Anchor Alarm Project


There is a parts list at the end of this article.

This device, intended for cruising boats, gives a less than 20 foot of drag alarm and will increase your peace of mind at anchor, especially at night in those anchorages where you don't trust the bottom. In tight locations, the electronic anchor alarm on GPS or LORAN devices cannot be set close enough to give an early warning without incurring frequent false alarms, especially on wind or tide changes. This device avoids most false alarms caused by tide and wind changes, even with 100 feet of anchor rode out, yet gives an alarm if your anchor drags a few feet. It works best in shallow water up to about 30 feet. It was marketed by West Marine in '93 through '95 however the cruising market did not have enough volume to support a catalog entry.

Caution: This is just a tool and although it is a significant improvement over nothing, it should not substitute for a full time, or periodical watch, in dangerous situations.

Alarm Picture 3KB

It works by dragging a position sensing line, with a 1lb lead sinker attached to its end, over the bottom as your boat swings at anchor. The line is attached to a hinged sensor supported on an outrigger arm on one side of your boat. The stainless steel hinge is arranged so that the moving part can only swing towards the bow. The hinge pin only permits fore and aft motion and motion aft is restricted by a fixed arm.

A small waterproof magnetic switch (burglar alarm type) mounted on the fixed arm, has its contact held open by a magnet mounted on the moving portion of the hinge arm. A 12 volt beeper, wired in series with the switch, can be located near your bunk to wake you. Motion of the boat from side to side or forward has no effect but if the anchor drags, allowing movement aft, the line will pull the hinge arm and magnet away from the switch and close the contact. Usually on a change of tide or wind, the boat rarely ever backs up. Motion is usually sideways as it swings around and the switch is not sensitive to this. Some boats, especially hyperactive race boats will move in all directions and may suffer from false alarms but most heavier cruising boats work fine.

A convenience offered in the commercial model included a radio link between the switch and the alarm for simple installation and stowing. Mount a small waterproof box containing the magnetic switch and a battery operated wireless door bell. Caution: Keep a rubber band around the hinge when not in use because if the magnet does not keep the switch "off" when the device is not in use the battery in the transmitter will be discharged rapidly.

Construction Details

There is a parts list at the end of this article.

Start with a pair of Rail Mounts. They are bolted firmly to each other to make a right-angle mount for the outrigger arm. The outrigger arm is necessary so the sensing line does not get hung up on your hull topsides and to allow free and clear positioning of the hinge assembly. About 12 to 18 inches clearance works OK so add 4" for the clamp area and add the distance from the deck stantion to the edge of the hull. For the outrigger arm, use a piece of aluminum tubing the same diameter as the stantion (so the same size Rail Mount can be used). PVC tubing is not stiff enough however it works OK if you insert a close fitting wooden dowel rod inside and glue on a pair of end caps, or glue the outboard end into an electrical junction box as shown in the photo.

Mount a stainless vertical plate to the outer end using a stainless U bolt and attach the hinge midway up the plate. Mount the magnet on the end of the hinge arm (on the side away from the backing plate) and mount the contact, or a waterproof box containing the door bell transmitter on the other side of the backing plate so the switch is held OPEN by the magnet when it is resting against it. Don't worry if the hinge doesn't open all the way due to screw heads etc.

Operating Instructions

In use, adjust the line attached to the end of the hinge so it is about 150% of the depth of the water. Longer reduces sensitivity, shorter increases it so adjust appropriately depending on the situation and experience. You can adjust the sensitivity of the switch itself by tilting it forwards and backwards so that there is enough weight on the magnet holding it closed to avoid false alarms due to wave motion but don't tip it back so far that the line can't pull it up far enough to trip the switch considering the angle of the line to the bottom. Depending on how your boat "fishtails" on its anchor rode, moving the attachment point forward or aft from midships may reduce false alarms. It will take some experience to know which settings are best for your boat under different conditions of wind, tide and waves.

Parts List

Here are sources and part numbers for the items required for constructing this project: