Stability. A brief discussion.

[Portage_Bay]

My thoughts on stability for coastal and inshore recreational trawlers. Especially boats without stability data available to the captain and that have been modified since build. In other words, many of us here on TF.

I see a lot of talk here by posters very knowledgeable about stability referencing Gz curve, Center of Gravity (CG), Center of Buoyancy (CB) and Angle of Vanishing Stability (AVS). All good data to be sure but many of us don’t have it.

This post is for those taking the first steps in understanding stability. Not for those already well versed in the subject.

I've seen some incorrect statements and some nonsense. An example of an incorrect statement is that CG must be below CB for a boat to be stable. Nope, simply not true. An example of nonsense is the idea stability can be assessed by timing a boat's roll at the dock in relation to it's beam. Umm, if there's any truth to that I'd be very interested in seeing the proof.

I was going to write a long post but have decided to suggest you read A Guide To Fishing Vessel Stability by Maritime New Zealand. A great deal of it applies to our coastal cruisers.

It’s well worth your time if you don’t want to read the PDF to look at the attached image. Think about how CB changes with angles of heel. Think about the risk of boarding seas and down flooding long before AVS is reached. Think about how your boat will respond to boarding seas and downflooding.

Understand that Gz, CB and AVS are often calculated for static conditions only. Flat water, no outside influences. Nothing like being at sea.

Consider that CB is not simply a calculation for a slice through the hull as all images I can find demonstrate. CB is for total immersed volume of the hull. The immersed volume changes, sometimes dramatically in a sea way.

There is a brief discussion of why stabilizers do not make a boat more stable. The discussion is about paravanes but in my opinon applies to any form of stabilization. All stabilization does is slow the roll for comfort. And if it slows the roll when the boat is upset it’s reasonable to assume that it slows the boat’s ability to right itself.

Consider a recreation boat designer's goals. Creating a boat with comfortable living spaces, much of it above deck. A boat capable of navigating shallow harbors, not a lot of hull in the water. A boat that has a comfortable roll. A stiff boat is uncomfortable because it has high Gz, a tender boat has low Gz and easily upset. The designer strives to find a balance between safety and comfort. Don’t mess with that too much by adding weight up high. Be careful removing weight down low as wall. Swapping a lead acid bank for lithium? You’re changing CG.

That leaves the unanswered question. Getting back to the reason for this post. What’s the owner / captain of an older boat lacking stability data that has been modified to do? Be conservative with weather and sea state conditions assessments. Don’t add any more weight up high.

 

[Roger Long]

Roll period can be very useful for rough initial determinations of GM but a rolling constant must be applied to the beam. This constant is primarily determined by the Radius of Gyration, or how far the mass of the vessel is distributed from the center of gravity. For example, a sailing vessel with the same rolling period and beam as a power vessel will have a different GM due to the weight of the rig. The same would be true for fishing vessels with, say, the same hull; one with a tall shrimper rig and the other rigged for something like long lining with little top hamper. Calculating the rolling constant directly is too labor intensive to be practical so it’s usually derived by comparing the roll periods of similarly configured vessels where the GM is also known. In the many stability tests I’ve conducted, most for USCG certification, the roll period was always measured, converted to a rolling constant, and recorded to contribute to the data base. For an individual vessel, it can be very valuable for tracking changes in GM with modifications. Most of the trawlers considered by this forum are similar enough that a known GM, beam, and rolling period for one could be used to determine, by rolling period, if another falls into the high, moderate, or questionable stability range. For more, see the sidebar I wrote on rolling in “Voyaging Under Power”.

There is a written version here of a lecture I used to give to college students as a simplified, no math, primer on the subject. It’s focused more on sail but much is still applicable to power vessels. Teaser: buoyancy and the metacenter are imaginary.

https://www.cruisingonstrider.us/Stability.htm

 

[Hippocampus]

Thanks so much Roger and PB.

 

[Portage_Bay]

Roger,

Thank you for this. Happy to be corrected in my misstatement by an expert such as yourself!

Reading your response jogs my memory of being able to purchase a device decades ago that monitored roll period real time. We didn't really trust it, we had good stability documentation done at build, updated after major re-configuration and fine tuned by inclining experiments as needed.

