Stabilisation at rest: Anti-roll tank

an interesting and pleasantly low tech solution, I wouldn't want all that water/weight up that high above my CofG, but if you've got plenty of righting moment spare and the space to disguise the box then it could be a great way of disrupting the rock and roll... I can't see top manufacturers wanting to add anything so simple and low cost though.
 
So it is using water high up on the boat to give a good moment to stop it moving.

I would imagine the forces required were massive but worth a look.

Could not see the testimonial site. Just took me I generic google groups.

Replacing teak on the fly bridge anyway. New tank I feel ! Under the rear seats at the back of the fly but given it's on the overhang I doubt it would take the lid !
 
From what I read, the owners chose the tank to be high up for convenience and the mathematician adjusted the calculations to suit. The trawler is 70ft and looks like steel, yet the tank needs less than a ton of water. On a boat my size we could be looking at just a few hundred kilos of water, indeed the tank could double as the fresh water tank, or even fuel. Fascinating
 
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jrudge said:
So it is using water high up on the boat to give a good moment to stop it moving. I would imagine the forces required were massive but worth a look. Could not see the testimonial site. Just took me I generic google groups.
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I don't think the forces do have to be massive. The idea is to damp the roll inputs with an opposite input that prevents the roll increasing. Think of a child on a swing - the energy input isn't much but if it is timed right it gradually increases the amplitude. If it's timed wrong, or deliberately input against the frequency, the swing slows down and stops. On our 40' boat we have a flopper stopper deployed off the side of the boat which does the same sort of thing with a small hinged plate that folds when the boat rolls towards it and opens as the boat rolls away, thus damping the "away" roll. The force it generates is quite small but enough to do the job.

Here's the text of the owner's blog posts in support of his ART:- 'ART, aka Anti Roll Tank, is doing as well as it did on day one in 2006. Nothing has been done to it including adding water......it still has the same 6 1/2" level I put in in 2006 in the British Virgin Islands. The tank has been looked into by many customs and boarding parties however. Just looks so enticing to them as a contraband carrier ;-).. Hard to believe, IMHO. Totally passive, no power used, nothing movable in it so I expect another 10 years before it should have a ruler stuck in it for another level check ;-) ....... Lots of folks have asked us how our anti roll tank, aka ART, performed. My crew said it was absolutely amazing. He would not have believed it if someone told him. An example is that we shut down once a day to check fluids in the engine/transmission. As most trawlers do Swan Song comes beam to the wind and seas when she loses way. So each day when we stopped we would sit parallel to the seas. ART just keeps on working and SS sits without rolling as the waves pass under us. Works as well in the small seas in the middle of the passage as well as the large ones at both ends of the passage. Underway it keeps most of the rolling under 5 degrees with an occasional roll towards 10 or so. The largest roll was a braking wave that hit us broadside just about 10 hours from Hawaii. Perhaps a 30 degree roll but I don't think any type of stabilization other than a Sikorsky heavy lift could help with those types of waves.

Bouba said:
From what I read, the owners chose the tank to be high up for convenience and the mathematician adjusted the calculations to suit. The trawler is 70ft and looks like steel, yet the tank needs less than a ton of water. On a boat my size we could be looking at just a few hundred kilos of water, indeed the tank could double as the fresh water tank, or even fuel. Fascinating
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Not sure whether you could use the tank for anything other than the stabilising fluid because the weight of fluid has to stay the same. But otherwise it does seem a tempting possibility for a new-build boat - especially if those heavy gyros people are fitting nowadays start failing and designers start hunting for an alternative.
 
Having struggled to read the rather amateur (bad) science, I can safely say "It cannot work" as stated.
The only possible outcome is to change the natural (pendulum) frequency of the boat. But there is something to be garnered here. Significantly changing a large weight far from CoG will alter natural frequency, so if there is a particularly bad input (wave height, speed, etc) then changing the amount of water in the tank will help.
 
