Daylight shape cylinder

For signallig and navigational purposes from sunrise to sunset. When visibility is poor, the navigation lights may be used as well.  

Used during daytime to indicate that a vessel is constrained by her draught. Which means that a power-driven vessel is restricted in her ability to deviate from her course because of her draught in relation to the available depth and width of the navigable water. For more information see Colreg rule 28.