Saturday, March 29, 2008

Boat Handling (Sternway)

Backing the boat up, making sternway, is almost completely inevitable, beyond just getting into and out of our docks, however, we often have to or want to make sternway for other reasons, as part of the jockeying around for turning tight corners (backing and filling), and sometimes just because the place we want to go to is behind us, but not far enough away to justify turning the whole boat around.
Some boats are very controllable under sternway, but even these don’t behave as well as they do when making headway. At the other end of the spectrum are boats whose sternway relegate the use of reverse motion to the bare minimum possible.

Sternway hull dynamics
Part of the problem is the design of the boat. It’s built to go ahead, and to meet the water with the point of the bow, or sometimes called stem. Going astern, it doesn’t track as well, and the bow may hang off to one side, the whole boat sliding through the water diagonally. This is especially so the bigger and flatter the transom, which is exactly the opposite, to a bow. Each quarter, where the side meets the transom, may be pointy, but not at all in the right place.
So, trying to push such a structure through the water, the transom may start to slip off to one side or the other. As it does, the whole boat turns, and there are some vessels in which this cannot be counteracted no matter how you steer against it.
Sometimes the more power and more speed helps, but just as often it can make things worse. Most of us have enough sense to take off power before the boat actually starts making doughnuts in the water, but that’s just what happens, with some vessels, if you leave them in reverse for too long.

Asymmetric propeller thrust
The thrust of a propeller is often asymmetric, especially in reverse gear. This is for reasons of inclined propeller shafts, and corkscrewing propeller discharge currents, and other factors. Suffice it to say that, while the effect is almost absent in some boats, in others it can be almost overpowering, some will not back in a straight line, because of asymmetric thrust, under any conditions.
The majority of propellers are right-hand, meaning that they turn clockwise, looking forward, when in forward gear. These right-hand propellers walk the stern to port, in reverse gear. The more you understand how your boat handles, you are less likely to do anything foolish. Furthermore, the asymmetric effect can sometimes even be made to work for you, such as swinging the stern to port.