Exhibit 4.21  SUPPLEMENTAL LETTER AGREEMENT  From:
  
  The Royal Bank of Scotland plcTo:
  
  Danaos CorporationTrust Company ComplexAjeltake RoadAjeltake IslandMajuroMarshall Islands MH 96960  14 April 2010  Loan Agreement dated 20 February 2007
(as amended and supplemented, the Loan Agreement) made between (i) Danaos
Corporation (the Borrower), (ii) the banks and financial institutions
listed in Schedule 1 thereto as lenders and (iii) The Royal Bank of
Scotland plc as swap bank, issuing bank, agent and security trustee in respect
of a revolving credit facility of up to US$700,000,000.  1BACKGROUND  1.1The Borrower and The Royal Bank of Scotland plc (in its various relevant capacities,
referred to below as RBS) wish to implement certain
matters as described in paragraph 3 below.  1.2A default has occurred under the Loan Agreement, specifically under Clause 9.1(e)(v) (which
constitutes an Event of Default under Clause 20.1(a) of the Loan
Agreement) and a potential default has occurred under Clause 9.9(b)(iv) of
the Loan Agreement.  1.3The rights of RBS in respect of the Event of Default and the potential
Event of Default respectively referred to in paragraph 1.2 above are hereby
expressly reserved (along with the rights of RBS in respect of any other Events
of Default or potential Events of Default, whether or not known to RBS) and the
entering into of this letter agreement shall not be, or be deemed to be, a
waiver by RBS of any of its rights. Such rights shall only be waived, if at
all, by the express agreement in writing of RBS.  2DEFINTIONS  2.1In this letter agreement words and expressions defined in the Loan
Agreement shall have the same meaning.  2.2In addition the following words and expressions shall have the meanings set
out below:  Cash Pledge No. 1means a Greek law pledge dated 29 July 2008
executed by the Borrower in favour of RBS in respect of the No. 1 Pledged
Account;  Cash Pledge No. 2means a Greek law pledge dated 2 December 2008
executed by the Borrower in favour of RBS in respect of the No. 2 Pledged
Account;


Cash Pledge No. 3means a Greek law pledge dated 19 October 2007
executed by the Borrower in favour of RBS in respect of the No. 3 Pledged
Account;  Effective Date
meansthe date on which the matters set out in
paragraph 3 below shall be implemented and come into effect, as evidenced by
the execution by RBS of the Effective Date Certificate (but not being a date
later than 9 April 2010, or such later date, if any, as RBS may agree in
writing);  Effective Date
Certificatemeans a certificate to be
executed by RBS subject to the terms and conditions of paragraph 4 below,
substantially in the form set out in Appendix A to this letter agreement;  No. 1
Pledged Accountmeans an account of
the Borrower maintained with RBS, Piraeus branch, having account no. 577411 and
designated RBS/Danaos Corporation US$700 million facility  Cash Deposit
Account;  No. 2
Pledged Accountmeans an account of
the Borrower maintained with RBS, Piraeus branch, having account no. 577411 and
designated RBS/Danaos Corporation Swaps;  No. 3 Pledged Accountmeans
an account of the Borrower maintained with RBS, Piraeus branch, having account
no. 577411 and designated RBS/Danaos Corporation plc;  Replacement
Securitymeans security for certain
of the obligations of the Borrower to RBS, secured, as at the date of this
letter agreement, by Cash Pledge No. 2 and Cash Pledge No. 3 (prior
to release of that security pursuant to the terms, and subject to the
conditions of, this letter agreement) to consist of:  (a)a guarantee of each Owner;  (b)a second priority or preferred mortgage in respect of each Mortgaged Ship
(and, on delivery, each ship which then becomes a Mortgaged Ship);  (c)a second priority general assignment in respect of each Mortgaged Ship
(and, on delivery, each ship which then becomes a Mortgaged Ship); and  (d)a second priority pre-delivery security assignment in respect of each
Approved Ship, Alternative Ship or Additional Approved Ship,  all
in such form as The Royal Bank of Scotland plc may approve or require;  Shares Securitymeans first priority security (by way of pledge
and/or charge, as applicable) to be granted by the shareholder of each Owner in
respect of the whole of the issued share capital of each Owner in favour of the
Security Trustee as security for the obligations of the Borrower under the
Finance Documents.  2.3With effect from the Effective Date all references in this letter agreement
to the Loan Agreement shall be to the Loan Agreement as amended and restated
pursuant to this letter agreement.  2.4The provisions of Clause 1.2 to 1.5 (inclusive) of the Loan Agreement shall
apply to this letter agreement with any consequential modifications.  3IMPLEMENTATION  3.1The matters set out below in this paragraph 3 shall, subject to paragraph
4.2 below, be implemented and take effect on the Effective Date.  2


