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Auto power

Jan 1, 2003

Whereas a depth sounder or GPS is just a black box, many boaters have a personal relationship with their autopilots. They give them names like Otto or Hal and treat them as members of the family. I admit that I have a personal regard for my own autopilot. Watching the little fellow perform in a cold rain from the comparative warmth of the companionway, it is difficult not to feel some rapport.

Autopilots do much to relieve the boredom and fatigue of steering and can often accomplish the task better than most helmsmen. They never get tired or inattentive and save fuel and time by steering the best possible course. Without a doubt the most useful piece of electronic equipment on board is a strong, reliable autopilot. Just as assuredly, the most worthless piece of gear on a boat is an autopilot that is not strong or reliable. I have heard numerous stories from owners of both types. The path to the perfect autopilot can have a few twists and pitfalls.

Most autopilot failures can be traced to water, electronic, and stress-related problems. Several years ago a large number of failures were caused by water reaching electronic or electrical parts. This is especially true for the cockpit-mounted devices. Fortunately, most manufacturers have improved their waterproofing techniques, and this type of failure is not as common. The Coast Guard definition of waterproof, according to CFR 45 subpart 110.20, requires that the object will withstand a 65-gallon-per-minute stream of water from a one-inch nozzle for five minutes, regardless of the angle of attack, and not leak. While this is not the same as submergence testing, it is probably sufficient for most autopilot use.

Electronic failure is in many cases closely linked to general quality of the parts and workmanship. For example, a 10-k-ohm resistor can have either a five-watt or 10-watt rating; they will both do the job, but the 10-watt component will last longer. Of course, the 10-watt resistor costs more and so does quality workmanship. Cost is not always a good yardstick, but usually low cost and high quality don't go together.

Failure today is most often caused by overstressing the unit, or just getting too small a pilot for the boat. Saving money by buying an undersized autopilot and rationalizing this purchase by reasoning that it will only be used in good weather is not a good idea. Face the factsit is in bad weather that most people do not wish to steer, and thus the undersized autopilot gets thrust into handling the boat under conditions far beyond its capabilities. Steering a boat in flat calm water is far different from steering the same boat in big seasthus the unit is often subjected to stresses that surpass its design criteria.Sizing an autopilot

Selecting a properly sized autopilot is dependent on the force needed to turn the boat. That force depends directly on the size of the rudder, the speed of the turn, and the speed of the boat moving through the water, which is, in turn, loosely dependent on the weight or displacement of the boat and barely related to boat length at all. However, many manufacturers recommend autopilot size based on boat size or displacement. Partly this is done because it is not a simple thing to size an autopilot properly.To size an autopilot properly it is necessary to know either the maximum turning moment or the maximum steering force and the distinction between these terms. The turning moment is the torque applied to the rudderpost that is needed to turn the boat. It is generally measured in length-force units such as foot-pounds. The steering force is that force needed by the helmsman to steer the boat, this time measured in just units of force, like pounds. The maximum steering force is set within a small limit on well-designed boats to be around 50 lbs. It is related to the turning moment through the mechanical advantage provided by the steering mechanism. Let's take a look at a couple of typical applications of mechanical advantage used for steering boats.

In small sailboats the steering force is applied by a lever called a tiller. You provide the pounds of force to the tiller, which turns the force into torque by providing a distance to the rudderpost. The longer the tiller, the greater the mechanical advantage of the tiller, but the farther the tiller end must be moved to effect a turn. On boats with wheel steering the mechanical advantage is provided by a combination of gears and levers. The force needed to steer gets smaller as the size of the wheel gets larger and the number of rotations required gets greater.

Now we begin to see the true benefit to mechanical advantage: Given enough mechanical advantage we can reduced the force to almost zero. The bad news is that, the more mechanical advantage, the greater the motion of the steering device, and therefore the more slowly the boat responds to the helm.

Mechanical advantage is not the only factor that has negative effect on response time. Many autopilots are incapable of moving the rudder through its full range of motion. For example, most linear units are mounted between 18 and 24 inches from the rudder post and extend between 10 and 18 inches. This combination of dimensions means that the drive units are only capable of turning the rudder 15° to 30°, rather than the normal 40° to 45° each side of center.

The response angle (ø) of the autopilot can be calculated by equation 1 Equation 1/2A is the operation stroke in inches and A is the distance from the rudder post to the attachment point in inches. As you can see, the farther away the drive unit is mounted from the rudder post, the smaller the response angle. The smaller the response angle, the smaller the thrust required by the autopilot, and the less control over the boat's course in extreme conditions. So, in the same way as mechanical advantage, you are again faced with the trade-off between response time and steering force. If your unit has both large mechanical advantage and small response angle, you are compounding your response-time problems. It is not too hard to see that some sort of compromise between speed and strength is needed. Part of that compromise depends on your boat and where your are going to sail it. If you have a boat that has good steering qualitiesi.e., one that takes little helm tending to stay on course and, once off course, gets back quickly with little helm adjustment, then response time is less important. If you are not going to encounter heavy conditions where the dynamics of wind, wave, and boat interaction require the autopilot to respond to changes quickly, it is again not as consequential.

As a general rule, wheel units and linear tiller-mounted units provide low response time, and linear direct-drive units provide the quickest response. Once the decision is made on the amount of response time required for your boat, then all that is left is to determine the required forces.Wheel units

The units that attach directly to the wheel supply between 7.5 and 30 foot-pounds of torque. To select the proper-size wheel autopilot, it is then necessary to assume, estimate, or measure the maximum steering force needed to turn the wheel. You can assume your boat is well designed and use 50 lbs, or you can make a few high-speed turns and estimate a figure. The maximum force would be applied with the rudder hard over while the boat is going top speed. If you feel you must have an exact measurement, the force can be measured by using an appropriately sized fish scale attached to the outer rim of the wheel.

