The term "vessel constrained by her draught" means a power-driven vessel which because of her draught in relation to the available depth of water is severely restricted in here ability to deviate from the course she is following.
As per the definition the CBD vessel is having an exceptional draft and in order to mobilize the vessel of such draft, the vessel needs to be power driven. Also, as per the Rule No 18 Responsibilities between vessels, the CBD vessel in certain circumstances have to be give - way vessel. In this situation, the CBD vessel has to take timely and effective action which can only be taken if the vessel is power driven.