A cable guide (110) comprises an upper face (120) and a lower face (130). The lower face (130) has a generally convex form so as to lie in contact with the flow channel (101) when fitted in the flow channel (101). The upper face (120) has a generally concave form and is shaped to minimise disturbance of the flow when fitted in the flow channel (101). A passage is defined between the lower face (130) and the flow channel having an entrance opening (113) at a first end (111) of the cable guide (110) and exit openings (114) on the sides of the cable guide towards a second end (112). By utilising a cable guide (110), disruption to the flow within the channel (101) by the entrance or exit of the cable (105) is minimised. Additionally, the guide (110) restricts the lateral movement of the cable (105) prior to entering opening (113), preventing the cable (105) lying across the direction of flow. In this manner, the cable guide (110) helps to reduce the disruption to the flow caused by the entrance/exit of the cable (105).