Free surface effect on fishing vessel with fishholds of 0.3B

Discussion in 'Stability' started by moose60, Sep 25, 2013.

  1. moose60
    Joined: Dec 2008
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    Location: Bellingham, WA

    moose60 Junior Member

    Hello All,

    I am familiar with the concept of a free surface effect and it's effect on vessel stability. In one of my fisheries we routinely operate with slack tanks. However, most of the boats have divided fishholds. Mine are full-height water-tight divisions (separate fisholds) that stretch about 0.3 of the beam of the boat each (3 total). The missing .0333 bits can be chalked up to insulation.

    I usually operate with the two side holds somewhere between 1/4 and 3/4 full. When filled, I then move to the center hold. My fish are stored in refrigerated seawater.

    My questions:

    1. How does the free surface effect of sub divided areas (such as my fishholds) affect stability compared to a case where the water can slosh from beam to beam?

    2. In another of my fisheries my holds are usually empty but I carry a large deckload of crab traps at times. This raises my center of gravity alot! I store as many as possible in the holds but that leaves 70% to go on deck. I usually partially tank (after pots go in fill about 1/2 with seawater) the two outside holds. This seems to help with the stern-down trim that occurs with the big stack on the stern and seems to alter the COG lower in the boat. Makes it feel lots better anyways.

    I did a roll test this fall to get an idea of initial stability before loading pots and came up with a GM of about 3'. I will retest after loading pots, and than after tanking but I'm also looking for some info from the experts.

    Other details: boat is GRP, about 36'x11', 4.5'd. fishholds each about 9'long, 3.5'wide and 4'deep.

    Thanks in advance,
    Byron
     

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  2. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    Good questions.....

    The subdivision is certainly better (if watertight) than leaving the hold open, but a proper study needs to be done to understand the real effects. Typically a curve of righting arms will be created, and then a curve of heeling arms (the heeling force from your tank loads) is deducted from the righting arms. Then we see if the boat still has adequate stability.

    Adequate stability is not something you can feel. There are hundreds of drowned fisherman's kids and wives who can attest to this.....Find out what it really is. This requires an inclining (to establish VCG) and a computer study based on the hull lines of the boat.


    This is a start, roll tests are not very accurate due to the use of a fudge factor which is a guessed approximation. An accurate GM will be found with an inclining experiment.
     
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  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Moose60, be very careful what you read and analyze it before you give it as good (even what I'm saying.)
    1. - Free surfaces in tanks to produce a similar effect like increased height c of gravity of the ship. In fact, the free surface correction has two parts, one to be deduced from the initial GM (climbs the vertical position of c. G.) and another, which also depends on the angle of heel, which is deducted from the value of GM each heel. The deduced amount, is directly proportional to the inertia of the free surface. Therefore, if you decrease the free surface, making longitudinal bulkheads as you have done, leading to three tanks, the free surface of each tank is greatly reduced and the inertia of each decreases more (The sum of the three inertia is less than the initial inertia). Do not infer anything righting arms (that's wrong), just the GM is lower (G appears to be higher) and therefore the curve GZ (righting arm) is different (as G's high increases, the GZ value decreases).
    2.-The aft ballast can have several beneficial effects on the boat:
    - Improves the trim, which can improve stability.
    - Can lower the c. to g. a percentage greater than the apparent increase height c. to g. due to free surfaces. Would have to calculate how high the liquid ballast offset both effects.
    3. - Do not know if what you've done is an inclining test or just have measured the balance period. Both systems can be used to determine the height of c. to g. of the vessel. The first one, if well done (which is not easy), is much more accurate.
     
  4. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Byron,

    Do you have a stability book for the vessel that is current and stamped approved by the USCG?
     
  5. moose60
    Joined: Dec 2008
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    Location: Bellingham, WA

    moose60 Junior Member

    Thanks for the input Gang!

    Ad Hoc: No. I bet that if you surveyed 100 small fishing vessels in the fleets I operate in 0 would have a stability booklet. They are not required for this size boat (should be), sooo boatbuilders ,in this area anyway, don't supply one. If you find a southeast AK gillnet boat with a stability book I will buy you lunch! The larger boats that I sell fish to (90'ish) do have stability books.

    Tad:Good info on the roll test vs inclining. I read up on the inclining test-looks doable. Too late for this season, but I will give it a whack after october when I have more time.
    I wanted to use the GM figure derived from my roll period test to help me with determining some kind of ballpark for the reduced stability when pots are stacked on the boat(re rolltest loaded). Not necessarily as an absolute or something to go to the bank with.

