Peter....... to your comment.....
"I figured we are looking at a low speed tug here and not a high performance race boat"The following is rather simplistic
............but clearly confirms an understanding of actual comparative rudder loadings between two differing vessels you mention
NB..... I have used imperial dimensions in the description .....them metricated by approximation
Example 1 4 inch square tug boat rudder = 100 x 100 mm, speed 5 km, mass 30 kg
100x100x5 30 = 1666 units of load on the rudder surface
Example 2
1 inch square speed boat rudder = 25mm x 25mm, speed 20 km, mass 7 kg
25x25x20 7 = 1785 units of load on the rudder surface
As you can see...... first thoughts can sometimes be a little
.......Derek