Hawaii interisland ocean cargo transportation firm Young Brothers Ltd. has committed to buying four new tugboats for close to $20 million apiece to replace much of its aging fleet.

The company announced Monday that it signed a contract to buy the boats from Louisiana-based Conrad Shipyard for about $80 million.

The new tugs are slated to be delivered in 2018 and 2019.

Young Brothers has nine tugboats — six for towing and three for providing assistance in harbors. The four tugs slated for replacement — Ocean Pathfinder, Hokulani, Moana Holo and Manuokekai — are 40 to 43 years old, the company said earlier this year.

The company said the new tugs, which are expected to be useful for 35 years, will lower crew expenses because they will provide faster transit, improve fuel efficiency and reduce downtime and maintenance needs.

“Our investment in these new tugs will directly support and further our commitment to frequent, reliable, affordable and universal service for decades into the future,” Young Brothers President Glenn Hong said in a statement.

Young Brothers plans to station the new boats on the neighbor islands and name the vessels after former captains.

The announcement about renewing part of the company’s tug fleet follows a general rate increase request earlier this year by Young Brothers to raise freight rates regulated by the state Public Utilities Commission, though the cost of the four new boats is not factored into the requested rate hike.

Young Brothers said in its rate increase application that it plans to ask the PUC to factor in the expense for the new ships at a later date.

The pending general rate increase request would boost Young Brothers’ revenue by about 4 percent, or $3.14 million. The PUC has scheduled public hearings for July to consider the requested general rate increase after the state Consumer Advocate raised concerns about a lack of detail in the rate hike application.

Young Brothers said it needs higher rates to pay for increased operating expenses in the face of demand that has risen only between 0.5 and 1.5 percent over the last three years.

Among the cited higher expenses are labor costs from the latest bargaining agreements with employee unions, vessel dry-docking costs and shared services expenses between Young Brothers and its parent company.

The Consumer Advocate said Young Brothers didn’t provide necessary information regarding the distribution of money among its affiliated companies and didn’t include a summary of expenses by island or month.

One area of expenses being examined by Young Brothers is its harbor-assistance tugboats. The company said in its application that it is considering getting rid of or repurposing its harbor-assistance tugs and getting out of this part of the cargo business, which isn’t regulated by the PUC.

“YB is examining the expense and future capital needed to support the harbor assist business and whether this part of YB’s operation is a core aspect of YB’s business,” the company said in the application.