A Question About Hull Characteristics

[Lee Jerry]

Originally Posted by Lee Jerry

An inclining experiment measures GM. From the vessel lines plan, you can calculate KM. (This can be done by hand but is usually done by computer model these days.) You can then calculate the KG through basic algebra on the earlier equation:
KG = KM - GM

Of course, in my haste I forgot to mention one detail (and too late to edit):
Technically, the incline experiment doesn't actually measure the GM. However, from the data collected in the experiment you can then derive it.

[valhalla360]

The problem is many of the things mentioned in isolation and in general terms are true.

In practice, other factors often mitigate.

Two big take aways you should look into:
- Form vs Dynamic Stability. A round bottom monohull has almost no Form stability. It's not until it's heeled over enough for the Dynamic stability imparted by the keel (or other ballast) starts resisting, that the boat stops rolling. On the other hand a Catamaran has tremendous Form stability. It will immediately resist any force that attempts to roll it. Similar effects can be applied to hobby horsing or yawing.
- Related to Dynamic Stability is the Roll Moment of Inertia. This is a measure of how much the vessel resists changes in angular momentum. The most famous example is a monohull that loses it's mast and suddenly wallows horribly because without the mast, the RMI is drastically lower. Generally, the boat won't capsize but it will sure feel like it. In smaller or more tender boats, even with the mast, if you pull the sails down, you may feel some of this effect. Weight far from the center of rotation is the best for increasing the RMI. Again, extreme examples would be a narrow round bottom monohull vs a wide beam catamaran.

[Lee Jerry]

Originally Posted by valhalla360

The problem is many of the things mentioned in isolation and in general terms are true.

In practice, other factors often mitigate.

Two big take aways you should look into:
- Form vs Dynamic Stability. A round bottom monohull has almost no Form stability. It's not until it's heeled over enough for the Dynamic stability imparted by the keel (or other ballast) starts resisting, that the boat stops rolling. On the other hand a Catamaran has tremendous Form stability. It will immediately resist any force that attempts to roll it. Similar effects can be applied to hobby horsing or yawing.
- Related to Dynamic Stability is the Roll Moment of Inertia. This is a measure of how much the vessel resists changes in angular momentum. The most famous example is a monohull that loses it's mast and suddenly wallows horribly because without the mast, the RMI is drastically lower. Generally, the boat won't capsize but it will sure feel like it. In smaller or more tender boats, even with the mast, if you pull the sails down, you may feel some of this effect. Weight far from the center of rotation is the best for increasing the RMI. Again, extreme examples would be a narrow round bottom monohull vs a wide beam catamaran.

This doesn't make much sense to me. Maybe you can explain better. In particular, I don't think you are using "dynamic stability" properly (or at least conventionally). The conventional definition of dynamic stability refers to seakeeping and maneuverability. Transverse stability has generally been a static thing (although this is changing slightly with computers).

Not sure what you consider "round bottom" (how round?) or "almost no form stability" (how low?), but a round bottom with a draft less than the radius of said bottom will have form stability.

I sure as heck don't know what you mean by "dynamic stability imparted by the keel." The keel has no real dynamic properties with respect to transverse stability (assuming a fixed keel). While it hopefully generates hydrodynamic lift (to offset leeway, and does not contribute to stability) and maybe some dampening in roll (not heel). In fact that lift creates a heeling moment, not righting. How does stability of a powerboat work without a keel?

W.r.t. stability, the keel weight (and other ballast) is factored into the CG, and once that is done have no other effect. When the boat heels, the center of buoyancy (CB) of the immersed hull shifts and starts countering the roll. It will do this as long as the CB is "above" the CG. It's exactly the same with a catamaran, many of which don't even have a keel. Their high form stability comes from the large shift in the CB due to the wide beam.

As for roll inertia, nothing you mentioned talks to stability. Roll inertia is more related to motions and comfort.

[Adelie]

Originally Posted by Lee Jerry

Not exactly.

