About the otter usv

[VastSeaL]

Dear Professor Fossen, I want to use the otter USV as a object of study, can i know the max current speed for sailing.

[cybergalactic]

You can find the information in the 'otter.m' file.

% Main data g = 9.81; % acceleration of gravity (m/s^2) rho = 1025; % density of water L = 2.0; % length (m) B = 1.08; % beam (m) m = 55.0; % mass (kg) rg = [0.2 0 -0.2]'; % CG for hull only (m) R44 = 0.4 B; % radii of gyrations (m) R55 = 0.25 L; R66 = 0.25 L; T_sway = 1; % time constant in sway (s) T_yaw = 1; % time constant in yaw (s) Umax = 6 0.5144; % max forward speed (m/s)

Here, Umax is the maximum speed when there are no currents. If you have currents close to 6 knots (approx. 3 m/s), the Otter USV will drift away. In practice, controlling the vehicle will be very hard if the currents are stronger than 1 m/s since you need stability margins. You can test this yourself by running the script 'SIMotter.m' and changing the current speed and direction in the script. The default values are:

V_c = 0.3; % Ocean current speed (m/s) beta_c = deg2rad(30); % Ocean current direction (rad)

[VastSeaL]

How to calculate the stability margins that mentioned in your answers? Thanks!

[cybergalactic]

The currents are stochastic, so performing a stability analysis of the vehicle dynamics is not straightforward. In the deterministic case (constant currents), generating a DP capability plot is the standard practice. This plot is obtained by setting the state derivative of the dynamic equation to zero and solving for the steady-state solution when you apply a constant current to the vehicle and, at the same time, apply the maximum thrust.

https://www.offshoreengineering.com/dp-dynamic-positioning/dp-capability-plot-example/

https://www.dnv.com/maritime/services/dp-capability-85124/

In practice, the vehicle must move faster than the ocean currents. You can simulate the vehicle and change the current speed and direction in the m-file. From this, you learn what kind of currents the vehicle can handle.