My Anti-roll Bag Experiment

[Benthic2]

I spec'd 6 inch pipes in my drawing but you can get it larger if you needed. [ I'm available for independent graphic artist projects if anyone is interested ] I've already lost all my math cred, but lets see if this is sound reasoning. We need to move 20 gallons of water in about 8 seconds. I think that's about 8000 gallons/hour. A bilge pump can can do that with a 2 inch diameter, so a 6 or 8 inch diamter pipe should be OK. However, I was envisioning a siphon effect to get the last of the water to the other side so too big a diameter might be a problem.

With the hump in the middle the water won't change sides until a certain amount of roll is attained, and then, once the boat leans enough to make the tube horizontal it should all go at once with a little siphon pull to help it.

Does the damping come from the conservatioin of momentum when the moving water is stopped, or is it the shifting center of gravity. I sort of figured it was the opposite of a kid on a swing set. The kid leans back and his center of gravity shifts back, so the swing moves forward. When he reaches the forward most point he leans forward so his COG moves forward adding energy to the swing back. I had imagined the water tank would be just the reverse of a kid trying to swing higher.....if the kid reverses his movements he will take energy out of the system instead of adding it. That's what I thought the water was doing, providing a damping effect to the moving COG of the boat.

 

[Delta Riverat]

The weight does not concern me. I have seating for 7 people up there.

 

[Simi 60]

Metronome
https://www.online-stopwatch.com/online-metronome/

Put weight down and speed up the beat/roll
Put weight up and slow the beat/roll......but go to high or heavy and it may not recover.

 

[Simi 60]

The weight does not concern me. I have seating for 7 people up there.

7 children or 7 giants

 

[Delta Riverat]

Hehe, 7 giants -

I have the big Todd captains chair at the helm and 2 bench seats along the sides that are 20 inches x 72 inches each

 

[SteveK]

The weight does not concern me. I have seating for 7 people up there.

seating is off topic but, member GFC posted this
seating for 12 but a capcity not to exceed 1500lbs

My earlier comment was not how many people you can fit on the flybridge in calm water orunderway, it was adding moving weight from side to side and how that affects the structure holding up the flybridge. Wear and Tear?

Tips from Boat US

 

[Delta Riverat]

I get it, I do and it is a big concern. If the water goes smashing into the structure you have impact loading and unloading. If it smashes into the tube ends I wonder how long the tube is going to hold.

Yeah, lots of stuff to wonder about.

 

[Marco Flamingo]

[QUOTE=Delta Riverat;1192532]I get it, I do and it is a big concern. If the water goes smashing into the structure you have impact loading and unloading. [/QUOTE]

Then don't think about the water that is always smashing (and unsmashing) into the front of your boat. It will keep you awake at night. Always maintain your speed under 2 knots to ensure impact loading is at a minimum. Or, maybe it is easy to build a hull (or a tank) that can handle a few hundred pounds of water smacking into it. My guess is most of us already have one filled with potable water or diesel somewhere onboard.

My anti-roll bag is surprisingly powerfull to look at, but remember, it is much like me running from side to side to stifle a roll. It can be done. The difference is that the water doesn't poop out after a few cycles. I can go to bed and it keeps on working.

 

[ksanders]

seating is off topic but, member GFC posted this
seating for 12 but a capcity not to exceed 1500lbs

My earlier comment was not how many people you can fit on the flybridge in calm water orunderway, it was adding moving weight from side to side and how that affects the structure holding up the flybridge. Wear and Tear?

Tips from Boat US

Your concerns have me thinking...

Yes the flying bridges of our boats are strong. We can have over a thousand pounds up there in people.

But... to correct the roll of a boat you are applying a lateral force to the flying bridge in a way that the engineers that designed the boat never thought of.

Similar forces if you installed a set of paravanes, but only attached them to the upper part of the flying bridge. in reality worse than that because the paravane arms would apply a force in a mostly downward direction, and the water tanks force is completely lateral in direction.

