Dear Boat Doctor,

I am installing an autopilot in my Compass 47, and I have some questions. I chose the Raymarine Evolution autopilot, specifically the EV400 core pack with a Type 2 Long drive unit.

My first question is about the drive unit. I chose the Type 2 Long drive based on the displacement of my boat. The design displacement of the Compass 47 is 29,500 pounds, but with all our cruising gear, we are at roughly 33,000 pounds. My displacement is right at the 33,000 pounds specification for the Type 2 Short drive, so I chose the Type 2 Long drive with its capacity of 44,000 pounds.

The Type 2 Long specifies a tiller arm radius of 14 inches. The Type 2 Short specifies a 10-inch tiller arm radius, which is the length of my tiller arm. Did I choose the wrong drive?

My second question is about the power cable requirements. My calculations show I need a six-gauge cable, but this seems huge. Is that correct?

Nathan Zahrt

S/V Ultima

Dear Nathan,

I think the Raymarine autopilot is a great choice. The system works well, and if you need help, you can find it in much of 

the world.

The Type 2 drives are confusing. The motor portion of the drive units are identical, but the Long model has a longer stroke (12 inches on the Short model and 16 inches on the Long model). The point of the longer stroke is to allow for a longer tiller arm radius. A longer tiller arm gives greater leverage to drive the rudder. The longer tiller arm allows the Long model to drive a higher displacement boat.

With your 10-inch tiller arm, you could have used the Long or Short model for your application. Your application is at the upper end of the drive’s capacity, your short tiller arm limits you to 33,000 displacement. If your boat were any larger, you’d need a longer tiller arm or a hydraulic drive.

I think the Long model was the right choice. As long as the drive physically fits the space, the extra stroke length won’t hurt you. From my experience, the long drive is more popular, so replacement parts are typically more available.

Your electrical question is one of length and current draw. The length of the cable run, and the device’s maximum current draw dictate the conductor’s size. Referencing the installation manual for your autopilot, Raymarine specifies 10-gauge wire up to 16 feet, eight-gauge wire to 23 feet, and six-gauge wire up to 52 feet.