Page 25 - 07

INTERCEPTOR DESIGN AND CONTROL FOR THE HIGH SPEED CRAFT 23

4. Mechanical Design of an Interceptor

Mechanical design of an interceptor starts with designating the main components and breaking
down these components to parts.

Main components are basically as follows (but may vary according to the design);

 Actuator (with linear or rotary actuating and could be pneumatic, hydraulic or
electric driven)

 Interceptor blade (best preferred to be light weight, most likely aluminum or a stiff
composite)

 Case or something similar to mount the interceptor to the hull and to embed the
interceptor blade

4.1 Actuator

All displacement of the interceptor blade is provided by the actuator. Therefore actuator
selection is important. Required criteria should be achieved with this selection, such as required
force, stroke length and speed.

Minimum actuating force should be calculated.

= 1 × × ν2 × × (4.1)
2

Lift force can be calculated according to the above formula (4.1).

= × (4.2)

Friction force between the interceptor blade and the bearing can be calculated with the above
formula (4.2). Minimum actuating force is equal to the calculated friction force.

Actuator should be selected according to designated force, stroke length that provides enough
area for required lift force and desired stroke speed. Also either the actuator should meet IP67
criteria or there should be a watertight casing covering the actuator.

4.2 Interceptor Blade

Interceptors (or also trim tabs) are sized according to the lift required for the vessel. The
required lift force has a direct effect on interceptor blade size and thereby the actuating force (as
aforementioned). By the Formula 4.1 required area for the interceptor blade can be determined.
Chord and span then will be designated according to limitations.

After designating the dimensions of the interceptor blade, the material and the thickness of the
blade should be designated. Material selection will affect the weight, thickness and the friction
against the bearings. Thickness can be calculated according to the below formula; (v = shear
force)

= 1.5 (4.3)

Sayı 7, 2016 GiDB|DERGi