Complex sentence: It is similar to the ubiquitous Mirror being a gunter rigged pram designed for a crew of two, except that it is a little smaller and usually rigged only with main and spinnaker.
Split into simpler sentences: It is similar to the ubiquitous Mirror, being a gunter-rigged pram designed for a crew of two. However, it's a little smaller and is usually rigged only with main and spinnaker.