Anti-Rolling Tank

Discussion in 'Stability' started by conceptia, May 24, 2010.

  1. mark775

    mark775 Guest

    Owners insist on paying for them because they don't know any better. I have installed four active roll stabilization fins. Two were on the Marjorie MorningWood (Star) or the Life of Riley, nee Pizzaz (I don't remember which) and were huge. Couldn't tell the difference before and after but the owner wanted to keep up with the Joneses (that's a joke - Joneses own Delta Marine where the work was done).
    "But 'superior systems' just popping out suddenly in a couple years?...odds are against that" - I know. I have been working on it for eight years and it can do little more than help with the rate of decay for large yachts.
    About all any system can do is moderate sychronous roll and this is where the "up to" whatever percentage comes into play - a vessel ought not have sychronous roll with headway on and most vessels exhibit inherant stability (like riding a bike) when moving anyway. As an example, a simple propeller change can make a boat roll less underway and there are, of course, many other variables.
    Also, the percentages relate in many cases to roll amplitude, which is only one component of roll. There is also period, accelerations (decelerations) and rate of decay.
    My point is that synchronous rolling (rolling in beam seas when the wave period is close to the natural roll period) is easily abated by many means - I can stop it on my boat (seriously!) by jumping back and forth opposite the exaggerated roll, by tanking down, or (I could, if I chose to) with a sail and it's rig providing inertial damping even with no headway due to the weight of the rig and its distance from the roll center (increased roll moment of inertia) even with light airs.
    In any event, those not versed in this should be informed that the lower of the percentages in the range of effectiveness in advertizing is an average of the effectiveness and even that is suspect. The uppper percentage is "we stopped a synchronous roll" - not that big of feat, especially considering that a well designed vessel doesn't do it while with way on.
    Mine is, instead, a system for small craft, particularly hard chine, "snap roll" (oddly, sometimes termed "excessively stable"), lightweight vessels, and since a vessel will have to maintain stability (for purposes of a classification or USCG rating) even in failure of the system, and my system is specifically designed for retrofit (initially) and has to be quite flexible in mounting, there is more to getting something to market than "I have an idea" and tack up a shingle. Yes, give it two years.
    Also, my take is that the absence of any add-on stabilization is superior to active fins for reasons of cost, lack of real-world benefit, complexity, use of valuable midship space, and failure proclivity.
    As for tanks, passive ones can only work in one sea-state tuned for one specific vessel. I feel that in some other sea-state that they would be non-effective or exacerbate a problem. Active ones (How?..they ahve a cross pipe at the top of an "U" tank with a valve tied to a vessel motion sensor? Wud pressurizing the air allow more positive response?) seem pretty tricky, with a lot of weight up high - might just as well stop the flop with some stable weight up there tuned to the prevailing sea state. What when it fails? The free-surface effect is a detriment in this instance and will add to a list.
     
  2. ancient kayaker
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    ancient kayaker aka Terry Haines

    I think the purpose of the vessel needs to be taken into acccount when considering an active roll control system. If the vessel is inherently stable and has good roll damping properties then the system is not needed. However, I imagine that the purpose of the vessel could dictate hull shape and balance to the extent where roll control is beneficial, even essential.
     
  3. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Looks like sometimes anti-roll tanks can be a very, very costly device. I've just read this story:

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

    P.S.
    Not a case against rolling tanks, just sharing an info found around. :)
     
  4. BMcF
    Joined: Mar 2007
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    BMcF Senior Member

    And fortunately for those of us who have installed many hundreds, active roll stabilization systems work so well that we sell most of them based on demonstrated performance and not the 'bling' factor.

    :p
     
  5. DennisRB
    Joined: Sep 2004
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    DennisRB Senior Member

    Someone screwed up bad with that one, but I only saw reference to normal ballast tanks.
     
  6. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Yes, that's correct. I haven't read it with due attention.
     
  7. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member

    Active rudder control typically reduces roll motion by 40% in practice in Naval vessels, it has the massive advantage of no additional drag devices, low complexity no extra weight or structure. It's lack of implementation outside of naval vessels is surprisingly low.

    Active fin stabilisers were in use in the early 1900's Large damped sliding masses and gravity-fluid systems around the same era. Large gyroscopes were used in some German Naval vessels from 1906 .

    It all runs around in circles except for active rudder control. That is more recent.
     
  8. larry larisky

    larry larisky Previous Member

    and sea sickness will still with us for ever. trying to control the nature of the sea is ludicrous. if people are not comfortable on a vessel, they can stay in land or take a plane.
    if someone take a look from an airplane at the vessel on the sea, they will see immediately how tiny they are, but the white top of the waves are huge.
    controlling that is taking the owner for a ride.
    i know some active one work, and stop working as fast they started.
    we had in one fishing vessel an articulate rudder. the captain was happy, so the thing must have been working for him.
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    Agreed, but that's not really what they are trying to do! If you have a commercial vessel and you are competing with other vessels, and stabilizers are the accepted norm, you have to fit them or get out of the business. Passenger vessels in particular, have always striven for comfort, even livestock carriers have stabilizers to deliver their cargo in optimum condition.
     
  10. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    just happened on this from a PUP forum, and thought it might have some interest in these discussions


    We had looked into these units when Swan Song was earlier in the stage of re-construction. At the time, according to the factory man in Japan, we'd need two of the mid-sized units to keep Swan Song from rolling less than 25 degree each side. If a sea state was at the level that we were rolling this much the power required to keep them both spinning was just over 8 KW. This all according to them. In a really nasty sea state they would need to be turned off as if they hit the stops often and with too much force they might self destruct. Suffering and surviving a knockdown wasn't in their vocabulary! All in all we opted not to go in that direction as the predictability of sea state isn't within our ability in making a passage.

