<PAGE>

                                                                  EXHIBIT 4.1(c)

                          SUPPLEMENTAL INDENTURE NO. 3

     This SUPPLEMENTAL INDENTURE NO. 3, dated as of December ___, 1996, is by
and among MORAN TRANSPORTATION COMPANY, a Delaware corporation (the "Company"),
MORAN TOWING CORPORATION, a New York corporation ("Moran Towing"), the
Guarantors listed on Annex I hereto (collectively, the "Guarantors"), MATC,
INC., a Delaware corporation, MEC I, INC., a Delaware corporation, and BPC I,
INC., a Delaware corporation, and FLEET NATIONAL BANK (formerly known as Shawmut
Bank Connecticut, National Association), as trustee (the "Trustee").

                              W I T N E S S E T H:
                              - - - - - - - - - -

     WHEREAS, the Company, the Trustee and certain of the Guarantors (other than
Seaboard Barge Corporation, a Delaware corporation ("Seaboard"), Petroleum
Transport Corporation, a Delaware corporation ("PETC"), and Moran Towing of
Delaware, Inc., a Delaware corporation ("Moran Delaware," and collectively with
Seaboard and PETC, the "New Guarantors"), which are becoming Guarantors
hereunder) are parties to that certain Indenture dated July 11, 1994, as amended
by Supplemental Indentures No. 1 and 2 (as so supplemented, the "Indenture"),
pertaining to the Company's 11-3/4% Series B First Preferred Ship Mortgage Notes
due 2004 issued under the Indenture (the "Notes");

     WHEREAS, after giving effect to the transactions and mergers described
below, the Guarantors listed on Annex I hereto will constitute all of the
Subsidiaries of the Company.

A.  Prior Dissolutions; Mergers
    ---------------------------

     1.  Prior Dissolutions
         ------------------

     WHEREAS, prior to or on the date hereof, the following Guarantors were
dissolved in compliance with Section 5.14 of the Indenture (as described below):
Belair Picton Co., Caribbean Barge Corporation, James River Towing Company, John
E. Moore Company, Lewes Curtis Bay Towing Co., Moran Salvage Company Inc., Moran
Tankship No. 1 Inc., Moran Tankship No. 2 Inc., Moran Tankship No. 3 Inc., Moran
Tankship No. 4 Inc., Moran Towing of Maine, Inc., Moran Container Corporation,
and Moran Towing of Puerto Rico, Inc.;

     2.  Transactions Involving Moran Towing & Transportation Co., Inc.
         --------------------------------------------------------------

     WHEREAS, pursuant to certain agreements and plans of mergers, effective
December 31, 1996, the following Restricted Subsidiaries shall merge with and
into Moran Towing &


<PAGE>

Transportation Co., Inc., a New York corporation ("MT&T"), which will be the
surviving corporation in such merger:


         Aberdeen Curtis Bay Co.
         Annapolis Curtis Bay Co.
         Curtis Bay Towing Company of Delaware
         Elkton Curtis Bay Co.
         Irvington Curtis Bay Co.
         Kirkwood Curtis Bay Co.
         Laurel Picton Co.
         Warwick Curtis Bay Co.

     WHEREAS, subsequent to such mergers, MT&T shall transfer all of its assets
to Moran Towing in exchange for a promissory note from Moran Towing, and then
subsequently MT&T shall transfer such promissory note to a newly formed
subsidiary of MT&T, Moran Delaware, after which MT&T will merge with and into
Moran Towing, all effective as of January 1, 1996;

     3.  Moran Towing of Miami, Inc./Moran Towing of Florida, Inc.
         ---------------------------------------------------------

     WHEREAS, pursuant to an agreement and plan of merger, effective December
31, 1996, Moran Towing of Miami, Inc., a Delaware corporation, shall merge with
and into Moran Towing of Florida, Inc., a Florida corporation ("Moran Florida"),
which will be the surviving corporation in such merger;