The forces applied are not like a person running back and forth because the person is simply weight, and the force of the water has the multiplying factor of decelleration. Thats why a seemingly small weight of water can exert enough force to dampen the roll of the boat. This is similar to why we wear seat belts in a car.

so... for me... while i really like the idea of roll reduction i think I'll pass on the water tank option. And... if I ever install paravanes I'll add the supporting structure to distribute the loads to the hull and not the flying bridge

 

[Simi 60]

My comment was aimed at if the 7 "giants" end up on the same side of the vessel at the wrong part of the cycle that it can end badly.

I'm sure it has happened before on commercial vessels where everyone has ran across to look at a whale

Here's one

https://www.theguardian.com/world/2...atching-boat-tragedy-say-canada-investigators

 

[SteveK]

Then don't think about the water that is always smashing (and unsmashing) into the front of your boat. It will keep you awake at night. Always maintain your speed under 2 knots to ensure impact loading is at a minimum. Or, maybe it is easy to build a hull (or a tank) that can handle a few hundred pounds of water smacking into it. My guess is most of us already have one filled with potable water or diesel somewhere onboard.

.........................

Marco. You are taking this personally which is not the intent. You are losing the discussion with your examples. The hull is designed to go thru the water and take the forces. The built in tanks are low along with the center of gravity.

Your proposal puts a side to side movement of approx 200 lbs above center of gravity thus applying a force laterally greater than 200lbs on the vertical supports holding up the flybridge. The vertical walls are not braced for side to side forces of a continuous nature to counter the effects of a beam sea.
Why do boats have recommended capacity underway and sitting at dock.
The question asked was will the joint of the wall to ceiling hold up under such constant sudden movement. Will the bulkheads? I know if it is uncomfortable to sit up top then I will move to lower helm. The thought of 200lbs rocking back and forth above my head scares me.

 

[Simi 60]

I know if it is uncomfortable to sit up top then I will move to lower helm.

I'm always amazed at the difference it makes when I go down a level on ours
What feels like "oh sh1t" up top feels like bugger all below.


Unfortunately, we have no lower helm so it's a short lived enjoyment.

 

[Bmarler]

I think the bag solution like Marco has demonstrated has less lateral force transferred to the boat than a solid structure would have. If the bag was unrestrained on the roof it wouldn’t launch itself off at the first roll. The deceleration force is somewhat contained within the bag. That’s not to say there isn’t some lateral force applied, but it’s less with a bag. That’s the beauty of Marcos design. There’s more down force than lateral force.
I like the idea of the portable solution. Roll it up and stow when not needed. If expecting to do some miles on the outside, deploy and secure.
There must be a smaller version of the bladders like the ones you see on semi trucks. That’s some tough material. Clamping the ends shut instead of sewing. I do that with fire hose used as pressure bags filled with air.
Placed above a structural bulkhead, I have the perfect spot. May try it sometime, if my project list were to suddenly drop to a manageable level.

 

[rslifkin]

I'm always amazed at the difference it makes when I go down a level on ours
What feels like "oh sh1t" up top feels like bugger all below.


Unfortunately, we have no lower helm so it's a short lived enjoyment.

Same situation here with the only helm placed a bit higher than ideal for comfort. At least ours is fairly far aft. But it's definitely amazing how different the motion can feel in different parts of the boat. Some sloppy confused seas yesterday definitely had me wishing for stabilizers of some kind. Wouldn't have fixed the pitching, but less roll would have been nice and would have allowed a course that didn't pitch so badly.

Things like helm placement are definitely a factor in stabilization seeming more important on some boats than others.

