Distribute values equally

Question

Each ship docks at a port. If too much ships arrive at a single port the crew gets overwhelmed.

How to return a result set so a port does not get overloaded? One possible solution and set based result would be:

SELECT shipnr, portnr FROM (
SELECT shipnr, portnr, DENSE_RANK() OVER(PARTITION BY portnr ORDER BY shipnr) AS N FROM ships
) AS a
ORDER BY N, portnr

However, this is not what I am looking for since at the end we get three ships arriving at F. Below is an example I did by hand of the result set I am looking for:

1   A
6   B
10  C
11  D
13  E
16  F
20  F
2   A
7   B
12  D
14  E
17  F
21  F
3   A
8   B
15  E
18  F
22  F
4   A
9   B
19  F
5   A

Since there are many port F it should return in groups of 2 so that specific port is not overwhelmed.

Answer 1

Ok, here is my revised approach, using NTILE. It actually gives roughly the same as your proposed output.

declare @ships table
(
shipnr int,
portnr varchar(50)
);

insert into @ships (shipnr, portnr)
values (1, 'A'), (2, 'A'), (3, 'A'), (4, 'A'), (5, 'A'), (6, 'B'), (7, 'B'), (8, 'B'), (9, 'B'), (10, 'C'),
(11,'D'), (12, 'D'), (13, 'E'), (14, 'E'), (15, 'E'), (16, 'F'), (17, 'F'), (18, 'F'), (19, 'F'), (20, 'F') , (21, 'F') , (22, 'F') 

DECLARE @PortNumber INT = (SELECT COUNT(DISTINCT portnr) FROM @ships) - 1

;WITH CTE_Ships AS
    (
    SELECT S.shipnr
        , S.portnr
        , NT = NTILE(@PortNumber) OVER (PARTITION BY portnr ORDER BY (portnr))
    FROM @ships AS S
    )

SELECT * FROM CTE_Ships ORDER BY NT, portnr