Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

opposite direction, then the rudders are

installed inboard of the shaft.

As with spade rudders, the leading edge

of the rudder blade should be angled slightly

aft to shed debris.

Boat Behavior in Turns

At the beginning, we described rudders as

acting like simple boards angled to the water

flow. We then went on to explain that rudders

are hydrofoils that generate hydrodynamic

lift to create side force. Some references wax

eloquent on determining the coefficient of lift

(CL) for a particular rudder-foil section shape

and then calculating the resultant normal

force(the force at right angles to the rudder

blade) at the predicted angle of attack based

on the CL. Though these calculations give

some engineers a feeling of comfort, this

method isn’t any more accurate than the

method that follows. This is because there

are so many unknowns:

• The water speed the rudder “sees” is
  difficult to calculate accurately as the
  rudder is partially in the wake of the
  hull, which reduces the apparent speed
  at the rudder. At the same time, the
  rudder is also in the slipstream of the
  propeller, which increases the apparent
  water speed at the rudder. Exactly how
  this balances out is impractical to
  calculate for most applications.


• The angle of attack that the water actu-
  ally makes with the rudder is almost
  always less than the rudder angle. This
  is because the boat skids sideways as it
  turns, and the entire hull turns as the
  rudder turns. The magnitude of these
  effects will differ between boats, at dif-
  ferent speeds on the same boat, with
  different loading and trim conditions,
  and in different sea conditions.


• Boat speed decreases during a turn.

Figure 10-14 shows the skidding effect

and how it differs between different boats

PART FOUR: RUDDERS AND STEERING SYSTEMS

Figure 10-14.
Boats “skid”
through turns