 

[mvweebles]

Interest discussion from Michael Kasten, NA. Includes comparisons of all types of stabilization including bilge keels/roll chocks

http://kastenmarine.com/roll_attenuation.htm

Passive Anti-Roll Tanks: According to published research in Marine Technology, in some sea conditions, with optimized tank / vessel design, roll reductions in both amplitude and acceleration on the order of 50% to 60% have been documented. In other sea conditions, the percentage of roll reduction appears to vary considerably. Vessel speed does not appear to be important to roll damping benefit. There does not seem to be any negative effect on vessel speed or efficiency, except of course for the added displacement required to carry the extra deadweight of the tank contents. Anti-roll tanks seem to vary in size from around 1.5% to around 2.5% of a vessel's displacement. If located higher, the overall weight may be able to be less, since the tank will have a greater effect due to being farther from the vessel's center of gravity. Similarly, if the tank is able to be the full width of the vessel, its effect appears to be greater and there may be the potential for a reduction in tank weight. Space requirements are very difficult for small pleasure vessels (say below 60 feet). Possible undesirable effects on stability, depending on the vessel (large free surface effect). Very unlikely as a retro-fit. Possibly noisy. Relatively complex to design correctly (therefore relatively expensive to design). Relatively inexpensive to build. Relatively simple in use.

 

[psneeld]

The WWII class icebreaker I rode from Antarctica and home to the US (a bunch of sea miles) had massive heeling tanks with gigantic pumps in the system to assist and "tune" the tanks according to the differing sea conditions.

They never really used them underway as the system as sophisticated as it was didn't help all that much. Icebreaker hull forms are known for bad roll.... the icebreaker in my avatar had 30-60 degree rolls one trip on the way to Dutch Harbor, AK after leaving Seattle.

The times they did use the system was when we got stuck up on top of some ice and couldn't get off.

They used the pumps to roll the ship back and forth and fire hoses shooting water under the hull to help the ship slide off easier.

I was curious to ask a million questions of the ship's engineer....thinking at some point I might use a similar system on one of my future boats.

I thought about, studied and started designing an anti-roll tank system for my 40 foot Albin about 11 years ago. I stopped after figuring the amount of weight needed to be effective for the sea conditions I wanted to use it in. Lower sea conditions requiring smaller water weight were not my goal..... when I stated to see water weights well into the hundreds of pounds and how poorly my cabin structure was constructed, I abandoned the idea. I also considered Simi's concern that the amount of resisting force necessary to slow the roll in really bad conditions, poorly calculated may lead to a bad situation which I wasn't willing to explore.

 

[Marco Flamingo]

Marco. You are taking this personally which is not the intent. The thought of 200lbs rocking back and forth above my head scares me.

I'm not taking things personally, I'm only relating my personal experience. I haven't seen any reason to be scared of water moving in a bag or a properly designed tank. I was just pointing out that we already have liquid tanks on our boats. And we have all had more than 200# moving around on our FBs. Having watched the water move back and forth in my bag, it is not like having a 200# bowling ball crashing back and forth. It's much more like having a 200# person quickly moving side-to-side (probably less point loading). I am assuming that most trawler FBs can handle that without requiring structural changes. Mine can.

I'm not looking for or expecting a system that will eliminate a 60 degree roll. I fully expect that a passive anti-roll tank would make things worse in a 60 degree roll. Maybe even a 45 degree roll. That's no reason for the common hair-on-fire dismissal of the concept. As I noted, a lot of boats have hundreds of pounds of crap on the FB that will exacerbate a 45 degree roll and think nothing of it. There is a reason to not worry: has anybody on TF experienced a 45 degree roll on the inside passage?

Since I've been watching my inclinometer app, the worst I've seen (cargo ship wake on the beam with me trying to maximize synchronous rolling) is 12 degrees on about the third roll. That's enough for unsecured things to hit the floor and is definitely increases the possibility of injuring people onboard. I'm not sure how the math would work, but it's possible that a 40% increase in stabilization results in a 95% reduction of onboard injuries. That would be a good justification for having 200# of water on the FB.

If one needs to invent positive ART scenarios, here's a good one. Having 200# of water overhead in a tank stabilized my boat such that my wife didn't fall overboard in rough seas. Play one fear against another. The wife overboard scenario scares me the most.