Roll period can be very useful for rough initial determinations of GM but a rolling constant must be applied to the beam. This constant is primarily determined by the Radius of Gyration, or how far the mass of the vessel is distributed from the center of gravity. For example, a sailing vessel with the same rolling period and beam as a power vessel will have a different GM due to the weight of the rig. The same would be true for fishing vessels with, say, the same hull; one with a tall shrimper rig and the other rigged for something like long lining with little top hamper. Calculating the rolling constant directly is too labor intensive to be practical so it’s usually derived by comparing the roll periods of similarly configured vessels where the GM is also known. In the many stability tests I’ve conducted, most for USCG certification, the roll period was always measured, converted to a rolling constant, and recorded to contribute to the data base. For an individual vessel, it can be very valuable for tracking changes in GM with modifications. Most of the trawlers considered by this forum are similar enough that a known GM, beam, and rolling period for one could be used to determine, by rolling period, if another falls into the high, moderate, or questionable stability range. For more, see the sidebar I wrote on rolling in “Voyaging Under Power”.

There is a written version here of a lecture I used to give to college students as a simplified, no math, primer on the subject. It’s focused more on sail but much is still applicable to power vessels. Teaser: buoyancy and the metacenter are imaginary.

https://www.cruisingonstrider.us/Stability.htm

 

[Roger Long]

Reading your response jogs my memory of being able to purchase a device decades ago that monitored roll period real time.

I was once involved with the design of some long liners that had anti roll tanks of a type invented by my partner. Instead of the patented design with a restriction in the middle, the tanks were simple rectangular tanks. By putting in just the right amount of water, about 6 inches, the breaking of the wave would slow the water movement in time with the rolling in such a way that the water and weight would be on the low side when the vessel rolled back. Effective.

We were shocked when the boats were in service to find most of them with the tanks, mounted on top of the pilot house, completely filled which put the vessels in dangerous stability territory. The reason was that the skippers, experimenting to find the right level, found that the roll just got slower and slower and the boat more comfortable as they filled the tanks so they just slugged them right up.


I left to start my own office about that time and installation of overflows at the proper level were being discussed. The slowest roll of all is one where the boat just goes over and doesn't come back. Counterintuitively, a boat on the verge of capsize will roll very little.

 

[PierreR]

An example of nonsense is the idea stability can be assessed by timing a boat's roll at the dock in relation to it's beam. Umm, if there's any truth to that I'd be very interested in seeing the proof.

That leaves the unanswered question. Getting back to the reason for this post. What’s the owner / captain of an older boat lacking stability data that has been modified to do? Be conservative with weather and sea state conditions assessments. Don’t add any more weight up high.

I am the source of the nonsense you have written off. Your question is totally unanswered because you have written off everything but a multi boat unit big buck test that is only good in the configuration tested and any changes to the boat including movable load makes it a mute point.

I would have posted the full description of the roll test but it's copywrite material. The Nature of Boats by Dave Gerr is a very good basic reference manual that covers it very well. Chapter 15 is "The Great Stability Mystery"

If my explanations going forward are still nonsense, so be it. You interpretation takes nothing away from me or my knowledge and boating experience.

Dave Gerr makes it clear that the roll test is very useful for comfort and of fair value for checking if you are approaching the dangerous ultimate stability point. He also make is clear that its a rough test to tell you cheap and dirty whether you can add weight up top.

For ultimate stability the roll test does not give you numbers. It gives you an idea if you need the full test or to exercise caution when making changes. He then goes on to state that the roll test is only useful for power boats that are not trying to carry sail and have fairly high initial stability.

In his words, if, when deeply rolled at the dock and the roll test reveals a ratio grater than 1.1 or if the roll is very slow near the roll extremes or, shows any tendency to hang before coming back, then you best think about not taking your boat in rough conditions or adding additional weight up top. Again this is only for power boats. Her recommends a full stability calculation if the above parameters are true.

Tad did not say the roll test was not useful. What he said was that from his point of view, the only way to be sure is the run a full stability test. I do not disagree with this statement at all. I asked him to clarify what kind of $ that might involve and he indicated that if you have line drawing and stability centers the calculation would perhaps only be a couple thousand dollars. Pulling lines from a hull and a static stability test are time consuming and would add substantially to that cost.

