UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  SCHEDULE 13D
                        Under the Securities Act of 1934

                                NATCO GROUP INC.
                                (Name of Issuer)

                                  Common Stock
                         (Title of Class of Securities)

                                   0001057693
                                 (CUSIP Number)

                                David Nierenberg
                               The D3 Family Funds
                               19605 NE 8th Street
                                 Camas, WA 98607
                                  360-604-8600
                  (Name, Address and Telephone Number of Person
                Authorized to Receive Notices and Communications)

                                  April 7, 2006
             (Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report
the acquisition that is the subject of this Schedule 13D and is filing this
schedule because of ss.ss.240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the
following box: |_|



--------------------------------------------------------------------------------
1    NAME OF REPORTING PERSON
     S.S. OR I.R.S. IDENTIFICATION NOS. OF ABOVE PERSON

     The D3 Family Fund, L.P. (David Nierenberg is president of the General
     Partner, which is Nierenberg Investment Management Company.)
--------------------------------------------------------------------------------
2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                 (a)  |X|
                                                                 (b)  |_|
--------------------------------------------------------------------------------
3    SEC USE ONLY


--------------------------------------------------------------------------------
4    SOURCE OF FUNDS*

     WC
--------------------------------------------------------------------------------
5    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
     PURSUANT TO ITEMS 2(d) OR 2(e)                                   |_|


--------------------------------------------------------------------------------
6    CITIZENSHIP OR PLACE OF ORGANIZATION

     Washington
--------------------------------------------------------------------------------
               7    SOLE VOTING POWER

                    804,365 common shares (4.3%)
               -----------------------------------------------------------------
  NUMBER OF    8    SHARED VOTING POWER
   SHARES
BENEFICIALLY        0
  OWNED BY     -----------------------------------------------------------------
    EACH       9    SOLE DISPOSITIVE POWER
  REPORTING
   PERSON           804,365
    WITH       -----------------------------------------------------------------
               10   SHARED DISPOSITIVE POWER

                    0
--------------------------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

     1,780,401 shares (9.4%)
--------------------------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES*  |_|


--------------------------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

     9.4%
--------------------------------------------------------------------------------
14   TYPE OF REPORTING PERSON*

     PN
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!



--------------------------------------------------------------------------------
1    NAME OF REPORTING PERSON
     S.S. OR I.R.S. IDENTIFICATION NOS. OF ABOVE PERSON

     The D3 Family Retirement Fund, L.P. (David Nierenberg is president of the
     General Partner, which is Nierenberg Investment Management Company.)
--------------------------------------------------------------------------------
2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                 (a)  |X|
                                                                 (b)  |_|
--------------------------------------------------------------------------------
3    SEC USE ONLY


--------------------------------------------------------------------------------
4    SOURCE OF FUNDS*

     WC
--------------------------------------------------------------------------------
5    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
     PURSUANT TO ITEMS 2(d) OR 2(e)                                   |_|


--------------------------------------------------------------------------------
6    CITIZENSHIP OR PLACE OF ORGANIZATION

     Washington
--------------------------------------------------------------------------------
               7    SOLE VOTING POWER

                    274,361 common shares (1.5%)
               -----------------------------------------------------------------
  NUMBER OF    8    SHARED VOTING POWER
   SHARES
BENEFICIALLY        0
  OWNED BY     -----------------------------------------------------------------
    EACH       9    SOLE DISPOSITIVE POWER
  REPORTING
   PERSON           274,361
    WITH       -----------------------------------------------------------------
               10   SHARED DISPOSITIVE POWER

                    0
--------------------------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

     1,780,401 shares (9.4%)
--------------------------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES*  |_|


--------------------------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

     9.4%
--------------------------------------------------------------------------------
14   TYPE OF REPORTING PERSON*

     PN
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!


