Anti-Rolling Tank

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

  1. BMcF
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    BMcF Senior Member

    No..active four-fin systems are installed on hundreds, though mostly megayachts, and the performance numbers like 89% are from real data taken during sea trials..25 years of sea trials.

    My definitition of 'useless' is rather arbitrary but again based on 25 years of testing at sea. A few percent reduction in motions from some passive solution, compared to double-digit reductions with active solutions is roughly how I define the demarcation between usefull and useless. The complete story is clearly much more complex...but the stuff of techniucal papers and not a forum post.
     
  2. dskira

    dskira Previous Member

    Could you make a synopsys, or a short description of the paper, will be interresting.
    Daniel
     
  3. apex1

    apex1 Guest

    A fish, commonly named "flopper stopper" reduces roll amplitude very efficient, but costs about a knot when sized right. The danger incorporated in such system is quite high. It can even sink a vessel when all the worst situations come together.

    A hydraulic stabilizer will cost between 0,3 and 0,5 knot when designed right for the average speed and seastate.

    A gyroscopic system (like the Mitsubishi, marketed by Ferretti) is not worth talking. Even in the recommended applications they have to use 2 or more of them to stabilize a 60ft boat to the same extend a fin system would. Forget it.......

    In larger sizes that goes much worse.

    Bilge keels and other static devices have as much believers as deniers, but no really influence on rolling in heavy sea. Maybe of some assistance, like hard chine, but that was it. Forget it.

    Anti roll tanks have been installed by van Lent in NL on a few Megayachts. They soon left it. It was not worth talking, even not when active and electronically controlled. De Vries did not even try it.
    None of the German yards (the world leaders in megayacht building) ever did think about it!

    It is a (fascinating) trap!

    Leave it.

    Regards
    Richard
     
  4. ancient kayaker
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    ancient kayaker aka Terry Haines

    Perhaps it would be helpful if the nature of the rolling was described, i.e., amplitude (X deg) period (sec), also midships station and other stations if available. These data could be related to the size of the vessel to estimate CoG to determine if additional weight above the CoG is safe.
     
  5. BMcF
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    BMcF Senior Member

    A good, if short and brutal synopsis, IMHO:D I would hasten to add though that Intering (now owned by RR I believe) makes roll tanks that certainly have worked well when properly integrated and Seakeeper, Inc. makes actively gimballed stabilization gyros that have clearly demonstrated some very impressive roll control performance.

    If I've learned nothing else in 25 years of ship motion stabilization, I have learned this: Take manufacturers undocumented claims of performance with a grain of salt AND view anecdotal reports of the good and the bad performance of any control solution with a wary eye. Too many good kits have been improperly specified and installed over the years and who gets the credit?..the manufacturer of course, not the NA, designer or yard that installed the gear.Converseley, as in any business, there are snake oil salesmen in ours too.

    Put another way..if Intering's roll tanks didn't/don't work well in many applications..how dey stay in business all these years?..;)
     
  6. dskira

    dskira Previous Member

    I was hopping you had a paper on the subject.
    Daniel
     
  7. mark775

    mark775 Guest

    "...and the performance numbers like 89% are from real data taken during sea trials"
    "...claims of performance with a grain of salt"

    No, you - 89% is just not true. Maybe they touched 89% in some harbor-perfect scenario but 89% is just not happening. - and you agreed above!
     
  8. BMcF
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    BMcF Senior Member

    that was an awfuly lame attempt at splicing two separate thread segments to derive or synthesize a new point.:cool:

    1. There are some very capable roll damping systems in service that reliably achieve nearly 90% roll motion reductions in conditions at sea where resonant roll motions are excited. Reductions of 60-70% are considered a minimium performance level. 2. There are some pretty lame collections of motion control hardware that don't work very well, if at all.