There is one situation where I think that an ART would likely cause a greater roll in a following sea. If conditions are such that the vessel surfs down a breaker and is skewed off to one side, when the bow drives into the trough there will be some "bow steer" causing the vessel to broach. It's a creepy feeling even without an ART. As you can see in the video, the vessel would lean to one side way longer than its natural roll period and the liquid in an anti-roll tank could make a bad situation worse. It is why ARTs have a quick drain (although Swan Song says in an old post that it has never been used even in a decade of open crossings). A better solution for me would be to avoid running at full speed with 6 foot breaking seas on my stern. Not too difficult where I boat.

 

[psneeld]

If you are suggesting some of the cautions mentioned or the skepticism as being "hair on fire"....

Believe me when say this would be one of the easiest DIY targets to shoot down.

Like so many things on TF, what works for one boat...works for one boat, one skipper....may not work for anyone else. That's not to say that posting it wasn't a great idea or help...but as other put out input.... that is just part of posting DIY stuff.

So yes you now have made it personal and I will let it go...but others that have issued some caution.... have done it in good faith.

Me.... for one with quite a bit of experience at sea agrees with the one post from Mr Weebles/Kasten Marine that any anti rolling tank might be great in a small window of sea conditions and useless or worse in others.

Not saying it can't be done nor attempted, just adding to the factual base. Limited results or dangerous situations may result if one has limited skills or experience.

Good luck with yours and your personal (not everyone's) boating but challenging others this way may get a flurry of input not to your liking.

 

[Delta Riverat]

If I used 1% of displacement I would have 420 pounds of water which is 50 gallons. Sounds about right.

 

[Blues]

In theory it should work if the periodicity of your slosher is a bit different than that of the wave train but you’d have to tune for conditions. What might go wrong? Maybe you’ll just puke all over the place, maybe the bridge will collapse (all that force has to go somewhere), or maybe the boat will turn over.
Me? I keep the weight as low as possible, oh and I’ve screwed up plenty which is why I instinctively ask what could go wrong.

 

[Hippocampus]

Marco this segment of your early post got me thinking

“ A 700# dinghy, two kayaks, a freezer, and 4 obese Americans on the FB is okay. 500# of water? No way!”

It’s common for ocean going sailboats to have an angle of vanishing stability of 130 degrees. Some rallies or races require 120 or at least 110. It’s very rare for recreational trawlers to have AVS of even 90. Those few are A rated and from the look of your boat would guess at most you’re B rated or worst. You posted about 10’ waves. Unless you are dead down wave or directly into the train you will corkscrew. Even beam to in those conditions you usually see a wave train with wind waves on top or 2 or more trains coming from different angles. In any of these situations 500# has a really meaningful impact on ultimate stability and can be the difference between a knockdown or even inversion.
The effect on righting arm is dependent upon weight and distance to meta center. That’s why oceangoing sailboats are so miserable to be on when dismasted in a seaway. Less losing the mast improves AVS but at a huge detriment to comfort. Boat is too stiff. Boat maybe at higher risk for downflooding as well given it may oscillate into an oncoming wave.
So your tank is a well proven and viable technique for decreasing roll. The physics are simple, its passive and of low cost. However it may make sense for a ship or other vessel that has been designed from scratch to have them and the AVS taken into consideration but I think it may be dangerous for the typical recreational trawler.
I would suspect most people would abandon the flybridge well before seeing 10’ waves so one would only be dealing with the 500# you stipulate. Even so would think just that 500# could be quite dangerous.
All active forms of stabilization (fins,rotors,gyros) have no impact whatsoever on AVS but tanks do. PS mentioned U shaped tanks as seen in ships. Typically they are in the hull with the weight entirely or mostly below the waterline. Even so with that placement and a shorter relative arm they can adversely affect AVS if not tuned correctly. From what I understand valving allows tuning to vary. Your system does not.
There’s a difference between how naval architects use the term “stability “ and how you and I use it when talking about comfort. There’s no way placing additional weight on the flybridge improves stability. Agree both tanks and even a non moving weight improves comfort. Even a non moving weight creates inertia.
If you’re in protected waters or never go out on really sporty days see your project as possibly safe. However even then before deploying would run this by a NA to see if it’s safe and up to what seastate it can be left filled with water.

