The most common and most likely answer to this crossword clue is the 7 plus 6 letter word BUTTOCK PLANES
B
U
T
T
O
C
K
P
L
A
N
E
S
Buddy explains!
The straight part
We know that "they help in measuring ships" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they help in measuring ships = buttock planes.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.