Straight shafts are perfect mechanical devices. They have predictable characteristics and no natural weak spots. They behave according to two principal parameters. Diameter and Material. The larger the tube diameter the stiffer it is and the more carbon is in it the stiffer it is, no a narrow glass shaft is flexible where a larger carbon shaft is stiff. In pure terms , stiff is good, there is no propulsive energy wasted in the flex. Olympic racers like stiff shafts. The opposite end of the scale are rodeo boaters. There life is all about shock impacts hyper extension, involuntary movements, unintendo's. These guys need a shock absorber between them and their blades to prevent injury. The shafts Rough Stuff use are heat formable epoxy glass shafts. Stiffer shafts are available on request.
There are 2 types of crank shaft, the Double Torque & Modified Crank, both are shown and explained below.
With regards to structural strength, any bent shaft no matter how it is constructed is inherently weaker than an straight shaft. This is because the fibres are bent rather than straight. Any fibre is at it's strongest when in tension, and not compression as in a crank shaft structure.
Double Torque Crank
There are only two valid reasons for bending and already perfectly good straight shaft.. one is to improve reach, for racing, and the other is to relieve the stress angle in your wrist. The double torque shaft improves reach by placing your hand behind the centre line of the shaft and hence the paddle blade further in front of your hand than you would ordinarily be able to reach. This unfortunately de-stabilises all of the strokes you do behind yourself ,i.e. pry strokes, reverse sweeps etc.
So for general paddling and rodeo this layout is not great. Still, relieves the wrist issue and is great for touring (sea paddling) and racing where reverse strokes are not an issue.
The big down side of bent shafts is that they can never be as strong as a straight shafts because of the bend. the difficulty is that the fibres tend to fold under compression on the back side of the shaft and this is far worse on braided shaft types rather than uni-directional fibre type. All crank shafts are going to be heavier than their straight counterparts for a given strength.
This type of paddle shaft neither improves your reach nor impairs your pry strokes. It is basically neutral just like a straight shaft. It does however greatly reduce the stress on your wrist. There are two principal stress components at work on your wrist. Firstly there is an up down movement associated with the feathering of your paddle. This is easy to reduce by reducing the feather angle of your paddle. The second component is a horizontal movement from side to side associated with reaching forward and back. This is much harder to reduce and is the far more erosive or damaging movement. Essentially, if you paddle every day you will eventually experience repetitive stress injury due to this movement. More mature individuals taking up paddling for the first time are the most susceptible group to suffer from this. If you are over thirty and you either have taken up paddling or are doing a lot more than you used to and have sore forearms, wrists, thumbs, sharp or dull pains in your shoulder that don't go away when you take a short break or return when you recommence paddling. Then you are probably suffering from tendentious or repetitive strain injury.
There are two way to get rid of it, the first is stop paddling (a non option!!), the second is to get a modified crank paddle shaft and to paddle gently until it goes away.
Setting the feather to 55° has been calculated and proven to be the optimum feather angle to reduce the associated strain on you back and shoulders.