Navigation lights are no help in broad daylight, since you only see them once it gets dark. Yet you still need to tell, in full daylight, whether the vessel ahead is an ordinary boat, part of a towing convoy, or a vessel restricted in her ability to maneuver. That is what shapes (day shapes) are for: simple black shapes hoisted on the mast or stay. This article explains the most important day shapes for pushed and towed convoys and for special vessels, a topic from the International Regulations for Preventing Collisions at Sea that comes up in both the German SBF Binnen (inland waters) and SBF See (coastal waters) exams.
The four basic shapes
Every day shape prescribed by the collision regulations is built from four simple black basic shapes:
- Ball
- Cone (point up or point down)
- Cylinder
- Diamond: two cones joined at their bases, point to point
Once you can reliably tell these four shapes apart, you can work out most combinations logically instead of memorizing every single one by heart.
Pushed convoys, towed convoys, and vessels made fast alongside
On inland waterways you regularly encounter convoys, meaning several craft travelling together. Three formations are distinguished:
- Pushed convoy (composite unit): A pusher vessel is rigidly coupled to one or more unpowered barges and pushes them ahead. The connection is so firm that the whole convoy behaves as a single vessel.
- Towed convoy: One or more craft are pulled behind a power-driven vessel on a towline, so the connection is not rigid but a rope.
- Vessels made fast alongside: Two or more craft are firmly connected side by side, for example to maneuver together.
For you as an observer, the key difference is how the formation appears at night: a rigidly coupled pushed convoy is treated under the collision regulations as a single vessel and therefore shows only one set of navigation lights. A towed convoy, on the other hand, consists of two separately recognizable units, the towing vessel and the vessel being towed, each showing its own lights. The article on navigation lights explains those light combinations in detail.
On large inland waterways such as the Rhine, you regularly meet pushed convoys made up of several barges, which makes them noticeably longer and wider than a single cargo ship. For you as a recreational skipper, that means giving these convoys plenty of room and assuming they have very little room to maneuver, even in situations where you would formally be the stand-on vessel.
The diamond shape for long tows
Towed convoys are subject to one additional rule that applies both day and night: if the length of the tow, measured from the stern of the towing vessel to the after end of the last vessel or object towed, exceeds 200 meters, both the towing vessel and the vessel being towed must exhibit an additional diamond shape where it can best be seen. The last vessel or object in the tow must also carry a diamond shape at or near its after end, so that a crossing vessel can tell exactly where the tow actually ends.
If a vessel or object being towed, say an unmanned pontoon, cannot for structural reasons exhibit the required lights or shapes, every possible measure must still be taken to illuminate it or at least make its presence known. In practice this means: if you spot a diamond shape on an otherwise unremarkable craft or object, it very likely belongs to a towed convoy, and you should keep well clear of the entire length of the tow, not just the visible towing vessel.
Recognizing fishing vessels
A vessel engaged in fishing shows a double cone by day: two cones with their points together, one above the other. This shape applies both to vessels trawling a net through the water and to other fishing vessels, for example those using gillnets or longlines. At night the two are distinguished by the color of their all-round lights, but by day the shape is the same.
For a fishing vessel that is not trawling (for example one using gillnets or longlines), if its outlying gear extends more than 150 meters horizontally, the vessel shows an additional cone, point upward, pointing in the direction of the gear. That tells you which side to give extra room, so you don’t run into nets or lines that have been paid out. Trawlers don’t carry this extra shape, since their net trails directly astern in their wake rather than out to one side.
Not under command and restricted in ability to maneuver
Two terms that are easy to mix up in the exam can be told apart clearly by their day shapes:
- Not under command (for example following an engine failure): two black balls, one above the other.
- Restricted in her ability to maneuver (for example while dredging, sounding, or towing a heavy convoy): three shapes in a vertical line, in the order ball – diamond – ball.
The mnemonic from the navigation lights article, where two red all-round lights at night mean not under command, carries over directly to the two balls by day. For a vessel restricted in her ability to maneuver, the diamond appears as the middle shape by day, just as a white light appears between the two red lights at night.
Dredging and diving operations
A vessel restricted in her ability to maneuver that is dredging or engaged in underwater operations, and whose work obstructs a fairway on one side, shows two further shapes in addition to the ball-diamond-ball on the affected side:
- Two balls, one above the other, mark the side on which an obstruction exists, meaning the side you should avoid.
- Two diamonds, one above the other, mark the side on which other vessels may safely pass.
Important for the exam: these are not distress signals. They simply indicate that the vessel’s ability to maneuver is restricted and that it therefore has priority over most other vessels, as described in the article on right-of-way rules.
Vessels constrained by their draught
A vessel constrained by her draught, meaning a large ship whose draught ties her closely to the available fairway, may exhibit an additional cylinder in addition to her normal lights, where it can best be seen. Other vessels should avoid blocking the safe passage of a vessel marked this way, since she has very little room to maneuver around obstacles.
At anchor and aground
Vessels that are not underway also have clear day shapes:
- At anchor: a single ball, hoisted where it can best be seen, usually forward.
- Aground: three balls in a vertical line, in addition to the anchor lights or anchor ball.
These two signals look similar at first glance but are easy to tell apart: a single ball simply means “I am at anchor,” while three balls stacked together mean “I am aground.”
Day shapes and sound signals belong together
Day shapes are only half the picture. In good visibility you see the shape, but in fog or heavy rain only your ears help. That is why exactly the same vessel categories, fishing vessels, vessels not under command, vessels restricted in their ability to maneuver, vessels constrained by draught, and vessels towing or pushing, give their own sound signal in restricted visibility, so they remain identifiable even without visual contact. The article on sound signals for the SBF exam explains how those signals are built and where the typical mix-ups between coastal and inland waters lie. Once you know the vessel categories from this article, the matching sound signals become much easier to remember.
How to approach this in the exam
A fixed routine helps with day-shape questions:
- Identify the basic shape: ball, cone, cylinder, or diamond?
- Count and order: one ball, two balls, or ball-diamond-ball?
- Place it in context: is this a towed convoy, a fishing vessel, or a vessel restricted in her ability to maneuver?
- Work out the consequence: who has priority under the right-of-way rules, and how much clearance do I need?
Here is an example: you see a vessel showing two cones, points together, one above the other. Step 1 tells you this is a double cone. Step 2 gives you the count: two cones in this specific arrangement. Step 3 places the shape in context as a fishing vessel. Step 4 tells you that a fishing vessel ranks above a power-driven vessel and a sailing vessel under the right-of-way rules, so you need to keep clear and also watch out for gear the vessel may have paid out.
With this approach you can reliably work through most exam questions on day shapes, even when the combination shown looks unfamiliar at first.
Conclusion
Day shapes translate the most important night-time light combinations into four simple daytime forms: ball, cone, cylinder, and diamond. Pushed and towed convoys, fishing vessels, vessels not under command or restricted in their ability to maneuver, and vessels constrained by draught, at anchor, or aground can all be identified unambiguously once you have internalized the basic shapes and their typical combinations. The Boatpass app lets you practice exactly these day-shape questions using the official ELWIS question catalog, until you can identify every combination correctly on the first try.