     WHEREAS, pursuant to an agreement and plan of merger, effective January 1,
1997, Moran Florida shall merge with and into Moran Towing, which will be the
surviving corporation in such merger;

     4.  Moran Mid-Atlantic Corporation
         ------------------------------

     WHEREAS, Moran Mid-Atlantic Corporation, a Delaware corporation, shall,
pursuant to a dividend which will be paid effective as of December 31, 1996,
distribute all of its assets (other than certain real property, which will be
retained) to Moran Towing and shall be renamed "Hampton Roads Land Co., Inc.",
effective December 31, 1996;

     5.  Petroleum Transport Corporation
         -------------------------------

     WHEREAS, each of the following Restricted Subsidiaries shall establish
wholly-owned Restricted Subsidiaries, which will be Delaware corporations, to
which such Restricted Subsidiaries shall contribute the vessels specified below,
effective December 31, 1996:

                                       2
<PAGE>

     Subsidiary                           Wholly-Owned Subsidiary  Vessel
     ----------                           -----------------------  ------

     Moran Atlantic Towing Corporation    MATC, Inc.               Maine
     Moran Enterprises Corporation        MEC I, Inc.              Texas
                                                                   Florida
     Barge Pennsylvania Corporation       BPC I, Inc.              Pennsylvania

     WHEREAS, after giving effect to such contributions, each of Moran Atlantic
Towing Corporation, Moran Enterprises Corporation and Barge Pennsylvania
Corporation (which will own the vessel Valentine Moran, which is not, and will
                                       ---------------
not be, a Mortgaged Vessel) will merge, effective December 31, 1996, with and
into Moran Towing, with Moran Towing being the surviving corporation in such
merger;

     WHEREAS, each of MATC, Inc., MEC I, Inc., BPC I, Inc. (which will own the
vessel Pennsylvania, which is not, and will not be, a Mortgaged Vessel),
       ------------
Seaboard Shipping Corporation and Moran Power Corporation shall merge, effective
December 31, 1996, with and into PETC, which shall be the surviving corporation
in such merger;

     6.  Other Mergers with and into Moran Towing
         ----------------------------------------

     WHEREAS, in addition to the mergers of MT&T, Moran Florida, Moran Atlantic
Towing Corporation, Moran Enterprises Corporation and Barge Pennsylvania
Corporation with and into Moran Towing, as described above, pursuant to certain
agreements and plans of merger, the Guarantors listed on Annex II (collectively,
the "Other Merged Guarantors") hereto shall merge (collectively, the "Mergers")
with and into Moran Towing, effective December 31, 1996, with Moran Towing being
the surviving corporation in such Mergers.

     7.  General
         -------

     WHEREAS, Section 6.03 of the Indenture provides that (a) a Qualified
Restricted Subsidiary shall have the right to merge with any other Qualified
Restricted Subsidiary provided that the Qualified Restricted Subsidiary which is
the surviving corporation shall execute a supplemental indenture (in a form
reasonably satisfactory to the Trustee) pursuant to which such surviving
corporation shall (i) expressly assume the obligations under the applicable
Guarantee of the merged Qualified Restricted Subsidiary which is not the
surviving corporation in such merger and (ii) confirm the due and punctual
performance of the Guarantee of such surviving corporation and every covenant in
the Indenture on the part of such surviving corporation to be performed or
observed; and that (b) a Restricted Subsidiary that is not a Qualified
Restricted Subsidiary shall have the right to merge with any other Restricted
Subsidiary which is not a Qualified Restricted Subsidiary provided that the
Restricted Subsidiary which is the surviving corporation shall (i) execute a
supplemental indenture (in a form reasonably satisfactory to the Trustee)
pursuant to which such surviving corporation shall

                                       3
<PAGE>

(1) expressly assume the obligations under the applicable Guarantee of the
merged Restricted Subsidiary which is not the surviving corporation in such
merger and (2) confirm the due and punctual performance of the Guarantee of such
surviving corporation and every covenant in the Indenture and the Collateral
Documents on the part of such surviving corporation to be performed or observed
and (ii) execute any instrument required by the Trustee pursuant to Section 3.4
of the applicable Ship Mortgage(s);

