Highline Rescue Boat Systems: A study of the load created by tethered rescue boats with respect to stream velocity, trim and hull size.

Abstract

Water rescue practitioners must balance the degree of risk associated with a given technique with the needs of the victim. One such technique, the focus of this study, consists of a rescue boat positioned within moving water by means of a high line. It has been shown to be useful for high risk rescues, often when the patient or subject is unable to assist with their own rescue. Despite the degree of risk, little empirical data pertaining to the resulting load (force) placed on the highline by a boat in moving water can be identified in the literature.
In the absence of contextual data, this study examines the research undertaken by the rope rescue community into the forces exerted by suspended loads, and argues that many of the resulting principles, especially the establishment of the load, are not applied during highline positioned boat operations. Therefore the research question of this thesis; ‘what force (N) will be induced by a boat positioned from a high line when deployed in moving water typically encountered during swift water rescue?’ is addressed by establishing the load under real-world operational conditions. In addition to this, the antecedents leading to a proposed worst case event (WCE) are also presented, and tested.
Testing was conducted within a flow-calibrated channel offering stream velocities typical of those encountered during water related rescues. Two contrasting craft were selected for the investigation as representative of those used during swiftwater rescues, a Eurocraft (large hull) and an MFC Rescue Sled (small hull). The experiment subjected the high line to a range of forces induced by the two craft over a range of stream velocities between 0.6 – 5.4ms-1. A load cell was utilised to collect force/time data during deployments to the current vector. The independent variables of trim (relative positioning of the load within the boat) and average stream velocity were investigated.
Under low flow conditions, when trimmed neutrally (the crew were positioned centrally), the Eurocraft induced the highest force on the high line (approximately 1.22kN at 0.6 ms-1 average stream velocity). When trimmed with the load placed to the rear (aft) or towards the front of the boat (bow trim) force was reduced on the high line. Conversely at higher stream velocities (4.5 – 5.4 ms-1) force values rose rapidly when bow or aft trim conditions were induced, and force was reduced in a neutral trim state.
The testing of the WCE induced a peak force of 3.42kN when the boat was deliberately flooded with water during the highest flow available (5.4 ms-1). The force associated with the WCE for a high line constructed with a track line mid-point angle tending to >120º is a theoretical force multiplication approaching that of the rating of the equipment.