The buoyancy vector of a hull acts perpendicular to the waterplane. When the hull is heeled a small amount, say half a degree, the buoyancy shifts to the side while still acting vertically, so this new buoyancy vector will cross the centerline at some height. For a small range of heel angles, say 2-4 deg, all of these vectors will cross at the same point; this point is called the metacenter, point M. The height of the metacenter above the baseline (point K) is the distance KM.

The various distributed weights of the vessel can be assumed to act at the center of gravity (CG), point G. (This a pretty common concept, so I won't go further.) The height of the CG above the baseline is the vertical CG, either VCG or KG.

The difference between the height of the metacentric (KM) and vertical center of gravity (KG) is the metacentric height, GM.
GM = KM - KG
If GM is positive, the vessel is stable at rest. If GM is negative, the vessel is unstable and will list or capsize.

An inclining experiment measures GM. From the vessel lines plan, you can calculate KM. (This can be done by hand but is usually done by computer model these days.) You can then calculate the KG through basic algebra on the earlier equation:
KG = KM - GM

The purpose of an inclining experiment is to determine the vertical center of gravity (KG or VCG) of the vessel.

Now, the type of keel / ballast has a direct effect on the VCG. Therefore, the keel / ballast has a direct effect on the GM and the initial stability. Some might say it has a great deal to do with it, since many sailboats would be unstable at rest without it.

In flat water for ALL angles of heel the buoyancy vector is vertical. When the boat is in the face of a wave the buoyancy vector is perpendicular to the water. If the boat is upright and on the face of the wave the buoyancy vector is driving the roll. A boat with high initial/form stability (generally associated with greater beam) is more likely to capsize.

A boat with higher gravimetric/ultimate stability (associated with heavier boat, lower total CG, higher B/D) is always trying to right the boat regardless of water plane until the boat passes AVS.

[Adelie]

Originally Posted by valhalla360

The problem is many of the things mentioned in isolation and in general terms are true.

In practice, other factors often mitigate.

Two big take aways you should look into:
- Form vs Dynamic Stability. A round bottom monohull has almost no Form stability. It's not until it's heeled over enough for the Dynamic stability imparted by the keel (or other ballast) starts resisting, that the boat stops rolling. On the other hand a Catamaran has tremendous Form stability. It will immediately resist any force that attempts to roll it. Similar effects can be applied to hobby horsing or yawing.
- Related to Dynamic Stability is the Roll Moment of Inertia. This is a measure of how much the vessel resists changes in angular momentum. The most famous example is a monohull that loses it's mast and suddenly wallows horribly because without the mast, the RMI is drastically lower. Generally, the boat won't capsize but it will sure feel like it. In smaller or more tender boats, even with the mast, if you pull the sails down, you may feel some of this effect. Weight far from the center of rotation is the best for increasing the RMI. Again, extreme examples would be a narrow round bottom monohull vs a wide beam catamaran.

By Dynamic stability I assume you mean stability relate to the ballast and center of gravity relative to the center of buoyancy. Generally this is called ultimate stability, I occasionally call it Gravimetric stability as that calls to mind the root of its action, gravity acting on the boat relative to center of buoyancy at significant angles of heel. Ultimate/gravimetric stability acts whether the boat is moving thru the water or not so it is a static phenomenon not a dynamic one.

Dynamic stability would involve changing forces on the hull or appendages due to water or air flow. Certain forms of hydrofoil (ladder foils) have built in dynamic stability.

You could consider roll moment inertia a form of dynamic stability but I think of it more as dynamic resistance.

[Lee Jerry]

Originally Posted by Adelie

In flat water for ALL angles of heel the buoyancy vector is vertical.

OK. Did I say otherwise?

When the boat is in the face of a wave the buoyancy vector is perpendicular to the water. If the boat is upright and on the face of the wave the buoyancy vector is driving the roll.

Waves are not involved in the discussion.

A boat with high initial/form stability (generally associated with greater beam) is more likely to capsize.