     WHEREAS, each of Moran Florida, Barge Pennsylvania Corporation and Moran
Towing of New Hampshire, Inc. is a Qualified Restricted Subsidiary, and Moran
Towing is a Restricted Subsidiary which is not a Qualified Restricted
Subsidiary;

     WHEREAS, notwithstanding Section 6.03 of the Indenture, which relates to
mergers of (i) Qualified Restricted Subsidiaries with other Qualified Restricted
Subsidiaries and (ii) Restricted Subsidiaries which are not Qualified Restricted
Subsidiaries with other Restricted Subsidiaries which are not Qualified
Restricted Subsidiaries, the Company and Moran Towing intend that the Mergers of
each of Moran Florida, Barge Pennsylvania Corporation and Moran Towing of New
Hampshire, Inc. with and into Moran Towing fulfill the requirements of the
proviso to Section 5.14 of the Indenture;

     WHEREAS, Section 5.14 of the Indenture provides that subject to Article 6
of the Indenture, the Company will do or cause to be done all things necessary
to preserve and keep in full force and effect (i) its corporate existence and
the corporate existence of each of its Restricted Subsidiaries, in accordance
with their respective organizational documents (as the same may be amended from
time to time) and (ii) its (and its Restricted Subsidiaries') rights (charter
and statutory), licenses and franchises; provided, however, that the Company
                                         --------  -------
shall not be required to preserve any such right, license or franchise, or the
corporate existence of any Restricted Subsidiary, if the Board of Directors of
the Company shall determine that the preservation thereof is no longer desirable
in the conduct of the business of the Company and its Restricted Subsidiaries
taken as a whole and that the loss thereof is not adverse in any material
respect to the Holders;

     WHEREAS, Sections 10.01(g) and (h) of the Indenture provide that the
Trustee, the Company, the Guarantors and a Subsidiary, as applicable, may amend
or supplement the Indenture without the consent of any Holder to make any
changes that do not adversely affect the legal rights of any Holder or to
supplement the Indenture to provide for mergers of Restricted Subsidiaries
pursuant to Section 6.03, and

     WHEREAS, the Company, Moran Towing and the Guarantors intend that this
Supplemental Indenture No. 3 fulfill the requirements of such Sections 5.14 and
6.03, and that Moran Towing assume the obligations under the Guarantees, the
Indenture and the Collateral Documents of the Merged Guarantors.

                                       4
<PAGE>

B.  New Guarantors
    --------------

     WHEREAS, the Company has organized PETC as a Subsidiary with the intention
that PETC become a Guarantor under the Indenture and a Restricted Subsidiary
that is not a Qualified Restricted Subsidiary pursuant to the terms of the
Indenture;

     WHEREAS, the Company has organized each of Seaboard and Moran Delaware as a
Subsidiary with the intention that each of Seaboard and Moran Delaware become a
Guarantor under the Indenture and a Qualified Restricted Subsidiary pursuant to
the terms of the Indenture;

     WHEREAS, Section 5.23 of the Indenture provides that any Person that was
not a Guarantor on the date of the Indenture may become a Guarantor by executing
and delivering to the Trustee, among other things, a supplemental indenture in
form and substance satisfactory to the Trustee, which subject such Person to the
provisions (including the representations and warranties) of the Indenture as a
Guarantor;

     WHEREAS, Section 10.01(h) of the Indenture provides that the Trustee, the
Company, the Guarantors and a Subsidiary, as applicable, may amend or supplement
the Indenture without the consent of any Holder to supplement the Indenture to
provide for additional Guarantors pursuant to Section 5.23; and

     WHEREAS, the Company and each of the New Guarantors intend that this
Supplemental Indenture fulfill the requirements of such Section 5.23, thereby
making each of the New Guarantors a Guarantor.