    I hear ya on the kids and wives. That part of why I'm even pursuing this. The math side of vessel stability is not much considered or discussed in these parts and when it is it is often dismissed. Those tenders that I sell salmon to in the summer? Some of them disregard their stability booklet regularly (at least that's what they tell me). I, on the other hand, want to get old...

    As for the computer study based on the lines of the hull-do you set up stations and measure the hull particulars at all ten stations? I doubt that drawings for this boat still exist. Perhaps you might know a source, being a BC resident/part of the nautical world up there. It's an Albion, built in Haney BC about 1980. Any leads are appreciated.

    Also, I'm quite a fan of your work. Nice lines with a real strong streak of practicality. I especially like your designs derived from the old halibut schooners.

    TANSL: Very interesting on the relationship between a free surface acting like a higher VCG.
     
  6. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    I suggest you hire a qualified person to do a thorough & comprehensive stability assessment. Adding ballast helps... maybe!. It also reduces your freeboard which can reduce the range of stability.
     
  7. TANSL
    Joined: Sep 2011
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    Location: Spain

    TANSL Senior Member

    You are quite right and especially with liquid ballast must be very careful how much you put. However, the boat should be prepared to bear the cargo hold full load, which also greatly reduces the freeboard.
     
  8. mchl
    Joined: Sep 2013
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    Location: France

    mchl MCHL Stabilitycalculation

    If your fishold is rectangular, if you divide it in two longitudinally, the free surface inertia, and then the virtual rise of VCG due to free surface effect, will be divided by 4. If you divide it in three, it will be divided by 9.
     
  9. moose60
    Joined: Dec 2008
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    Location: Bellingham, WA

    moose60 Junior Member

    mchl: That's what I was looking for! Thanks. I figured that there might be a simple way to generalize this.
     

  10. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    Moose...

    Doubtful you will find any drawings of the boat (if there ever were any).

    The inclining is simple, the really important thing is to measure floatation of the boat (where the waterline is exactly) and to avoid any free-surface effect by making sure all tanks are either right full (pressed up) or right empty.

    Basic hull measuring is...

    Not difficult at all, just requires a bit of care. You need a string that's longer than the boat, some painters masking tape, 25' tape measure, a medium length carpenters level, sharpi marker, plumb bob, and a clipboard with pen and paper. A nice tool is a self-leveling cross-line laser level. Some places rent these or you can buy one for about $150 plus a mounting pole. A nice afternoon and a willing helper....you're set.

    On your boat you have two surfaces to measure, the bottom (horizontal) and the topsides and bow (vertical). Two separate methods are used. The string is stretched out tight parallel to the keel at 18" off centerline on the side you will measure (only do one side of the boat). It can be at any height that's convenient but at least 10-12" below the hull bottom. Use the tape to mark stations every 2.5' or 3' from bow to stern. Mark those stations "A" through whatever.

    The string is your "base" line and positioned at "base" vertical height. You will measure everything relative to the baseline and base height. I usually set up level planks or blocking beyond the ends of the boat and just drive some nails in these to tie the string to. The base string will move outboard in even increments (say 12 or 18") after each buttock line measurement.

    So to start the measuring you work along your base string from one end to the other, taking measurements from the string (baseline) up vertically to the hull surface and recording them (this is part of the reason for the helper). The hand level or plumb bob are used in conjunction with the tape measure to get a vertical measurement. If you like you can mark the stations on the hull using the hand level, eyeball, and sharpi.

    Do three or four buttock lines in this way, moving the base string outboard after each series of measurements along the boat. The last series of bottom measurements will be of the chine or chines. Use the plumb bob to get widths of the chine from your base line, and then have the helper hold the tape on the chine while you use the level to get chine height above base.....

    Then switch to horizontal measure mode. In this case the string moves vertically outside the boat (still parallel with the keel line) in steps that represent waterlines. Using the level and tape measure from the string to hull surface at each station and work your way up to the sheer. Again use the plumb bob held by your helper along the sheer to get width and height from the base line. The topsides is where the cross line laser is nice, it projects a line of light horizontally (waterline) and vertically (base line) at the same time. Due to the hull curve you need to set it up twice, beyond the bow and stern, and raise it in increments just like the string.

    That's a start.......
     
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