Roll Inertia?

[er9]

does a monohull (fin keel) with a higher ballast/displacement ratio have greater roll inertia versus a monohull with a lighter ballast displacement ratio (fin keel) or vice-versa?

to me it seems a monohull with a higher ballast/displacement ratio has a greater roll inertia due to its more severe 'whipping' motion as it snaps back from heeling at greater angles but i may be misunderstanding.

also i would assume keel design plays a large part as a longer or full keel would slow this force not to mention keel depth etc...?

[Don C L]

Hmmm, by roll inertia do you mean "righting moment" or resistance to rolling. Resistance to rolling will probably be the initial stability which is a factor of hull shape and beam. Righting moment will be a function of the distance that develops between the center of gravity and center of buoyancy as the boat begins to heel and isn't too much until you are over 20 degrees or so. Here's a quick simple description, https://www.sailboat-cruising.com/righting-moment.html.
The ballast to displacement figure comes in as it shows where the center of gravity is as a result of the location of the ballast. Also are we talking about a boat that is sailing or just rolling at anchor?

This discussion may be helpful:
https://www.boatdesign.net/threads/t...-moment.61860/
Note that the righting moment is the product of the displacement of the boat and the arm, the distance horizontally between the center of gravity and the center of buoyancy.
Now it seems you may be asking about how fast a boat will right itself, heavy vs. light displacement, fin vs. full keel. I think it will be that the greater the righting moment, the faster it will right itself as a rule, regardless of keel, but I'll defer to a naval architect there on righting speed.

[Breaking Waves]

Originally Posted by er9

roll inertia

roll inertia has a specific technical meaning = how difficult it is to change the rotational velocity of the object around a given rotational axis.

The rotational inertia of an object depends on its mass. With greater mass creating greater inertia. It also depends on the distribution of that mass relative to the axis of rotation. When a mass moves further from the axis of rotation it becomes increasingly more difficult to change the rotational velocity of the system.

so, everything else equal - yes deeper bulbed keel will create more roll inertia

[a64pilot]

Originally Posted by Breaking Waves

roll inertia has a specific technical meaning = how difficult it is to change the rotational velocity of the object around a given rotational axis.

The rotational inertia of an object depends on its mass. With greater mass creating greater inertia. It also depends on the distribution of that mass relative to the axis of rotation. When a mass moves further from the axis of rotation it becomes increasingly more difficult to change the rotational velocity of the system.

so, everything else equal - yes deeper bulbed keel will create more roll inertia

This is correct, but remember inertia works both ways, it both will help keep one rolling once it starts, but it will also resist it to begin with.

I believe the inertia is the biggest different between a power boat and a sail boat rolling moment wise, the sail boat has of course a lot more inertia, and yet it rolls a lot less.

However a deeper bulbed keel isn’t create more inertia, if I understand it one purpose of a deeper bulbed keel is to achieve the same level of inertia with less mass, making the boat sail the same, but be lighter so it accelerates better, has less wetted area so should be faster etc. Also of course the keel is a higher aspect ratio so less drag, and that smaller surface area keel will have some effect on rolling of course too
But a shallow draft full keel boat can have the same inertia, but it will take considerably more mass
Remember WAM, weight X arm = moment

[Breaking Waves]

Originally Posted by a64pilot

However a deeper bulbed keel isn’t create more inertia, if I understand it one purpose of a deeper bulbed keel is to achieve the same level of inertia with less mass

it is why I said 'everything else equal'. if you start simultaneously messing with total mass and mass distribution it is impossible to give an answer without a specific case calculation. If you only change one variable at a time (like mass, or mass distribution) and hold others equal, you can answer.

[a64pilot]

Originally Posted by er9

does a monohull (fin keel) with a higher ballast/displacement ratio have greater roll inertia versus a monohull with a lighter ballast displacement ratio (fin keel) or vice-versa?

Maybe, it depends on the arm of the ballast, less ballast further out has the same effect

to me it seems a monohull with a higher ballast/displacement ratio has a greater roll inertia due to its more severe 'whipping' motion as it snaps back from heeling at greater angles but i may be misunderstanding.

Yes, but there are other factors like length of mast or masts and weights aloft as well as what the arm of that ballast is
also i would assume keel design plays a large part as a longer or full keel would slow this force not to mention keel depth etc...?

In theory yes, but maybe not as much as you might think, unless there was a lot of rolling then the larger surface area keel would work to some extent like a flopper stopper

Oh, and on edit it seems to me that hull design is at least as important to resisting rolling and heeling as how much ballast there is, a narrow beamed wine bottle shaped hull will roll much easier than a flatter bottom, wide beam hull.

[a64pilot]

Originally Posted by Breaking Waves

it is why I said 'everything else equal'. if you start simultaneously messing with total mass and mass distribution it is impossible to give an answer without a specific case calculation. If you only change one variable at a time (like mass, or mass distribution) and hold others equal, you can answer.

