A low emission submersible engine is part of a multipurpose aquatic device, such as an aquatic craft or an electrical generator. A propulsion system having a propeller is coupled to the submersible engine to produce a swimmer propulsion machine and a generator is attached to the engine to produce an electrical generator. The submersible engine receives air from a snorkel that extends up above a water line. The aquatic device has a fuel reservoir which may receive liquid fuel or may receive a fuel cylinder. Floatation chambers keep the aquatic device afloat with the engine submerge. The exhaust assembly and exhaust ports from the engine may also be submerged and the exhaust ports may provide additional thrust. A propulsion system housing integrates the drive shaft and propeller, clutch coupler, exhaust conduits and water flow conduits and is detachably attachable to the engine.

A methane transportation system is provided. The system may include a methane source configured to dispense methane at a first location, and an underwater vehicle. The underwater vehicle may include a propulsion system configured to transport the underwater vehicle underwater from the first location to a second location and a vessel defining a storage chamber configured to receive water and the methane from the methane source. The storage chamber of the vessel may have a pressure exceeding one atmosphere and a temperature during transport from the first location to the second location sufficient to form methane clathrate in the storage chamber. The system may further include a methane receiver configured to receive the methane released from the storage chamber at the second location. Related methods are also provided.

A methane transportation system is provided. The system may include a methane source configured to dispense methane at a first location, and an underwater vehicle. The underwater vehicle may include a propulsion system configured to transport the underwater vehicle underwater from the first location to a second location and a vessel defining a storage chamber configured to receive water and the methane from the methane source. The storage chamber of the vessel may have a pressure exceeding one atmosphere and a temperature during transport from the first location to the second location sufficient to form methane clathrate in the storage chamber. The system may further include a methane receiver configured to receive the methane released from the storage chamber at the second location. Related methods are also provided.

A selectively submersible vessel includes a combustor, a Stirling engine thermally coupled with the combustor, an electric generator mechanically coupled with the Stirling engine, a rechargeable battery electrically connected with the electric generator, a compressor electrically coupled with the rechargeable battery, a ballast tank fluidly coupled with the compressor, a snorkel fluidly coupled with the compressor, a controller electrically connected with the rechargeable battery, at least one sensor electrically connected with the controller, a data storage device electrically connected with the controller, and a data communications device electrically connected with the controller.

A selectively submersible vessel includes a combustor, a Stirling engine thermally coupled with the combustor, an electric generator mechanically coupled with the Stirling engine, a rechargeable battery electrically connected with the electric generator, a compressor electrically coupled with the rechargeable battery, a ballast tank fluidly coupled with the compressor, a snorkel fluidly coupled with the compressor, a controller electrically connected with the rechargeable battery, at least one sensor electrically connected with the controller, a data storage device electrically connected with the controller, and a data communications device electrically connected with the controller.