Getting an accurate number is not as easy as it would appear because a rapid, high-speed turn on some boats is dangerous, and a slow turn reduces the speed of the boat because the rudder acts as a brake. Several trials should be made in both directions, and the maximum force measured should be the one used for selecting the unit. Since these units are generally sized by torque, remember to convert the steering force measured above into a torque by multiplying it by the radius of your wheel. Because it is not practical to install a unit that must work at its maximum, it is prudent to oversize the unit somewhat, particularly if the response angle is close to the full rudder angle.Linear-drive units

The thrust on linear-drive units will vary from 80 to more than 1,200 pounds. Where that force is applied determines the amount of torque available to turn the rudder. Most linear units extend between 10 and 18 inches and are mounted between 18 and 24 inches from the rudderpost. This means that when the linear-drive unit is attached directly to the rudderpost it produces from 120 to 2,000 foot-pounds of turning moment. The wide variance in thrust is necessary because some of these units are attached to the tiller and others directly to the rudderpost. As we saw earlier, using the mechanical advantage of the tiller reduces the thrust requirements on a linear unit. Tiller linear drives.

Arriving at the amount of thrust needed by the autopilot to steer a tiller-controlled boat is relatively simple. Remember that the steering force and response angle are less at the forward end of the tiller. As you move aft along the tiller toward the rudder, the steering force increases. The steering force again can be estimated or measured. If you decided to measure the force, be sure the measurement is taken at the point where the unit is to be mounted, not at the end of the tiller. For estimates or measurements taken other than at the mounting point, multiply that force measurement by the distance to the rudder post and divide it by the distance from the rudder post to the attachment point. Direct linear drives.

For those linear units that are to be installed belowdecks on boats controlled by wheels, determining the force required is a little more difficult. It is necessary to go from steering force to torque on the rudder shaft and then back to force on the autopilot. First, count the number of turns (N), and the angle swept (S) by the rudder going from lock to lock. The mechanical advantage (M) of the wheel can be calculated by the following formula: Equation 2R/S, where R is the wheel radius in feet.

The torque on the rudder shaft is then equal to the mechanical advantage (M) multiplied by the maximum wheel steering force (F) estimated or measured above. To calculate the thrust required by the linear unit, it is necessary to know where the unit will be mounted. Remember the strength of these units is measured in pounds of thrust (T) and not torque. To change thrust into torque, a lever arm (A) is required. The lever arm in this case is the distance that the drive unit is mounted away from the center of the rudder post. The thrust needed by the autopilot can be found by using equation 3. Equation 3/A Where A is the lever arm in feet.Hydraulic drives

For those fortunate enough to have hydraulic steering, the sizing of the autopilot is simplified, assuming the architect designed the steering system properly. All that is needed is the displacement of the main steering ram or, on power-assisted systems, the displacement of the servo ram in cubic inches. Given that information the rest of the system is easily selected. Remember the use of a servo has the same effect as mechanical advantage in that it reduces power requirements while slowing response time.Power consumptionThe final consideration in sizing an autopilot is power consumption, especially for sailboats on which the unit may need to run on battery power for a considerable time. Power consumption, or input power, is measured in watts or amperage at a specified voltage and is related to output power by a factor called efficiency and the duty cycle of the unit. The efficiency factor is commonly between 0.5 and 0.8. Because the output power depends on the combination of force (pounds) generated by the unit, where it is applied (feet), and how fast the unit will respond (seconds), it is physically impossible to get a low-amperage unit that provides high-output torque and fast response time. However, all is not lost, because total power consumption also depends on the amount of time the unit is actually operating, or the duty cycle.

Because of this dependence on duty cycle, power consumption, or input power, is commonly reported in two different formats: stand-by power, which is the minimum power draw, or some sort of duty cycle power. The stand-by power is not as important as the duty power. When comparing units, be sure that the duty cycle is similar and voltage is similar. If the voltage is different, remember that power is equal to voltage multiplied by amperage (P=IR). If the duty cycles are different they can both be converted to 100% duty for comparison by dividing the duty power (or amperage) by the duty cycle (percentage).

When the drive unit is operating, the current draw depends on the force and speed needed to turn the rudder and varies between 0.5 and 12.5 amps. That is to say, within reason, in any given condition a small unit will require as much power as a large unit to turn the same boat. The total input power depends more on how often the drive unit operates rather than the size of the unit.

The actual duty cycle on any given boat will vary, and it depends on the manufacturer, nature, and the boater. The manufacturer can reduce the operation time by making the unit smarter, nature by producing mild weather, and the boater by operating efficiently. Not much can be done about the weather, but the boater can do several things to reduce the duty cycle. A sailboat that is difficult to steer is one that is poorly designed, poorly tuned, or poorly sailed. In all these cases the autopilot will be required to operate continually to adjust the course. Provided the boat is well designed, the duty cycle can be reduced by getting a smarter unit, tuning the rig properly, and setting the sails so that the boat almost steers itself. Most good books on sailing discuss sail balance to get maximum performance or speed from the boat. Adjusting the sails to steer the boat uses the same principles but with a different result.

Thus we come to the end of the discussion that I hope will allow you to develop a strong and lasting relationship with your autopilot.


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