Righting moments for offshore yachts

Discussion in 'Boat Design' started by Tussock, Nov 19, 2014.

  1. Tussock
    Joined: Sep 2014
    Posts: 47
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Christchurch, New Zealand

    Tussock Junior Member

    What do the designers out there consider to be a satisfactory righting moment? Do you guys have a minimum you aim for when designing an ocean-going yacht?

    While we're going, how about an AVS?

    Cheers,
    Bryan
     
  2. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 206, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Look up Delenbaugh angle for the ability to carry sail.

    Then look up 'Yacht STIX' ( Stability Index ISO 12217 Part 2) which will give you an idea for how the limit of stability relates to seaworthiness for intended use.

    Graham Radford has a good page on stability here:

    http://www.radford-yacht.com/stablty1.html
     
  3. Tussock
    Joined: Sep 2014
    Posts: 47
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Christchurch, New Zealand

    Tussock Junior Member

    Thanks... I can calculate all sorts of values for my own boat, but I'm more interested in what others consider to be minimum requirements for offshore use. I'm a sailor and not a designer... Trying to get to grips with some numbers before finalising a design.

    Cheers,
    Bryan

    Edited, for hoof-in-mouth.
     
  4. Alik
    Joined: Jul 2003
    Posts: 2,900
    Likes: 237, Points: 73, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    ISO12217-2 as a minimum. And above that, up to Your experience and expected boat's exposure to the elements.
     
  5. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 206, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Thinking of the MCA guidelines mentioned by Radford, have a look at this thread: http://www.boatdesign.net/forums/stability/grimalkin-good-design-51378-2.html#post705181

    There are a couple of issues with LPS_ AVS, one is the time for a capsized craft to re-right, the other is the likelihood of a capsize following a wave induced knockdown. The latter is called the "Knockdown Recovery Factor". It's well indicated in the area of the RM curve from 90 degrees to LPS.
    Note that you'll often see GZ curves and GZ must be multiplied by Displacement to get a basic RM for comparison between various craft.

    Wolfston have shown that all craft can be knocked to 90 degrees by a significant beam on breaker regardless of design, (there is so much energy in the breaking jet). Whether a knockdown proceeds into a full inversion depends on several design factors but the most significant ready reckoner is based on a simple calm water righting moment curve.
     
  6. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,003
    Likes: 207, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    Mike and Alik have pretty much hit the nail on the head. There is no one absolute value for any given boat--each boat will have its own desirable minimum. Half of stability is controlled by the vertical position of the center of gravity (VCG) and the other half is controlled by hull shape (VCB and BM). Generally, lower center of gravity is better than higher center of gravity, which translates into more area under the positive side of the righting moment curve than under the negative side, and the higher the value of AVS. This is a case, generally, of more is better. I have designed boats with AVS = 180°, but that's rare. On a sailboat, no one wants an AVS of 90°--that really would be unsafe. As Alik suggests, follow the ISO 12217 standard to see what your boat would need as a minimum for Class A sailing (open ocean).

    Eric
     
  7. TANSL
    Joined: Sep 2011
    Posts: 6,205
    Likes: 293, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Please note that the ISO 12217 standard is only valid for boats between 6 and 24 m in length.
     
  8. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,003
    Likes: 207, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    Which is the vast majority of boats, and certainly in the range of what most people would want to take offshore with limited crew.

    Eric
     
  9. TANSL
    Joined: Sep 2011
    Posts: 6,205
    Likes: 293, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Yes, Yes, you forgive Mr. Sponberg. But when we speak of a term as general as "ocean-going" yatchs should narrow terms.
     
  10. Tussock
    Joined: Sep 2014
    Posts: 47
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Christchurch, New Zealand

    Tussock Junior Member

    Thanks all. I have a hull shape that gives an AVS of 160 degrees, and it returns to a stability of zero at 180 degrees, but I'm fiddling with the length. As length has such a profound effect on moments, beyond knowing that more-is-better in terms of stability, it's an open question as to how much is enough.

    There's good information in the sources suggested above, cheers all.
     
  11. TANSL
    Joined: Sep 2011
    Posts: 6,205
    Likes: 293, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    In general it is sufficient that the ship meets all stability criteria. It is not normal to study how much more stability must be given.
    When we speak of transverse stability, the moment calculated is in relation to the longitudinal axis of the vessel. The moment about this axis is proportional to the length of the waterplane but is proportional to the cube of the breadth. That is, as we all know, the breadth provides stability.
     
  12. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 206, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member


    160 degrees is quite high, it's excellent but it may come at the expense of reduced initial stability.

    I think it's important to note for knockdown recovery that it's the area of the RM curve from 90 to LPS that's more significant than LPS alone.

    Although 160 is close to some recommendations. Prof Nomoto at Osaka who did a lot of work on yacht stability has always been a vocal proponent of 150 degrees as the minimum LPS for an ocean going sailboat. At 150 degrees and above there is no stable inverted period for a moderate hullform, and he showed that such a capsized craft immediately returns upright from the rotaional energy imparted in the initial capsize.

    If you have an LPS of 150 degrees and a good robust RM curve past 90 degrees then you won't gain much, often by reducing habitability adopting a hullform with a higher LPS but of equal energy within the RM envelope.

    With an accurate 3d CAD model of the hull and a simple inclining experiment it's quite simple to detrmine the centre of gravity and the displacment, and then to generate an accurate RM curve. A lot of owners are shocked to find their imagined LPS is significantly less than the designers figure. This is due to the C.G. being raised significantly by the accumulation of gear and equipment not originaly calculated in the weights and moments study.
     

  13. Tussock
    Joined: Sep 2014
    Posts: 47
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Christchurch, New Zealand

    Tussock Junior Member

    That's another of those truisms that's utterly obvious, ONCE someone has pointed it out.... Thanks Mike.

    And thanks to all - good info.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.