The squat and lift of a vessel changes as the speed changes. For some vessels, if the squat/lift is not accounted for, significant errors are introduced into the soundings. In this section, you can specify up to 10 speed-draft value pairs. During merge, the difference between the instantaneous draft and the static draft is computed and the final depth compensated.
Draft is measured in metres/feet, and speed is measured in knots. All draft values should be relative to the same reference, but the actual reference is not important. The first Speed-Draft pair must correspond to a state where no draft correction is necessary. The delta draft values, relative to the initial draft, are computed and used to correct observed soundings.
This formula is used to correct soundings for dynamic draft:
Depth = observed depth – waterline + delta draft.
Delta draft is computed from the present vessel speed, as derived from the navigation system.
You can also create your own dynamic draft model and load Delta Draft information directly which will override the Dynamic Draft table in the HVF.
1. Click Dynamic Draft in the sensors list box to highlight the selection and display the dynamic draft data fields.
2. Type data as needed in the following fields:
• Date: The year and Julian day of the current time stamp for the draft table.
• Time: The hour and minute of the current time stamp for the draft table.
• Apply: Select Yes to apply the table during merge (or No to not apply the table).
3. Click Edit in the Error field.
A dialog box containing a table with up to 10 speed-draft pairs is displayed.
4. Click inside the Speed cell and type a speed value.
5. Click inside the Draft cell and type a corresponding draft value.
6. Continue entering speed /draft pairs as needed.
7. Click OK.
The speed-draft table is saved in the HVF.