With a very similar working principle to Kaplan units, these turbines have an horizontal shaft.
This arrangement is optimal when paired with a penstock, and usually save lot of excavation volumes compared to a Kaplan unit.
Likewise Kaplan units, they well fit with low head (from 2 to 16 meters) and relatively high flow situations, showing a very wide high efficiency field thanks to their double regulated mechanisms: accordingly to the available flow, the distributor and the runner blades (from 3 to 5) take a position following a collimation curve, allow to obtain the maximum power production.
Are available also single regulating Tubular turbines, cheaper but with worst efficiency when working different from design flow. Each turbine should be calculated accordingly to the site characteristics (head and maximum flow), looking for the optimal combination of shaft speed and overall efficiencies.