3.2The Borrower shall on the Effective Date, in order to reduce its debt service
costs under the Loan Agreement, apply all amounts standing to the credit of No. 1 Pledged Account, No. 2 Pledged
Account and No. 3 Pledged Account in prepaying Advances. In respect
of such prepayment it is agreed or acknowledged (as the case may be):  (a)that the requirements of clause 9.6 of the Loan Agreement are hereby
waived;  (b)that the requirement of clause 9.5 of the Loan Agreement for prepayment to
be made on the last day of an Interest Period applicable to the Advance prepaid
are hereby waived on condition that the Borrower, in accordance with clause
9.11(a) of the Loan Agreement, pays any sums payable under clause 22.1(b) of
the Loan Agreement;  (c)that the amount prepaid shall be applied towards prepayment of Tranche B;  (d)that the Borrower is permitted so to apply in prepayment amounts standing
to the credit of No. 1 Pledged
Account by virtue of the terms of the Loan Agreement (clause 4.9(c)(ii)) (as in
effect at the date of this letter agreement) and the terms of Cash Pledge No. 1;  (e)that the Borrower is not, without the consent of RBS, permitted so to apply
in prepayment amounts standing to the credit of No. 2 Pledged Account and No. 3
Pledged Account but RBS is prepared to allow such amounts so to be applied in
prepayment subject to and upon the terms and conditions of this letter
agreement;  (f)that Cash Pledge No. 1, Cash Pledge No. 2 and Cash Pledge No. 3,
respectively, secure (prior to the release of that security pursuant to the
terms, and subject to the conditions of, this letter agreement), inter alia,
the obligations of the Borrower under the Loan Agreement and the other Finance
Documents;  (g)that on the Effective Date RBS shall release its security constituted by
Cash Pledge No. 1, Cash
Pledge No. 2 and Cash Pledge No. 3 in order to allow the matters
referred to above in this paragraph 3.2 to be implemented; and  (h)that amounts prepaid pursuant to this paragraph 3.2 shall be available to
be re-drawn by the Borrower subject to and upon the terms and conditions of the
Loan Agreement as supplemented, amended and restated by this letter agreement.  3.3On or before the Effective Date the Borrower shall procure that the
shareholder of each Owner shall grant the Shares Security and shall procure the
satisfaction of, the applicable provisions of paragraph 4 below in respect of
the Shares Security.  4CONDITIONS PRECEDENT  4.1The occurrence of the Effective Date, as evidenced by RBS execution of the
Effective Date Certificate, shall not occur until RBS has received the
following documents and evidence in form and substance satisfactory to it:  (a)corporate authorities of the Borrower and the Owners and the shareholders
of the Owners respectively authorising the execution of this letter agreement
and the Shares Security;  (b)the execution and delivery of the Shares Security, together with all items
required to be delivered pursuant to the terms of the Shares Security and any
required registrations, notices or other matters required pursuant to or in
connection with the Shares Security;  3


(c)such legal opinions from lawyers appointed by RBS (in whatever applicable capacity)
as it shall require in respect of this letter agreement and the matters
referred to herein; and  (d)evidence, by way of a certificate from the Borrower, that the
representations and warranties contained in paragraph 8 below are true and
correct as at the Effective Date.  4.2If RBS, in its discretion, executes the Effective Date Certificate
notwithstanding that it has not received certain of the documents and evidence
required to be delivered in accordance with paragraph 4.1 above, the Borrower
shall deliver, or procure the delivery of, the outstanding items within such
period as RBS shall specify, and failure to deliver the same within such
specified period shall constitute an Event of Default.  4.3Any agreement of RBS, pursuant to paragraph 4.2 above, to defer the date
for delivery of any of the documents or evidence required to be delivered in
accordance with paragraph 4.1 above until after the Effective Date shall not,
for the avoidance of doubt, result in the consideration provided by RBS (in
whatever applicable capacity) for the deferred documents or evidence being past
consideration.  5RE-DRAWING  5.1Itis
confirmed and agreed (including, without limitation, with reference to clause 4.9(c)(ii) of
the Loan Agreement as in effect at the date of this letter agreement) that the
amounts prepaid pursuant to this letter agreement may be re-drawn by the
Borrower for the purposes described in paragraph 5.2 below subject to and upon
the terms and conditions of the Loan Agreement and that such terms and
conditions include delivery of the documents described in Part B of
Schedule 3 or, as the case may be, Part C of Schedule 3 of the Loan
Agreement in form and substance satisfactory to the Agent and its lawyers in
respect of the applicable Approved Ship, Additional Approved Ship or
Alternative Ship, as the case may be.  5.2The purposes for which the amounts prepaid pursuant to this letter
agreement may be re-drawn by the Borrower are in outline as follows (as more
particularly set out in the amended and restated Loan Agreement to take effect
pursuant to this letter agreement):  (a)to finance a portion of the remaining pre-delivery and delivery contract instalments
of the Alternative Ships (ie Sundong hull no. S4005 and Hanjin hull nos. N-219,
N-221 and N-222);  (b)to finance all of the remaining pre-delivery and delivery contract
instalments of Additional Approved Ship A (ie Jiangnan hull no. H1022A); and  (c)as to the balance available to be drawn (approximately $16 million) for
such purpose as RBS may approve (as more particularly set out in the amended
and restated Loan Agreement to take effect pursuant to this letter agreement).  6AMENDMENT AND RESTATEMENT  6.1The Loan Agreement shall, as at the Effective Date, be amended and restated
in the form set out in Appendix B to this letter agreement (on condition that
it reflects accurately the provisions of this letter agreement and all previous
amendments of, or supplements to, the Loan Agreement, to the extent not
inconsistent with, or superceded by, this letter agreement), subject to such
minor  4


