Gulfweed Keel

Discussion in 'Sailboats' started by garren, Aug 19, 2011.

  1. garren
    Joined: Mar 2009
    Posts: 31
    Likes: 1, Points: 8, Legacy Rep: 11
    Location: Billings, Montana

    garren Junior Member

    Thanks Tad and Paul. You guys are great.

    Gary
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 481, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Tad's great, I'm just mediocre.
     
  3. sharpii2
    Joined: May 2004
    Posts: 2,044
    Likes: 206, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    Can't resist the temptation to add a comment or two.

    A few years ago, I was reading "The Boats They Sailed In", the chapter about the original Sea Bird ketch. The author wondered how 750 lbs of ballast was sufficient for a 5,000 lb boat. In the book, there was a tiny lines drawing of the boat, including the keel. Based on that, I calculated how much the keel displaced and discovered the ballast was just more than sufficient to counteract the keel's buoyancy. It added little to the boat's initial stability, but plenty to it's self righting capability, once the keel started coming out of the water. Then, not just the metal at the bottom is counted as ballast, but the huge deadwood as well. I calculated that the entire keel, including solid deadwood, weighed roughly 1,200 lbs, which does not seem all that modest for a 5,000 lb boat. That comes to about 25% ballast, as opposed to only 15%, which the metal alone would indicate.

    My guess is the centerboard version of the boat (with the same amount of ballast) would actually be stiffer, but keel version would be safer.

    Now days, we are so used to keels that are so short that they are all, or mostly solid metal, and have such high specific gravity that they add stiffness (initial stability) as well as ultimate stability. Not necessarily so with some of the classics from yesteryear.
     
  4. garren
    Joined: Mar 2009
    Posts: 31
    Likes: 1, Points: 8, Legacy Rep: 11
    Location: Billings, Montana

    garren Junior Member

    That's interesting and enlightening. Thanks for taking the time to post.

    Gary
     
  5. Tad
    Joined: Mar 2002
    Posts: 2,307
    Likes: 191, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    It isn't enough to ensure positive self-righting under all conditions. Read Voss on the 1911 voyage of the Sea Queen eastward from Yokohama in the Pacific. At one point she turned bottom up and stayed there until righted by another huge breaking wave.


    What counts when the boat is on her side is how far G (center of Gravity) is below B (center of Buoyancy). In a low-sided, small cabin trunk boat (such as Sea Bird or Gulfweed) B can't go far, so there is little righting energy.

    This makes no sense to me.....
     
  6. garren
    Joined: Mar 2009
    Posts: 31
    Likes: 1, Points: 8, Legacy Rep: 11
    Location: Billings, Montana

    garren Junior Member

    Tad,

    Would you please expound on your response to the last comment? Would a hull of this nature (single chine with a CB) be more safe (whatever that means) with or without external ballast? I guess I'm wondering about the likelihood of a capsize and filling with water before righting (if it rights at all).

    Would moving the cabin sides out to the topsides generally improve its buoyancy and resistance to capsize?

    Gary
     
  7. sharpii2
    Joined: May 2004
    Posts: 2,044
    Likes: 206, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    I don't see why. By just adding the ballast to the bottom of the boat, you are lowering the CG more than you are lowering the CB. With the deadwood's buoyancy added down low, you may be subtracting RM, at low angles of heel, rather than adding. Especially if the whole keel/deadwood system ends up at or near neutral buoyancy.

    Once the keel/deadwood assembly come out of the water, i think you would agree, the ballast, at the very least, has more leverage than the ballast bolted onto the bottom of the hull.

    At least, that's my reasoning
     
  8. Tad
    Joined: Mar 2002
    Posts: 2,307
    Likes: 191, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    Gary,

    It's generally accepted that (in a given hull) a lower center of gravity provides a greater range of positive stability. To lower the C of G use denser materials down low, lead or steel outside is good. For the centerboarder a ballasted lifting keel would be best, if the keel could be pinned in place (down).

    Some would say it's safer to eliminate the ballast completely and add a couple more hulls for safety.......;)

    Raising freeboard or increasing beam will increase righting arm or righting moment, but the boat will no longer be a Gulfweed.

    Sharp....

    I think your reasoning is now clear. With centerboard rather than full keel the canoe hull will float lower at a given displacement(minus keel volume). Due to raked ends and flaired sides I (inertia of waterplane) will be higher for the centerboarder at low angles. This advantage will mostly disappear when the boat is on her side. A rigorous scientific study could document this but I doubt the average sailor would see it IRL......
     

  9. philSweet
    Joined: May 2008
    Posts: 2,414
    Likes: 240, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    As a guess, and it's only a guess, I suspect the designer's primary concern with this boat is to keep the design waterline where it is drawn. This sort of hull with sweeping chines is not very tolerant of large changes in displacement. It will have a sweet spot, and I think the designer wants you to sail there. The absolute performance is presumably secondary to the feel of the boat when properly weighted. On a boat such as this, I would adjust the rig to match the keel choice and keep the waterline where it is drawn.
    Or, if you are feeling froggy, adjust the lines of the chine to suit the revised keel choice.
     
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.