I know you understand, I was trying to get him to understand, cause his question was all about displacement / ballast ratio.
Now I’m no sailor so I may be wrong, but I don’t think that ratio takes into account the arm of the ballast, just merely the mass, and if it doesn’t, well then put politely it doesn’t tell you much really. My shoal draft full keel would probably take at least twice the mass to achieve the same effect as a deeper bulb keel boat.
Plus I don’t think it takes into account the mass or arm of the mast(s), and I’m, fairly certain it doesn’t hull design.
The ratio would be a lot more helpful if it were displacement / ballast moment.
We all look for simple answers, it’s our nature and what we want, especially if we are shopping for things, we like to compare the “numbers” to try to pick the superior product, but sometimes the numbers aren’t telling us as much as we think they are, and sometimes they are deliberately misleading.

So if I understand correctly, ballast / displacement ratio doesn’t really tell you all that much, unless of course as you say you can compare two completely identical boats with different masses of ballast

[svHyLyte]

Originally Posted by er9

does a monohull (fin keel) with a higher ballast/displacement ratio have greater roll inertia versus a monohull with a lighter ballast displacement ratio (fin keel) or vice-versa?

to me it seems a monohull with a higher ballast/displacement ratio has a greater roll inertia due to its more severe 'whipping' motion as it snaps back from heeling at greater angles but i may be misunderstanding.

also i would assume keel design plays a large part as a longer or full keel would slow this force not to mention keel depth etc...?

What you are describing is the Moment of Inertia" of the yacht which is a function of the distribution on the mass of the boat multiplied by the square of the distance of the center(s) of mass from the center of gravity or, in the case of a boat, the "Metastatic Centroid", or the center of roll. The various elements that make up the total mass of the boat are determined and the distance of their "centers of mass" are squared and summed. The Moment of Inertia is the resistance of a body to a change in its angular velocity. In a yacht, the Moment of Inertia is an important measure of stability, roll resistance, but there is also the roll resistance induced by the shape of the hull and appendages such as the keel being forced through the water laterally which would generate a resisting force to rolling (one reason why full keel boats are considered to be more sea kindly than fin keelers).

As an aside, the importance of the Moment of Inertia to roll resistance was well understood in the age of "Tall Ships" when, in a heavy cross sea that would induce rolling, some sea captains would hoist heavy sacks of coal or ballast stones to their mastheads to "stabilize" their 3 or 4-mast ships.


FWIW...

[Adelie]

More ballast and placed deeper will have slightly higher roll inertia.

The biggest effect on roll inertia is the mast. The mast accounts for about 1/2 the roll inertia of a boat. Read the appendix of “Fastnet, Force 10” for more about this.

Want to increase roll inertia, hoist a moderate weight up the mast, 5-10lb.

[a64pilot]

Hoisting a weight up the mast will of course raise the center of gravity, and destabilize the boat, stability in this case meaning the speed at which the boat will attempt to return to stasis.
However raising the CG while destabilizing the boat will also slow its rolling period and may make it more comfortable.
Cruise ships as an example have a high CG, which of course is destabilizing, but apparently the slower roll rate is more comfortable to the passengers.

Remove the mast on a boat and it will become much more stable, but it’s motion as opposed to being smooth and slow will become fast and almost violent and very uncomfortable.

[TeddyDiver]

^^ what Adelie said above!
What comes to more modern sailboats it's the hull form and the roll dampening characteristics playing also significant role.

[Adelie]

Originally Posted by a64pilot

Hoisting a weight up the mast will of course raise the center of gravity, and destabilize the boat, stability in this case meaning the speed at which the boat will attempt to return to stasis.
However raising the CG while destabilizing the boat will also slow its rolling period and may make it more comfortable.
Cruise ships as an example have a high CG, which of course is destabilizing, but apparently the slower roll rate is more comfortable to the passengers.

Remove the mast on a boat and it will become much more stable, but it’s motion as opposed to being smooth and slow will become fast and almost violent and very uncomfortable.

Raising a moderate weight up the mast will decrease stability by a small amount but will NOT destabilize the vessel.

Counter-intuitively removing the mast will increase STATIC stability but decrease capsize resistance for monohulls. For monohulls capsize is a function of breaking waves which is a dynamic event. Roll inertia is what resists this dynamic force, not static stability.

Multihulls are different and wind action alone can capsize them.

[FAST FRED]

The most important for comfort is the rate at which the boat stops tolling in one direction and begins to roll the other way.


While a round bottomed boat will probably roll further its slow rebound is fine. A lightweight square box can be a Vomit Comet.

[a64pilot]

Decreasing stability, is by definition destabilizing.
It doesn’t have to be to unsafe levels, for instance I used to adjust the aircraft we made to the absolute max limit of rearward center of gravity when we built them, that was done purposely to destabilize the aircraft, because stability is the enemy of maneuverability, and you want a crop duster to be maneuverable, not stable.

[Pauls]

Adelie and Fred got it right. Inertia is the resistance of a body to change. It is not righting moment. They are two different things.

Inertia resists the force of a wave striking a boat. When a wave strikes a boat on its beam a low inertia boat will roll rapidly from that strike, while a high inertia boat will move more slowly. Roll moment of inertia is a primary factor in the resistance of a monohull to capsize from wave strike.

Older boats with heavier displacement and in particular with heavier rigs have higher moments of inertia. Lightweight hulls and lightweight rigs have less. This is an example of the trade offs inherent in yacht design.