You have no justification to say this based only on the beam / GM. You need a lot more information to make any determination on when any boat will capsize let alone compared to another.

A boat with higher gravimetric/ultimate stability (associated with heavier boat, lower total CG, higher B/D) is always trying to right the boat regardless of water plane until the boat passes AVS.

This is either a tautology or is likely wrong - either:
"all boats are trying to right themselves until they're not"
or
I don't know what "regardless of the water plane" means.

[Adelie]

Originally Posted by Lee Jerry

OK. Did I say otherwise?

No, I misread that part of your post.

Originally Posted by Lee Jerry

Waves are not involved in the discussion.

This is a cruising forum for boats that travel coastally and offshore, implicitly waves are involved, I just made it explicit.
If we were discussing racing boats in protected waters that would be a different discussion, and a different forum.

Originally Posted by Lee Jerry

You have no justification to say this based only on the beam / GM. You need a lot more information to make any determination on when any boat will capsize let alone compared to another.

There is generally higher form stability when the beam of a boat increases. Form stability drives rolling in waves, it tries to right the boat about half the time and overturn it the other have. The Capsize Screening Formula (CSF) was developed following the Fastnet 1979 disaster. The derivation of the formula is described and explained in "Desirable and Undesirable Characteristics of the Offshore Yachts" by a CCA technical committee in the years immediately following. The CSF makes it clear in the formula that that increasing beam increases the likelihood of capsize in breaking waves.

Originally Posted by Lee Jerry

This is either a tautology or is likely wrong - either:
"all boats are trying to right themselves until they're not"
or
I don't know what "regardless of the water plane" means.

OK, let me rephrase less broadly, at moderate to high angles of heel (less than extreme angles past 90*) ultimate or ballast stability will be trying to right the boat, until the angle of the waterplane approaches vertical.

Contrarywise, form stability will be trying to roll the boat in the direction the wave is moving until the boat reaches the crest of the wave, at which time the form stability will reverse and try to right the boat.

[Bowdrie]

Originally Posted by Adelie

There is generally higher form stability when the beam of a boat increases. Form stability drives rolling in waves, it tries to right the boat about half the time and overturn it the other have.

True.
This is a vid of an early S&S Swan 43, with deep hull sections and not excessive beam, and one of the findings of the committee was born out.
That boats with deeper "wineglass" sections will roll INTO the wave face, whereas the shallow sectioned arc-bottom boats with quite large beams like the typical Bennies/Jennies roll AWAY from the wave face, and in extreme cases are much more likely to be knocked down or rolled.
This was during the Katie storm, gusts were reported as being over 100mph at the Needles and in the high 70s just about everywhere else.
The Swan is snugged down, a spitfire jib and trysail and a BIG one hits her.
You can watch the mast vs horizon to get an idea of the roll.
She reacts and recovers in a good way without excessive drama, but I'm glad I wasn't aboard.

https://www.youtube.com/watch?v=cp0qvM98pZY

[Lee Jerry]

Originally Posted by Adelie

This is a cruising forum for boats that travel coastally and offshore, implicitly waves are involved, I just made it explicit.
If we were discussing racing boats in protected waters that would be a different discussion, and a different forum.

Of course cruising boats sail in waves. But this discussion is about stability. Stability analyses start as static analysis in flat water. Most end there too. And it's in flat water that the stability factors we talk about (metacentric height, righting arm, etc.) are derived.

There is generally higher form stability when the beam of a boat increases. Form stability drives rolling in waves, it tries to right the boat about half the time and overturn it the other have. The Capsize Screening Formula (CSF) was developed following the Fastnet 1979 disaster. The derivation of the formula is described and explained in "Desirable and Undesirable Characteristics of the Offshore Yachts" by a CCA technical committee in the years immediately following. The CSF makes it clear in the formula that that increasing beam increases the likelihood of capsize in breaking waves.