     NOW THEREFORE, the parties agree as follows, for the benefit of each other
and for the equal and ratable benefit of the Holders of the Notes:

     Section 1.01    Defined Terms.  Capitalized terms used in this Supplemental
                     -------------
Indenture but not defined herein shall have the meanings given such terms in the
Indenture.

     Section 2.01  Acceptance by Trustee.  The Trustee accepts the modifications
                   ---------------------
of the Indenture hereby effected only upon the terms and conditions set forth in
the Indenture as supplemented by this Supplemental Indenture No. 3.  Without
limiting the generality of the foregoing, the Trustee shall not be responsible
for the correctness of the recitals contained herein, which shall be taken as
statements of the Company, and the Trustee makes no representations and shall
have no responsibility for, or in respect of, the validity or sufficiency of
this Supplemental Indenture No. 3.

                                       5
<PAGE>

     Section 2.02  Construction.  This Supplemental Indenture No. 3 is executed
                   ------------
as and shall constitute an instrument supplemental to the Indenture and shall be
construed in connection with and as part of the Indenture.

     Section 2.03  Ratification.  Except as modified and expressly amended by
                   ------------
this Supplemental Indenture No. 3, the Indenture is, in all respects, ratified
and confirmed and all the terms, provisions and conditions thereof shall be and
remain in full force and effect.

     Section 2.04  MT&T
                   ----

     Prior to its merger with and into Moran Towing, MT&T hereby agrees as
follows:

     (a) to assume the obligations of the following Restricted Subsidiaries
under the Guarantees of such Restricted Subsidiaries:

         Aberdeen Curtis Bay Co.
         Annapolis Curtis Bay Co.
         Curtis Bay Towing Company of Delaware
         Elkton Curtis Bay Co.
         Irvington Curtis Bay Co.
         Kirkwood Curtis Bay Co.
         Laurel Picton Co.
         Warwick Curtis Bay Co.; and

     (b) that MT&T confirms the due and punctual performance by MT&T of the
Guarantee and the Collateral Documents of MT&T and/or such Restricted
Subsidiaries and every covenant in the Indenture and the Collateral Documents to
be performed or observed by MT&T and/or such Restricted Subsidiaries.

     Section 2.05  Moran Florida
                   -------------

     Prior to its merger with and into Moran Towing, Moran Florida hereby agrees
as follows:

     (a) to assume the obligations of Moran Towing of Miami, Inc. under the
Guarantee of such Guarantor; and

     (b) that Moran Florida confirms the due and punctual performance by Moran
Florida of the Guarantee of Moran Florida and/or Moran Towing of Miami, Inc. and
every covenant in the Indenture to be performed by Moran Florida and/or Moran
Towing of Miami, Inc.

                                       6
<PAGE>

     Section 2.06  Transfers of Barges to Newly-Created Wholly-Owned
                   -------------------------------------------------
Subsidiaries; Subsequent Merger with and Into PETC
--------------------------------------------------

        (a) Prior to the merger of each of the following Restricted Subsidiaries
with and into PETC, each of the following Restricted Subsidiaries listed in the
column entitled "Wholly-Owned Subsidiary" below hereby agrees to be bound by and
subject to all the terms of the Indenture (including representations and
warranties) as a Guarantor, including without limitation the provisions of
Article 3 of the Indenture, as if such Restricted Subsidiaries were a signatory
to the Indenture. In addition, prior to, or concurrently with, the contribution
of the following Mortgaged Vessels to each of MATC, INC. and MEC I, INC., each
of Moran Atlantic Towing Corporation, Moran Enterprises Corporation, MATC, Inc.
and MEC I, Inc. shall execute and deliver a Ship Mortgage, or an assumption to
ship or fleet mortgages, as the case may be, with respect to the vessels set
forth opposite their names below (provided that BPC I, Inc., which will own the
vessel Pennsylvania, which is not a Mortgaged Vessel, will not execute a Ship
       ------------
Mortgage or an assumption to a ship mortgage), which Mortgaged Vessels are
presently subject to First Preferred Fleet Mortgages in favor of the Trustee:

