Features February 1, 1999 Issue

Offshore Log:
Robertson AP 300CS on Trial. ‘Bob’ Earns His Keep

After sailing from the US to Venezuela with a Monitor windvane, Calypso tests her new autopilot. It proves its mettle in 8-foot, following seas.

Our trip from Venezuela to Bonaire was the first real test of “Bob,” our Robertson AP 300CX autopilot. He passed with flying colors.

Although I finally completed installation of the autopilot in Trinidad back in July, our first test of the unit—a 220-mile light-air sail from Trinidad to Puerto La Cruz, Venezuela in late September, 1998—suffered from an installation glitch.

Our Robertson autopilot control unit, computer and rudder feedback mechanism are mated to a Whitlock SN251PSM drive motor, which is spline-coupled directly to one of the bevel boxes in our Whitlock Mamba steering system. This powerful Whitlock motor is normally used on boats up to about 50 feet, and gives a comfortable margin for our 40-footer. The clutch on the 1/4-hp. motor draws about 2.2 amps, which is very close to the overload value of the Robertson’s clutch control circuit. In fact, we kept getting a clutch overload alarm on our trip to Venezuela.

Phil Quartararo of PYI, who supplied our steering system and who has frequently overseen installation of the Whitlock/Robertson autopilot combination, instantly diagnosed via e-mail the source of our problem. He suggested that we use a relay in the clutch control circuit to reduce the current draw at the computer, taking the operating current for the clutch from the main power feed to the autopilot.

Miraculously, the Radio Shack in Puerto La Cruz had just the right relay in stock. We bought two, believing that for about $7, a backup was a good idea. (Now you know why our boat weighs so much; we have backups for virtually everything except the hull.)

To our great relief, this fix was exactly the right call.

Our trip from Venezuela to Bonaire was a 250-mile reach and run, mostly in breezes of 20 to 30 knots, with following seas up to about 8 feet, tough work for an autopilot. Normally, we would use the Monitor windvane in these conditions, but this trip was meant to be a test of the autopilot, and test it we did.

Following seas are a hard test for any steering mechanism—human, mechanical, or electrical—but the Robertson AP 300 and Whitlock drive unit worked perfectly, steering the boat at least as accurately in those conditions as a person could manage. In fact, disengaging the autopilot to see just how tough the steering was showed that the autopilot was better than we were.

I was concerned that the extreme rolling conditions of downwind sailing in heavy air could confound either the Robertson compass or the computer steering circuitry, but this was not the case. When the boat started to roll, the autopilot seemed to apply only enough pressure to hold the boat on course, rather than oversteering the way a human helmsman might.

At this time, we are underutilizing the capabilities of the autopilot. It is not interfaced with any other system component. Eventually, we will interface the autopilot with our Northstar 941XD GPS to permit steering to routes and waypoints. For now, a compass bearing works just fine.

The only downside to this hybrid system is relatively high current draw. Current draw of the Robertson control circuitry is about .75 amp, giving a total constant base draw of about 3 amps with the motor at idle. To this must be added the consumption of the drive motor. While steering, total system draw is anywhere from 4 to 7 amps. Over a 24-hour period in these fairly difficult conditions, the autopilot drew an average of about 4.5 amps or 110 amp-hours per day. It goes to show why many offshore cruisers like to use windvanes for long passages, using the autopilot for motoring and motorsailing, when current draw is a non-issue.

The AP 300CX was one of Practical Sailor’s top ten gear picks for 1997, and our own experience aboard Calypso so far confirms that it is still a great piece of equipment.


Contacts- Robertson AP 300CX autopilot, Simrad, 19210 33rd Ave. W., Lynnwood, WA 98036; 206/778-8821. Whitlock drive motor, PYI, PO Box 536, Edmonds, WA 98020; 206/670-8915.

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