Isla San Francisco in the Sea of Cortez is a lovely spot to hang out when the weather is settled. The only problem is that the charts leave something to be desired in the way of detail (as is the case with much of this area). Even the Google image (above) does not reveal much.

This is the "vector" chart detail of the approach (the US DMA charts are no better). It is situations like this which prompted us to install a Furuno CH 270 SONAR on Wind Horse.

Here we are closing with the anchorage from the south. The SONAR (above) is showing a clear path ahead, but there is something near the surface just a quarter of a mile off our starboard bow (dense red image). The radar screen on the right will put this into perspective. If you refer back to the Google image at the top, you can see the hooked reef which is attached to the southern end of the island. That is giving us the SONAR return. Notice that this reef does not show up on radar.

Now we have worked our way into the anchorage. The boat is facing north and you can see the reef to port and the beach behind us in both SONAR and RADAR (including the reef which shows up better from this side). The SONAR is being operated in horizontal mode in this and the previous images.

With the hook down we want to take a good look around us for bottom obstructions, so we switch to the vertical SONAR image. This vertical slice through the water is looking about 30 degrees to port, towards open water. The boat is represented by the inverted triangle to the left. If you look closely you will see that the bottom is sloping away from us, getting deeper as it goes.

Here we are looking off the aft port quarter. About half way across from the left side of the image you see the bottom come up in a solid mass. That is the reef.

Same image as above, but this time with the radar to orient you to the scene.

We’re getting more comfortable with the SONAR and now feel that if we had to, we could navigate blind into a situation like this, using just the SONAR – going very slowly.

And the official chart of Isla San Francisco? Not a lot of detail, as previously mentioned – and the island is somewhat off its charted position!