Given the cost of a full stability test, if my project of adding weight is under $15k I am going to run the roll test. It's far better than a guess.

How I would do that for my boat is, load the boat in the worst case scenario to what I expect too encounter if I am totally not paying attention. Get as may people as I can to roll the boat at the dock and time the seconds for three consecutive tests and compute the average.

If the roll ratio is greater than 1.1 or I sense any form of hanging, I will think hard and long about adding or subtracting weight. If it passes, I will add enough weight to more than compensate for the changes I want to make and repeat the test. it is passes, I will not feel to bad about making changes such as Solar panels, enclosures, hard tops and davits up high.

I thank Tad for giving me an idea of how to determine the cutoff when a full test is warranted. My boat is currently indoors on level cement and the boat is leveled to the water line. Good time for me to pull the lines so I have them. It's not difficult but tedious and far easier with two people.

In the mean time, I do not feel like I have no options but a wild ass guess from looking a the boats profile. For myself, your question is answered. I am curious is someone else has anything better other that paying the big bucks.

For those who want to know I would suggest getting Gerr's book, "The Nature of Boats". Beyond basics, there is "Skene's Elements of Yacht Design" and "Preliminary Design of Boats and Ships" by Hamlin. If you don't remember enough calc, you will find them hard to read.

I am not in the slightest insulted if everyone thinks I am nuts. If you do then get in line behind my wife.

 

[Portage_Bay]

I do not think you are nuts. And if you see above I have been corrected by Roger Long, another expert on stability.


My apologies if my post offended you.

I am not in the slightest insulted if everyone thinks I am nuts. If you do then get in line behind my wife.

 

[Roger Long]

I would have posted the full description of the roll test but it's copywrite material.

Here's how to do it: (Gawd forbid any copyrighted material should ever appear on the Internet.)


Loosen the lines and push the vessel free of dock and fenders.
Push or step on and off the rail until you see a little movement and then keep pushing in rhythm with it until you get as much roll going as you can. It's easier than you think if there is no wind. I've gotten good rolls going on vessels over 100 feet long.


Stop pushing. The measurement isn't valid if being forced. Sight something on the boat and time as many rolls as you can all the way from one side to the other and back. Divide by the number of rolls.

I was once sitting with some crew on the rail cap of a big schooner. We had our feet on the pier talking to someone ashore and I was leaning against a davit. I started pushing and soon had a really good roll going. No one noticed what I was doing and were looking around mystified by the action in the calm harbor. I told them and, since I was there to do a stability test the next day and they saw how easily I could get a vessel over 100 feet long rolling hard enough to hear creaking in the rigging, they were very helpful and attentive when I did the test.

 

[Delfin]

A bit more expansive explanation of how to conduct a stability roll test.


https://assets.publishing.service.gov.uk/media/5a7ba08040f0b645ba3c58a9/draft-guidance-note.pdf

 

[Roger Long]

A bit more expansive explanation of how to conduct a stability roll test.


https://assets.publishing.service.gov.uk/media/5a7ba08040f0b645ba3c58a9/draft-guidance-note.pdf

The Brits are good at this sort of thing. Our Gulfstar 43 has a roll period of about 4.5 seconds and a beam of 14 feet or 4.27 meters so just about right on their line. Lots of freeboard though and no transom door with big glass windows opening up into an aft cockpit to become the bow if power is lost and she starts running down wind. She rolls a bit more than a lot of trawlers but I'd consider her safer in that situation than most of the sedan configuration.

 

[PierreR]

Roger I will bet you can get that Gulfstar rolling and I am sure you get much better mileage than my Hatteras 42 LRC with twin Detroits.

My roll period is pretty snappy but I have only done it when I was in a very stable load condition.

 

[Marco Flamingo]

The last post in my "anti-roll bag" post here on TF shows the use of a phone app to easily and accurately measure the roll period over time. My goal was to see whether my anti-roll bag affected the roll period by changing (decelerating) the roll at a certain point. Got distracted by going south for the winter and can't link the thread because I'm not at my regular computer. But still following this stuff with interest.

 

[Marco Flamingo]

A bit more expansive explanation of how to conduct a stability roll test.


https://assets.publishing.service.gov.uk/media/5a7ba08040f0b645ba3c58a9/draft-guidance-note.pdf

Interesting that it says to measure maximum beam at deck height to conduct the test. Then it says to make a calculation as to a point above the waterline to not exceed during the roll test. That point is well below deck height. So what is special about deck height if the test never goes near it? My beam at deck height is .3 meters greater than at WL.

