Fin keel with ballast bulb — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/fin-keel-bulb/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Design rules and figures from the cited sources; example geometry by Xem Studio
Catalog version: 2026-10-20

Sizes: see table below
- 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.

parameter  value or range  source
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)

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Wikipedia: Bulb keel — web — Publisher's page; dimensions are facts — https://en.wikipedia.org/wiki/Bulb_keel
- onemetre.net: The IOM class — web — Publisher's page; dimensions are facts — https://onemetre.net/OtherTopics/IOMclass/IOM.htm

Free files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/fin-keel-bulb/index.json
API: https://xemstudio.com/api/geometry?object=fin-keel-bulb