The ability to place coiled tubing into a long horizontal well is limited by friction and buckling. A water-hammer tool incorporates a self-piloted poppet valve that converts the kinetic energy of the fluid moving though the coil into water-hammer pressure pulses that reduce friction and applies an end load that pulls on the coil. Water-hammer tools allow routine entry into horizontal sections over 10,000 feet long, representing a significant increase over operation without these tools. These tools incorporate an internal fluid bypass to control the force applied to the bottomhole assembly. An external bypass may also be provided to allow higher flow rates for well circulation. A numerical model of coiled tubing injector weight for coiled tubing well intervention with a water-hammer tool is presented. The model includes the effects of fluid bypass and calculates the maximum feed rate at which a water hammer will be effective for extending the reach of coil. The model is available in spreadsheet format and may be used for job planning and parameter sensitivity analysis. The predicted effects of water-hammer impulse magnitude, fluid bypass, friction coefficient, flow rate, well inclination, and dogleg severity on horizontal reach are discussed. The results of the numerical model are compared with a sample of case histories from over 12,000 extended reach well interventions. These case histories confirm the extended reach capabilities of water-hammer tools and that reducing feed rate below the predicted maximum allows greater extended reach.