An open horizontal heat pipe consisting of a condenser, an adiabatic section, and an evaporator is used to study boiling of R-ll on a surface covered with a porous wick. Porous metal wicks having two different thicknesses and with and without channels cut in their surfaces are examined. For these types of surfaces the heat flux increases very dramatically with increases in excess temperature once boiling is initiated. At an excess temperature of a few degrees Kelvin this rapid increase in heat flux stops and a more gradual increase is observed. It is postulated that this represents a transition from what is termed nucleate boiling where the evaporator surface beneath the wick is exposed primarily to liquid and film boiling which results when a vapor film blankets the surface. The experimental results illustrate that the performance of the evaporator can be improved significantly by cutting channels in the porous wick. In this study the heat flux at which the transition from what is termed nucleate boiling to film boiling was increased by 200% by cutting channels in the evaporator wick.