of sense.

Edited October 27, 2011 by Neil2

[bizzard]

The way to think about these issues is that the c of buoyancy is an upward force, the c of gravity a downward force. These forces must be equal otherwise the boat would sink or float into the air.

 

So in a floating boat, the c of g must be above the c of b. What happens when a boat heels is that the c of b shifts (depending on how much more hull is in the water, or how much less) but the c of g stays put. It's the relationship between these two forces that determine whether the boat stays upright or not, imagine a lever with the forces at either end. Once the c of b is above the c of g the boat wants to tip over. With a c of g very low down when you think about it even when the boat heels the leverage caused by the opposing forces isn't as strong as it would be with a high c of g.

 

In reality it's much more complicated. If you think about an extreme example of a boat with a wheelhouse, (albeit it would have to be watertight), heeling over to the point at which the house is in the water, the boat might go from being in danger of capsizing to becoming stable again. I believe some sea rescue vessels use this design principle. A narrowboat could be in a similar position. There's been a lot of debate on this and other forums about narrowboats at sea and there is an issue about stability because narrowboats have a very high c of g compared to say a blue water yacht. But if you could make the superstructure lighter and make it watertight, you could have a very stable construction. At least as regards side to side movement. Fore and aft is a different story...

 

Boats that are designed for inland use tend to have internal ballast as it isn't as crucial to have a low c of g in the form of a heavy keel, it's more useful to be able to shift ballast around depending on loading requirements. One of the drawbacks of modern NB contstruction is that it is usually very difficult, to make changes to the ballast once the boat is fitted out, which sort of defeats the object. That being the case you might as well have the ballast where it can do more good, on the outside ie the base plate. Which is why there's been a move towards heavier and heavier bottom plating. Apart form the cost, it makes a lot of sense.

I reckon our old mate Claudia will grasp that explanation ok.

[john6767]

Not sure I can go with some of your points

 

The way to think about these issues is that the c of buoyancy is an upward force, the c of gravity a downward force. These forces must be equal otherwise the boat would sink or float into the air.

c of g and c of b are not forces, they are points in space about which forces act/pass through. Clearly if the boat is at rest the actual forces are equal (Newton).

 

So in a floating boat, the c of g must be above the c of b. What happens when a boat heels is that the c of b shifts (depending on how much more hull is in the water, or how much less) but the c of g stays put. It's the relationship between these two forces that determine whether the boat stays upright or not, imagine a lever with the forces at either end. Once the c of b is above the c of g the boat wants to tip over. With a c of g very low down when you think about it even when the boat heels the leverage caused by the opposing forces isn't as strong as it would be with a high c of g.

I can not see why the c of g must be above the c of b for a boat to float, the location of these points does not affect if a boat floats or not, only its stability. A Google search tells me that some sail boats are designed with the c of g below the c of b to provide stability.

 

In reality it's much more complicated. If you think about an extreme example of a boat with a wheelhouse, (albeit it would have to be watertight), heeling over to the point at which the house is in the water, the boat might go from being in danger of capsizing to becoming stable again. I believe some sea rescue vessels use this design principle. A narrowboat could be in a similar position. There's been a lot of debate on this and other forums about narrowboats at sea and there is an issue about stability because narrowboats have a very high c of g compared to say a blue water yacht. But if you could make the superstructure lighter and make it watertight, you could have a very stable construction. At least as regards side to side movement. Fore and aft is a different story...

Not sure it really is more complicated, it just the location of the c of g and c of b is it not? Using a narrowboat example if you put the ballast on the cabin roof rather than the base plate, the g of g would be raised, and the boat would be more unstable (I guess you could argue about the definition of unstable here, but for this purpose it is how easy it rolls). With the ballast on the base plate the c of g will be lower than if it were on the roof, and you must be able to get to the point adding more ballast on the base plate that the c of g goes below the c of b. I can not see why that can not happen, even if it's not a good thing.

 

Boats that are designed for inland use tend to have internal ballast as it isn't as crucial to have a low c of g in the form of a heavy keel, it's more useful to be able to shift ballast around depending on loading requirements. One of the drawbacks of modern NB contstruction is that it is usually very difficult, to make changes to the ballast once the boat is fitted out, which sort of defeats the object. That being the case you might as well have the ballast where it can do more good, on the outside ie the base plate. Which is why there's been a move towards heavier and heavier bottom plating. Apart form the cost, it makes a lot of sense.

This I can agree with

 

 

 

[claudia]

Thanks all, from this I hope I am right in saying the further the boat is in the water the stabler it is.

[WotEver]

Thanks all, from this I hope I am right in saying the further the boat is in the water the stabler it is.

Yeah

[Neil2]

John 6767 is right - I was reminded of that Groucho Marx line "this man may look like an idiot, he may talk like an idiot, but don't let that fool you, - he really is an idiot".

