Think rudders are simple? Nope—it takes complex geometry to produce maximum running efficiency.

Rudder Mysteries

If you don’t want your boat to take you somewhere you don’t want to go, you should know a thing or two about rudders.

Rule of thumb: incorporate a little bit of toe-in on the rudders based on outboard-turning props.

Rule of thumb: incorporate a little bit of toe-in on the rudders based on outboard-turning props.

A while back, Capt. Bill Pike and I were discussing our nautical misadventures, trying to one-up each other with unusual, often unexplainable, occurrences we’d managed to survive: Racing sloops on the rocks, dockings so disastrous they were covered by The New York Times, rescues by the U.S. Navy shore patrol—the usual stuff. Then Bill recalled testing a new boat a couple of years ago, when the boat tried to make its escape. Seems that when Bill cranked on the throttles, as he is wont to do, the boat decided to stop answering the rudders and took off port and starboard, willy-nilly, heedless of the wheel. Throttled back, she regained her composure. What the heck?

Back at the dock, with the boat in the slings, the culprit was quickly identified: the rudders were toed-out about half an inch; the designer had spec’d that they be toed-in by the same amount. A quick adjustment of the tie bar put things right, and the boat subsequently behaved just as you’d expect at all speeds.

What’s toe-in and toe-out? Rather than the rudders running straight fore and aft, parallel to the boat’s centerline and to each other, they are angled in or sometimes out just a bit. Most designers consider toe-in to be when the leading edges of the rudders are closer together than the trailing edges. But sometimes there’s a problem of definition: One man’s toe-in is another’s toe-out.

Why Aren’t Rudders Straight?

You’d think if you wanted a boat to go straight, the rudders ought to be straight, too. Not so, said Chris Critchett, a naval architect with Michael Peters Yacht Design: The rule of thumb is to incorporate a little bit of toe-in on the rudders, based on outboard-turning propellers on a V hull. It’s a result of how the water flow coming off the propeller hits the rudder: It makes most rudders want to aim inboard a little bit. But today many propellers are in tunnels, and that can change the rules.

Adjusting rudders often means following a sea trial with mathematics.

Adjusting rudders often means following a sea trial with mathematics.

Powerboat rudders on planing boats are relatively short, explained naval architect Dave Gerr, and operate in the top half or two-thirds of the slipstream from the propeller. The rudder’s natural tendency is to follow the water flow; with outboard-turning props, the pressure tends to rotate the rudders outboard, i.e., toe them in. “Mike Peters is one of the few guys who specify rudder toe,” said Gerr. “That’s one reason his boats are so good.” According to Gerr, 90 percent of conventional powerboats have rudders aligned with each other and with the centerline. “It’s cheaper, quicker and easier, and works for most people,” he said. But properly toed-in rudders reduce drag, give crisper steering and more speed. Only thing is, he added, “there’s lots of stuff going on under the boat, and nobody totally understands it.”

How much toe is enough? Both Critchett and Gerr agree that the best way to set the rudders is to run the boat in a straight line, at moderate speed, and disconnect one rudder. Let it free-trail and mark the angle it takes. Repeat with the other rudder and set the tie bar to lock in those angles. “It’s a game of just mess with it and see what works best,” said Critchett. “Do your sea trials to set up the steering. Once it’s dialed in, it doesn’t have to change.”

Ackermann’s Angle

That’s not all, if you want your rudders to be tuned even better. Think about your car for a minute: When you go around a curve, the inner front wheel traces a circle of less radius than the outer wheel, the difference being the distance between the wheels. The correct steering angle is therefore different for each wheel, the inner one needing more angle to follow the tighter radius. Same with a twin-screw boat—in turns, the inboard rudder should be cranked over just a little bit more than the outer rudder. But the wheels on a car, and the rudders on most multi-engine boats, are connected with a tie bar, so how can the steering angles be different for each wheel/rudder?

Way back in 1817, Rudolph Ackermann, an English agent for German carriage-maker Georg Lankensperger, came up with a simple arrangement of steering arms and tie bars that solved the geometry problem. Both Lankensperger and Ackermann patented the setup, in Germany and England respectively. According to the Royal Society of London however, the mechanism was actually invented by Erasmus Darwin, grandfather of Charles, in 1759.

Determining rudders’ optimal toe-in geometry equals improved performance.

Determining rudders’ optimal toe-in geometry equals improved performance.