OK, at least I now know where you're getting it from. But your giving the CSF way, way too much credit. It uses only two variables (beam and displacement) and ignores all the others (length, draft, ballast ratio, underbody form, etc.). It might be useful to compare two boats of similar design, but it cannot make a generalization about all boats. At least not an accurate one (even if that was the intent).

OK, let me rephrase less broadly, at moderate to high angles of heel (less than extreme angles past 90*) ultimate or ballast stability will be trying to right the boat, until the angle of the waterplane approaches vertical.

Contrarywise, form stability will be trying to roll the boat in the direction the wave is moving until the boat reaches the crest of the wave, at which time the form stability will reverse and try to right the boat.

I think it might help if you define what you mean by "ultimate or ballast stability" and "form stability" before I comment further. I may know what you mean, especially for the latter term which I have heard, but I've never heard the former one.

[Don C L]

I think intuitively a beamier and flatter hull form has more for the sea surface (waves) to work on (more form stability) versus a narrower hull that is relying more on ballast for stability (more ballast stability) stability that doesn't really kick in until you are at 15 degrees or more. It's too bad that camera in the video was mounted and not handheld by someone who kept the horizon level.
A beamier boat in flat water has considerable buoyancy on the downwind side at work to right the hull, but that buoyancy is lost once on the face of a wave. The uphill side of the hull will still have its buoyancy resisting righting at that point. If the wave is breaking then the center of gravity can more easily overtop the center of buoyancy and the boat is capsized.

[Adelie]

Originally Posted by Lee Jerry

Of course cruising boats sail in waves. But this discussion is about stability. Stability analyses start as static analysis in flat water. Most end there too. And it's in flat water that the stability factors we talk about (metacentric height, righting arm, etc.) are derived.

This discussion started with a discussion of stability, there was no statement about it being limited to static stability; but even if there was, too bad, conversations drift. Since there is a very common misconception that stability is the same as capsize resistance. They are related but are not the same.

Originally Posted by Lee Jerry

OK, at least I now know where you're getting it from. But your giving the CSF way, way too much credit. It uses only two variables (beam and displacement) and ignores all the others (length, draft, ballast ratio, underbody form, etc.). It might be useful to compare two boats of similar design, but it cannot make a generalization about all boats. At least not an accurate one (even if that was the intent).

Am I giving it too much credit? Yes, all those other things affect capsize resistance, but for most monohulls beam and displacement tend to scale with length & scantlings & ballast and even grossly hull shape. So it is a good proxy to look at just beam and
displacement. This they don't scale together it is likely that capsize resistance will increase or decrease depending on which increases more as the boat scales up.
In Europe, there is STIX. I've looked at the formula and much of the part dealing with capsize resistance is based on the area under a vessel's stability curve and it's AVS. Since monohull capsize is a dynamic event dependent requiring breaking waves and stability is a static phenomenon there is an incorrect understanding of what resists capsize. STIX looks very much like the product of a committee where everybody's pet theories got tossed in and weighted and people with a vested interest in selling boats got to put in area under the area under the stability curve which does not cast wider boats in a bad light and wider boats sell better.

Originally Posted by Lee Jerry

I think it might help if you define what you mean by "ultimate or ballast stability" and "form stability" before I comment further. I may know what you mean, especially for the latter term which I have heard, but I've never heard the former one.

Form stability depends on the lateral movement of the center of buoyancy in response to small changes in heel. Catamarans are an extreme example of this. As the boat heels one ama immerses more and the other less, the net center of buoyancy moves significantly towards the hull that is more immersed. With multihulls the center of mass may be above the center of buoyancy, as the boat heels due to wind the center of mass moves slightly to leeward but the center of buoyancy moves even further. As soon as the windward ama comes out the water the lateral movement of the center of buoyancy almost stops and righting moment begins to decrease significantly.

