“Spoil the ship for a ha'porth of tar”
What it means
To ruin something valuable by refusing to spend a trivial amount on finishing it properly.
Where it comes from
A ha'porth is a halfpennyworth, the smallest quantity of anything that could be bought with a coin. The force of the phrase depends on the gap between that and whatever is being lost.
How sure are we?
Almost certainly nothing to do with boats. The earliest recorded forms speak of losing a hog for a ha'porth of tar, hog being a dialect word for a yearling sheep, and tar being what shepherds smeared on cuts and shearing nicks to keep the flies off. Ship appears to be a corruption of sheep in dialect pronunciation, and once the word had shifted, the maritime explanation — caulking a hull and losing the vessel for want of a little pitch — arrived to make sense of it. Both readings work. Only one of them is old.