 

[Delfin]

I suspect the presence of stabilizer fins might affect the results of a roll test, but I certainly don't know for sure.

 

[PierreR]

I do not think you are nuts. And if you see above I have been corrected by Roger Long, another expert on stability.


My apologies if my post offended you.

No you did not insult me, that is very difficult to do.
My frustration stems from being clearly misunderstood in the other thread. Its clear to me that you were one of the one's getting what I said wrong.
The problem is that language itself is a bit of a mystery to me and when I am misunderstood by most, I know that I likely did not choose my words correctly to convey my actual thought. So, my frustration is not clearly communicating what was actually in my head. That is not your issue, its mine.

 

[Hippocampus]

Perhaps Tad will help us here. I’ve noticed some boats are very tender initially but stiffened right up and contrapositive as well. I walk away thinking theres a bit of a disconnect between initial stability and ultimate. Believe this is somewhat borne out looking at Gz curves and calculations of comfort quotients. “Brewer’s Comfort Ratio is: Displacement in pounds/ (.65 x (.7 LWL + .3 LOA) x B1.333). “
Think boats float on the water or in it to varying degrees. One of the most uncomfortable passages I took was on a Chris White racing tri. However given its extreme beam never worried for a second about it flipping. Great sea boat but jerky. Much more worried on some series production cruising cats.
Would think even at rest rolling chocks or fins with no motion of the boat would create turbulence so might effect a roll test. Would think any increase in submerged lateral plane would slow roll as well. But minimal effect on AVS.
Look at the roll test as showing more about the comfort than safety. It’s not practical for us to get more than 15-20degees of roll while in a slip for most of our boats.

 

[Roger Long]

It’s not practical for us to get more than 15-20degees of roll while in a slip for most of our boats.

Roll period is independent of roll angle. Think of a pendulum. You only need to produce enough roll to be able to see when a cycle starts and stops and time it. If you watch closely, 2-3 degrees will give you a measurement.


You are way off base on the initial vs ultimate business but too much to go into here. This might help:
https://www.cruisingonstrider.us/Stability.htm

 

[Delfin]

Perhaps Tad will help us here. I’ve noticed some boats are very tender initially but stiffened right up and contrapositive as well. I walk away thinking theres a bit of a disconnect between initial stability and ultimate. Believe this is somewhat borne out looking at Gz curves and calculations of comfort quotients. “Brewer’s Comfort Ratio is: Displacement in pounds/ (.65 x (.7 LWL + .3 LOA) x B1.333). “
Think boats float on the water or in it to varying degrees. One of the most uncomfortable passages I took was on a Chris White racing tri. However given its extreme beam never worried for a second about it flipping. Great sea boat but jerky. Much more worried on some series production cruising cats.
Would think even at rest rolling chocks or fins with no motion of the boat would create turbulence so might effect a roll test. Would think any increase in submerged lateral plane would slow roll as well. But minimal effect on AVS.
Look at the roll test as showing more about the comfort than safety. It’s not practical for us to get more than 15-20degees of roll while in a slip for most of our boats.

Given the formula for calculation of stability based on period divided by beam, if the period is less (tender boat) than the stability will be greater than a vessel with a slower period (stiff boat) if the beam is the same. I consider Delfin to be a 'tender' boat, but as heel angle increases the effect of most of her weight being near or below the waterline, the gravitational force of the keel increases, and stability along with it. Make sense?

Confounding this calculation would be masts and fins, both of which would seem to increase roll period. If you've ever seen how a dismasted sailboat rolls you'll know what I mean.

 

[Nick F]

Lots of theoretical stuff here, but what I would really like is to see how the GB42 (a very popular boat, and what I have) looks against, say, the CE criteria.

Does anyone have any info relative to this?

Does anyone have righting arm curves (or KZ, or whatever) for the GB42?

 

[SeaDogAK]

There is a brief discussion of why stabilizers do not make a boat more stable. The discussion is about paravanes but in my opinon applies to any form of stabilization. All stabilization does is slow the roll for comfort. And if it slows the roll when the boat is upset it’s reasonable to assume that it slows the boat’s ability to right itself.

