1. An outboard motor comprising means adapted for attachment to the stern of a boat and a propulsion unit connected to said attachment means for vertical and horizontal swinging movement relative to said attachment means, said propulsion unit including a rotary internal combustion engine including a first housing assembly comprising mEans defining aligned first and second trochoid shaped rotor cavities and means defining a first water jacket system adjacent to said first and second rotor cavities and having inlet and outlet ports, said first housing assembly further comprising first and second housing members respectively including first wall means at least partially defining said first and second rotor cavities and second wall means at least partially defining first and second water jackets forming a part of said first water jacket system, said first water jacket including said first water jacket system inlet port and said second water jacket including said first water jacket system outlet port, said first housing assembly further comprising an intermediate member located between said first and second housing members and including wall means further defining said first and second rotor cavities and additional wall means defining a third water jacket communicating between said first and second water jackets and forming a part of said first water jacket system, a second housing assembly comprises means defining aligned third and fourth trochoid shaped rotor cavities and means defining a second water jacket system adjacent to said third and fourth rotor cavities and having inlet and outlet ports, said second housing assembly comprising third and fourth housing members respectively including first wall means at least partially defining said third and fourth rotor cavities and second wall means at least partially defining fourth and fifth water jackets forming a part of said second water jacket system, said fourth water jacket including said second water jacket system inlet port and said fifth water jacket including said second water jacket system outlet port, said second housing assembly further comprising an intermediate member located between said third and fourth housing members and including wall means further defining said third and fourth rotor cavities and additional wall means defining a sixth water jacket communicating between said fourth and fifth water jackets and forming a part of said second water jacket system, and means securing together said first and second housing assemblies, a lower unit rigidly supporting said engine and including a gear box normally partially submerged in water, a propeller rotatably supported by said gear box and operably connected to said engine, a water pump driven by said engine and having an inlet communicating with the water, and a water conduit communicating between said pump and separately with each of said first and second water jacket system inlet ports.

5. An outboard motor comprising means adapted for attachment to the stern of a boat and a propulsion unit connected to said attachment means for vertical and horizontal swinging movement relative to said attachment means, said propulsion unit including a rotary internal combustion engine including a first housing assembly comprising means defining aligned first and second trochoid shaped rotor cavities and means defining a first water jacket system adjacent to said first and second rotor cavities and having inlet and outlet ports, said first housing assembly further comprising first and second housing members respectively including first wall means at least partially defining said first and second rotor cavities and second wall means at least partially defining first and second water jackets forming a part of said first water jAcket system, said first water jacket including said first water jacket system inlet port and said second water jacket including said first water jacket system outlet port, a second housing assembly comprising means defining aligned third and fourth trochoid shaped rotor cavities and means defining a second water jacket system adjacent to said third and fourth rotor cavities and having inlet and outlet ports, said second housing assembly comprising third and fourth housing members respectively including first wall means at least partially defining said third and fourth rotor cavities and second wall means at least partially defining third and fourth water jackets forming a part of said second water jacket system, said third water jacket including said second water jacket system inlet port and said fourth water jacket including said second water jacket system outlet port, and means securing together said first and second housing assemblies, a lower unit rigidly supporting said engine and including a gear box normally partially submerged in water, a propeller rotatably supported by said gear box and operably connected to said engine, a water pump driven by said engine and having an inlet communicating with the water, and a water conduit communicating between said pump and separately with each of said first and second water jacket system inlet ports.