Where Does Ballast Differentiate From Displacement?

[David M]

From a stability and trim perspective, ballast changes a vessels center of gravity. It does not matter where it is placed if it changes a boats center of gravity. Ballast can also be placed higher to reduce a snap roll on a vessel, producing a more comfortable ride.

There is a misconception that the CG has to be below the center of buoyancy. Not true, stability comes from the shifting of a vessels center of buoyancy as it rolls. This creates a moment arm which has the effect of righting the vessel back to vertical. Therefore, for ballast to increase stability (righting moment) it does not necessarily have to be placed below the waterline.

A vessel does not "feel" where the waterline is located. It only knows where the CG is respect to the center of buoyancy.

[Blue Stocking]

Originally Posted by DefinitelyMe

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.

I agree, you are correct. I was interpreting the OP question in reference to a simple displacement hull, not a modern variation such as water ballasted design, for instance. Going in that direction, a catamaran, such as a Tornado, flying a hull, with crew on trapeze, actually has its ballast entirely above it water-borne hull section.

[barnakiel]

It is also worth noting that some thick GRP (as well as some steel) hulls are so heavy that they actually have little 'ballast' (meaning little lead or iron added at the lowest extremity of the hull).

This set-up tends to produce very soft boats.

b.

[Capt Rottnest]

Originally Posted by barnakiel

It is also worth noting that some thick GRP (as well as some steel) hulls are so heavy that they actually have little 'ballast' (meaning little lead or iron added at the lowest extremity of the hull).

This set-up tends to produce very soft boats.

b.

But a "soft" boat has a slow roll in a rolly anchorage while a stiff boat (CoG much lower than CoB) snaps abruptly an anchor. Its the job of the NA to get the compromise right for a cruising boat so the crew can sleep. Also an uncomfortable boat can be cured to a certain extend but replacing the mast with a heavier one. Useful to know for owners seeing the light & converting to junk rig.

[barnakiel]

Originally Posted by Capt Rottnest

But a "soft" boat has a slow roll in a rolly anchorage while a stiff boat (CoG much lower than CoB) snaps abruptly an anchor. Its the job of the NA to get the compromise right for a cruising boat so the crew can sleep. Also an uncomfortable boat can be cured to a certain extend but replacing the mast with a heavier one. Useful to know for owners seeing the light & converting to junk rig.

+1

Was it on Voss or Slocum who hoisted a bag of potatoes up the mast?

Yes, when mass is distributed more evenly rather than concentrated at the lower extremity, a boat will have a slower roll. BUT it will be a deeper / longer lasting roll too - a boat that rolls to 30 degrees each way for twenty minutes after that bloody tugboat passed you by is a hard place to live in.

I like stiff boats: cats and racers with light hulls/rigs and 50% ballast - yes, they are a bit jerky BUT they settle down nearly as soon as that tug wave rolls away.

Another aspect is that such low ballast / heavy hull configs tend to give false impression of boat's stability in rough seas. They may show gentle motion and lull the crew into a 'she is doing just fine' mode. Then one wave is bigger than others and that's the end of the story. BUT, off course, this is an extreme case, not likely to happen in any professionally designed, professionally built boat.

Cheers,
b.