A bill to amend Section 506 of the Merchant Marine Act, 1936, to permit permanent payback of construction-differential subsidy.

Amends the Merchant Marine Act, 1936 to allow a vessel for which a construction-differential subsidy (CDS) has been paid to operate in the domestic trade under specified conditions which include the repayment of a proportionate share of such subsidy by the owner to the Secretary of Transportation.

Allows vessels which have been engaged in the domestic trade for two and one half years and which were built without a CDS to file a notice of harm (a notice that CDS-built vessels are harming such vessels to the point where they no longer can compete) with the Secretary. Authorizes the Secretary to terminate the operation of the CDS-built vessel if the notice of harm is granted.

Allows CDS-built vessels with three years of uninterrupted service in the domestic trade to operate permanently in the domestic trade if a proportionate share of CDS repayment is made.