# Fin keel with ballast bulb

> Why do racing yachts and RC sailboats use a thin fin with a bulb?

- Category: Boat design › Sailboat appendages
- Kind: Standard part
- Basis: Design rules and figures from the cited sources; example geometry by Xem Studio
- Page: https://xemstudio.com/objects/fin-keel-bulb/ · JSON: https://xemstudio.com/api/geometry?object=fin-keel-bulb · Plain text: https://xemstudio.com/objects/fin-keel-bulb/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); files Xem Studio Design License (free to use and sell from, not to redistribute as files: https://xemstudio.com/license/)
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** True size, with no printing tolerance added.

## 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.

## Sizes

| 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) |

## Where the files are

- Wikipedia: [Bulb keel](https://en.wikipedia.org/wiki/Bulb_keel) — web — Publisher's page; dimensions are facts
- onemetre.net: [The IOM class](https://onemetre.net/OtherTopics/IOMclass/IOM.htm) — web — Publisher's page; dimensions are facts

## Free STEP/STL (1 parts, Xem Studio Design License, Xem Studio)

Index: https://xemstudio.com/files/hardware/fin-keel-bulb/index.json · license: https://xemstudio.com/files/hardware/fin-keel-bulb/LICENSE.md

- fin-keel-bulb-appendage: [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) — fin keel bulb: 420 mm deep, 60 mm chord, NACA 00-series foil, bulb Ø40 × 180 mm

## Questions

**Does the IOM rule limit fin shape?**

No; fin material and thickness are free (onemetre.net).

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