They are pierced at intervals with openings fitted with hinged flaps called freeing ports, to allow the escape of any water shipped in a sea-way.
Zigqeloziwe kwizithuba ezibhekele ngokulinga neyo imingxunya yafakwa ihenjisi ezibhenguzaye ekuthiwu lizibuke elikhululekileyo, ukuvumela ukuphuma kwamenzi ebenyene ngaxeshikweni inqahowe iselwandle.