    I have seen and worked with both 1 and 2. I made no attempt to relate the two in my earlier post; quite the contrary in fact. As with anything, some education and research is required before making purchase decisions.;)
     
  9. mark775

    mark775 Guest

    "in conditions at sea where resonant roll motions are excited." Yes, it stopped some flopping. My point exactly is that under an ideal scenario where the boat's moment matched the wavelength thay got 89% reduction. You are being a salesman!
     
  10. BMcF
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    BMcF Senior Member

    OK..what else will reduce a vessel's motions by 90%?....and by numbers in double-digitis under nearly any/all other conditions aside from worst case?

    I'm not a saleman; I perform vessel motion and habitability assessments. The results of those are almost always presented in the form of polar plots, showing what motion parameter limits are exceeded, and to what degree active or passive changes will improve/expand an operational envelope before limits are exceeded. That kind of assessment can cover everything from the differences between hull types to the effects of a wide range of stabilization add-ons. Fortunately, there are quite a few more active solutions available these days than there were a couple decades ago. And properly selected and configured flume tanks are still in that mix of solutions too.
     
  11. Tackwise
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    Tackwise Member

    Having a similar problem as conceptia, I have been following this discussion with interest. To briefly explain the background:
    For a newly built coaster (L*B*T=100*13*5.4m) the yard advised against the placing of bilge keels. The reason being:
    1) Due to the small radius of the bilge, the bilge keel would have to be small (maximum 200mm) as not to extend outside the ship. The effect of the bilge keel against rolling would therefore be negligable.
    2) Due to the small radius of the bilge, the bilge itself would have an anti rolling effect similar to a bilge keel.

    I believe the two above arguments have some merit. However because the crew of the ship have been complaining on the rolling of the ship, we have to re-evaluate the placement of the bilgekeels.

    My first assumption is still that the placement of bilge keels along approx 40m of hull) will have a negligable effect on the rolling, and probably a more significant effect on the overall resistance. However I am lacking the data to substanciate this claim, and hoping you can help!

    PS as you may guess, active roll damping is out of the question due to ..... costs!
     
  12. mark775

    mark775 Guest

    Now, it's 90%? BM, you need to be needed too much. I just wouldn't want people spending money on the damned things - and your livelyhood depends on it. Watch in about two years for another superior system.
     
  13. conceptia
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    conceptia Naval Architect

    Tackwise, you are right. Even I thought of bilge keels, but the effect on the roll motion is very negligible.
    One of our competitors had advised the client to convert the ballast tanks in the extremities to U-tanks to reduce roll motions. Actually they went on to design the whole system, but the client was not happy with the amount of structural modifications it required. I was hoping for using a tank in the midship area, but found the cargo FO tank in place. :(
    I feel there will be a loss in the directional stability when the U-tanks are situated at the extreme ends of the vessel. (expecting comments from the Pros). Also torsion is a big concern.
    The stabilizing fins has to be at midship region, but as i told there are FO tanks over there. I'm still thinking for a idea, with this thread as one among my home works.
     
  14. BMcF
    Joined: Mar 2007
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    BMcF Senior Member

    I have no clue what that paragraph meant or if you had a point. Quite a few companies have, for many decades now, provided a range of active stabilizers solutions and a lot of them work extremely well (hence why owners insist on paying for them, eh?)

    The range of solutions keeps increasing over time as does their effectiveness. Hmm...that sounds an awfull lot like the evolution of just about any technology, doesn't it?:p But 'superior systems' just popping out suddenly in a couple years?...odds are against that.:)

    I'm a neophyte in the world of active ship stabilization; I've only designed and installed mybe 50 or so systems on various vessel types over the last 25 years. But I've been fortunate to have worked with some darned clever folks whose experience goes back a lot farther than my quarter century worth..and I've been even more fortunate in the extent to which those folks helped me along in my earlier years and were willing to share what they knew.
     

  15. dskira

    dskira Previous Member

    Don't be modest :cool:
    Daniel
     
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