That’s true of passive stabilization, like paravanes, but wouldn’t it be different for active stabilizers? Fins, if I understand right, sense the roll when the boat is in motion and apply a force in the opposite direction to damp the roll. Which should do more than just slow the rolling motion, if I understand the physics right.

 

[Portage_Bay]

As I'm learning by reading the responses to my OP there is a lot I need to learn. So, take this next as opinion only. Really mostly questions.

Stabfilizers can't change CG. Stabilization reduces roll, reduced roll should limit changes in CB. However each stabilizing system on a boat has limits in how much roll it can compensate for. Once the limit is reached what happens? Does your boat continue to increase it's roll? I'll say it does. Then how does the system react to being 'out of range'? How does the system sense roll and react to roll? I have no idea. Can you be sure the system will act in a way to help the boat right itself?

There are simply too many unknowns for me have confidence stabilizers increase stability. That said I don't have a lot of experience with stabilization. Some with paravanes, more with active fins. They do dramatically increase comfort. But do they increase absolute safety? I don't think so.

Roger Long has been great in his contributions to this thread and starting his own Rolling, Rolling. Maybe he'll provide his thoughts on the subject.

That’s true of passive stabilization, like paravanes, but wouldn’t it be different for active stabilizers? Fins, if I understand right, sense the roll when the boat is in motion and apply a force in the opposite direction to damp the roll. Which should do more than just slow the rolling motion, if I understand the physics right.

 

[Hippocampus]

Roll period is independent of roll angle. Think of a pendulum. You only need to produce enough roll to be able to see when a cycle starts and stops and time it. If you watch closely, 2-3 degrees will give you a measurement.


You are way off base on the initial vs ultimate business but too much to go into here. This might help:
https://www.cruisingonstrider.us/Stability.htm

Thanks for the education. I expressed myself badly. I understand about pendulums. Father was the president of the national watch and clock collecters Association for awhile. He delighted in sharing his knowledge so he taught me about that subject. Still hull shape and weight placement matter to my understanding. Been on high aspect bulbed fin keeled boats with little form stability. They tend to be quite tender at modest heel angles but very stiff and remain safe at higher. Thought the effect produced by the bulb (or bob in a clock) increases as it deviates from the null. When the gravitational force is directly below the pivot point effect is absent. As it deviates more effect is seen. Longer the arm greater the effect.more weight greater the effect. For clocks and torsion spring watches longer the arm slower the swing if equal input from a weight or spring as propulsive force and input gearing. To my understanding boats are more complicated as there’s not just one input causing deviation and not just one input aimed at restoring the boat to level or pendulum to neutral. . For clock pendulums there’s the gravitational force on the bob (mainly) once you start a swing and the force of the spring or weight maintaining a continuing swing. Even with the weight pulled up or spring fully wound until you start the swing nothing happens. Its a latent force. Simple system. One force in. One force setting the period. For boats there’s more forces causing deviations and more forces against deviation from level. Yes there’s a host of clever modifications of the escapement to keep the release of the force applied to the balanced wheel or pendulum the same period but it’s a simpler closed system c/w a boat. For boats forces causing deviations are constantly changing. Forces resisting deviations are as well. What I’ve been responding to has been a lack of appreciation of the complexities of some discussions about stability. What type of stability? Moving or still? Exactly what you mean when you say stability? Does it matter to you how quickly the vessel returns to level or what degree it’s stable when inverted or at 90 degrees?
You know infinitely more about this subject than I do. I’m neither facile with the language of NAs nor fully grasp the impact of the physics involved. Continue to look forward to you helping us understand these complexities.

 

[Mobcat]

Its pretty simple imo, if you start with a good design for example a classic trawler style displacement hull, then if you add weight up top you add weight down below and as close to the keel as possible, we added a whole other level to a ex Tasmanian crayfish boat of 45ft a extra bedroom living space and fly bridge and all we did was put about 4 tonne of second chain down In the sole under beds and as close to the keel as possible, and I actually believe now that the boat is more stable than ever in seas, we approximate that the roof top extension weight was around 2 tonne but we also hold another 1000lt of water now to

 

[Delfin]

As I'm learning by reading the responses to my OP there is a lot I need to learn. So, take this next as opinion only. Really mostly questions.

