A retractable centreboard that slides out to act as a keel(noun)
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Use "daggerboard" in a sentence
"In extreme winds it is necessary on some boards to change the daggerboard or centerboard."
"Jon and Ben managed to roll over the top at the start of the run, but overstood the finish and let Andy and Tom through, until a broken daggerboard cost them the race."
"They reported that they dragged the long line for a while before finally cutting it, bringing nearly 10 metres on board and suspect there is some minor damage to the daggerboard which may require repair."