 

[Marco Flamingo]

Sorry about the hair-on-fire comment. I was referring to threads I had read on other sites. Blues now brings up one of my favorites ("the boat will turn over"). I don't know the condition of all of the boats TF, so maybe 200# would collapse some of their flying bridges. My comment was just intended to challenge some fears. I don't mean to discourage comments and theories, in fact I encourage them. I need my ideas challenged and everyone gets to decide their own comfort level.

Hippocampus said: "All active forms of stabilization (fins,rotors,gyros) have no impact whatsoever on AVS"

If the active stabilization system is operating perfectly. If some little computer/electrical/hydraulic doodad malfunctions or gets out of sync, it can sink a vessel. The MV Zenobia is one example. Sitting in a quiet harbor, Zenobia's form of active stabilization malfunctioned and it sank. Good thing it was in port.
When an active stabilization fin is torn off, thus breaching the hull, that can effect vessel stability (until it settles on the bottom, to make it a real hair-on-fire comment). What would happen if an active gyro got 180 degrees out of sync in heavy seas? We wouldn't know because there wouldn't be any survivors (again, my hair-on-fire analysis).

But my point is all forms of stabilization can have some unfavorable effect under certain conditions. I've had various computer/electrical/hydraulic failures onboard over the years and will in the future, which makes me very interested in a simple system that's "activation" relies only on gravity. Gravity has never failed me. And I'm willing to suffer what some might call "maximum effectiveness" for simplicity.

Delta River Rat: Most of the studies I've read are looking at a liquid weight of 1-2% of vessel weight. For me, 2% would be 340#, which is at least 100# more than I have ever tried. My trials were at just over 1%. Of course placement, tank design, liquid choice, vessel roll period, desired effectiveness, etc. would dictate the weight needed.

I'm not sure how much the available research on the placement of a tank on the deck of a RORO cargo ship is valid when applying it to the flying bridge of a small trawler. Therefore I figured that with limited time/cost investment I could do my "research" with just sea trials and a bag. My bag will easily hold another 11 gallons, bringing the weight up to 2%. I'll post the results if I survive.

 

[Hippocampus]

