Experimentally, during fast wave (FW) radio frequency (rf) heating in DIII-D L-mode discharges, strong acceleration of neutral beam (NB) deuterium beam ions has been observed. Significant effects on the n/m = 1/1 sawtooth stability are also seen. Simulations using the Monte-Carlo Hamiltonian code ORBIT-RF, coupled to the TORIC full wave code, predict beam ion tails up to a few hundred keV, in agreement with the experiment. The simulations and experiment both clearly show a much greater efficiency for 4th harmonic FW heating than for 8th harmonic heating. Simple analyses of the kinetic contribution to the ideal magnetohydrodynamic (MHD) potential energy from energetic beam ions generated by FW heating yields reasonable consistency with the observations. A more detailed analysis shows a more complicated picture, however. Other physics effects such as geometry, plasma rotation, and the presence of a free boundary, play a significant role.