Ultimate or ballast stability is the stability derived from the center of mass of the boat being below the center of buoyancy. From 1855 to sometime in the 1900s the Thames Rating system encouraged very narrow vessels, the more extreme labelled "lead mines". An example was Evolution which had a 50'9" LWL, a beam of 6.4' and a draft of 10' with the ballast concentrated in a bulb. Because of the very narrow beam there is almost no lateral movement of the center of buoyancy, stability was almost entirely a function of the angle of heel and distance from the center of mass to the center of buoyancy. Because stability is dependent almost entirely on angle of heel there is almost no righting moment until the boat has heeled significantly, say 10-15*.

Modern monohulls use both form and ballast/ultimate stability so they can resist the initial heel by lateral movement of the center of buoyancy and the later heel by the increasing heel acting thru the lever arm of the center of gravity below the center of buoyancy.

Let's go back to CFS and the research that led to it. When they did tank testing of models in breaking waves they noticed that boats without masts were more likely to capsize than the same boats with masts. Hulls that they could get to capsize consistently in a certain size of breaking wave, stopped as soon as a mast was added. A hull without a mast has a lower center of gravity, more area under the stability curve, a higher peak righting moment and a larger AVS. And it was much more likely to capsize. What does this tell you? It tells me that stability is not the primary means of resisting capsize.

While capsize resistance is not completely divorced from the stability curve, it is a minor component. What it the major component? Roll moment of inertia. While a mast is somewhere around 1-2% the mass of a boat, it is responsible for something approaching 50% of the roll moment of inertia. Having figured this out the researchers went and looked at the boats that had capsized and those that hadn't and saw sufficient data to confirm.

In the 1979 Fastnet race the smallest vessel to finish the race was a Contessa 32: beefy mast, plenty of ballast, relatively narrow beam. Longer boats than that were capsized or withdrew.

If you want to break the CSF you need to build a boat with a heavy mast, a very light hull with a moderate beam and very deep draft with a small to moderate ballast in a bulb on the bottom. But nobody builds that kind of boat, if you are building a light hull with a deep bulb keel then you are racing and will go all the way with a wide beam for rail meat to hold the boat down and the lightest mast you are comfortable with so you can carry sail as long as possible without reefing. You may even go with a scow hull to maximize form stability within the required dimensions for a box rule.

[Don C L]

Reducing weight aloft as most racers do, reduced the RMI, so one can imagine that, once at a crest, the hull that can be snapped over the fastest by the breaking wave is the most vulnerable to having its CG top its CB. The extra second or two a higher RMI can resist that snap could make the difference perhaps?

I also think freeboard, something not brought up yet, is a liability in these conditions as well. I would RATHER a wave wash over the boat than slam a higher freeboard. Most of the discussion of boat stability from the standpoint of seaworthiness must be done in a rough sea.

[Don C L]

This video is widely circulated and touted as a boat in trouble but really they were just trying to sort out some problem below from what I have read elsewhere. This deep, narrow hull is not rolling that wildly considering she is running, and in fact at the end, once she turns and is broad reaching instead of running, you can see how she is more stabilized and the hull allows the swell pass under her without adding much to the heel. Not the best video to demonstrate it, but

https://www.youtube.com/watch?v=Nf7FddPO5QM

[Adelie]

It's not just at the crest but all the way past the preceding trough. As the boat approaches the trough, the backside of the preceding wave is heeling the boat towards the oncoming peak. The boat with lower beam and/or higher roll moment of inertia leans into the oncoming wave longer and is slower to roll with the wave so it is less likely to reach to point of no return and more likely to reach to crest where form and ballast stability both work to right the boat.

It's like a tuned damper in a building to resist earthquake inputs. Increased beam makes the boat more in tune with the wave frequency and the increased displacement increasing the roll moment of inertia detunes the roll of the boat from the period of the wave.

[Don C L]

Originally Posted by Adelie

It's not just at the crest but all the way past the preceding trough. As the boat approaches the trough, the backside of the preceding wave is heeling the boat towards the oncoming peak.

I understand that logically, and agree, but you must be sailing in a lot shorter period seas than I am!