surface the entrainment elements or fins 2 are machined into the blade tip surface and normally that part of the blade tip surface from which the shroud 1 is formed these elements or fins 2 can be specifically machined at any desired angled shape or height in order to provide the best performance for cooling of the blade tip surface through air bled or leaking through the passages 5 alternatively entrainment elements could be separate components appropriately secured to the blade tip generally a consideration of the rotational speed will be taken such that the elements or fins 2 will be substantially perpendicular to the coolant flow leakage swirl direction as the blade rotates and as indicated previously will have a height above the opening 14 of the passages 5 in order to further enhance the heat transfer within the channels between the entrainment elements 2 ripple or trip strips or other heat transfer co efficient augmentation structures can be provided for heat exchange with the coolant flow by the present invention a surface cooling technique is provided to ensure adequate cooling of the blade tip as indicated previously surface cooling has the advantage of a lower weight addition along with reduced stress levels and manufacturing costs in comparison with specific provision of a large number of coolant passages and openings in the actual blade tip structure wall thickness in such circumstances the width 6 of the blade tip can be thinner as this thickness need not incorporate the passages and openings for coolant release whilst maintaining sufficient structural integrity to resist distortion or disintegration in use in short the entrainment elements or fins 2 allow increased cooling efficiency with surface released coolant flow by entrainment and retaining that.