Dougal Tolan said:
Having struggled to read the rather amateur (bad) science, I can safely say "It cannot work" as stated.
The only possible outcome is to change the natural (pendulum) frequency of the boat. But there is something to be garnered here. Significantly changing a large weight far from CoG will alter natural frequency, so if there is a particularly bad input (wave height, speed, etc) then changing the amount of water in the tank will help.
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I did ask in my OP whether it would work or not so I won't be surprised if it doesn't - the fact that this seems to be a unique application is perhaps evidence that it does not work. But I'm not sure I understand what you are saying. If the device changes the natural (pendulum) frequency of the boat dynamically using the initial charge of water in the tank (not by changing the amount of water) then it might have the effect of adding and subtracting energy sufficient to dampen the roll - just like my swing analogy. And IIRC the description specifically refers to the pendulum frequency.
The thing that I don't understand is how the baffles inside the tank can modify the surge of water from one side to the other with sufficient accuracy to achieve the desired effect without risking the opposite effect of amplifying the roll. Usually we don't want fluids surging in tanks on board boats - a fairly small amount of water sloshing around on the car deck caused the Herald of Free Enterprise to capsize in a few minutes.
By the way, I'm very familiar with the idea of adding weight away from CofG to alter natural frequency. When our 40' trawler yacht was built the ballasting was done with the assumption that a specific weight of tender would be carried on the (fairly high) boat deck. The boat rolls slightly more when we are at anchor, the tender has been launched, and the weight is no longer there. But the flopper stopper deals with that.
 
TwoHooter said:
Usually we don't want fluids surging in tanks on board boats - a fairly small amount of water sloshing around on the car deck caused the Herald of Free Enterprise to capsize in a few minutes..
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Surely that's exactly what would happen if you put all that weight high up on the boat.
If I get too many people up on the flybridge I feel nervous about it .
Not to mention the extra windage from a bloody great tank.
Was the article published on 1st April?
 
The book "Voyaging Under Power" (Beebe, Umstot) compares this method with other systems such as flopper stoppers, active fins & gyros. They seemed to not be dismissive of the system, although it seems that it is essential to have a method of very quickly draining the water content. It does have to be designed by an expert engineer. http://amzn.to/2wwAKG6
 
Whilst I agree with the weight of water being a ton and high up, surely it is more than compensated for by the fuel and the engines? Amplification of roll occurred to me as well.
 
The baffles ensure the water within 1or 1-1/2 rolls of the hull the water up top is on the" wrong side "
Essential features are Obviousely the amount of bafflement , height above the cog , width and the mass of water .
Mass being critical ,hence the guy bragging about 61/2 inches deep .
Too deep or too little ( leak -evaporation or looks near enough eg 6 inches or 7 inches then -----
As the swing example
Your legs will get in sync with the swing and extend the roll rarther than impede it .

That's a D boat ,but the same principles apply to a P boat say a Manhatten 70 or a Priny 67 @ anchor .

My issue Is not scepticism over it working ,it does with the correct maths and correct internsl bafflement ,it's the
Cyclical on off forces on the fly superstructure basically shaking something loose --read GRP stress cracks .water leaks from seals cracking in the FB ,windows etc -not the actual tank itself ,knock on secondary leaks weather getting in .
So this cannot be a "home made " retro job with help from Southampton Uni maths dept for the calc s .
SS and Priny need to in on this @ build to reinforce the fly structure .
Sticking 1/2 a ton or a ton of static jet ski , tender ,crane , wet bar etc is all well and good they do that stuff and it's screwed down does not move ,but then adding another 2-4 tons that move ( cubic meter water weighs a 1000kg,) -will IMHO without substantial reinforcing ,
Knacker /stress crack /flex the FB

Aesthetics - none issue it could be under a sun pad .
It's probably much easier just to pluck a gyro or stabs off the shelf for a 70 ft FB bulder of a planing boat ,plug n play --and pass the £ on to the punter + margin .
Lighten the FB structure and Bulid in the process .--back to known quantities , with little comeback .
 