     Subsidiary                           Wholly-Owned Subsidiary  Vessel
     ----------                           -----------------------  ------
     Moran Atlantic Towing Corporation    MATC, Inc.               Maine
     Moran Enterprises Corporation        MEC I, Inc.              Texas
                                                                   Florida
     Barge Pennsylvania Corporation       BPC I, Inc.              Pennsylvania

        (b) After giving effect to the merger of the Restricted Subsidiaries
listed in Section 2.06(a) with and into PETC, PETC hereby agrees to be bound by
and subject to all the terms of the Indenture (including representations and
warranties) as a Guarantor, including without limitation the provisions of
Article 3 of the Indenture, as if PETC were a signatory to the Indenture.

        (c) Concurrently with the merger of the Restricted Subsidiaries listed
in Section 2.06(a) with and into PETC, PETC shall execute and deliver Ship
Mortgages, or assumptions to ship or fleet mortgages, as the case may be, with
respect to each of the following Mortgaged Vessels (provided that PETC, which
will own the vessel Pennsylvania, which is not a Mortgaged Vessel, will not
                    ------------
execute a Ship Mortgage or an assumption to a ship mortgage with respect to such
vessel):

            Maine        (Official No. 571982)
            Texas        (Official No. 630729)
            Florida      (Official No. 623034)
            New Jersey   (Official No. 523392)
            Rhode Island (Official No. 537350)
            Connecticut  (Official No. 999754)

                                       7
<PAGE>

     Section 2.07  Seaboard; Moran Delaware. Each of Seaboard and Moran Delaware
                   ------------------------
agrees to be bound by and subject to all the terms of the Indenture (including
representations and warranties) as a Guarantor, including without limitation the
provisions of Article 3 of the Indenture, as if Seaboard and Moran Delaware were
signatories to the Indenture.

     Section 2.08  Moran Towing.  Moran Towing hereby agrees as follows:
                   ------------

        (a) to assume the obligations of MT&T, Moran Florida, Moran Atlantic
Towing Corporation, Moran Enterprises Corporation, Barge Pennsylvania
Corporation and the Other Merged Guarantors under each of the Guarantees of
MT&T, Moran Florida, Moran Atlantic Towing Corporation, Moran Enterprises
Corporation, Barge Pennsylvania Corporation and the Other Merged Guarantors;

        (b) that Moran Towing confirms the due and punctual performance by Moran
Towing of the Guarantee and the Collateral Documents, to the extent applicable,
of Moran Towing and/or MT&T, Moran Florida, Moran Atlantic Towing Corporation,
Moran Enterprises Corporation, Barge Pennsylvania Corporation and the Other
Merger Guarantors and every covenant in the Indenture and the Collateral
Documents, to the extent applicable, to be performed or observed by Moran Towing
and/or MT&T, Moran Florida, Moran Atlantic Towing Corporation, Moran Enterprises
Corporation, Barge Pennsylvania Corporation and the Other Merged Guarantors; and

        (c) Concurrently herewith, Moran Towing has executed and delivered
assumptions of Ship Mortgages in compliance with Section 3.4 of the Ship
Mortgages with respect to the Mortgaged Vessels heretofore owned by MT&T, Moran
Atlantic Towing Corporation, Moran Enterprises Corporation and the Other Merged
Guarantors.

     Section 2.09  Exhibit F.  Effective as of the date of this Supplemental
                   ---------
Indenture No. 3, Exhibits F-1, F-2 and F-3 to the Indenture shall be replaced
with Annex I attached hereto.

     Section 2.10  Counterparts.  This Supplemental Indenture No. 3 may be
                   ------------
executed in any number of counterparts; each signed copy shall be an original,
but all of them together represent the same agreement.

     Section 2.11  Governing law.  This Supplemental Indenture No. 3 shall be
                   -------------
subject to the governing law and choice of forum provisions of Section 13.09 of
the Indenture.

                                       8
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture No. 3 to be duly executed as of the day and year first above written.


