Power & Motoryacht – September 2019

Rudder Mysteries

BOATYARD

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

By Mike Smith

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 design-

ers 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.
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
S most people,” he said. But properly toed-in rudders reduce drag, give
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