The bottom topography of ridged sea ice differs largely from that of other sea ice types. The form drag on ridge keels plays an important role affecting sea ice drift and deformation. We have carried out laboratory experiments and numerical simulations for a ridge model in a flume in order to better understand the characteristics of the form drag. The experimental setup covered both laminar and turbulent conditions. The local form drag coefficient of a keel, Cd, varied with the keel depth h and slope angle α in the turbulent regime. The numerical model extended the experimental results to independence of the water depth in order to achieve an analogy for ocean conditions. The results showed Cd= 0.68ln(α/7.8),R2= 0.998, 10˚ ≤ α≤ 90˚, Cd ranging from 0.14 to 1.66, when keel depth is much smaller than mixed layer depth. In the Arctic Ocean, keel slope angles are within the range of 10˚–50˚ where Cd increases monotonously and becomes the dominant part of the total ice-water drag coefficient when α ≥ 20˚. When h/Lr (the ratio of keel depth to spacing) was high (h/Lr>0.01), the ratio of air-ice to ice-water drag coefficient first decreased and then increased with α and reached the minimum at α ≈ 30˚. The variation of Cd with α (10˚–50˚) affects the momentum transfer of drifting sea ice, and we suggest that Cd under ridged sea ice to be tuned to 0.14–1.26 in multi-category sea ice models.