I agree with DWB, when going astern with a single shaft boat, usually from stationary, prop steering or prop walking is the main influence on where the boat goes. At this stage there is little to no control authority exerted by the rudder. Prop walking will tend to take the stern one way in particular. Viewed from the stern, when moving ahead a right handed prop will want to send the stern to starboard, if you leave the rudder alone, in no wind.... As when moving slowly from stationery the prop is pulling some water down the starboard side of the hull, and pushing some water up the port side. This is easily cancelled out with rudder input as there's a water flow over the rudder. So its not an issue if noticed at all.

Astern its the opposite direction, a pull to port, but no water flow to speak of over the rudder blade. So no control authority, the boat goes where it wants to.

I agree with Dave that rudder position relative to the prop is another factor, further away being better in terms of control, but it still needs a flow, so maybe slightly better control once there is a little speed built up, with all the risks that entails.

I think hull form is going to contribute too, as in the hulls grip on the water, probably related to prop size and pitch, so the most tricky boat to control astern is going to be a faster and flatter V planing hull, with a relatively large and coarse prop, and a rudder thats quite close.

Add in wind, and it's anyone's guess where the boat will go.

Twin shafts with counter rotating props on separate but mixed channels is the way to go...!

Have i just alienated all you Swordsman builders..!!!?

@zooma I've got reverse on all my builds but don't use it very often, when I do it's usually because I've been approaching the jetty too fast and have to use it as a brake! As you have found the steering in reverse is usually pretty poor.

@zooma I believe that moving forward, the prop pushes the stream of water past the rudder, which then directs the stream in whatever direction the rudder is pointed.
When reversing, the prop pulls the water towards it, making the water streaming past the rudder very much diminished.

The reason your boat wants to gently go left is due to what is called "prop steer". Meaning the direction of the prop rotation pulls the rear of the boat slightly (in your case, to the left).
I'm sure there are more technical explanations, but I believe my description is basically correct. I'm open to any more informed input.

As to why some steer better than others when reversing, I'm sure it depends on the relationship of rudder to prop. Also, some boats have a nozzle either attached to or fully functioning as the rudder. The nozzle is more effective at directing water flow.

Dave B