Mast Length and Stability

Discussion in 'Sailboats' started by Zora, Aug 13, 2019.

  1. Zora
    Joined: Aug 2019
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    Location: Eire

    Zora Junior Member

    Hi all,

    I have a steel hull Dick Koopmans (11 metres on deck) and i am hoping to get some opinion on mast length and how i could counter it with ballast etc.

    The boat is built to the attached drawing, however her original rig was lost. The alternative was a larger rig that is currently on the boat. I am still fitting out the boat (i bought a bare hull), so it is off its waterline as it is (no tanks yet), and when the wind picks up any bit the boat heels a little in her berth. In bad weather it heels up to approx 30 degrees. We have had the boat out sailing but keep it reefed and it sails mostly fine, although i haven't been out in much bad weather. I measured the mast to be a little over 17 metres off the deck.

    when i had her registered, the tonnage surveyor calculated her displacement at 11.91 tons. The boat currently weighs a little over 10 tons due to little interior or fitted systems.

    My main question is whether this extra mast length can be easily countered by added ballast to the keel base (possibly welding a plate to the bottom of the keel)

    I have probably left out important information and i understand it's not a simple answer. I just want to come up with some form of plan, whether that be cutting the rig or adding ballast.

    Thanks
    Rhys

    PS. here is the project in video form - Sailing Yacht Zora https://www.youtube.com/channel/UCZ05SYliiCbF_0mLNTI4IQg?view_as=subscriber
     

    Attached Files:

  2. M&M Ovenden
    Joined: Jan 2006
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    M&M Ovenden Senior Member

    Hi Rhys,

    I think the first thing you should do is a inclining test to determine the current center of gravity - I'm assuming you don't have any stability curve information. If you had the boat lines you could then draw this out in software to calculate the hydrostatics , and could see the changes with additional ballast. Murielle knows more about this stuff than me....

    My first boat was a Contessa 26 (folkboat) and it would heel over in any little puff - it had little static / form stability. However at one point it would just lock in and really take some wind to heel any further. Have you tried this on your boat ?

    Cheers,
    Mark
     
  3. Zora
    Joined: Aug 2019
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    Location: Eire

    Zora Junior Member

    Thanks for the reply Mark, unfortunately i have very little actual information on the hull design.

    Interestingly enough, i have noticed that at a certain angle of heel she does dig in a lot better. Usually it's just a few degrees and it takes far more wind to push her over any further.

    This is her under a reefed main and full Genoa in about 5 knots of breeze.

    [​IMG]

    Some nasty weather came through over the weekend, and she was heeling about 15/20 or so degrees more than other boats in the marina. An Etap 32i that was berth next to me was very stable in comparison.

    Do you have any more information on inclining tests? I have heard of them, i will do some googling to see if i can find any methodology.

    On one hand, i really like the extra sail area. On the other hand, i'm not sure i'll be as enthusiastic about the taller mast in a serious blow or a large swell.
     
  4. M&M Ovenden
    Joined: Jan 2006
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    Location: Ottawa

    M&M Ovenden Senior Member

    The inclining test involves ballast (friends), a plumb bob, bucket of viscous liquid to dampen plum bob (dish soap I've read is handy) and beer. Google will tell you the rest.

    I wonder if you could find a NA student that needs a project.....

    Cheers,
    Mark
     
  5. Men
    Joined: Aug 2019
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    Location: UK

    Men Junior Member

    I did some fast crude measurements on the plan you sent, and it seems that the original mast is abt 14.5m above deck. That means the 17m is a lot higher than it should. By adding extra ballast you would increase the forces on everything involved, rigging, stiffeners, chainplates as all design calculations were based on the original mast dimensions and righting moment. I wouldn't go that way. I believe the safest and best way to go would be to fix your mast at the right height.
     
  6. Milehog
    Joined: Aug 2006
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    Location: NW

    Milehog Clever Quip

    Just finishing the fit out will stiffen her up as she settles to her designed waterline. How much? That is the question. I suggest you load her to the weight you expect her to be at completion and decide from there.
     
  7. MikeJohns
    Joined: Aug 2004
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    Location: Australia

    MikeJohns Senior Member



    Have you contacted the designer ?

    Contact Information http://www.dickkoopmans.nl/over_ons/contact.html
     
  8. Zora
    Joined: Aug 2019
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    Location: Eire

    Zora Junior Member

    I had an email conversation with him, and he didn't voice any specific concerns. Any further investigation on his part would have incurred (and rightly so) pretty extensive fees. I do have her original sail plan, and after speaking with riggers and other engineers i think that shortening the mast will probably be the wisest course of action. I'm quite sure that cutting 2 or 3 metres from the bottom of the mast won't cause massive problems. I need to do more investigation but from what i can see so far it will just be a case of moving the mast hardware up. The boom is pretty high as it is so that can be moved up slightly less, and i can cut the bottom off the main up to the first reef. The spreaders as they are, are very high off the deck, so moving those down by cutting from the bottom won't be a big deal.

    You can see how massive it is from this photo!

    [​IMG]

    As advised above, i'll do this work once the boat is complete and make my decision then. Appreciate the replies, i'll report back.
     
  9. Zora
    Joined: Aug 2019
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    Location: Eire

    Zora Junior Member

    Thanks for all the replies, much appreciated
     
  10. Men
    Joined: Aug 2019
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    Location: UK

    Men Junior Member

    Keep also in mind that in some standards, you need to avoid having holes at the middle 50% between spreaders. this could be a consideration if you cut your mast 3 meters and the old spreader holes end up in the middle of the panel between the new spreaders. Make sure to discuss this point with the people you will be consulting.
     
  11. Zora
    Joined: Aug 2019
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    Location: Eire

    Zora Junior Member


    Thanks Men. I had actually not planned to move the spreaders. I think that removing the bottom 2 or 3 metres of the mast and moving everything up shouldn't have too much of a detrimental effect on the angle of the shrouds to the spreaders, as the shrouds are almost straight at the moment in the lower section, and I should be able to sort the angle by adjusting the spreader.

    A further question I have is how critical is the length of the lower section when compared with the middle and upper section? Is there some critical ratio or rule that determines the location of the spreaders? It's a double spreader rig. My fear is that if I cut the bottom of the mast, I'll affect the strength of it. I just can't think of an engineering reason as to how.
     

  12. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    The position, number and length of the spreaders determines the angles that form the stays with the vertical and, therefore, the value of the forces on them and on the mast.
     
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