A practical procedure for flotation and stability tests

Discussion in 'Boat Design' started by Smarten, May 27, 2006.

  1. Smarten
    Joined: May 2004
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    Location: Rome

    Smarten Junior Member

    Hi,

    My boat is a 57’ semi displacement hard chined trawler of about 61000 lbs. of displacement.

    At the end of refitting project, now under execution, I calculated the boat would be heavier of about 6500 lbs not evenly distributed (most at stern and on the flybridge).

    Just before to make the last decisions, I am planning to put the boat in the water and simulate the target weights distribution to see what happen.

    Practically I would like determine the new static waterline and understand if I will need modify some solution or to add some ballast to counteract a (more than probable) negative stern static trim (not sure if we have any conventions about I mean too heavy at stern).

    Additionally I would like to determine some boat figures as Pound for Inches and, if possible, how the Trim change moving loads at bow or at stern to carry out a sort of loading manual.

    Probably a lot of others useful information also regarding stability are deducible.

    Can you suggest any practical procedure to follow in order to get as much as possible info from this kind of test?

    Thank you and regards

    Lino
     
  2. Guillermo
    Joined: Mar 2005
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    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

    Lino,
    First of all you'd need some kind of hull's lines drawings, at least approximate. If you do not have the proper thing, you should obtain at least seven station's profiles by yourself. Bow should be one of those stations (area = 0) and stern other to get the whole of the picture. The remaining five, equallly spaced in between.
    You'd need to put the boat ashore over an horizontal and even floor, floatation line parallel to floor and sidewards leveled. With the help of two lines of wooden boards over the floor, parallel to the boat's center line, and some strings, you can get hull's measures by triangulation, section by section. Just measuring one side of the boat is enough because of symmetry (assumed).
    Then, with sections duly scaled down to paper, you may work out section's areas for every draught, hull volumes and all hydrostatics, just armed with a hand calculator and using the simple Simpson's rule, no computers needed. You should draw the hydrostatic curves with data obtained, for easier use.
    With the floatation test you mention, measuring draughts (or freeboards) forward and aft, you can go into the hydrostatic curves and get all data needed.
    (I'm asuming no big trims, so just wit one set of hydros would be enough)
    You can also use this useful calculator: http://web.nps.navy.mil/~me/tsse/NavArchWeb/1/module6/simpson.htm
    although you'd need 11 stations instead of 7.
    If you want to go computers, you may use the free Archimedes program:
    http://www.naval-architecture.co.uk/
    Afterwards you can perform the stability test and then, when knowing lighship condition center of gravity's position, you can obtain your boat's stability curves.
    Or, easier, you may hire an NA to do the job.
    Cheers.

    (P.S.: For a quick course on Naval Architecture, thoroughly visit: http://web.nps.navy.mil/~me/tsse/NavArchWeb/index1.htm)
     
  3. Smarten
    Joined: May 2004
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    Location: Rome

    Smarten Junior Member

    Folks,

    Guillermo, thank you very much for your suggestion and useful links.

    I have the drawings in fact I got from the shipyard the Technical Documentation they submitted for CE certification. This is pretty exhaustive stuff although I would not commit entirely about the accuracy due some strange and repeated "errors" in the Stability Calculation that suggested me they are using the same generic file for any boat they submit to our EU “very collaborative” certification bodies
    Apart these malicious considerations all the geometric data I checked were correct.
    Unfortunately also here but for different reasons if I have to remark that my boat is slightly different from the others produced. It was in fact elongated at stern of about 4 feet
    Also if at this time I didn't yet get as carry out the corrections I will do this after and for sure, despite my intellectual curiosity, I will call a naval colleague to check my results.

    What I want check for now is the location of waterline against the designed and perform some stability test to check the figures I am a bit sceptical about. So the plan it would be:

    * Mark the waterline in the target conditions modifying planned gears installation if found unsatisfactory
    *Adding a weight around 3000-3600 pounds and move aft and forward the supposed GC trying to find where the boat will really maintain the initial asset. This in order to locate the true GC and Pound for Inch ratio.
    *Move the weight at bow and at stern to compute the angle of trim for a given momentum.
    *Shift this weight laterally getting the heeling angle with a pendulum (it would be theoretically less than 4°)

    I envision to use a pendulum of given length (about 6 feet) for the heeling angle. But what about the trim angles? I don’t think this solution will be good too due the little angles I aspect to have. Any specific suggestion about?

    Is this test set reasonable or I am wrong wasting time and money?

    Regards

    Lino
     
  4. Guillermo
    Joined: Mar 2005
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    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

    Lino,
    First, I would have had appreciated if you had identify yourself as having specific formation on these subjects, as I'd wouldn't have posted such elementary suggestions.

    I wouldn't do the test as you say. Having hull lines plan, I'd better add the extra 4 feet at the stern, digitalize them, compute all Hydros, KN curves, etc., perform an inclining experiment in as close to lightship condition as possible (your intended pendulum length seems correct, but for test weights I'd begin with 2500 pounds, trying to get no more than a 2º-3º angle), and then obtain GZ curves for whatever load condition desired, as well as trims, etc.

    Guessing GC by just moving randomly around a weight (If I understood you correctly), seems quite weird to me. And if you previously do not perform all hull data calculations, I don't see how you are going to get an accurate position of the GC from the inclining experiment.

    Well, there is an approximate method to roughly estimate its vertical height and also GMo, that we use for small fishing boats, but you'll need anyhow some basic submerged hull and floatation measurements.

    Cheers.
     

  5. ABoatGuy
    Joined: Aug 2004
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    Location: LeftCoast

    ABoatGuy Member

    Smarten,

    Whoever did the CE work, in all likelyhood, has the stability software to do this sort of a job. It you have the added weights and the old floation (freeboards or drafts), it would take about 10 minutes to answer your questions.

    Although people make mistakes, I have found most notified bodies to be a little more diligent then to allow the use of generic hulls, although there may be some out there!
     
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