Macquarie Bank is proving itself to be an infrastructure investment machine in need of constant feeding, even when the investment merits are dubious.The bank's latest investment is the purchase of 100 per cent of the Isle of Man Steam Packet Group Limited from Montagu Private Equity for £225 million. The bank will chip in £40.6 million in equity, with the balance of the purchase price presumably debt.In the media release, Macquarie said it would "explore a range of options for its holding, including the potential transfer to a Macquarie managed fund within the next six to 12 months."This document with dead-pan delivery asserts that the Isle of Man ferry monopoly "fits with the Macquarie Group's strategy of investing in essential service infrastructure businesses."The Steam Packet ferry service (with several roll-on, roll-off ferries servicing four ports in Britain and Ireland) is one of those ever-lasting small-market monopolies. In this case, the chief achievement of Montagu, the vendor, has been to negotiate the extension of the monopoly to 2020, with the option of an extension to 2026.Meanwhile Westpac's Hastings Fund Management is rumoured to be a bidder for British ports operator PD Ports. The Independent and the Daily Telegraph report on the speculation.