These durable track ends are machined from a single piece of aluminum. The integral 40 mm Carbo Airblocks with free-running Delrin ball bearings allow for easy adjustment when you move the car in a strong breeze - which is when you want the traveler to work at its best.When deciding on a purchase system, expect the load on the control line to be approximately 20% of the load on the mainsheet block. For most cruising boats, this will mean designing a 4:1 (or even higher) purchase system to keep control line loads in the reasonable 30 to 45 lb range. Harken purchase recommendations when using 27 mm traveler control blocks: Use a 2:1 purchase for boats with sail area under 125 ft (12 m) Use a 3:1 purchase for boats with sail area from 125 ft to 200 ft (12 m to 19 m) Use a 4:1 purchase for boats with sail area from 200 ft to 250 ft (19 m to 23 m)