Stabfilizers can't change CG. Stabilization reduces roll, reduced roll should limit changes in CB. However each stabilizing system on a boat has limits in how much roll it can compensate for. Once the limit is reached what happens? Does your boat continue to increase it's roll? I'll say it does. Then how does the system react to being 'out of range'? How does the system sense roll and react to roll? I have no idea. Can you be sure the system will act in a way to help the boat right itself?

There are simply too many unknowns for me have confidence stabilizers increase stability. That said I don't have a lot of experience with stabilization. Some with paravanes, more with active fins. They do dramatically increase comfort. But do they increase absolute safety? I don't think so.

Roger Long has been great in his contributions to this thread and starting his own Rolling, Rolling. Maybe he'll provide his thoughts on the subject.

Stabilizers can't really increase stability. They just counteract roll via a motion sensor that detects the direction of roll and actuate fins to counter that motion, or via a gyro instead of fins that resist roll. Stability is inherent in the vessel's design and where mass is distributed within that design.

Delfin is a tender boat because of her wineglass hull, but with a steel hull, aluminum superstructure and a majority of weight at or below the waterline, stiffens as roll increases. A month at sea this summer gave her stabilizers a workout, but I doubt we experienced more than 5 degrees roll the whole time, even with predominant 6' seas coming abeam. In those conditions, the 9 sq ft fins were routinely pegged, but other than jerkiness as waves stuck from the beam she was always upright.

 

[Roger Long]

Its pretty simple imo,...

Yes. Quite simple

The center of gravity of a typical trawler will usually be between 80 and 90 percent of the hull depth amidships. Depth is measured from the fairbody line to the main deck at midships. The fairbody is where the line of the flattish portion of the hull next to the keel projects to the centerline. Center of gravity is the "G" in GM. The "M" is largely a function of beam but, more precisely, the moment of inertia of the waterline plane and displacement.

If you add weight up high, as you did, and want to maintain the same center of gravity, the critical number is the weight multiplied by the distance from it's center of gravity to the G of the vessel as a whole. Divide this number by the distance from G to where you can install the ballast and you will know how much ballast needs to be added. If you want to be exact, you need to have done an inclining experiment and have hydrostatic data on the hull to have a good fix on the center of gravity.

Presuming you have maintained the center of gravity position, the boat will now be heavier due to the modifications and ballast. This will increase the displacement, underwater volume, so the Metacenter, M, will come down unless the hull has an unusual amount of flare to increase waterplane moment of inertia as underwater volume increases. Most vessels will have M lower as they are made heavier.

So, GM will now be slightly decreased along with righting arms. This will increase the amount the vessel will heel due to side wind force but this will be somewhat offset by the greater displacement. Since you have added windage surface area, there will be more wind force offsetting that change back in the other direction.

Freeboard will have been reduced by the additional weight. This means that, for the same center of gravity location, the range of stability will be reduced and thus the angle the vessel can recover from if knocked far down by something like a breaking wave from the side. OTOH, if the additional superstructure along with the prior above deck structure can be considered watertight at least for the period of time it would take the vessel to recover from a knockdown, then the range of stability might be increased. Few power vessels have enough range of stability though that a second tier level of buoyance would ever come into play.

There are two possible reasons why your vessel now feels more comfortable. One is that GM has simply been reduced so that the vessel is rolling slower. It's counterintuitive but, the slower a vessel rolls, the less ability it will have to reduce side wind force and the shorter it's range of stability is likely to be. You can have a boat feel much more comfortable and also be much less likely to recover from extreme heel angles. From the little information you provide, it sound plausible that you have pretty much maintained the center of gravity and the slower rolling is more a function of adding weight well spread out from the center of gravity which also slows the roll. You likely have a vessel which is slightly less recoverable from rare and unlikely extreme heel. However, given the low probability of such events for a powerboat and the probable range reduction, IF you have not raised the center of gravity significantly, the safety of the vessel is probably not significantly compromised.

I'd want to have an inclining test and detailed calculations though if I were responsible for modifications of the extent you described.

See, simple.

 

[guy with a boat]

Yes. Quite simple …

See, simple.

Great explanation. Thank you.

 

[Kit_L]

Roger, please post more here!