# Spade rudder

> What is a spade rudder and why do racing boats use it?

- 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/spade-rudder/ · JSON: https://xemstudio.com/api/geometry?object=spade-rudder · Plain text: https://xemstudio.com/objects/spade-rudder/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 spade rudder hangs free of the keel with no skeg, so it acts as a pure foil and can be balanced with area ahead of the stock (Good Old Boat). Its stock must resist bending as well as twisting.
- Balance cuts helm load but too much makes steering hard to control: old practice capped it near 10%, modern rudders reach 17% (Good Old Boat, quoting designers).
- Spades run fat, round leading edges to resist stall; typical thickness 10-12% (Stewart).
- The file is a reference example built by Xem Studio from the numbers above (NACA 0012 per Perry; size illustrative.). Sizes the sources do not fix are illustrative; scale the example to your own vehicle.

## Sizes

| parameter | value or range | source |
|---|---|---|
| Balance area | up to ~17%; ~15% on boats over 45 ft (Perry) | Good Old Boat |
| Max thickness position | 30% chord | Good Old Boat |
| Thickness ratio | 10-12% typical (Stewart); 10-18% (Wurmfeld) | Good Old Boat |
| Favoured foil | NACA 0012 (Perry) | Good Old Boat |
| Rudder area | ~1.4% of sail area (Tanton) | Good Old Boat |
| Max rudder angle | 35-40 deg | Good Old Boat |

## Where the files are

- Good Old Boat: [The rudiments of rudders](https://goodoldboat.com/the-rudiments-of-rudders/) — web — Publisher's page; dimensions are facts

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

Index: https://xemstudio.com/files/hardware/spade-rudder/index.json · license: https://xemstudio.com/files/hardware/spade-rudder/LICENSE.md

- spade-rudder-appendage: [STEP](https://xemstudio.com/files/hardware/spade-rudder/spade-rudder-appendage.step) · [STL](https://xemstudio.com/files/hardware/spade-rudder/spade-rudder-appendage.stl) — rudder spade: 300 mm deep, 60 mm chord, NACA 00-series foil

## Questions

**How much balance?**

Around 15%, rarely above 17% (Good Old Boat).

## Fits with

- [Fin keel with ballast bulb](https://xemstudio.com/objects/fin-keel-bulb/)
- [Skeg-hung rudder](https://xemstudio.com/objects/skeg-hung-rudder/)
- [Daggerboard](https://xemstudio.com/objects/daggerboard/)
- [Centreboard](https://xemstudio.com/objects/centreboard/)
- [Canting keel](https://xemstudio.com/objects/canting-keel/)
- [Rigid wing sail](https://xemstudio.com/objects/wing-sail/)