MORAN TRANSPORTATION COMPANY


By: /s/ Jeffrey J. McAulay
   -----------------------------------
Name:
Title:    Vice President


MORAN TOWING CORPORATION


By: /s/ Jeffrey J. McAulay
   -----------------------------------
Name:
Title:    Vice President


THE GUARANTORS LISTED ON ANNEX I HERETO


By: /s/ Alan L. Marchisotto
   -----------------------------------
Name:
Title: Secretary As to each of the Guarantors
       listed in Annex I, as Vice President,
       unless otherwise stated.


MATC, INC. (to be merged, as described in Section 2.06)


By: /s/ Alan L. Marchisotto
   -----------------------------------
Name:
Title:    Secretary

                                       9
<PAGE>

MEC I, INC. (to be merged, as described in Section 2.06)


By: /s/ Alan L. Marchisotto
   -----------------------------------
Name:
Title:    Secretary


BPC I, INC. (to be merged, as described in Section 2.06)


By: /s/ Alan L. Marchisotto
   ----------------------------------
Name:
Title:    Secretary


FLEET NATIONAL BANK,
as Trustee


By: /s/ Susan T. Keller
   ----------------------------------
Name:
Title:    Vice President

                                       10
<PAGE>

                                                                         ANNEX I

                                   GUARANTORS
                                   ----------


                 Guarantors after Giving Effect to the Mergers
                 ---------------------------------------------
                  and the Other Transactions Described Herein
                  -------------------------------------------

A.  Restricted Subsidiaries
--  -----------------------

    1.  Restricted Subsidiaries which are Not Qualified Restricted Subsidiaries
    --  -----------------------------------------------------------------------

Moran Towing Corporation
Petroleum Transport Corporation

    2.  Qualified Restricted Subsidiaries
    --  ---------------------------------

Florida Towing Company
Curtis Bay Towing Company of Pennsylvania
Curtis Bay Towing Company of Virginia
Moran Insurance Company Limited
Moran Towing of Texas, Inc.
Moran Shipyard Corporation
Jakobson Shipyard, Inc.
Moran Barge Corporation
Portsmouth Navigation Corporation
Hampton Roads Land Co., Inc.
Moran Services Corporation
Moran Bulk Corporation
Moran Towing of Delaware, Inc.
Seaboard Barge Corporation


B.  Unrestricted Subsidiaries
--  -------------------------

None


<PAGE>

                                                                        ANNEX II
                            OTHER MERGED GUARANTORS
                            -----------------------

         Merged Guarantors which are Qualified Restricted Subsidiaries
         -------------------------------------------------------------

Moran Towing of New Hampshire, Inc.

              Merged Guarantors which are Restricted Subsidiaries,
              ----------------------------------------------------
                   but not Qualified Restricted Subsidiaries
                   -----------------------------------------
Amy Moran Inc.
Berkley Curtis Bay Co.
Chesapeake Barge Corp.
Chestertown Curtis Bay Co.
Commodore Point Co.
Drummond Point Co.
Easton Picton Co.
Hampton Curtis Bay Co.
Judy Moran Inc.
Moran Bucksport Corporation
Moran Coal Company, Inc.
Moran Energy Corporation
Moran Somerset Corporation
Moran Towboat Corp.
Moran Trade Corporation
Naticoke Curtis Bay Co.
Perryville Curtis Bay Co.
Perth Amboy Tugs, Inc.
Portland Towboat Company, Inc.
Portsmouth Curtis Bay Co.
Queenstown Curtis Bay Co.
Sykesville Curtis Bay Co.
Towboat Betty Moran Corp.
Tug Cathleen E. Moran Corp.
Tug Cynthia Moran, Inc.
Tug E.F. Moran, Jr. Inc.
Tug Elizabeth W. Moran, Inc.
Tug Eugene F. Moran, Inc.
Tug Harriet Moran, Inc.
Tug Helen B. Moran, Inc.
Tug Junior Moran, Inc.
Tug Mary Moran, Inc.
Tug Nancy Moran, Inc.
Williamsburg Curtis Bay Co.