A new mezzanine vehicle deck will be made this autumn for Manannan and installed before each TT, accommodating an extra 300 motorcycles each way every day.
It will also be installed for 2015's Isle of Man Festival of Motorcycling, which includes the Classic TT and Manx Grand Prix.
During this year's TT, the Steam Packet carried more than 12,100 motorcycles each way, an increase of 8 per cent on the 2013 figure, and demand for travel to next year's TT is already substantially higher than this time last year.
Steam Packet chief executive Mark Woodward said: 'We have invested heavily over the years to significantly expand our vehicle capacity for the TT.
'Our vessels and shore staff work round-the-clock to maximise capacity for the event and this year we invested in MVArrow and other charters. However, in recent years we have seen motorcycle numbers for the TT and Festival of Motorcycling grow by some 25%, and we know there is an increasing demand to bring motorcycles to the Island for these events.
'This additional capacity for motorcycles is another direct benefit of the long term charter of MV Arrow as it allows us to move high vehicles away from Manannan and onto the Ben-my-Chree, while high passenger vehicle space is freed up on the Ben by moving freight to the Arrow.'
He added that the company had looked at a variety of ways to meet demand to ensure as many fans as possible could go to the TT, but additional short term charters were not available or economic.
'Manannan already carries an impressive 1,000 motorcycles per day each way and this mezzanine deck will increase her capacity to around 1,300 each way,' he said.
'In the short term, this will allow for more motorcycles to be carried on the most popular crossings and we anticipate an additional 1,500 to 2,000 visiting motorcycles during the 2015 TT. In the longer term, this additional capacity will make us more self-sufficient as it has proved so difficult to source suitable charter vessels for such short periods and the investment will help us meet future demand for some years to come.
'There has been a marked increase in demand for the Festival of Motorcycling since the launch of Classic TT, and we will be able to play our part in supporting the ongoing growth of that event and the Manx Grand Prix by using the new deck to increase capacity for the festival.'
The extra mezzanine deck will lead to a restriction on the height of vehicles Manannan can carry during the TT and Festival of Motorcycling periods, but extra high vans and similar vehicles will be accommodated on Ben-my-Chree instead.
The intention is to release the additional motorcycle capacity for TT2015 deposit bookings at 8.30am on Monday, July 28.
To enable the additional capacity to be added to the reservation system, sailings for the TT2015 period (Wednesday, May 27, to Monday, June 15, 2015) will be closed from midday on Sunday, July 27, 2014, in preparation for release at 8.30am the next day.