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Here's a post that just popped up on the Nordhavn Owners Group from one of the members who owns an N62. I have his permission to re-post it here:

I’ve been extolling the virtues of Anti-Roll Tanks from the first day I had experience with one.
I was involved (as Superintendent) in the installation of a 160 Te anti-roll tank, 20m above the waterline, on a North Sea Diving Support Vessel.
Before the project began, we tried extending her bilge keels. In dry-dock, the bilge-keels were widened so they stuck proud of the vessel about 1m, for about a 50m run along the vessels belly. It certainly helped during transit, but did nothing while at rest at sea (Vessel operating in DP, Dynamic Positioning).
The next try to get the vessel from rolling so much, was the anti-roll tank installation at the quayside. This installation virtually transformed the way the vessel performed. Ergo = I’m a believer.
Any vessel can retro-fit an anti-roll tank, but they tend to look unsightly done afterwards...... done right at the design stage, you would never know it was there.
It’s a completely passive way of stabilizing the vessel with no electronics required.
I think it does take quite a bit of engineering to get the baffles inside the tank “positioned just right” but once done, it’s a fit-it and forget-it stabilization method. The only tweeking required is the volume of sea-water that is pumped into the tank. But an operator would soon learn the volumes-to-conditions, and make their own chart.
 
TwoHooter said:
The thing that I don't understand is how the baffles inside the tank can modify the surge of water from one side to the other with sufficient accuracy to achieve the desired effect without risking the opposite effect of amplifying the roll.
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I think thats the disadvantage of the system. IMHO it can only work if the boat has a predictable roll period and the tank system is designed for that roll period otherwise, as you say, there is the risk that with an unpredictable roll period, the weight of the water amplifies the effect of the roll instead of countering it. It could be that the kind of boat to which the tank is fitted, which I assume to be a D boat, has a range of roll periods in a variety of sea conditions which can be predicted but I'm not sure that, say, the roll period of a P boat could be similarly predicted.

I believe there are water ballast type stabiliser systems fitted to commercial vessels which operate on a similar basis as this or are controlled by pumps. Maybe a pump type system could work with a leisure vessel but then when you start fitting pumps, sensors and control systems as well as a tank, I guess the cost is getting into the territory of fins and gyros which are, of course, far more space efficient

Interesting thread though
 
If you are at anchor the movement of the boat will be relatively small compared to a beam sea whilst under way. You roll a few degrees and want to stop that. Tanks like this could indeed work and hence their use on an oil platform.

Now put out to sea and get caught side on by a wave which tilts you over far more. Would I want a ton and a half at the highest point of the boat? No I bloody well wouldn't. I know how different it feels with 8 people on the flybridge of a 50 foot boat. The benefit of traditional stabilisation such as gyros is that not only do they seek to dampen any movement they also make the vessel fundamentally more stable by adding weight to the lowest part of the boat. It's like fitting another half an engine !

Henry :)
 
henryf said:
If you are at anchor the movement of the boat will be relatively small compared to a beam sea whilst under way. You roll a few degrees and want to stop that. Tanks like this could indeed work and hence their use on an oil platform.

Now put out to sea and get caught side on by a wave which tilts you over far more. Would I want a ton and a half at the highest point of the boat? No I bloody well wouldn't. I know how different it feels with 8 people on the flybridge of a 50 foot boat. The benefit of traditional stabilisation such as gyros is that not only do they seek to dampen any movement they also make the vessel fundamentally more stable by adding weight to the lowest part of the boat. It's like fitting another half an engine !

Henry :)
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The physics may be correct under certain conditions, but I for one would certainly not want the cog of my boat be raised any higher, if at all possible, common sense I figure. I have read the book Passage Making, still not too impressed about this water tank high up business esp for my p boat.
 
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One common view seems to be that this isn't a good idea for planing boats, about which I know nothing - I've never even been on one except for our RIB. On our 27 tonnes displacement boat you could put 20 people on the boat deck and flybridge and not notice any stability issues.
I think there's a misunderstanding about the positioning of anti-roll tanks. I believe they can go anywhere - they don't have to be mounted high on the boat. The shape and baffle arrangements are adjusted according to where the tank is positioned.
Concerning rolling at anchor, in a displacement boat this can be a major issue, hence our use of the flopper stopper when there is a swell. We anchored overnight in South Haven of Skomer Island recently and without the flopper stopper it would have been untenable. We simply won't run the generator at night, it's very well silenced but when we are somewhere like Skomer we do not want any noise at night (except the birds), so even if we could fit a gyro it wouldn't be the answer. An anti-roll tank does sound appealing but no way would we attempt a retro-fit.
 
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