Settings / Anchor Monitor
The Anchor Monitor can (1) estimate the location of the anchor on the seabed and (2) trigger an alarm when your it detects Own-Ship moving too far from that location.
This monitor improves on the simple alarm most mariners are accustomed to. When setting your anchor, the monitor estimates the actual location on the seabed where the anchor is set. It does this by accounting for the GPS antenna's position relative to the bow, your current heading, and the scope you have let out. After the anchor is set, the estimated location is displayed, along with the predicted circle of swing and alarm circle.
The extra accuracy can be helpful when you wake up in the middle of the night and check if your anchor is dragging. You can clearly see the location and orientation of your vessel relative to the original estimated anchor location and swing circle. Improved accuracy may allow use of a smaller alarm radius, so you can get an earlier warning when dragging begins. It can also be helpful when maneuvering your vessel's bow above the anchor to raise it.
All calculations are based on the location where the anchor (normally) attaches to your vessel, the bow. The Monitor uses your vessel's size, position of GPS antenna, (as specified in the Vessel True-scale settings) and heading to identify the location of your bow. It works best when using your vessel's GPS with a fixed antenna location on your vessel. When using an mobile device, of course results will vary as the device is moved within the vessel.
If all this sounds complicated, that is fine. You can use a value of zero for Scope. Then the Alarm Distance can be used in similar manner to a basic anchor alarm. You may also want to use a zero scope in more complex anchoring situations, such as when using multiple anchors.
- The advanced features for tracking the location of your anchor are intended to improve the accuracy of information available to help determine if the anchor is dragging. There are still many sources of error.
- This feature is primarily focused on helping to determine if the anchor is dragging. As such, it attempts to track movement of the vessel's bow, where the anchor is normally attached. In particular, the dashed alarm circle on the Navigate display indicate the maximum swing of the bow.
- The term scope is used here somewhat loosely. Here it is intended to correspond to the horizontal distance of the bow from the anchor. This does not account for the vertical drop to the seabed and so may be somewhat less than the actual amount of chain/rode that has been let out.
- If you are using an iPad, you may also want to enable the Use Internal Heading Sensor. Keep the iPad facing forward toward the bow.
- For best results, we recommend setting the size of Own-Ship vessel and location of your cockpit (if you are using your iPad's GPS) or the location of your GPS antenna (if you are using an NMEA feed from your vessel). This allows SEAiq to infer the position of the bow and factor that into initial anchor placement and alarms.
We recommend setting the anchor position during the normal process of anchoring, at the point when the anchor has been physically set into the seabed and Own-Ship is backed up on it. The rode should be somewhat straight and the bow pointed toward where the anchor is set. At that point you should also know approximately how much scope is out.
- Set the Scope Radius according to how much chain/rode you have let out. As discussed above, this value is really the horizontal swing radius and should be somewhat lower to account for the depth in which you are anchoring.
- Switch the Set Anchor switch to ON. SEAiq will then estimate the location of the anchor in the seabed based on Own-Ship's current position, shape, heading, and the scope you specified above. This also enables the alarm below.
- If necessary, adjust the Alarm Length to indicate the additional distance beyond the Scope Radius should cause an alarm to be triggered. This amount can be used to account for inaccuracies in the Scope Radius, GPS, and other factors. If you find you are getting "false alarms," you may want to increase the Alarm Length. In many cases, this value can remain the same between uses.
- Set the Enable Alarm switch to ON. An alarm will now be generated when the bow moves more than the sum of the Scope Radius and Alarm Length from the estimated location of the anchor.
- If using the alarm, check that the Keep Awake setting is ON.
The Navigate tab will show:
You need to be at most 1:20,000 scale for circles to be shown.
- A black anchor symbol at the estimated location of the anchor.
- Two black dotted circles. The inner circle indicates the scope radius which is the expected range of the bow. The outer circle shows the swinging radius of the stern.
- A dashed circle for the alarm radius. An alarm will fire if the bow moves beyond this circle.
After setting the anchor, you can change it by single-tapping on the anchor to select it and dragging it to a new location. You can also graphically adjust the scope and alarm distance by tapping them and dragging them to a new distance.
If an alarm is generated, you will first want to ensure the vessel is safe. If you feel the alarm was generated unnecessarily, you may want to increase the Alarm Length.
- Set Anchor: Enabling this sets an anchor monitor.
- Enable Alarm: Enable this if you want an alarm generated if Own-Ship moves too far from where the anchor was set.
- Scope: This is the estimated scope or swing radius currently used for your anchor.
- Alarm Length: This is the extra distance the bow can move beyond the scope, beyond which an alarm is generated.
- Use Shackles: (Only SEAiq Pilot) Enable this if you want to use Shackles for anchor-related units.