Saturday, March 29, 2008

Small Boats

Discharge Current
When going ahead, the rudder is in the propeller’s discharge current. This explains the better steerage afforded by forward propulsion than by reverse, in reverse gear, not only is propeller thrust worse, but there is no discharge current flowing from the propeller over the rudder (the suction current has a much weaker effect), so there is little to amplify the rudder’s effect.

Many propellers are designed to enhance their forward propulsive efficiency. The blades are pitched, raked and cupped to give the best possible bite with the boat in forward gear, which, after all, is where the transmission spends most of its time. So, you may need to use reverse gear, perhaps when you need to stop, or to develop enough sternway to allow the rudder to steer.
However, use of reverse gear brings with it the problems of asymmetric propeller thrust and poor sternway hull dynamics. There is a bit of a balancing act required, and the internal contradiction of needing lots of power, in reverse gear, which then causes undesirable side effects, explains the limitations of sternway steerage.

Wind and Current
A boat which does handle well in reverse will also be steerable in a current. A boat crabbing in a cross current, for example, is going straight through the water. It’s just that the water itself is moving, so the boat goes diagonally over the bottom. But if the boat can steer well when making sternway in still water, it can steer equally well when making sternway in moving water.

Air in motion turns the boat, and a brisk breeze can yaw it forcefully enough that countering it with rudder or outdrive just doesn’t work. The simplest case, backing into the wind, often works well. The exact opposite, reversing with the wind, is uncontrollable in some boats, the bow blows off, and even if you can swing it back head to wind, it immediately blows off to the other side.
In between is making sternway in a beam wind. One would think that a sternway steerable boat would also do fine, but experience shows otherwise. It is a very stable vessel which allows control under this situation. For example, say you’re in a cross wind, which then blows the boat a little downwind. You would have to correct for this by turning the stern upwind. Except that now, if the blow is heavy, the bow may weathervane downwind. Even if not, the boat in a cross wind has to move diagonally through the water as well as over the bottom, so it’s usual docile sternway characteristics may be upset by not being allowed to track directly fore and aft.

Stern Way
To know and to understand the limitations of your particular boat, and to experiment, on the water, with its sternway characteristics. Spend a few minutes, now and again, doing training such as figures-of-eight. Try wide ones and tight ones, slow and fast, and in winds and currents of varying magnitude. Use power gently, and then vigorously. Try staying under power, and try coasting. You will undoubtedly gain a better feel for your boat.
There never is a conclusion to boat docking, and how much more interesting boating is as a result. In the event that any of us ever thinks we have sternway all figured out, an occasion will arise which causes us to realize that there is always more to learn.