 

The c of g can be below the c of b, but I remember reading somewhere that the only vessels where this applies are submarines... But yes I suppose ther might be extreme examples of "surface" vessels. I think I am right in saying the vast majority of boats have their c of g above the c of b and I'm sure this is the case with narrowboats.

It helps me to think about these centres being "forces" but I appreciate that isn't correct and is misleading.

 

The reason I referred to adding superstructure as complicating the issue is because on the one hand you might assume this would raise the centre of gravity and therefore be a bad thing, but if this is offset by a beneficial shift in the centre of buoyancy as the boat heels, it might be a good thing. You would need some pretty fancy computer software capable of factoring in the weights of different materials etc. to reach a conclusion.

 

It's all a bit academic anyway when we are talking about narrowboats on canals, but I do find it interesting that someone (claudia) immediately notices a change in the stability of his boat by taking out some of the ballast. In a nb weighing many tons "dry" I wouldn't have thought it would make so much difference.

[davidc]

taken from one of my old army notebooks

 

 

metacentre, also spelled metacenter, in fluid mechanics, the theoretical point at which an imaginary vertical line through the centre of buoyancy intersects another imaginary vertical line through a new centre of buoyancy created when the body is displaced, or tipped, in the water, however little.

 

The centre of buoyancy of a floating body is the point about which all the body parts exactly buoy each other, in other words, the effective centre of the displaced water. The metacentre remains directly above the centre of buoyancy regardless of the tilt of a floating body, such as a ship. When at rest on even keel, the vessel’s centre of buoyancy is directly below the centre of gravity as well as below the metacentre. (The centre of gravity is the point in a body about which all parts of the body balance each other.) When a vessel tilts, one side displaces more water than does the other, and the centre of buoyancy moves and is no longer directly under the centre of gravity; but regardless of the amount of the tilt, the centre of buoyancy remains directly below the metacentre. If the metacentre is above the centre of gravity, buoyancy restores stability when the ship tilts. The stability increases with the distance between metacentre and centre of gravity, called the metacentric height. If the metacentre is below the centre of gravity, the boat is unstable, and a tilt results in capsizing.

 

 

hope this helps ???????????????????????????????????

 

[bizzard]

CLAUDIA!!! Does this help?.

[Neil2]

Ooooooooeeeerrrrr.......

 

I'm gonna do it again I think, but I'm not sure if it is as simple as the metecentre being below c of g results in a capsize. I'd be more convinced if that had come from a Navy notebook...

 

I think you can still have a positive righting lever with the metacentre below the c of g, it's hard to explain without drawing a picture but you can construct a hypothetical situation where the opposing forces of buoyancy and gravity are still working to right the boat, but the metacentre is well and truly below the c of gravity.

 

I could be wrong....

[davidc]

Ooooooooeeeerrrrr.......

 

I'm gonna do it again I think, but I'm not sure if it is as simple as the metecentre being below c of g results in a capsize. I'd be more convinced if that had come from a Navy notebook...

 

I think you can still have a positive righting lever with the metacentre below the c of g, it's hard to explain without drawing a picture but you can construct a hypothetical situation where the opposing forces of buoyancy and gravity are still working to right the boat, but the metacentre is well and truly below the c of gravity.

 

I could be wrong....

 

 

http://en.wikipedia.org/wiki/Metacentric_height

 

if it sounds like duck

and walks like a duck

its duck

 

 

and never trust a sailor

from a royal engineer

Edited October 29, 2011 by davidc

[Pentargon]

The reason I ask is I have removed the very large poo tank from the front which made it stand very high at the front so I removed several large pieces of steel from the engine room, this leveled it nearly but I thought it now seems to rock from side to side much more.

Claudia. I'm going to climb out my own [well-greased] barge pole now to tell you in a few words how to organise 'ballast'. Firstly what you have done above is brilliant. You took out a heavy tank from the bows and a bunch of steel plate? from the stern to 'balance'. That has LIFTED the boat in the water. You have reduced Claudia's DRAFT and that means you can go in shallower water or carry more load.

 

Check down the weedhatch for how much water is OVER the tip of any one prop-blade set vertical. If that depth is equal to or more than the RADIUS of your prop, it's not likely to "cavitate". In reality that number could be down to half a radius and be fine, unless you are trying to drive the guts out of the boat. And then you deserve all the cavitation that comes your way.

 

Put a spirit level on the prop-shaft and in an ideal world it should be dead level with the boat afloat. Reason is that the stern tube is (usually) parallel to the waterline boot-top. If these two items are ok, forget about 'stability'. "Stability" is only for salt-sea sailors driving well-overloaded boats and for them has a canalboat but needs to get a life. I've checked through all of Jim Shead's 36' Springers and the drafts are all over the place from 16" to 30" so I'd be happy to have my boot-top anywhere it lies on whatever waterline I get. Over.