The Isle of Man Steam Packet Company's fastcraft Snaefell is to go out of service for repairs early next month.
The work has been timed to start on 7 September after the busy school holiday period is over.
Manannan will be used to cover all scheduled Liverpool and Belfast services while repairs are carried out.
Both crafts are currently only running on three out of four engines as they have both encountered problems with one of their engines.
As a result, crossing times to Liverpool take three hours rather than two-and-a-half.
Chief executive Mark Woodward said he hopes the company will keep disruption to a minimum and apologised for any inconvenience caused.
He said: "The approach we have taken was selected as it causes the least disruption to passengers and avoids the need to cancel sailings.
"Repairs to Snaefell have been scheduled to commence after the busy school holiday period.
"All scheduled Liverpool and Belfast fast craft sailings will be completed by Manannan during this time."
Manannan is expected to undergo repairs in October.