    As most of you know we went the route that few have taken and for the life of me I don't know why. An Anti-roll tank, ART, is our only roll reducing system on Swan Song. A totally passive device with no maintenance to date, 5 years, and only one moving part, water.

    I wish I could say exactly what the numbers are for roll reduction but I can't. What I can say is that in 15,000 odd nm we have never rolled more than 30 degree and perhaps on 20-25. This includes a tough patch coming into Hawaii last year in waves that were occasionally in the 30' range. We can sit with no way on in 8-10 ft. seas with very little movement. So little that you can work on things and not be chasing tools all over the place. This is a very comforting fact especially when you have to shut down the engine for daily checks at sea.

    For some reason Naval Architects hate them. They are afraid that they will somehow encourage a capsize or something similar. We have found all of this is bunk. Swan Song is stiff and has a high righting moment but it is round bilged and likes to roll and once started would go on for 12-15 rolls as a minimum. We tested all of this with full inclination tests, etc. Our boat parameters were fed into the computers that Dr Bass up in Newfoundland runs with his software and came up with the design tuned to our boat. $10,000 later it was constructed and put in place on top of Swan Song's Pilothouse. 1500 lbs. of water over our heads ;-)

    Everyone who has ever been aboard either on the hook or underway is astounded by how Swan Song behaves in the water. At first they are puzzled and then as they watch other boats nearby whether they are sailboats with the masts waving in the sky or power boats showing lots of bottom paint they realized that Swan Song doesn't hear the music that the other boats are dancing to. As Seahorse John (now departed) once said in Bequia after an evening on board with the ferryboats passing 50' away without spilling the wine, "I thought you must be aground as you aren't rolling like the others". Then as he watched us round the point heading north out of the harbor the next day waiting for the first African tradewind swell to pin us down Swan Song just went on her merry way. "**** I got to get that system".

    Bob Phillips, Another Asylum, has the same system done by Dr Bass. He is the one that sold me on it. Check with Bob as I think he'll compliment just about everything I've said.

    I can't compare if paravanes would be as good but I don't think so especially at slow speeds, stopped or in shallow water plus they do require effort. Active fins are nothing but trouble long term and because of our size, propensity to roll, and slow speed the size of a system for Swan Song is large and in the order of $75K with all the attendant maintenance over the years. We would already be due for an overhaul. So for us the option was the paravanes or ART. Glad we made the choice we did and have never looked back.

    Also as to significant wave height measurement, significant wave height is the average of the highest 1/3 of the wave. The actual wave height can be as much as 3 times that on occasion and from my experience usually is. Our height of eye is 12' off the water sitting in the pilothouse and beam waves up to this height aren't comfortable but offer no problem on the beam. Even an occasional crest coming aboard just tosses you sideways. Bigger waves that this and we are either taking them further forward or further aft. Dead down we are fine even in that 30' stuff near Hawaii. 20-30 degrees off our stern is our weak point. We get both the pitch and a roll so the corkscrew motion gets uncomfortable pretty quickly

    Dave & Nancy
    Swan Song
    Roughwater 58
     
  11. MikeJohns
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    MikeJohns Senior Member

    Specifications:

    Length: 58'
    Beam: 13.8'
    Draft: 5.6'
    Displacement: 64,000 lbs.


    Brian

    I have some severe reservations about a system implemented this way. Also Passive anti roll tanks have a nasty habit of getting out of phase in some headings and actually amplifying the roll. Active tanks are a lot better.

    On top of the wheelhouse..........With the tanks weight included they are putting close to a ton on the wheelhouse roof! ( look at the picture below). That's rather alarming if the vessel wasn't actually designed for it.

    And they left the water in, in rough seas................:!:

    That's the problem with vessels that don't need stability books or operator training. Those tanks should be drained quickly in many conditions. One of the worst features of these passive tanks is that any list from any reason is immediately reinforced. This can be from something as simple as a wind load.

    I suspect that what's making their vessel appear more stable is actually the reduction of stability (reduced GM) and a higher roll gyradius. This works for vessels at anchor, just fill a tank on your wheelhouse and you'll immediately feel a difference and often completely stop the rolling you were experiencing.

    I'd lay money on very similar characteristics if they just had one tank or a fixed mass.

    I'd like to see the GZ curve with the tanks full. People should be aware that these passive tanks have been implicated in the loss of vessels when not drained in time.
     

    Attached Files:

  12. Tantalus
    Joined: Mar 2010
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    Tantalus 1963 kauri cruiser

    Back to static bilge keels: what is better? Long shallow keels, or shorter deeper keels?
     
  13. watto99
    Joined: Feb 2011
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    Location: australia

    watto99 New Member

    Hi
    I know that this is a long dormant string, but it's a subject that is very much alive
    In my own search for this information I tracked down a report and bought it on line for $40 from the US naval archiect association.
    It's well worth the money and answers the questions about benefits and optimum design of bilge keels etc
    Here's the title/ authors
    Roll Damping on Two New England Trawlers: An Experimental Study
    Clifford A. Goudey 1 and Madan Venugopal 2
     
  14. brian eiland
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    brian eiland Senior Member

    The photo you posted is too small to see the tanks. Could you make it bigger?

    I have a very interesting update by the owner of Swan Song that I will be posting soon.
     

  15. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    A few days ago I got this private message update from the owner of that vessel Swan Song




    This is the original posting I had found on another forum,...
     
    jorgepease likes this.
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