pair of crossed spring-lines between the boats as well as breast lines at both ends. Then the two boats don't yaw apart as much as they otherwise would, which makes them more controllable.
Inevitably there will be some turning moment imposed by the drag of the non-powered boat. Mike Stevens Before President got Kildare, we had a modeller bring working models of President and Northwich to an AGM and this effect was very obvious, such that the pair could only be turned one way - the other way the momentum of the butty defeated the engine. The solution on the real boats was to put Kildare on the side (I think the left) to which President turns because of the paddlewheel effect when slowing down. The result is that paddlewheel effect and butty momentum largely cancel each other out. I think practice is also to maintain a fairly constant speed between locks and to do as much acceleration and braking as possible while the boats are in the locks and can't turn. I've also used this approach when breasted up in my own boat. Of course the paddlewheel effect is much less potent in a diesel boat than in a steamer. Sean
