Hi Toesupwa,
The pivot point must be counted as the last point the the tow rope is in contact with the tugs structure and as you correctly point our your towing point (Staple) is aft of the units. If you look at the configuration of any single, twin or ASD propelled tug you will find that the towing point is just aft of midships, mush further forward that on your vessel.
The easiest way to explain i feel is in terms of vessel rudders. These have developed over the years and there are three basic types which have been used over the past century or so;
Fully Balanced. This is when the rudder stock is almost in the centre of the rudder blade allowing equal forces to act on the area ahead of the pivot point and equally on the area aft of the pivot point. The advantage of this is that you require a very small torque to turn the rudder, but the disadvantage is directional instability as the rudder will have a tendency to wander due to the equal forces being appled as it passes through the water. In tug terms this would be great for a harbour tug with her towing point midships as she would be very maneuverable and would be able to pivot on her tow line with minimum rudder appled. This direction instability always gives the danger or turning the tug over (where the tow rope comes across the beam at right angles almost midships at the towing point) and this is why a gog or gob rope is sometimes used to transfer the towing point further aft.
Semi Balanced. This is where the rudder stock is approximately 1/3 back from the leading edge of the rudder blade. This increases the turning torque required but gives much better direction stability. This is the type of rudder used in all merchant vessels with a conventional propulsion package built today. In tug terms this is what most single/twin/ASD tugs have. This gives good maneuverability for use in harbours but still excellent direction stability for towing at sea (i.e. in a straight line).
Non Balanced. This is where the rudder stock is located right on the leading edge of the rudder, whilst giving the best directional stability it gives the worst turning performance and requires the most torque to turn the rudder blade. This is the category that your tug falls into. When you are towing over aft your tug will not pivot around the towing point at all. The entire tug will have to force the complete length of her hull through the water to change direction which will lead to using all engine power to position your tug and not the tow, this will lead to total loss of control of the tow.
You have made such a nice job of her top sides and she looks like she will be a cracking model, i would just suggest that you think about the option of placing the towing winch on the bow and towing through the lead on the bow as most ASD tugs to to avoid disappointment with the performance of your finished model.
Cheers.
Tugchief.