Roll Inertia?

[Ken Fry]

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?

Impossible to say. Depends upon the distribution of masses.


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.

The severity of whipping motion (or snapping back) has to do with many factors, including hull shape, distribution of masses, size of keel viewed from abeam, any appendage augmentation (wings, bulbs, etc). I've never sailed on a keel boat that "snapped" back, actually -- even dinghies roll back at a pace slower than "snapping"... granted this is a personal interpretation. Roll damping is different than roll inertia, which is different than righting moment.

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...?

Hard to know what you mean by "slow this force." I guess you mean slow the motion. A full keel of a given mass will have less depth than a fin keel. Then a comparison (in roll rate) can be made only if all the numbers are taken into account (roll moment, roll inertia, roll hydrodynamics). The tip of a deep keel will travel at a higher speed (in rotation) and that higher speed creates greater drag (and therefore roll damping) than there will be along the bottom of a full keel, because drag goes up with the square of velocity. So if one boat uses (as a keel) a four by eight sheet of iron with its length vertical and another uses the same sheet with its width vertical, the roll damping is much greater for the first, even though its "tip" is half the length. (because the tip is travelling at twice the speed, meaning 4 times the drag, acting on a longer lever. )

This effect is seen in dinghies. Even though a centerboard has very low mass, the roll rate is slowed (the boat seems less twitchy) when the board is down mainly because of the hydrodynamic effects, rather than mass effects.

In short: there is no single ratio or feature that describes how a boat responds to roll (or pitch or yaw.) Numerous factors come into play. For that reason, Marchaj's book "Seaworthiness, the Forgotten Factor" is rather long.

[Don C L]

Originally Posted by Cal31

Thank you all for this wonderful discussion! I sail a Cal31 that rolls terrifyingly fast in small, short period waves coming from the beam to stern. I really like the boat but hate that such small waves throw the boat around so badly. The mechanics of this problem seem to be at the heart of this discussion.

I am at the point of either selling the boat or trying some method to reduce the roll rate. If I understand the discussion correctly, my options are to add some weight aloft or add weight to the keel. Is this correct?

Again, thank you.

You can experiment but IMO, in spite of the discussion, I wouldn't start adding too much weight aloft. What you are describing sounds like more a feature of your boat's hull shape and displacement in response to short period waves which will cause that discomfort in lighter displacement boats like yours. If this is occurring running or broad reaching then this is a 3 dimensional rotational wallowing. The added weight aloft will initially resist motion but once it gets going, unless there is an opposite motion, like returning upright on the back side of the wave, it will, as mentioned before increase amplitude, or how far the boat will want to go. Weight aloft is usually a bad thing for sail boats. In this discussion it is interesting to consider that a relatively small weight positioned at the correct height, could offer the inertia to counteract motion a particular wave period is causing. At least that is my take on it. More weight in your keel would be my choice, but that is not an option. I don't know how deep your bilge is, but I know of folks who have added bricks of lead in the bilge for stiffness. In this case, since the weight wouldn't be very low or deep, it wouldn't contribute too much to roll inertia unless you added quite a bit, and then you are just asking your hull to plow a bigger furrow in the sea without much benefit to stability. Your boat is on the beamier side, shallow and relatively light, so my thinking is it is just a feature of your particular design in those particular seas. A beamy boat at the dock feels stiffer, but that beam gives waves more leverage to act on too once sailing. It is actually one reason I like the way my skinny boat responds while sailing; there is mostly pitch, not roll, that is until I head downwind! When I am out with friends on their Cat 27, the motion feels more bouncy, far more controlled by the hull's response to the waves and I get a queasy stomach. If you are motoring in steep swells, keep the mainsail up. It will offer resistance to rolling.

[betwys1]

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

You had me until the last sentence. The moment of inertia does depend on the mass versus distance to the roll axis - but the roll axis is a whole 'nother variable! <g>

[Ken Fry]

Originally Posted by Don C L

You can experiment but IMO, in spite of the discussion, I wouldn't start adding too much weight aloft.

I agree with Don.

