26 C.F.R. § 2.1-21
A new vessel constructed otherwise than under the provisions of title V of the Act, and not purchased from the Administration must, within six months from the date of the construction contract, or within the period of any extension, be completed to the extent of not less than 5 percent as estimated by the Administration and certified by it to the Secretary of the Treasury. In case of a contract covering more than one vessel it will be sufficient if one of the vessels is 5 percent completed within the six months' period from the date of the contract or within the period of any extension, and so certified. All construction must be completed with reasonable dispatch as determined by the Administration. If, for causes within the control of the taxpayer, the entire construction is not completed with reasonable dispatch, the Administration will so certify to the Secretary of the Treasury. For the effect of such certification, see § 2.1-23 .
26 C.F.R. §2.1-21