Active systems can be easily and quickly turned off. Not so much with passive systems. Both fish and tanks are passive and present a much higher risk of negatively effecting ultimate stability. Hence the Canadian government notice concerning fish and the use of valving systems and dewatering systems for tanks in ships.
Some basic realities. When Magnus, fins or gyros are working they have no positive effect on AVS but neither do they have a negative effect. The one exception is gyros if they reach the limit of procession and and the software isn’t programmed to deal with it. Even then the operator can turn it off and fix the gyro so it doesn’t proceed until it winds down. A simple button push. For fins the off position is neutral. Again there’s two options proper software and simply a button push to turn it off. Same with Magnus. You can never say never but the reality is having either fins, Magnus or gyros contributing to a knockdown or inversion is so negligible that neither rating organizations, governmental bodies nor insurance companies have imposed any restrictions on any vessels having any of these comfort systems onboard. Sea state restrictions are not effected by their presence.
For roll inhibiting tanks this is not true as in the presence of no failure and with them operating nominally they can have an adverse effect on stability. Before they are installed and deployed a careful review of their effect on stability should be calculated by by a skilled professional (usually an N.A.). Yes there are mechanisms and design elements to decrease the risks but some degree of risk remains.
A simple thought experiment. You have a “B” boat and put on a closed tubular bag or two tanks joined by tubing. It weighs 500#. The rate of fluid transfer from one side to the other changes with internal baffles, angle of heel, rate of acceleration of heel, pipe size or other factors. A simple bag with no internals will transfer water and weight quite rapidly so will be in synchrony with the roll or extremely close to it. If in synchrony can augment the roll. Even worst can increase the risk of a harmonic. With your design once fabricated and installed the various rates of transfer cannot be modified. Assume some mechanism to slow rate of transfer is employed. A system different than yours. As you know from experience when you’re in 8’ers waves aren’t all 8’ nor do they all come from the same direction. Inevitably unless you are always exactly 90degrees ( no roll just heave and pitch) you will be caught wrong footed and your system would then increase roll. Unlike active systems it’s not if but when. On ships with adequate Gz curves and because size does matter when in a seaway the occasional bigger roll isn’t a big deal. For your non “a” rated boat it is.
Roll modifying tanks have been around forever. It’s a very old concept. But it’s in the ash heap of history for good reasons. As said above for recreational trawlers doing this as a aftermarket modification I’d be extremely cautious. I’d want some way to rapidly empty the bag regardless of angle of heel. I’d want some way modify rate of transfer. But I’d most importantly want to know from a professional source that its installation would not so adversely affect Gz, AVS and behavior as to present a significant risk.
Remember for these calculations a single pound at 60’ equals 60lbs at a foot. My flybridge is about 20’ from my meta center for weight. Believe that center is around a foot above my engine. On my boat there’s a sign thats very specific about how much weight can be brought to or stored on the flybridge safely mounted by the builder at the stairs to it. . We get off the flybridge in a seaway.

 

[Hippocampus]

Interesting about the RO RO. Wiki says it was ballast tanks. Don’t think that has anything to do with stabilization for comfort but everything to do with AVS and Gz. . Also maiden voyage. Seems kinks not worked out.

https://en.wikipedia.org/wiki/MS_Zenobia

Some years back read a treatise about roll inhibiting tanks by a Canadian professor. Very scholarly and long but end of day discovered a lot more complex concerns than you would first think. I have no idea what kind of boating you do or the details of your boat. I like your inquisitive attitude and support your experimentation . My only point is without knowing what that 2% means to your boat you don’t know its impact.

 

[Marco Flamingo]

Some years back read a treatise about roll inhibiting tanks by a Canadian professor.

That could have been Dr. Bass. He is now a retired professor of mathematics from Memorial University of Newfoundland. He wrote one of the most thorough studies where he got 3 "identical" fishing boats to use different stabilization systems. In a follow-up article I read that the captain on the boat with the ART no longer used it. One issue was that crew members were overfilling the tank to make it more comfortable when working on deck or sleeping. Once underway, the captain was surprised (not in a good way) about the ship's handling and stability. Too much of a good thing is something to watch out for.

Not sure if Dr. Bass is still around and what his opinion of ARTs is now.

 

[Marco Flamingo]

A simple thought experiment. You have a “B” boat and put on a closed tubular bag or two tanks joined by tubing.

That's the situation I hope to avoid. Two tanks connected by a tube, especially if there is some type of mechanical valve system, creates something different than a free-surface ART. Mine (in a bag) isn't quite free-surface, but more so than having a connecting tube.

Until one watches the free-surface tank in action, it is difficult to imagine the effect on the vessel of just a surface wave in the tank. Here is a grainy video that shows the basic motion. The video "pours in the exact amount" to nullify a particular wave period. Real life isn't so simple.

For smaller rolls, the surface wave in the tank appears to be the only thing bouncing back and forth. I don't think two tanks connected by a subsurface tube would continue to have the same effect as the free surface wave. Each tank would have a surface wave bouncing inside. For larger rolls (larger than in the video), one side of a regular tank completely empties out, as it does in my bag. It looks completely different than a surface wave, but appears to be about the same timing and have the same calming effect.

One of the "unrealities" in the video is the depth of water used in the tank. If to scale on a 50' boat, the water in that tank would likely be 2 feet deep. The tank on Swan Song held 6.5 inches of water. My bag is about 3 inches deep. There is only a real surface wave to about a 3 degree roll. After that, the "tide is out" on the high side and the mass is moving back and forth.

