Stability and ballast

[Iceman]

I am new to boating and have a query about the stability of my boat.

I am currently renovating the former ice boat EREBUS (29', 5'8", 2'9").

Although I have never seen her in the water, from the photo of her in the past, she seems to sit very low. I have found some, extremely heavy, iron weights in the hull for ballast. There is also an Ailsa Craig RF1 amidships.

 

I realise that, by design, she will rock due to the narrow profile and rounded hull shape.

 

Does it need to sit so low to be stable?

Can you over ballast a boat causing a pendulum effect?

 

Any comments will be welcome.

Thanks

Edited July 9, 2011 by Iceman

[steelaway]

I am new to boating and have a query about the stability of my boat.

I am currently renovating the former ice boat EREBUS (29', 5'8", 2'9").

Although I have never seen her in the water, from the photo of her in the past, she seems to sit very low. I have found some, extremely heavy, iron weights in the hull for ballast. There is also an Ailsa Craig RP1 amidships.

 

I realise that, by design, she will rock due to the narrow profile and rounded hull shape.

 

Does it need to sit so low to be stable?

Can you over ballast a boat causing a pendulum effect?

 

Any comments will be welcome.

Thanks

 

Hi

 

Without sounding like I know anything about these boats.

 

2'9" is quite a deep drought, many canals are only 3or4 ft deep, so I dont think you should make it any deeper.

 

Sitting low in the water is not usually a problem on canals

 

Being a ice boat it could well have been designed to rock to help break the ice.

 

Hope this helps

 

Alex

[Iceman]

I do not want to make it deeper.

I would like it to be shallower but am concerned about increasing the instability.

 

As I mentioned in the post, I understand that they are designed to rock.

[steelaway]

I do not want to make it deeper.

I would like it to be shallower but am concerned about increasing the instability.

 

As I mentioned in the post, I understand that they are designed to rock.

 

 

The bit about 'can you over ballast etc' has me confused.

You will have to suck it and see

 

Alex

Edited July 9, 2011 by steelaway

[Iceman]

The bit about 'can you over ballast etc' has me confused.

You will have to suck it and see

 

Alex

I was told by a surveyor that too much ballast can cause a pendulum effect if the boat rocks.

I'm as confused as you

[blodger]

I am new to boating and have a query about the stability of my boat.

I am currently renovating the former ice boat EREBUS (29', 5'8", 2'9").

Although I have never seen her in the water, from the photo of her in the past, she seems to sit very low. I have found some, extremely heavy, iron weights in the hull for ballast. There is also an Ailsa Craig RF1 amidships.

 

I realise that, by design, she will rock due to the narrow profile and rounded hull shape.

 

Does it need to sit so low to be stable?

Can you over ballast a boat causing a pendulum effect?

 

Any comments will be welcome.

Thanks

 

The way a boat rolls is tied in with how much is the metacentric height in ship stability. I am sure there will be something about it on the internet if you google it. It is something you had to work out on cargo ships each voyage. Lowering the centre of gravity gives you a stiffer, quicker roll hence ballast low down a bit like filling double bottom and deep tanks when sailing empty. The metacentre point is a function of the underwater volume and is raised IIRC by the cube of the beam. To put it simply a pencil will roll more than a coffin

[Iceman]

The way a boat rolls is tied in with how much is the metacentric height in ship stability. I am sure there will be something about it on the internet if you google it. It is something you had to work out on cargo ships each voyage. Lowering the centre of gravity gives you a stiffer, quicker roll hence ballast low down a bit like filling double bottom and deep tanks when sailing empty. The metacentre point is a function of the underwater volume and is raised IIRC by the cube of the beam. To put it simply a pencil will roll more than a coffin

Thats what the wife said !!

[Theo]

Thats what the wife said !!

 

The shape of the hull is crucial in establishing stability. With a vertically sided hull an angle of heel produces a very marked shift towrds the low side in the position of the centre of bouyancy. It is this shift which produces the righting moment which brings the boat vertical once more. This happens even though the centre of buoancy is below the centre of gravity, a state in which you would expect the boat to be unstable.

 

The ice boat has a circular (or nearly so) cross section so as it heals there is no shft in the centre of buoyancy. I would guess, then, that the centre of gravity would have to be below the cntre of bouyancy which would require lots of ballast low down. Yes the boat would roll like a pig but isn't that what it was designed to do?

 

Nick