Where Does Ballast Differentiate From Displacement?

[endoftheroad]

Does my subject title make any sense?

At what point in a boats displacement or design technically be specified as ballast? There must be a defining point.
It can't be everything below the waterline, could it?

[hellosailor]

Ballast is a purposely installed weight or mass with no other purpose.

Displacement refers to the hole you've made in the water, and how much water you've put someplace else.


So the title sounds more like "How is a donut different from a donut hole?" to me.

[Sailmonkey]

Ballast is weight added to a hull for stability. So if you have a 15000 lb displacement but carry 5000 lbs of ballast, then the rest of the hull weighs in at 10000 lbs.

[endoftheroad]

I knew I didn't know how to ask this question.
I know wht displacement is and I know what ballast is.

I'm thinking more of what a manufacture or an architect can specify as ballast.
If the keel is already heavy and they claim they added 5,000 ballast and then you have a lighter boat below the waterline and they add 7,500.
Now I'm really confused.

Building a keel heavier than necessary because you know you have an issue with having to add alot of ballast for the design.
Well, you are already adding ballast then. And then comes in the big drop of say 5,000lbs. So you may have actually added 6,000lbs of ballast.

[zeehag]

if the keel weighs in at 6000 pounds, that is your ballast.

your displacement is the weight of the amount of water your boat relocates.

my ericson with a 9000 pound ballast keel displaces 11,500 pounds of water.

[hellosailor]

Ballast traditionally is moveable and removable. Ever walk down a cobblestone paved street? Those were often ballast stones that were put in the hold of ships making a return trip without heavy cargo. Unload stones, load cargo, make trip. No cargo for the return trip? You needed ballast stones for stability, you threw them away (or sold them as pavers) at the other end.

You'll also find fixed ballast on production boats like the J/24, where a couple of pounds of lead plate or brick may be glued down here or there to make the boat sit level on her lines and ensure it meets the minimum weight requirement for the class. This compensates for variations in construction.

"Sandbaggers" used bags of sand as moveable ballast, hauled from one side ot the other as the boats tacked. Most racing boats today use crew on the rail as moveable ballast for the same reason. That's all ballast, and incidental to the displacement of the boat.

[Celestialsailor]

Originally Posted by hellosailor

Displacement refers to the hole you've made in the water, and how much water you've put someplace else.

Ahhh...this would explain rising Oceans...

[Pyrate]

Ahhh...this would explain rising Oceans...

Yep, more and bigger boats these days

[roverhi]

Originally Posted by zeehag

my ericson with a 9000 pound ballast keel displaces 11,500 pounds of water.

78% ballast to displacement ratio. That must be some stiff E-35 you've got there but think you'd have to move the waterline up about 3'.

[TaoJones]

Originally Posted by zeehag

if the keel weighs in at 6000 pounds, that is your ballast.

your displacement is the weight of the amount of water your boat relocates.

my ericson with a 9000 pound ballast keel displaces 11,500 pounds of water.

Originally Posted by roverhi

78% ballast to displacement ratio. That must be some stiff E-35 you've got there but think you'd have to move the waterline up about 3'.

That's just a typo, roverhi. KD knows that the ballast on her E35 Mk II is 5000 pounds.

Originally Posted by Sailmonkey

Ballast is weight added to a hull for stability. So if you have a 15000 lb displacement but carry 5000 lbs of ballast, then the rest of the hull weighs in at 10000 lbs.

Well, the hull had better weigh quite a bit less than 10,000 pounds, Sailmonkey, because any more weight or any instability after you've already matched the displacement is going to swamp the boat!

TaoJones

[mellowseas]

end of the road,
I also have a W 32. All that fiberglass at the turn of the bilge and below is not considered ballast. So the 7000 pounds of lead or steel in the keel is ballast but all that heavy fiberglass it rests in is not calculated as ballast. How it all is figured out I leave to William Atkins.

[Blue Stocking]

All weight projecting below a horizontal plane passing thru the centre of buoyancy, could be considered ballast. All weight above that plane needs to be balanced by the ballast. Look at the curve of areas on a lines drawing. If it is not a symmetrical bell shape from bow to stern, the boat will change trim as it lists. That is because weight is moving fore or aft.

[Sailmonkey]

Originally Posted by TaoJones

Well, the hull had better weigh quite a bit less than 10,000 pounds, Sailmonkey, because any more weight or any instability after you've already matched the displacement is going to swamp the boat!

TaoJones

If the hull is adequately designed, it will displace more with a payload added. A vessel that has 5000 lbs of ballast and displaces 15000 lbs does just that, it displaces 15k lbs of water to float.

[zeehag]

thankyou tao-- dont have the stats in front of me , as ercison is in sd....but ye get the idea....i know one of my boats has 9000 pounds ballast--must be this one...

[DefinitelyMe]

Originally Posted by Blue Stocking

All weight projecting below a horizontal plane passing thru the centre of buoyancy, could be considered ballast. All weight above that plane needs to be balanced by the ballast. Look at the curve of areas on a lines drawing. If it is not a symmetrical bell shape from bow to stern, the boat will change trim as it lists. That is because weight is moving fore or aft.

Hmmm, i disagree that ballast necessarily needs to act below the centre of buoyancy. As mentioned in an earlier post, moving crew from one rail to another is moving ballast. Boats with water ballast tanks are likewise. In fact, the full tanks are on the high side and therefore considerably above the centre of buoyancy. I think ballast can be loosely defined as any mass with the primary function of altering the centre of mass.