I will take some pictures of the Curlew’s hull (underside) this week if Eddie is at the club.

Meanwhile - does anyone have a link to the Curlew plans - or maybe have a PDF ?

This could help us to understand why this hull steers so well when being driven in reverse.

Bob.

Excellent description TD. I hold the opinion that all boaters should learn to dock a single screw inboard boat. Master the technique you describe & docking a twin screw vessel or an outboard becomes easy.

Excepting twin screw boats, all boats going astern will have a strong tendency to veer in one direction or the other. That tendency is dictated by the propeller, whether it is a right hand or left hand rotating prop. The amount of veer is strongly influenced by the pitch of the propeller. The pitch creates the swirl or walk of the prop. That is the sideways force created by the prop especially on the stern of the boat. Remember that boats steer from the stern, not the bow. Prop wash over the rudder moves the stern to one side or the other, pivoting the boat around the bow.
Now picture a boat, with the prop at the stern, rudder at the bow. The pivot point is still the bow, but now the entire boat is trying to follow the rudder with just the flow of water over the rudder created by movement of the boat. It does not work well. There were exceptions, but they did not last.
The key to steering a single screw boat going astern, is to master the technique of utilizing the prop walk. Spend some time at the docks and watch fishermen manouver . Their technique might be best described as "back and fill". They will back down until the stern just begins to swing, then apply ahead throttle with a kick. The kick pushes the stern around in the direction they want. Repeated with skill, they will dock faster and more precisely than a twin screw boat.
So, in a nut shell, while rudder size does influence the results, it is far more the skill of the operator that dictates the astern line of the vessel. Sorry to be so long winded😊

Looks like the rather splendid looking Curlew might have a nice keel to help keep it stable, reduce the port or starboard drift. As a displacement hull, maybe a less coarse pitch on the prop than a planing hull too. Will be interesting to see the rudder and prop placement for sure.

Or maybe there are some cunning bow and stern thrusters... ;-)

Bob you need a 'push you pull me boat ' pointed at both ends Col

Thanks for all the technical information, it is very much appreciated.

What does surprise me is that my fellow club member Eddie, run his model power boat under total control in reverse for an extended period of time.

Just showing off (of course) but he did prove beyond any doubt that he had total control of his boat and he was able to steer it in any direction whilst running backwards !

Next week, I will take a look at the size of the rudder etc to see if I can learn anything to explain why his boat steers as well in reverse as it does when driving forwards.

Bob.

(I realized after posting this that I'm repeating what other folks have said, but I'm going to leave it up - my apologies for the repeat)

First, rudder size has a lot to do with it. Motorboats typically have small rudders, and since 90% or more of the rudder authority comes from the prop wash hitting it directly, when the boat is going backwards a small rudder is comparatively ineffective. Compare the rudder on a typical motorboat to that on a typical sailboat - a sailboat rudder will be a significantly larger because it has to steer the boat without prop wash - and only works when the boat is moving at a reasonable speed. That said, my real sailboat (a 34' keelboat) would absolutely, positively, NOT steer in reverse at any speed or throttle setting, large rudder aside.

On the other hand, my ST tug, which has a big rudder, will steer in reverse IF she's going fairly fast in reverse, if the throttle is about 1/2-2/3, an if you have enough time to wait for her to respond.

Second - twin screw boats generally should steer, if sluggishly, in reverse because the prop wash hits both sides of the hull symetrically (assuming counter-rotating props). It is noticeably more difficult to steer if both screws rotate in the same direction (or all 3 rotating in the same direction, as in WWII US PT boats!) - steering using differential throttles should still work, though.

On the other hand, when a single screw boat is traveling in reverse, the prop wash hits the hull instead of the rudder - and it pushes the hull. However, it only hits the hull on one side due to the prop rotation. That pushes the boat sideways - hence the prop walk when reversing.

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