amendments,
including as may be necessary to correct manifest errors, as may be agreed
between RBS and the Borrower prior to or after the Effective Date. In the
absence of any such minor amendments, the Loan Agreement shall as at and from
the Effective Date take effect in the form set out in Appendix B to this letter
agreement.  6.2For the avoidance of doubt, the amendment and restatement of the Loan Agreement
is to give effect to amendments to the Loan Agreement and for ease of reference
and shall not constitute a novation of the Borrowers obligations under the
Loan Agreement.  6.3The Borrower shall, at the request of RBS, on or after the Effective Date
promptly execute an original counterpart of the amended and restated Loan
Agreement in the form set out in Appendix B to this letter agreement, subject
to such minor amendments as may be agreed between RBS and the Borrower, prior
to or after the Effective Date. This paragraph 6.3 is without prejudice to the
operation of paragraph 6.1 above but any amended and restated version of the
Loan Agreement separately executed pursuant to this paragraph 6.3 shall take
precedence over the form attached to this letter agreement as Appendix B.  7CONTINUED EFFECT  7.1Save as expressly amended by or pursuant to the terms of this letter
agreement, the Finance Documents shall continue in full force and effect and
all security constituted by the Finance Documents shall continue to secure the
obligations expressed to be secured thereby as such obligations may have been
amended and/or supplemented and/or restated by or pursuant to the terms of this
letter agreement.  7.2With effect from the Effective Date all references in the Finance Documents
to the Loan Agreement shall be read and construed as references to the Loan
Agreement as supplemented and/or amended and/or restated pursuant to this
letter agreement.  8REPRESENTATIONS AND WARRANTIES  8.1The Borrower represents and warrants to RBS (in whatever applicable
capacity) as follows:  (a)the Borrower and each Owner has the corporate capacity and has taken, or
will by the Effective Date have taken, all corporate action and obtained all
consents necessary for the execution and delivery of this letter agreement and
the implementation of the matters referred to herein;  (b)the execution by the Borrower and each Owner of this letter agreement and
the implementation of the matters referred to herein will not involve or lead
to a contravention of:  (i)any law or regulation;  (ii)the constitutional documents of the Borrower or any Owner; or  (iii)any contractual or other obligation which is binding on the Borrower or any
Owner.  8.2The representations and warranties contained in paragraph 8.1 above shall
be restated as at the Effective Date by reference to the facts and
circumstances then applicable.  5


9REPLACEMENT SECURITY  9.1Each Owner shall execute and where applicable register, and the Borrower
shall procure that each Owner shall execute and where applicable register, the
Replacement Security by not later than the earlier of:  (i)the date falling 60 days after the date of this letter agreement; and  (ii)any date on which the Borrower enters into a comprehensive debt
restructuring agreement with its major bank creditors.  In
connection with the execution and registration of the Replacement Security the
Borrower shall procure that there is delivered to RBS such corporate
authorities and legal opinions as RBS shall approve or require.  10MISCELLANEOUS  10.1The provisions of clause 29 (Notices) of the Loan Agreement shall apply as
if they were expressly incorporated herein.  10.2No-one who is not a party to this letter agreement shall have any rights
under it by reason of the Contracts (Rights of Third Parties) Act 1999.  10.3This letter agreement may be executed in any number of counterparts.  10.4If any provision of this letter agreement, or the matters to be implemented
pursuant to its terms, is or subsequently becomes void, unenforceable or
illegal under any applicable law, that shall not affect the validity,
enforceability or legality of the other provisions of this letter agreement, or
the matters to be implemented pursuant to its terms.  10.5Without prejudice to the provisions of Clause 21 of the Loan Agreement, the
Borrower shall on demand pay to RBS the amount of all expenses incurred by RBS
in connection with this letter agreement and its implementation in excess of
£5,000.  10.6This letter agreement, and any non-contractual obligations arising under
it, shall be governed by and construed in accordance with English law and the
provisions of clause 35 (Law and Jurisdiction) of the Loan Agreement shall
apply to this letter agreement as if they were expressly incorporated herein.  10.7This letter agreement is agreed to be a Finance Document for all purposes
of the Loan Agreement.  Please confirm your
acceptance to the foregoing terms and conditions by signing the acceptance and
acknowledgement below.   Yours faithfully/s/ A. RodopoulosA. RODOPOULOSfor
  and on behalf ofTHE ROYAL BANK OF SCOTLAND plc(in
  all relevant capacities)  6


