DIY Sailing Computer for Dinghies and Keelboats

Modern sailing rewards sailors who understand data-driven performance. Whether you are a competitive dinghy sailor optimizing trim during roll tacks or a keelboat weekend racer chasing the front of the fleet, knowing your heading, COG, SOG, VMG, and heel angle in real-time helps you make faster, smarter decisions on the water.

Traditionally, this meant installing expensive, dedicated sailing computers like Tacktick, Sailmon, or B&G systems, which often require through-hull sensors, NMEA networks, and boat-specific mounting. These systems, while precise, are cost-prohibitive for many sailors and impractical for dinghies.

However, by using a rugged, waterproof smartphone and a reliable holder, you can build your own sailing computer without expensive or invasive installations. Combined with the right Android app, this setup can deliver real-time performance metrics with instant and integrated values and audio output, allowing you to focus on sailing instead of your display.

This DIY approach is:

• Affordable
• Portable between boats
• Easy to implement without wiring or hull modifications
• Customizable with software tailored to your needs

Hardware: The Smartphone

Why Use a Rugged, Waterproof Phone?

Your daily smartphone is not ideal for saltwater environments due to:

• Salt spray damage
• UV exposure
• Impact and vibration during tacks and gybes
• Battery drain from GPS and sensors during long training days

Rugged waterproof smartphones, such as the Ulefone Power Armor series, are perfect for the task:

• IP68/IP69K and MIL-STD-810H certification: can handle immersion, spray, shock, and drops.
• Large, bright displays: readable in daylight while sailing.
• Long battery life: essential for all-day training and regattas.
• Glove and wet-hand usability: many rugged phones support these modes.

Importance of Sensors and Their Quality

To measure:

• Heading (magnetic compass)
• COG and SOG (via GPS)
• VMG (derived from COG/SOG and target/course heading)
• Heel angle (using gyroscope/accelerometer)

You need a phone with:

• High-quality GNSS chipset (GPS, GLONASS, Galileo) for accurate speed and position.
• Magnetometer (compass sensor): for stable heading information.
• Accelerometer and gyroscope: for smooth heel angle readings and better filtering against wave-induced noise.

Important: Some budget rugged phones lack a gyroscope, which reduces the quality of heel angle measurement. For example, certain CAT and low-end Ulefone models may not include a gyroscope; always check the sensor specs before purchasing.

Recommended device example: Ulefone Power Armor 19T: Good screen brightness, reliable GNSS, magnetometer, accelerometer, gyroscope, and barometer with 9600mAh battery for extended use.

The Holder: Secure Mounting

You need a secure, adjustable, and UV-resistant holder:

• For dinghies, the following mounting points can be considered:
  • below the boom
  • daggerboard case
  • under the boom with Velcro and bungee to reduce risk of snagging
• For keelboats, use a rail-mounted RAM X-Grip or Railblaza phone holder.

I personally prefer to mount it over the dinghy rudder which means that I am also able to use the smartphone for video recording. My smartphone holder is built out of aluminium pieces and 3d printed parts, fitting perfectly on top of the rudder. The smartphone holder itself I bought online, it is supposed to be waterproof (smartphone holder for bikes).

Also Consider:

• Anti-glare screen protectors.
• Quick-release for rapid retrieval in case of capsize.
• A small lanyard to prevent losing the phone during an unintentional capsize.

Software: The Sailing App

Your phone needs an app capable of illustrating sailing-relevant metrics in a high-contrast, easily readable interface with audio output.

Key Metrics to Display

1. Heading (Compass Heading)
   Indicates the boat’s instantaneous heading using the magnetometer. Useful for:
   1. Holding a consistent upwind course.
   2. Practicing steering to telltales or luffing angles.
2. COG (Course Over Ground)
   Shows your actual track over the ground, factoring in current and leeway. Allows sailors to:
   1. Compare intended vs. actual course.
   2. Evaluate the effect of current on tactics.
3. SOG (Speed Over Ground
   Indicates actual speed relative to the earth’s surface, measured by GPS. Useful for:
   1. Tracking performance during maneuvers.
   2. Evaluating effects of trim adjustments.
4. VMG (Velocity Made Good)
   Shows the effective speed toward a waypoint or wind direction, allowing optimization for upwind or downwind angles. Essential for:
   1. Performance tuning.
   2. Tacking angle decisions.
5. Heel Angle (Boat Heel)
   Shows the angle of heel using the accelerometer and gyroscope. Why it matters:
   1. Too much heel increases leeway and reduces rudder efficiency.
   2. Optimal heel varies by boat type (e.g., 10–15° for some keelboats).
   3. Sail trim and crew position can be adjusted for best heel and speed.

Instant vs. Integrated Metrics

Real-time values fluctuate due to wave motion and wind gusts, while integrated (averaged or damped) metrics allow you to evaluate trends in boat trim, course holding, and performance.

For example:

• Instant SOG: See how a puff or trim change immediately affects speed.
• Integrated SOG (10-second average): Track stable performance for tuning sails and trim.

This approach:

• Helps remove noise from wave-induced speed spikes.
• Allows focus on maintaining consistent VMG and heel rather than chasing transient data fluctuations.

Audio Output

Why audio matters:

• Constantly looking at a screen while sailing can compromise your awareness and trim.
• Audio readouts allow you to keep your eyes on the sails and water while still receiving feedback.

Examples of useful audio feedback:

• “VMG increasing” or “VMG decreasing.” or "VMG 4.5 knots"
• Periodic readout of heel angle.
• SOG pings when speed increases or drops below a threshold.

Some apps allow:

• Bluetooth audio to waterproof earbuds.
• Vibration or beep alerts when heel exceeds a set angle to prevent broaching or inefficient heel.

Recommended Apps for Android

• SailingMetrics
  • Free to use
  • Displays key metrics with configurable audio output   
• Sail Racer Pro
  • Displays heading, COG, SOG, VMG, and heel with graphical and numerical readouts.
  • Polar integration for performance optimization.
• SailFreeGPS
  • Free or low-cost.
  • Simple, large-number displays.
  • Heel angle shown using device sensors.
• RaceQs (for post-race analysis)
  • Records tracks and heel for later 3D replay.
  • Less useful for live feedback but invaluable for analyzing maneuvers and trim post-sailing.
• Kwindoo
  • Useful for regatta tracking with live position, though not specialized for heel monitoring.

Comparison with Existing Sailing Computers

Key takeaways:

• For dinghy sailors and training optimization, the smartphone approach offers incredible flexibility.
• For serious offshore racing or integrated wind/depth data, dedicated sailing computers still have an advantage.

Benefits of a DIY Smartphone-Based Sailing Computer

• Affordability: Most sailors already have a phone, and a rugged secondary device is inexpensive compared to marine instruments.
• Flexibility: Use across multiple boats without reinstallation.
• Rich data feedback: Easily integrates GPS, heading, heel, VMG, and audio outputs.
• Instant and integrated metrics: Improve your trim in real-time while building a library of data for post-training analysis.
• Ease of upgrades: App updates deliver new features without needing new hardware.
• Lightweight and compact: Especially beneficial for dinghies, skiffs, and small keelboats where weight and simplicity matter.
• Great for solo sailors: Heel alerts, speed increases, and VMG changes can be heard while your hands remain on the tiller or sheets.

Practical Tips for Implementation

• Battery Management: Rugged phones with large batteries are ideal, but keep a small power bank in a waterproof pouch for extended sessions.
• Calibration: Calibrate the compass and ensure the phone is mounted away from metal to avoid interference.
• Sunlight visibility: Use maximum brightness and anti-glare film if needed.
• Mount securely: Use additional safety lines to prevent losing your phone in a capsize.
• Preload maps and tracks: Useful if your app supports visual course navigation.

Conclusion

Using a rugged waterproof smartphone as a DIY sailing computer offers an accessible, effective way to bring real-time performance metrics, including heel angle, heading, COG, SOG, and VMG, to your sailing without the high costs of traditional systems.

By focusing on:

• The right hardware (rugged phone with necessary sensors)
• A secure, low-profile holder
• Purpose-built sailing apps
• Audio integration for hands-free feedback
• Data logging for continuous improvement

you can supercharge your training, refine your trim, and sail faster, all without breaking the bank.

This DIY approach aligns with modern sailing’s spirit: using available technology to improve your skills while keeping the focus on the joy and challenge of sailing itself.