{
 "version": "2026-10-20",
 "license": "Dimension sheets: CC BY 4.0. Standard and published-size files: CC BY 4.0. Xem Studio's own designs (files_license = Xem Studio Design License): free to use and sell things from, not to redistribute as files — https://xemstudio.com/license/. Other publishers' files: their own license, linked beside them.",
 "slug": "fin-keel-bulb",
 "name": "Fin keel with ballast bulb",
 "kind": "standard",
 "category": "Boat design",
 "group": "Sailboat appendages",
 "year": null,
 "question": "Why do racing yachts and RC sailboats use a thin fin with a bulb?",
 "tags": [
  "vehicle design",
  "sailboat appendages",
  "rc design",
  "aerodynamics"
 ],
 "aliases": [
  "bulb keel",
  "fin keel",
  "keel bulb",
  "rc sailboat keel",
  "IOM keel",
  "ballast bulb",
  "fin and bulb",
  "torpedo keel",
  "lead bulb rc yacht",
  "keel draft"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": false,
 "free_files": 1,
 "url": "https://xemstudio.com/objects/fin-keel-bulb/",
 "sheet_txt": "https://xemstudio.com/objects/fin-keel-bulb/sheet.txt",
 "sheet": {
  "basis": "Design rules and figures from the cited sources; example geometry by Xem Studio",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "A bulb keel puts the ballast in a teardrop at the bottom of a high-aspect fin, as low as possible for maximum righting moment (Wikipedia, Bulb keel). The thin fin supplies lateral lift with little drag.",
   "The penalty is draft, so bulbs suit deep, thin keels and are not used where shallow water matters (Wikipedia).",
   "The IOM RC class limits the bulb to 2500 g of lead and 420 mm draft on a 1000 mm hull with 4000 g minimum displacement (onemetre.net).",
   "Used on: International One Metre (IOM), International 110.",
   "The file is a reference example built by Xem Studio from the numbers above (Depth = IOM max draft; chord and bulb shape illustrative (IOM fin/bulb shape free).). Sizes the sources do not fix are illustrative; scale the example to your own vehicle."
  ],
  "table": {
   "columns": [
    "parameter",
    "value or range",
    "source"
   ],
   "rows": [
    [
     "IOM hull length",
     "1000 mm",
     "onemetre.net"
    ],
    [
     "IOM max draft",
     "420 mm",
     "onemetre.net"
    ],
    [
     "IOM min displacement",
     "4000 g",
     "onemetre.net"
    ],
    [
     "IOM max bulb",
     "2500 g lead (62.5% ballast ratio)",
     "onemetre.net"
    ],
    [
     "International 110 bulb",
     "300 lb cast iron",
     "Wikipedia (Bulb keel)"
    ]
   ]
  }
 },
 "sheet_text": "Fin keel with ballast bulb — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/fin-keel-bulb/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)\nBasis: Design rules and figures from the cited sources; example geometry by Xem Studio\nCatalog version: 2026-10-20\n\nSizes: see table below\n- A bulb keel puts the ballast in a teardrop at the bottom of a high-aspect fin, as low as possible for maximum righting moment (Wikipedia, Bulb keel). The thin fin supplies lateral lift with little drag.\n- The penalty is draft, so bulbs suit deep, thin keels and are not used where shallow water matters (Wikipedia).\n- The IOM RC class limits the bulb to 2500 g of lead and 420 mm draft on a 1000 mm hull with 4000 g minimum displacement (onemetre.net).\n- Used on: International One Metre (IOM), International 110.\n- The file is a reference example built by Xem Studio from the numbers above (Depth = IOM max draft; chord and bulb shape illustrative (IOM fin/bulb shape free).). Sizes the sources do not fix are illustrative; scale the example to your own vehicle.\n\nparameter  value or range  source\nIOM hull length  1000 mm  onemetre.net\nIOM max draft  420 mm  onemetre.net\nIOM min displacement  4000 g  onemetre.net\nIOM max bulb  2500 g lead (62.5% ballast ratio)  onemetre.net\nInternational 110 bulb  300 lb cast iron  Wikipedia (Bulb keel)\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Wikipedia: Bulb keel — web — Publisher's page; dimensions are facts — https://en.wikipedia.org/wiki/Bulb_keel\n- onemetre.net: The IOM class — web — Publisher's page; dimensions are facts — https://onemetre.net/OtherTopics/IOMclass/IOM.htm\n\nFree files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/fin-keel-bulb/index.json\nAPI: https://xemstudio.com/api/geometry?object=fin-keel-bulb\n",
 "sources": [
  {
   "publisher": "Wikipedia",
   "title": "Bulb keel",
   "url": "https://en.wikipedia.org/wiki/Bulb_keel",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "onemetre.net",
   "title": "The IOM class",
   "url": "https://onemetre.net/OtherTopics/IOMclass/IOM.htm",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  }
 ],
 "xem": {
  "status": "free-files",
  "sheet_free": true,
  "mesh_price_usd": null,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "spade-rudder",
  "skeg-hung-rudder",
  "daggerboard",
  "centreboard",
  "canting-keel",
  "wing-sail"
 ],
 "fits": [],
 "dimensions": "exact (nominal, no printing tolerance)",
 "dimension_rule": "All geometry on Xem Studio is exact (nominal, true size) with no printing tolerance. Where a part meets another part, the object lists its mating features (feature, exact size, what it mates with, fit class: press, slide, run, socket, clamp or designed). Apply your process's allowance to those features only; 'designed' gaps are part of the design and stay as drawn.",
 "mating": [],
 "faq": [
  {
   "q": "Does the IOM rule limit fin shape?",
   "a": "No; fin material and thickness are free (onemetre.net)."
  }
 ],
 "files_license": {
  "name": "Xem Studio Design License",
  "holder": "Xem Studio",
  "url": "https://xemstudio.com/license/",
  "note": "Xem Studio's own design: free to use, print, modify and sell things made from it; not to be republished or redistributed as files."
 },
 "offers": [
  {
   "file": "fin-keel-bulb-appendage",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "Xem Studio Design License",
   "step": "https://xemstudio.com/files/hardware/fin-keel-bulb/fin-keel-bulb-appendage.step",
   "stl": "https://xemstudio.com/files/hardware/fin-keel-bulb/fin-keel-bulb-appendage.stl",
   "note": "fin keel bulb: 420 mm deep, 60 mm chord, NACA 00-series foil, bulb Ø40 × 180 mm"
  }
 ],
 "markdown": "https://xemstudio.com/objects/fin-keel-bulb/index.md"
}