Derivation of "ketch"
Ketch was a "catch" or fishing boat (ketch from Middle English cache, from cacchen, "to catch"). The mizzen is bigger than seen on a yawl in order to hold the bow (front) of a boat toward the wind and oncoming waves. The mainsail at the front of the boat would have been dropped and the mizzen trimmed tight on the centreline. Set up this way most boats will point directly into the wind in a reliable way. It is also possible to ease the mizzen slightly to allow the boat to move slowly forward.
In a fishing boat this attitude allows the nets to be handled without the boat becoming "broadsides" to the waves allowing them to break over the sides of the boat. Fishnets can then be handled without putting the boat at risk.
For enough sail area to propel a fishing boat the mizzen mast has to move forward toward the middle of the boat which allows its sail to be bigger without upsetting the sail balance or distribution.
A "Ketch Rig" is simply the rig that matches the function of a "Ketch" or "Catch" or fishing boat.
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