We hereby acknowledge receipt of the above
letter, confirm our agreement to the terms of the same and hereby undertake to
be bound thereby   /s/ I. Prokopakis
  
  /s/ D. AndritsoyiannisI. PROKOPAKIS
  
  D. ANDRITSOYIANNISfor
  and on behalf ofDANAOS CORPORATION  Date:  14 April 2010  We hereby confirm and
acknowledge that we have read and understood the terms and conditions of the
above letter agreement and agree in all respects to comply with and implement
the applicable provisions of the same and confirm that the Finance Documents to
which we are a party shall remain in full force and effect and shall continue
to stand as security for the obligations of the Borrower under the Loan
Agreement (as amended and/or supplemented and/or restated by or pursuant to the
terms of the said letter agreement)   /s/ Zoe Lappa
  
  /s/ Zoe LappaZOE LAPPA
  
  ZOE LAPPAfor
  and on behalf of
  
  for and on behalf ofFEDERAL MARINE INC.
  
  BOXCARRIER (NO. 5) CORP./s/ Zoe Lappa
  
  /s/ Zoe LappaZOE LAPPA
  
  ZOE LAPPAfor
  and on behalf of
  
  for and on behalf ofCELLCONTAINER (NO. 1) CORP.
  
  CELLCONTAINER (NO. 3) CORP./s/ Zoe Lappa
  
  /s/ Zoe LappaZOE LAPPA
  
  ZOE LAPPAfor
  and on behalf of
  
  for and on behalf ofTEUCARRIER (NO. 5) CORP.
  
  MEGACARRIER (NO. 1) CORP./s/ Zoe Lappa
  
  /s/ Zoe LappaZOE LAPPA
  
  ZOE LAPPAfor
  and on behalf of
  
  for and on behalf ofMEGACARRIER (NO. 2) CORP.
  
  MEGACARRIER (NO. 3) CORP.  7


/s/ Zoe Lappa
  
  /s/ Zoe LappaZOE LAPPA
  
  ZOE LAPPAfor
  and on behalf of
  
  for and on behalf ofMEGACARRIER (NO. 4) CORP.
  
  MEGACARRIER (NO. 5) CORP/s/ Zoe Lappa
  
  /s/ Zoe LappaZOE LAPPA
  
  ZOE LAPPAfor
  and on behalf of
  
  for and on behalf ofCELLCONTAINER (NO. 4) CORP.
  
  SPEEDCARRIER (NO. 7) CORP./s/ Zoe Lappa
  
  /s/ Zoe LappaZOE LAPPA
  
  ZOE LAPPAfor
  and on behalf of
  
  for and on behalf ofCELLCONTAINER (NO. 6) CORP.
  
  SPEEDCARRIER (NO. 6) CORP./s/ Zoe Lappa
  
  /s/ Zoe LappaZOE LAPPA
  
  ZOE LAPPAfor
  and on behalf of
  
  for and on behalf ofSPEEDCARRIER (NO. 8) CORP.
  
  CONTINENT MARINE INC.Date:    14 April 2010  8


APPENDIX A  From:
  
  The
  Royal Bank of Scotland plcTo:
  
  Danaos
  CorporationTrust
  Company ComplexAjeltake
  RoadAjeltake
  IslandMajuroMarshall
  Islands MH 96960  [·]
2010  Supplemental Letter Agreement dated [·] 2010 (the Letter Agreement)  We refer to the
Letter Agreement. Words and expressions defined in the Letter Agreement have
the same meanings when used herein.  This is the Effective
Date Certificate and we hereby confirm that the Effective Date is [·]
2010.   Yours
  faithfullyfor
  and on behalf ofTHE ROYAL BANK OF SCOTLAND plc  9


APPENDIX B  [Amended and Restated Loan Agreement]  10