Lead replaced steel for improved handling? Hmmmm

Discussion in 'Stability' started by Salmoneyes, Jan 13, 2019.

  1. Salmoneyes
    Joined: Sep 2018
    Posts: 86
    Likes: 1, Points: 8
    Location: Southern Oregon

    Salmoneyes Junior Member

    Id like your "opinions" on this solution, specifically as I feel like the solution did not address the real problem. Thats "my" opinion.

    Quick background for those not familiar with our project.

    12m steel pilot house ketch
    13' beam
    6'3" draft (plans called for 5.5')
    20 ton displacement (plans call for 13.5)
    8000 lb lead Ballast (plans call for 7500 lb steel marbles)

    Masts were lowered from the plans for some reason, and the main boom shortened to accommodate the steering station on deck.

    So the boat is heavier than designed, causing it to sit deeper, drawing more water and the sail plan was reduced.

    The builders claim it handled poorly, so their solution was to remove the steel marbles and replace with lead plus additional 500 pounds, ultimately, lowering its center of gravity. I was given no other information than that, so I do not know if we are talking weather helm, or what "handled poorly" means. I was told however it sailed beautifully and was circumnavigated after the change and had a top speed of 10 knots running with a Twizzle rig. The builder thinks it sailed so well, he would not recommend changing anything. Unfortunately, to me thats like saying my 2 wheel drive truck has no traction in the snow so I filled the bed with sand and now its great. Why buy a 4 wheel drive...

    The question I have is:
    What is the real culprit behind the need for lowering CG which improved its handling?

    If the sail plan was reduced both horizontally and vertically, and the boat sits in the water deeper, I would have thought the CG would be lower than the design called for originally. The designer is Bruce Roberts, so its not "poorly designed" and the build strayed from the plans anyway, which means all bets are off.
     

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  2. JamesG123
    Joined: Mar 2015
    Posts: 542
    Likes: 61, Points: 28, Legacy Rep: 10
    Location: Columbus, GA

    JamesG123 Senior Member

    Yeah when a builder starts coloring outside the lines, its not really built to a design anymore, more like "inspired by". lol. I guess you have to take their word for it that they got it dialed in for the configuration it was built.
     
  3. Salmoneyes
    Joined: Sep 2018
    Posts: 86
    Likes: 1, Points: 8
    Location: Southern Oregon

    Salmoneyes Junior Member

    I like that... I will start using that. " our boat is a Bruce Roberts inspired" yada yada yada.

    Ok James,,, give me your thoughts on why lowering the CG on a build that had a lower than planned CG helped handling.

    I have a basic understanding when talking about righting arms and the metacentric height and their relationship to stability. Im still wrapping my brain around moving ballast for to aft and can not help wonder if it was more an issue of where the weight was placed originally?

    What I think I understand is the same as my 2 wheel drive truck analogy. Without weight in the bed, a 2 wheel drive truck lacks traction and can slip, or slide. With the ballast in the keel to far forward and a light stern, is it possible the boat was slipping, requiring more attention on the helm?
     
  4. Salmoneyes
    Joined: Sep 2018
    Posts: 86
    Likes: 1, Points: 8
    Location: Southern Oregon

    Salmoneyes Junior Member

    Here are some numbers Ive come up with based on some online calculators and rough math

    LOA = 40'
    LWL = 34.833'
    BEAM = 13'
    DISP =40,000 LBS
    SAIL AREA =950
    D/LWL = 423
    HULL SPEED =7.91
    SA/D =13
    LWL/BEAM =2.68
    MOTION OF COMFORT =55.33
    CAPSIZE RATIO =1.52
    POUNDS/INCH =1618
    RM30 = 79KNM
     

  5. JamesG123
    Joined: Mar 2015
    Posts: 542
    Likes: 61, Points: 28, Legacy Rep: 10
    Location: Columbus, GA

    JamesG123 Senior Member

    Pretty much what you said. All things being equal, lower CG improves stability. You can smother a lot of bad behavior by dumping lead on it. And if you've "designed" your way into a corner, that can be the only economical way of fixing it.

    I was going to add the old saw, " and if it an't broke..." but then since you have the boat stripped down in a refit, I guess now is probably the best time she will ever have of having her ballast trim fixed.
     
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