BC Ferries is converting its pair of Spirit-class vessels to a dual-fuel system that uses liquefied natural gas, estimated to be half the price of marine diesel.

The Spirit of Vancouver Island and Spirit of British Columbia are the workhorses of the Tsawwassen-Swartz Bay run, carrying a quarter of the fleet's passengers and vehicles and accounting for about 15 per cent of the fuel used.

The conversion will be part of their mid-life refit, aiming to save $9 million a year in fuel costs for their remaining 27 years of service. It is scheduled to be completed by the spring of 2017.

BC Ferries president Mike Corrigan had previously rejected the idea of LNG conversion, saying it was too costly. BC Ferries awarded contracts this summer for construction of three medium-sized ferries designed for dual fuel.

The refits will also include a low-friction hull coating and modifications to the bow and stern to reduce drag.

The refit will include electrical and passenger service modifications and "increase ancillary services net income," BC Ferries said in a statement.

Transportation Minister Todd Stone announced last year to put slot machines on ferries.