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.
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.
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.
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.
Here are sources and part numbers for the items required for constructing this project:
- Rail Mounts:- Helm brand "Quick Mount Rail Clamps", West Marine Use Model 153262 for 7/8", or 153270 for 1" to match the size of your rail and use the 7/8" size for the outrigger. We used 1" for all and provided a strip of 1/16" neoprene gasket to use as packing on a 7/8" stantion.
- Outrigger Arm:- Taco brand "Bright Anodized Aluminum" tube. West Marine Model 136440 (6 feet but cut a piece about 30") or heavy duty PVC conduit with a wooden dowel inserted for stiffness.
- For the radio link mode you can purchase the radio door bell at your local WalMart, KMart or Radio Shack store. The Radio Shack one is a little more expensive but it has the convenience of terminals for a remote push button so you don't have to open it up and solder on leads.
- You can get a "burglar alarm" type magnetic switch from Radio Shack. You need one that is Normally Open when the magnet is against the switch. Be very careful of the terminology here. In normal electrical circuits the type of switch is normally described in the "unactivated" state so a "normally open" push button would be one that closes when you push it, however in burglar alarm circuits, the normal condition is when the magnet is in fact operating the switch (in this case to hold it open!) so double double check your purchase - the switch must be OPEN when the magnet is beside it.
- A "Gate" type stainless hinge is the best shape with the long arm about 5 to 7" and a hinge pin about 2" long. This may be harder to find. We made our own for the production run (download drawing) but this approach may be too expensive for the hobbyist. We still have a few available while stocks last. (ALL GONE).
- You will need a 1 pound lead fishing sinker, use a teardrop shape to minimize catching on the bottom. Use a suitable fishing line for attachment, keeping in mind that the top end needs to be adjusted easily in length for the water depth.
- The waterproof box to hold the transmitter and switch can be a PVC electrical box with a gasketed cover, available from your local hardware store.
If you don't want to use the radio link, wire 12 volts, through a fuse or circuit breaker, to the switch, then to a beeper, then to 12 volts negative. Again Radio Shack stocks various electronic beepers suitable for use on 12 volts.