User's Manual

FutureWaves™ User Manual
FutureWaves™ v1.7.0.1
43
Why does the Ship Motion Sensor pitch/roll differ from my Clinometer?
Short answer:
A Clinometer is not a roll sensor.
Long answer:
A clinometer measures the direction of maximum acceleration.

For static systems, this aligns with the gravity vector and gives you the incline.

For dynamic (accelerating systems) a clinometer does not measure the pose of the body
— only the angle of the acceleration vector
When measuring at a height above the center of rotation, you get acceleration due to the
rotational motion and the moment arm. Given a system with a center of gravity at position A,
the sway acceleration at B for a roll angle (θ) around the point A at a frequency (ω) is:
𝑦󰇘 = 𝜔

𝜃𝐻
Measured direction φ of maximum acceleration for inclinometer at A:
𝜑

= 𝜃
Measured direction φ of maximum acceleration for inclinometer at B:
𝜑

= 𝜃 + atan󰇡
𝑦󰇘
𝑔

󰇢 ≅ 𝜃 +
𝑦󰇘
𝑔

≅ 𝜃1 + 𝜔

𝐻
𝑔


For a 8s period with H=20m we get:
𝜑

= 2.26𝜃
The FutureWaves™ ship motion sensor readings are corrected to motion at the ship Center of
Gravity.