The original rules are focussed on buying a ship new, with finance. It assumes a secured loan, or mortgage, but conceivably you could look at other arrangements - the Subsidised Merchant route is a variation of it. You potentially could secure the loan on another asset, or have such means as to be able to negotiate an unsecured loan at a higher interest rate. But in most cases it's cheaper and more sensible to use the ship itself as the asset.