We are in the BVI now and I have used iSailor on my phone the whole time without a need for the onboard chart plotter. The app is intuitive and accurate. I have also used an app called Anchor that is also intuitive and accurate. However, make sure you set the anchor point correctly and the range so you don't get inadvertent alarms. Our alarm went off at 1:30 am in White Bay (Jost) the other night. Let's just say that I amost needed to change shorts after being awakened by the alarm on this app! Not a good feeling for a first time bare boater on anchor the 2nd night of the trip.