Gulfweed Keel

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

  1. garren
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    garren Junior Member

    I was looking at Hanna's Gulfweed plans last night and noticed he designed three keel configurations - two without CB and one with CB. The full-depth keel requires 1100 pounds of outside ballast. His intermediate keel is a foot shorter but requires the same amount of outside ballast.

    I'm curious to know how are you able to reduce draft by a foot and not change outside ballast, or apparently anything else, and maintain the sailing characteristics of the full-depth keel?

    Thanks - Gary
     
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  2. frank smith
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    frank smith Senior Member

    There must be different rigs , or different intended usages . That is a guess .

    BTY Gulfweed is a pretty little boat , but Imo , the sheer need some work .
     
  3. Tad
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    Tad Boat Designer

    You can't..........
     
  4. frank smith
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    frank smith Senior Member

    Then what would the changes in sailing characteristics be?
     
  5. Tad
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    Tad Boat Designer

    frank...

    Well I don't know the details so I can only guess.....

    Generally the shallower ballast keel will mean less stability, the centerboard may or may not make up the loss of lateral area in the shallower keel....in theory you could build a centerboarder that would be better in some conditions....board up off the wind and less wetted surface, board down on the wind with a deeper and more effective foil......or you could build a ballasted board to correct the loss of stability but it has to be pinned down.....

    The intermediate keel will have less stability and wetted surface plus make more leeway......but to most cruisers the effect will be almost un-noticeable......
     
  6. frank smith
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    frank smith Senior Member

    Thanks , I also think the decrease in displacement on the shallow keel could be made up with internal ballast . Just a thought .
     
  7. garren
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    garren Junior Member

    Thanks Tad for your insight. Would increasing external ballast on the intermediate keel reduce the instability? If so, is there a formula to determine the necessary additional ballast?

    Gary
     
  8. Tad
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    Tad Boat Designer

    Jack Hanna was no dummy but he was human and did make mistakes or adopt odd thinking on occasion......I don't know what his thinking was in this case.....

    At any rate we do not know if there is a "instability" problem to fix....and no there is no simple formula to "fix" stability. For the average layman most stability issues are subjective....the boat "feels" tender or stiff....

    While I could make lots of modifications to the underbody of the Gulfweed and improve her performance such that she would be a different boat, in general I recommend building as designed (or find a different design)......if there proved to be a stability issue a couple of lead cheeks lag bolted to the keel would help.....but building lighter hollow spars and a foam cored cabintop would do the same thing and make her a faster boat(lower total displacement).

    Her actual stability with the various keels could certainly be studied, center of gravity for each model established and that data fed into a computer 3D hull model to generate righting arm and VMG curves. That's what NA's do, but the difference in ultimate stability is probably (for the two outside ballasted versions) something around 5 degrees.......
     
  9. garren
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    garren Junior Member

    I have been looking a little more at Hanna’s “How to Build Gulf Weed – A Little Cruiser Designed Especially for the Amateur Builder” and his iteration of keels for this design. Both the deepest keel design (4’ draft) and the intermediate keel (3’2” draft) incorporate 1100 pounds of outside ballast with mention in the narrative that additional ballast will be needed inside. No outside ballast is shown for the centerboard version (2’4” draft) nor is there any mention of inside ballast for the centerboard version in his “How to . . .” narrative.

    Since this design is “especially for the amateur builder,” how would an amateur determine whether any outside ballast is required with the centerboard version and, if determined to be none, should the amateur assume inside ballast would include the 1100 pounds shown in the other keel configurations plus some additional unspecified quantity of inside ballast to float on the DWL?

    Any and all insight is appreciated.

    Gary
     
  10. PAR
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    PAR Yacht Designer/Builder

    They wouldn't, you trust the plans.

    The plans should be fairly specific (I don't have them in front of me), but as a rule assumptions aren't a good idea. You work with the appendage configuration you want and don't transplant any information from the other setups.

    Simply put, it's likely the shoaler versions have less stability then the deepest keel version, but we don't know what the AVS is on this boat, in all three configurations, which may be sufficient enough to get away with the same ballast amount in all three versions.
     
  11. garren
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    garren Junior Member

    Thanks Paul. Not wanting to go through with something like this totally blind, how much effort would it take a professional to determine whether external ballast is necessary for the CB version and other factors like AVS?

    Gary
     
  12. Tad
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    Tad Boat Designer

    External ballast is definitely desirable, it will produce a safer and more seaworthy boat, also a stiffer and therefore faster sailing boat. Ideally it would be a lead pancake (in the CB version) about 2" high running from station 4 to station 8, make it as wide as required to get 1000-1200 pounds.
     
  13. PAR
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    PAR Yacht Designer/Builder

    I'm not sure you need to go to these troubles Gary. I don't know the design, but you'd have to assume Hanna knew the stability qualities of each and though the ballasted fin is going to be better, the centerboard version may be acceptable, unless of course racing another fin keel Gulfweed.

    Can you post the different profiles? It is had to believe the CB version has no ballast and it's reasonable to assume it'll need at least the same amount, though the pancake Tad mentions, may look more like a forked or slotted sausage. You also could attach it as a stub keel, with a slot, possably lowering the CB pivot bolt and case heights too.
     
  14. garren
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    garren Junior Member

    [​IMG]

    These are the keel profiles as previously posted by Tad. I have this page as part of the "plans." There are other sheets with more detail of the keel options if that would be helpful. Doesn't show outside ballast for CB keel though.

    Thanks for everyone's help.

    Gary
     

  15. PAR
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    PAR Yacht Designer/Builder

    I would reshape the bottom of the shoal keel to remove the "drag, adding a shoe of lead, to accommodate the ballast. Naturally,this wedge shape hunk of metal would need some minor volume calculations, so you can get the weight where you want it, but nothing difficult. The resulting new keel bottom profile will let her stand straight up, rather then bow down, if drying out on a tide (with props of course) or blocked up on the hard.
     
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