If you wanted to add weight aloft, you'd want to firmly affix it to the mast, so that it cannot flail around. But weight aloft is generally a bad thing, and such weight will reduce the limit of positive stability angle. It will, however, increase rotational inertia, and therefore slow movements. But it will also increase the amplitude of some motions, and may cause undesirable fore- and-aft pitch responses. You will change the resonant frequency of the system, in a way that will may reduce one problem but cause another.

Adding area to the keel could help, but that is no small task. Likewise, a keel bulb (end plate, winglet, etc) could help but that is a big deal too, and requires some analysis, engineering and re-engineering.

Many people like the sailing qualities of the Cal 31, so perhaps you could modify the way you are sailing her. Sailing with more sail up and more heel might reduce your problem, as would sailing on a slightly different point of sail in the conditions that bother you. Shifting weight fore-and-aft and side-to-side can help, too. Talking with other owners could be helpful.

Reshaping the hull should have the most dramatic effect -- but, of course, that's not feasible.

Buying another boat is an option -- but you'd need to sail under many conditions to see what you like and do not like -- presumably when you test sailed the current boat, you liked it.

Many people like the motions of a light catamaran, while many others prefer the motion of a narrow heavy displacement monohull, so to each his or her own. Most sailors will find something about a boat that is sub optimal -- then you either deal with it or move on.

[er9]

Originally Posted by Ken Fry

In short: there is no single ratio or feature that describes how a boat responds to roll (or pitch or yaw.) Numerous factors come into play. For that reason, Marchaj's book "Seaworthiness, the Forgotten Factor" is rather long.

much thanks. this has turned into an interesting discussion. so much to learn. ordering the book now...

[Cal31]

Don C L and Ken Fry, thanks for the words of wisdom. Not sure why I didn't see your responses earlier. On reflection, I think the Cal 31 is happiest when it is manually steered and driven hard with both main and jib up. Without wind force working on the sails, the hull is just too light and beamy to comfortably negotiate short period waves. Requires hand steering and frequent course correction to best manage conditions, even if the conditions are moderate. I am now mostly single-hand sailing, even on long passages, and try to use the autopilot to reduce fatigue. Just doesn't work out as much as I'd like.

Again, thanks for the wisdom.

[Don C L]

Originally Posted by Cal31

Without wind force working on the sails, the hull is just too light and beamy to comfortably negotiate short period waves.

If it is any consolation I think that is true of most, if not all, boats of similar design. Light to go faster and beamy to have more initial stability (and room inside for the boat show.) I used to sail Catalina 32s and 36s and they were responsive to the seas in all three axes which I did not find comfortable, but as long as there was plenty of wind I was ok with them usually (though I did have a bad case of vertigo once that was no fun.) I also sailed a Downeast 38 ( a heavier, long keel) and that had a significantly different motion and I felt fine with that, but it was certainly not as fast especially in light airs (but it had a bigger engine) So everything is a trade-off. I'd say learn to love more the speed and pointing ability and agility your boat has and then the negatives won't seem so bad anymore. And get out into deeper water where the swells aren't so steep and close together!

[skenn_ie]

Sailing boats roll less when leaning over. Electronic steering needs a programmed damper so that it doesn't fight the natural corkscrewing through the waves. Wind-vanes (I believe) are better at that than electronics, then use the GPS just to wake you if the boat goes significantly off course.

[Cal31]

Originally Posted by Don C L

If it is any consolation I think that is true of most, if not all, boats of similar design. Light to go faster and beamy to have more initial stability (and room inside for the boat show.)

For sure, she will run like a scalded dog even in light air. The extra room inside, I have to admit, is nice when aboard for months. When younger, I used to live for the passage and not the destination. Traveling further offshore, while it might take longer when coastal cruising, has always appealed to me and might be just the ticket to avoiding some of the high frequency chop.

Interestingly, when younger I sailed similar boats (Catalina 27 and 30, Hunter 33, Cal 35) and never gave a thought about the roll. So, it could be me and not the boat.

[Don C L]

I never gave a thought to much of anything either when I was young!

[Pauls]

Long, narrow boats, in addition to being sea kindly, can be fast.