Somebody noted above that controlling the forces generated from the weight shift in a bag would be different than in a tank. Probably, but I'm not sure how much difference it makes. The end of my bag appears to "pop open" when the majority of the water rushes into it. If the bag ends weren't contained by my coaming, it would likely scoot across the deck. But the arrival of the water isn't enough to pop open my bag of 18 oz. VCP with poorly made DIY seams. It is more akin to the power of somebody dumping a bucket of water on you (the water having only fallen a couple feet). That's why I related it to the force of water striking your hull when underway. Not a big deal to address structurally. That's force number one.

Force number two in my experiment is that there is now an additional #200 of dead weight to one side just after that side is rising. That also alters the roll period and stifles synchronous rolls a perceivable amount. Small effect for some rolls, surprisingly effective for for others.

I assume that any FB structural concerns are not because of the dead weight, as anybody with a boat similar to mine has had more than 200# on the FB. In fact, the FB by itself weighs way more than my 200# bag. As a guess, I'd say my structure weighs >1,500# from the deck up to the top of the mast (10.5 feet above the deck) . I know that all that FB weight can really whip back and forth when rolling, say a 10 degree roll from a cargo ship wake. I had never even considered that my FB might collapse because I have windows. I would be concerned if I had 4 adults on the FB during such a wake, but for the people, not the structure.

Those 4 adults (700#) would be higher above the CG than my 200# ART. In a big wake, they might also be bouncing around like the water in the tank, but even if they could hold on, I don't see why their mass is less dangerous from a stability point of view than #200 of water that shifts side to side.

To complicate the issue, my experience with the bag shows that it reduces the degree of the roll, dampens it, and probably reduces roll period (still haven't found a recording inclinometer app). But the math is intricate. In addition to the above example of 2,200# (people and super structure) cycling through 20 degrees (10 degree roll each way) in 1.7 seconds, I have have a 200# anti-roll bag. So I would have 2,400# aloft (people, structure, ART) and, based on my experiment, the mass aloft would only cycle through 12 degrees (6 degree rolls) in a longer period (1.8 seconds?). A fun math project (for someone else) that could result in an ART actually reducing the lateral strain on the FB structure. Although it is counterintuitive at first glance, adding an additional 40# of water could further reduce the strain on the FB.

 

[Hippocampus]

The video "pours in the exact amount" to nullify a particular wave period. Real life isn't so simple

Yup there’s the rub. Wave period is constantly varying. Even if the waves are exactly the same period will vary with angle of attack. In fact that’s why changing course is a good technique if the boats natural roll period is in a harmonic with seastate.

Vid is instructive. Amplitude of the waves quite small. The rectangular box seems to be heavily weighted as ~3/4 is under the waterline. There’s no overhead view of the box but suspect Pb or Fe object was placed on the bottom. Every boat is different depending upon weight and where it’s placed. Some are tender but have a high AVS or stiff will a low AVS as they depend on form stability. All these details affect roll amplitude, frequency and comfort.

Seems weights involved and heights of that weight doesn’t present a danger but don’t know that without calculation’s having been done. Does seem from what you say only a modest benefit in a tightly defined set of circumstances.

Another technique with modest but noticeable benefit which is passive and doesn’t effect stability negatively is rolling chocks.

Seems you’re having fun. Possibly not endanger it yourself so have a good time. For those thinking about using anti roll tanks for cruising boats that may see significant weather please engage the services of a NA or suitable professional to make sure it’s safe. ‘Bye

 

[guy with a boat]

Slight sidetrack on the topic of AVS. Makes sense that fins/gyro etc won’t change the AVS, but would they not resist the roll (yes) and reduce the likelihood that the vessel reaches AVS? Stabilization produces substantial forces to oppose the roll and I don’t know how that wouldn’t be of value in keeping the boat right side up as opposed to testing AVS limits. What am I missing?