                                       3


--------------------------------------------------------------------------------
1    NAME OF REPORTING PERSON
     S.S. OR I.R.S. IDENTIFICATION NOS. OF ABOVE PERSON

     The D3 Children's Fund, L.P. (David Nierenberg is president of the General
     Partner, which is Nierenberg Investment Management Company.)
--------------------------------------------------------------------------------
2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                 (a)  |X|
                                                                 (b)  |_|
--------------------------------------------------------------------------------
3    SEC USE ONLY


--------------------------------------------------------------------------------
4    SOURCE OF FUNDS*

     WC
--------------------------------------------------------------------------------
5    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
     PURSUANT TO ITEMS 2(d) OR 2(e)                                   |_|


--------------------------------------------------------------------------------
6    CITIZENSHIP OR PLACE OF ORGANIZATION

     Washington
--------------------------------------------------------------------------------
               7    SOLE VOTING POWER

                    71,153 common shares (0.4%)
               -----------------------------------------------------------------
  NUMBER OF    8    SHARED VOTING POWER
   SHARES
BENEFICIALLY        0
  OWNED BY     -----------------------------------------------------------------
    EACH       9    SOLE DISPOSITIVE POWER
  REPORTING
   PERSON           71,153
    WITH       -----------------------------------------------------------------
               10   SHARED DISPOSITIVE POWER

                    0
--------------------------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

     1,780,401 shares (9.4%)
--------------------------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES*  |_|


--------------------------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

     9.4%
--------------------------------------------------------------------------------
14   TYPE OF REPORTING PERSON*

     PN
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!


                                       4


--------------------------------------------------------------------------------
1    NAME OF REPORTING PERSON
     S.S. OR I.R.S. IDENTIFICATION NOS. OF ABOVE PERSON

     The D3 Offshore Fund, L.P. (David Nierenberg is president of the General
     Partner, which is Nierenberg Investment Management Offshore.)
--------------------------------------------------------------------------------
2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                 (a)  |X|
                                                                 (b)  |_|
--------------------------------------------------------------------------------
3    SEC USE ONLY


--------------------------------------------------------------------------------
4    SOURCE OF FUNDS*

     WC
--------------------------------------------------------------------------------
5    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
     PURSUANT TO ITEMS 2(d) OR 2(e)                                   |_|


--------------------------------------------------------------------------------
6    CITIZENSHIP OR PLACE OF ORGANIZATION

     Bahamas
--------------------------------------------------------------------------------
               7    SOLE VOTING POWER

                    395,630 common shares (2.1%)
               -----------------------------------------------------------------
  NUMBER OF    8    SHARED VOTING POWER
   SHARES
BENEFICIALLY        0
  OWNED BY     -----------------------------------------------------------------
    EACH       9    SOLE DISPOSITIVE POWER
  REPORTING
   PERSON           395,630
    WITH       -----------------------------------------------------------------
               10   SHARED DISPOSITIVE POWER

                    0
--------------------------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

     1,780,401 shares (9.4%)
--------------------------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES*  |_|


--------------------------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

     9.4%
--------------------------------------------------------------------------------
14   TYPE OF REPORTING PERSON*

     PN
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!


                                       5


--------------------------------------------------------------------------------
1    NAME OF REPORTING PERSON
     S.S. OR I.R.S. IDENTIFICATION NOS. OF ABOVE PERSON

     The D3 Family Bulldog Fund, L.P. ((David Nierenberg is president of the
     General Partner, which is Nierenberg Investment Management Company.)
--------------------------------------------------------------------------------
2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                 (a)  |X|
                                                                 (b)  |_|
--------------------------------------------------------------------------------
3    SEC USE ONLY


--------------------------------------------------------------------------------
4    SOURCE OF FUNDS*

     WC
--------------------------------------------------------------------------------
5    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
     PURSUANT TO ITEMS 2(d) OR 2(e)                                   |_|


--------------------------------------------------------------------------------
6    CITIZENSHIP OR PLACE OF ORGANIZATION

     Washington
--------------------------------------------------------------------------------
               7    SOLE VOTING POWER

                    234,892 common shares (1.2%)
               -----------------------------------------------------------------
  NUMBER OF    8    SHARED VOTING POWER
   SHARES
BENEFICIALLY        0
  OWNED BY     -----------------------------------------------------------------
    EACH       9    SOLE DISPOSITIVE POWER
  REPORTING
   PERSON           234,892
    WITH       -----------------------------------------------------------------
               10   SHARED DISPOSITIVE POWER

                    0
--------------------------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

     1,780,401 shares (9.4%)
--------------------------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES*  |_|


--------------------------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

     9.4%
--------------------------------------------------------------------------------
14   TYPE OF REPORTING PERSON*

     PN
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!


