Sternway

— by Charles T. Low, author of Boat Docking
— for By-the-Sea [ 1998 January ]

Backing the boat up, ‘making sternway’, is almost completely inevitable, unless you are the proud owner of a ‘drive-through’ slip (and I do know a boater who has one)! Beyond just getting into and out of our docks, however, we often have to or want to make sternway for other reasons, commonly as part of the jockeying required 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 idiosyncracies relegate the use of reverse motion to the bare minimum possible.

Why do they behave this way? Is there anything we can do about it?

Sternway hull dynamics — Part of the problem is simply the design of the boat. It’s built to go ahead, and to meet the water with the point of the bow, or more accurately, along the sharp angle of the 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 somewhat diagonally. This is especially so the bigger and flatter the transom, which is exactly the opposite, conceptually, to a pointy 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 phenomenon cannot be counteracted no matter how you steer against it.

Sometimes the application of more power and/or more speed helps, but just as often it only aggravates the situation.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 which we shall reserve for another forum. 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. At least if 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 when wanting to swing the stern to port.

Reverse Coasting — The obvious way to get around asymmetrical propeller thrust is not to use the propeller. However, unless you want to sail the boat backwards (some boats can!), or paddle it, or pole it like a Venetian gondola, you’re probably going to need to use power. The key, then, is to minimize using reverse gear — use it very gently, and for the minimum time necessary to produce or to sustain sternway. True, you need enough speed through the water to allow the rudder to steer (unless you have an I/O), but the rudder has more of a fighting chance if it isn’t required to overcome asymmetric propeller thrust.

‘Prop walk’ aside, the hull itself, even if it tracks poorly when going astern, may be more docile when going very slowly, disturbing less water and so experiencing fewer adverse hydrodynamic effects in return.

Discharge Current — When going ahead, the rudder is in the propeller’s discharge current. This partly explains the better steerage afforded by forward propulsion than by reverse — in reverse gear, not only is asymmetric 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.

Propeller optimization — 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 the bulk of its time. The corollary often is (as in my own beautiful boat) that reverse gear lacks power. So, you may need to use reverse gear enthusiastically, perhaps when you need to ‘put the brakes on’, or to develop enough sternway to allow the rudder to steer.

However, vigorous use of reverse gear brings with it the problems of asymmetric propeller thrust and poor sternway hull dynamics, as mentioned above. Clearly, there is a bit of a balancing act required here, and the internal contradiction of needing lots of power, in reverse gear, which then causes undesirable side effects, explains much of the limitations of sternway steerage.

Wind and Current — A boat which does handle well in reverse (and a substantial proportion of boats do, despite all of the foregoing!) 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. The helmsman must be able to conceptualize this very clearly, but if the boat can steer well when making sternway in still water, it can steer equally well when making sternway in moving water.

Not so in a wind. 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 out well enough. 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 here, 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. Fine, 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.

This was discussed in greater detail in last month’s article, ‘Wind’, but the upshot is that sternway in a wind is difficult. If your boat behaves well under these conditions, great. (Mine doesn’t!) This can be a nuisance, but again, it’s much better to know about it and avoid it, if necessary, than to be caught unawares and lose control of your vessel.

Skippering the Stern Way — Having digested all of the preceding information without undue gastrointestinal upset, you are quite correct to ask “Now what?” The “what” is (i) to know and to understand the limitations of your particular boat, and (ii) to experiment, on the water, with its sternway characteristics. Spend a few minutes, now and again, doing exercises such as figures-of-eight. Try loose 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 (disregard the funny looks from passers-by), and at an accelerated rate than just from the ‘general experience’ method, valuable though that too is.

Conclusion — There never is a total conclusion to boat docking, and how much more interesting boating is as a result. In the unlikely 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.

And always learning comprises a large part of the joy of close quarters maneuvering.

The End
Sternway — for By-the-Sea [ 1998 January ]
Copyright © 1998 ctLow

About the book — Boat Docking (Close Quarters Maneuvering for Small Craft) is a recently published book about how to dock a boat! It contains many concrete examples of boat docking, from the elementary to the advanced, and also has chapters discussing the theory of close quarters maneuvering.

  This is a softcover publication, 7"x9" (18x23 cm), 88 pages in length, with roughly 25,000 words and about 140 clear, simple illustrations. (That's a lot of illustrations!) It's a comfortable (winter) evening's read, or a ready on-board reference and discussion document.

  The distillation that has resulted is not, to our knowledge, reproduced elsewhere. This book is tightly focused only on slow-speed handling of small craft, and as such it is very complete and thorough. It has an excellent balance of technical analysis and practical boat-docking recipes.

Charles T. Low, the author of Boat Docking, is a recreational boater in the Thousand Islands on the St. Lawrence River. He and his family spend much of their summer vacations, weekends, and many evenings, on the water. Charles likes the beauty and tranquility of the islands, and also enjoys boating — an excellent match of process and product!

Visit the Boat Docking web site for further information.