 

[fgarriso]

Watch this video it covers passive and active stabilizers.

 

[Hippocampus]

Slight sidetrack on the topic of AVS. Makes sense that fins/gyro etc won’t change the AVS, but would they not resist the roll (yes) and reduce the likelihood that the vessel reaches AVS? Stabilization produces substantial forces to oppose the roll and I don’t know how that wouldn’t be of value in keeping the boat right side up as opposed to testing AVS limits. What am I missing?

To my understanding short answer is no. It’s also no for Magnus and for gyros. I’ve been knocked down. Several times while dinghy racing but also on very rare occasion (2 times) on large cruising/racing sail. Perhaps not comparable to power but would point out one thing. Static stability is static stability. That’s a reflection of weight distribution and form stability while not moving. Dynamic stability is the stability when the boat is in motion and varies with motion in all three axises. It’s dynamic stability that maybe more important for us.

There’s also the impact of wind and it’s negative effects on righting arm. Except when still or at very low speed while overtrimmed the knockdowns I’ve experienced were very sudden. What happens is you’re at maybe 20 degrees and forces in balance. A wave (or gust) exceeds the force required to tip the boat and down you go suddenly You don’t typically slowly roll on to your side. With power you don’t have a rig/sails but wave and wind are still in play. The gyro, Magnus, active fin is already working pretty much as hard as they can to keep you upright. Once the inverting force is greater over you go. What always interested me is center boarders with Pb ballast in the bottom of the canoe body or contained in a short keel stub while having the board up actually do better in survival conditions.

Unlike a stabilized power boat or a high aspect fin keeled boat they (board up center boarders) will stay parallel to the wave face and slide down it sideways. High aspect fin keeled sail or stabilized power will attempt to align to gravity and stay upright. This means the side facing the wave is closer to apparent surface of the wave. This in turn increases the likelihood of being overwhelmed. Beyond increased downflooding risk waves move forward but also in a circle so as the boat moves sideways it may also rotate on the X axis and knock down or invert.

In short active stabilization works until it doesn’t. Once force exceeds its ability only inherent stability counts in the moments before knock down. AVS is measured or calculated in the static state. Forces in the dynamic state are more complex.

Fins work by the negative pressure on their upper sides while moving forward. Once there’s enough quick rotation of the vessel on the X axis of the vessel there will be turbulence generated by both the leading and trailing edges. It may slow the knock down or inversion but not prevent it. Same with rolling chocks/bilge keels or even a fin. They will not prevent a knockdown. Once the boat is no longer on the surface but in the wave significantly or totally active stabilization isn’t going to meet the needs. There’s a reason for non Dashew style boats facing the wave train either with engine or sea anchor is the traditional survival technique. Appropriately constructed boats can use a JSD (Jordan series drogue) which has the major advantage of the boat moving in the direction of the wave train so less likely to breakup.

Active tanks may prevent these catastrophic results but only if you can pump water fast enough the weight is greater on the high side and the freeboard is sufficient. That means ships not boats. Even then very difficult to achieve.

Multihulls are limited to an AVS of ~90 degrees (some are much worst) being nearly entirely dependent upon form stability. Once again daggerboards or swing foils up increase survival from what I understand. They allow the boat to slide. Here again size matters so most sailors and NAs want a LWL of ~45’ to allow sufficient beam and force in the righting arm. Whereas monos(both power and sail) can recover from a knock down multihulls can’t. Even with escape hatches surviving while on top of an inverted multi is problematic in anything force 7 or above one would think. Better to stay upright.

Has regards comfort and so called comfort quotient weight and size matter. A heavy full displacement boat in general if drawn by a good NA floats in the the water and not on it. OP seems interested in comfort. Most of us are on SD. We diligent avoid serious weather. For us ,good Lord willing, we won’t see such weather on our SD coastal boats. But we still need to pay attention to how any modification we make may change static and dynamic stability.