                                       6


Item 1.     Security and Issuer.

            Common stock in NATCO GROUP INC., 2950 NORTH LOOP WEST, SUITE 700,
            HOUSTON, TX 77092

Item 2.     Identity and Background.

            The D3 Family Funds are Washington State limited partnerships, whose
            principal  business is investing in the equities of public micro-cap
            issuers.  The D3 Family Funds  consist of: D3 Family  Fund,  L.P, D3
            Family  Retirement  Fund, L.P, D3 Children's  Fund, L.P, D3 Offshore
            Fund,  L.P., D3 Family Bulldog Fund, L.P.  Located at 19605 N.E. 8th
            St.,  Camas,  Washington  98607.  No convictions  or  administrative
            proceedings as described in 2 (d) and (e).

Item 3.     Source and Amount of Funds or Other Consideration

            Source of funds is money invested in the partnership by their
            partners.

Item 4.     Purpose of Transaction

            On April 7 we acquired Natco  convertible  preferred  stock which we
            can convert into a 9.4 % stake in the company.

            We made this investment because we believe Natco is an under-valued,
            under-owned  and  under-followed   company  whose  excellent  growth
            prospects,  strong  management and governance,  improving  operating
            results,  and leading  competitive  position in a fragmented market,
            ultimately will be more widely recognized and appropriately  valued.
            Natco is pursuing a sensible  growth strategy of  consolidating  the
            domestic market and penetrating foreign country markets.  The global
            imbalance  between oil and gas supply and demand,  the maturation of
            the  largest  oil and gas fields  around the world,  and  increasing
            environmental regulation drive Natco's organic growth opportunity.

            While the preferred  shares we own entitle us to a board seat, we do
            not intend to  exercise  that right.  We believe  Natco has a strong
            board and a serious commitment to good corporate governance,  making
            it unnecessary for us to take a seat at the table.

            We also have the  unilateral  right to convert  our  preferred  into
            common stock at any time.  We already have begun  conversation  with
            Natco  about what would be the most  mutually  beneficial  way to do
            this,  and we  expect  our  dialog to  resume  in May,  after  Natco
            announces its first quarter results. In our view,  conversion of all
            preferred  shares into common  would  benefit  Natco's  shareholders
            three ways:  Natco would have a conventional  public company balance
            sheet,  higher  earnings per share,  and greater  liquidity  for its
            stock.

Item 5.     Interest in Securities of the Issuer.

            (a,b) D3 Family Funds own and have sole voting and dispositive power
            over 1,780,401, 9.4% common shares of NTG.

            (c)



            Fund                                 Transaction Date     Shares Bought    Price
                                                                           
            D3 Family Fund, L.P.                         4/7/2006           804,365    25.36
            D3 Family Retirement Fund, L.P.              4/7/2006           274,361    25.36
            D3 Children's Fund, L.P.                     4/7/2006            71,153    25.36
            D3 Offshore Fund, L.P.                       4/7/2006           395,630    25.36
            D3 Family Bulldog Fund, L.P.                 4/7/2006           234,892    25.36


            (d)   N/A

            (e)   N/A

Item 6.     Contracts, Arrangements, Understandings or Relationships with
            Respect to Securities of the Issuer.

            N/A

Item 7.     Material to be Filed as Exhibits

            N/A


                                       7


                                    SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, each of the
undersigned certifies that the information set forth in the Statement is true,
complete and correct.

April 10, 2006                             /s/ DAVID NIERENBERG
------------------                         -------------------------------------
                                           David Nierenberg
                                           President
                                           Nierenberg Investment Management
                                           Company, Inc., the General Partner of
                                           The D3 Family Funds


                                       8