Digital Readout Windlass

The purpose of the project is to be able to display the amount of chain rode that has been released when anchoring. When anchoring a boat, an anchor does not hold a boat by the weight of the anchor, rather by the flukes of an anchor digging into the bottom. If the rode (the material connecting the anchor to the boat) is not long enough then motion of the boat may pull the anchor out of the bottom. For typical conditions a chain rode should be 3-4 times the depth of the water. Markers on a chain rode are hard to see, and if using an electrical windlass to lower the anchor, the operator may be too far from the chain to see the markers. Having this digital readout is also advantageous when raising the anchor and knowing when to expect it to cross the final bow roller.

This project consists of three parts:

A modified Simpson-Lawrence 900GD Windlass.
An interface unit that monitors the windlass electronics.
The display unit.

Windlass Modifications
Pictures of the below description can be found at the link to the left. A standard Simpson-Lawrence 900GD electric windlass was purchased, totally disassembled and degreased. The final drive had two holes bored in it (3/4" as I remember). Into these holes a Plexiglas plug was press fit. In the center of the plug is a magnet. In a cavity under the final drive gear another piece of Plexiglas was machined to fit. In this Plexiglas a Hall Effect sensor was mounted so the magnets in the final gear would pass over it. Embedded in a depression in the Plexiglas was a 74HC14 hex inverter to provide additional drive power for the output of the Hall Effect sensor. Three wires, in a single cable, provide power (+5 volts DC), ground, and signal output. This cable exits through the bottom of the windlass near the power wires for the windlass motor. The windlass was reassembled, refilling with grease. The hole with the new wire was filled with 3M 101 sealant. The windlass was mounted as normal to the bow of my sailboat.

Interface Unit
The interface electronics are mounted in a box high in the anchor well where it should not get wet under normal circumstances. It contains a 5v switching regulator, a PIC16F873 and a MAX232A. The microprocessor (uP) monitors the pulse output from the windlass, the up and down signals to the windlass power relay, and RS-232 signals from the display unit.

The interface program uses the additional hardware placed inside the 900GD windlass. This consists of a hall effect sensor and two magnets on the final drive gear, placed 180 degrees apart. With 5/16 inch chain this neatly works out to 12 inches per revolution of the final gear, or a resolution of 6 inches per magnetic pulse sensed by the hall effect sensor. At no load, the output from the windlass is a 20 ms pulse (positive) every 200 ms. The initial design called for two hall effect sensors placed in the windlass with a small angular separation. Determining which pulsed first would determine the rotational direction of the windlass. This idea was superceded by a simpler concept of monitoring the Up & Down lines to the windlass relay for direction, and monitoring one of the hall effect sensors for rotation. The rode length is saved in uP EEROM; for non-volatile storage. The EEROM is updated after a change in rode length, and the length has not changed in the last 5 seconds The program incorporates a de-bounce routine on the hall effect sensor which may not be needed as the sensor includes a built in hystersis.

When on, the windlass interface outputs the current rode length at one-second intervals. The output is RS-232 (19.2k-8-n-1). The interface also receives three commands via RS-232, HOLD, CONTINUE and ZERO from the display unit. When the HOLD command is received, the interface will ignore the Up & Down lines and any changes to the hall effect sensor. This is so the windlass capstan can be used without affecting the length of the chain rode. A CONTINUE command has the interface resume normal operation of monitoring the windlass. A ZERO command will set the rode length to 000.0 feet.

Display Unit
The display unit electronics consists of a 5v switching regulator, a PIC16F783, a MAX232A, 4 LEDs with hexfet drivers, and four push button switches. This unit is mounted in the cockpit.

The display unit communicates with the interface unit which monitors extra circuitry placed in a 900GD windlass, plus the up-down switches to the windlass relay. The windlass interface and the windlass display communicate via rs-232. The windlass interface typically outputs the current rode once per second. The windlass display show the current rode on a 4 digit LED display. The windlass display also has a row of 4 spst momentary contact switches. These are used to send commands to the windlass interface. The output to the interface is RS-232 (19.2k-8-n-1). Button one is the HOLD/CONTINUE control that will tell the windlass interface to ignore any motion of the windlass (for such as using the capstan rather than the gypsy). One push of the button issues a hold command - the LED display shows "hold". Pushing the button again issues a CONTINUE command to the windlass interface, and the display once again shows the current rode. The second button issues a RESET command to the windlass interface, setting the rode to 000.0. The third button increases the brightness of the LED's using the on board PWM. The fourth button decreases the LED brightness. The LEDs are updated every 5 mS, for a multiplexing rate of 20 mS. The TMR0 timer is used in internal interrupt mode to generate the 5 mS.

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