Page 238 - Vetus

HOW TO DETERMINE THE CORRECT VETUS STEERING

Various combinations of boat speed, rudder blade surface area and balance sections apply a variety of forces on steering systems.
Furthermore the dynamic influences of wind and currents cause steering systems to be continuously used under sometimes harsh
conditions.
A skipper is dependent on the steering system and therefore it must be reliable under all circumstances. The design of the steering
system determines how rapidly the vessel responds to helm movements. Fast light vessels react quickly to small rudder movements,
while a slow, heavy displacement vessel will usually be set up to require more wheel movement for a given change of course.
A thoughtful calculation of a steering system is therefore essential.
This chapter explains how the appropriate steering system can
be determined for any boat.
Make your choice from a wide range of steering wheels and
steering systems in this chapter.

RUDDER TORQUE
The choice of the correct cylinder is determined by the rudder torque in Nm (or kg). The rudder torque is the determining factor
(Torque = force x lever). To ascertain the correct rudder torque, only the maximum speed of the vessel, the surface area of the
rudder blade and the maximum rudder angle (in degrees) are of importance. Information such as length of boat and engine power
are irrelevant. With a few exceptions, the rudder performs best with a maximum rudder angle of 35º to either side. Contrary
to what is sometimes claimed for rudders with normal dimensions, a larger rudder angle does not enhance the manoeuvring
capabilities of a vessel.

Your VETUS customer support representative will be pleased to provide you with recommendations for all steering system
components, based on the maximum speed of the vessel and a dimensioned sketch or the rudder (provided by you).
The formula to determine the rudder torque:
M (torque) = F x b (per rudder)
In other words: the force F, which is applied to the rudder (given in Newton = N), is being
multiplied by the lever ”b”, being the distance between the centerline of the rudder stock and
the centre of pressure which lies on the line X-Y.
F (the force applied to the central line XY) – taking into consideration a maximum rudder angle of
2 x 35º – is constituted in the following manner:
F = 23.3 x A x v in Newton (N), or: F = 2.33 x A x v in kgf.
2
2
A = total surface area of rudder blade in m .
2
v = speed in km/hour.
A rudder without balance section requires the formula:
b = 0.37 (1.21 ft.) x c (in metres);
Rudder without A rudder with balance section calls for the formula:
balance section b = (0.37 (1.21 ft.) x c) - e (in metres).
Calculation example of one rudder with balance section
The maximum speed of the boat is 16 km/hour (v); the total width of the rudder blade is 57 cm (c);
the width of the balance section is 9 cm (e); the height of the rudder blade is
100 cm (h).
2
F = 23.3 (1.67 lbf) x 0.57 (1.9 lbf) x 1.00 (3.3 lbf) x 16 (92 lbf) = 3400 N (340 kgf)
b = (0.37 (1.21 ft.) x 0.57 (1.87 ft.)) - 0.09 (0.3 ft.) = 0.12 (0.39 ft.) m.
Therefore, the rudder torque amounts to 3400 x 0.12 = 408 Nm (41 kgm) (848 x 0.403 = 342 ft.lbs.).
So, the VETUS hydraulic steering to be selected in this case is model MTC52. With a twin rudder
installation, the required torque is 2 x 408 Nm = 816 Nm, which makes model MTC125 the one to
choose. We recommend that you consult VETUS for an accurate calculation. We also calculate the
effects of the propeller wash, as well as the torque when going astern. Because smaller vessels tend
to respond quite sharply to the rudder commands, the maximum rudder torque is not used and a
Rudder with balance
section reduction of 10 to 20% off the calculated maximum torque is quite acceptable most of the time.
Be careful: some manufacturers of hydraulic steerings have already taken such reduction into
account when stating their capacity (torque). We, at VETUS, are of the opinion however, that the
choice of whether or not such reduction should be applied, is exclusively the option of the naval
architect.
All VETUS steering systems meet the CE ISO 8848 standard
238 Steering systems - Want to know more? Visit www.vetus.com