Whatever. The basic principle of Ackermann steering is to toe-in the steering arms on a carriage or a car, or the tillers on a boat (independent of toe-in or toe-out), a certain number of degrees, depending on the vehicle. Thanks to geometry, when the steering is put hard over, the inside wheel, or rudder, is turned more than the outside one, by half as many degrees as the toe-in of the tillers. Dave Gerr recommends a toe-in angle of 10 degrees for boats that have a beam of 10 feet or more, and 7 degrees for narrower craft. Catamaran sailors, who want minimum rudder drag in tight maneuvers, sometimes toe-in their tillers as much as 20 degrees.

Toe-in geometry is more complex with cars than with boats, of course, since the rear wheels come into play. With boats, there are only the rudders to worry about, and if things aren’t adjusted precisely, the only effect is turbulence around the rudders and a little extra drag—no big deal. But using the Ackermann principle to fine-tune steering can add that little bit extra to the boat’s handling. It’s easy to do when setting up the steering: It only requires cutting the keyways on the rudder posts 10 degrees, give or take, inboard from the centerline. Once installed, the rudders behave in much the same way as any other conventional, tie-bar steering system.

The End of the Tie Bar?

You probably won’t find many powerboats that make use of the Ackermann principle these days, and most of the preceding likely will be moot in a few years, as boatbuilders turn more to electronic steering with individual rudder control. Eventually, the tie bar will be as outdated as the rumble seat. If each rudder can turn independently, it can assume the most efficient angle whether running straight or cranked over in a hard turn. All that’s needed is intelligent control, which comes from a computer that crunches data input from various sources and determines how much to deflect each rudder. Toe-in or toe-out is adjusted from the helm for maximum straight-line speed; the computer handles the rest.

Viking Yachts was ahead of the curve with this, developing their VIPER steering in 2007. VIPER stands for Viking Independent Programmable Electro-hydraulic Rudder, and how it works is all in the name. Electric impulses from the wheel are sent to controllers, which activate hydraulic steering cylinders to turn the rudders. In the latest VIPER incarnation, the controllers are integral with the cylinders. There’s no tie bar. Viking senior mechanical engineer Eric Metz said that some skippers add toe-in to optimize steering, but Viking’s practice is to set toe for straight-ahead speed, to get the best performance. Users can easily set toe from the display at the helm, and it can be changed underway. The helm display shows effective rudder angle on an upper screen, but dig deeper and you’ll find the angle of each rudder too.

Since VIPER, similar steering systems have been introduced, including SeaStar’s Optimus, with similar capabilities. But Metz said the main difference with VIPER is its power source: While other systems rely on standalone 12-volt or 24-volt hydraulic pumps, Viking installs a hydraulic pump on each engine in a twin-engine powerplant. On some boats, the pumps also power thrusters, windlasses and other gear, or they can be dedicated to VIPER. Either pump can operate the system, so there’s redundancy if one dies; if both pumps go, and there’s still an engine running to take you somewhere, the VIPER helm can act as conventional hydraulic steering and activate the rudders directly.

But, added Metz, modern steering still relies on the basic principles. And he recommended Dave Gerr’s articles and books as excellent sources of information for those who want to understand how things work under the water. “You still need to get the rudder size right, and put it in the right place,” he said.

Maybe that includes adding a little toe-in, too, unless you want your boat to take you somewhere you don’t want to go. Next time you see him, ask Capt. Pike what that feels like.

2018 Boatyard rule

Steering Without a Wheel

Most high-tech, “smart” steering systems rely on hydraulics, but Volvo Penta has come up with something different: Fully electric steering, part of the company’s inboard joystick system. It arises from the same technology that animates IPS drives. According to a Volvo Penta engineer, the system uses electric-motor-driven actuators to turn the rudders, rather than hydraulic cylinders. Electric steering lets the user adjust both steering-wheel friction and the number of turns lock-to-lock. Or the wheel can be dispensed with completely, and all steering done by joystick. In docking mode, the system controls the rudders and the bow thruster simultaneously to provide precise helm response.

In some installations, depending on vessel size and rudder loads, one steering actuator can be linked to two rudders via a conventional tie bar arrangement. Larger vessels with higher steering loads use one actuator per rudder; there’s no tie bar, but the actuators are synced electronically to ensure the rudder turn rate is consistent from actuator to actuator. The twin actuators turn at the same rate and at the same angle as if a mechanical tie bar linked them together. They do not replicate an “Ackermann” steering system and cannot be toed-in or adjusted on the fly.

All-electric systems have advantages: There’s no hydraulic oil to top-up, and no high-pressure oil lines to leak. And what’s cooler than to sit at the helm with one hand on the joystick, the other on the engine controls, and drive the boat with minimal movements of each paw? It’s totally 21st century and is perhaps the direction steering will be moving in the future.

This article originally appeared in the September 2019 issue of Power & Motoryacht magazine.