Significantly increased propulsive efficiency is one of the key benefits of IPS, but that increase has been made possible only by a fundamental rethink of propulsion principles. About the only thing IPS has in common with conventional shaftdrive is that it uses fully submerged propellers rather than jets or surface-piercing propellers.
IPS uses two forward-facing contra-rotating propellers per unit. The superior efficiency of twin contra-rotating propellers over a single larger diameter one was appreciated many years ago, and Volvo's Duoprop outdrive is the established manifestation of that theory. Because they operate in clear water, propellers work better in tractor (pulling) form than in pusher mode but shaftdrive installation naturally ensured that the pusher principle has remained dominant in the marine world.
IPS uses two forward-facing contra-rotating propellers per unit. The superior efficiency of twin contra-rotating propellers over a single larger diameter one was appreciated many years ago, and Volvo's Duoprop outdrive is the established manifestation of that theory. Because they operate in clear water, propellers work better in tractor (pulling) form than in pusher mode but shaftdrive installation naturally ensured that the pusher principle has remained dominant in the marine world.
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