"Motorsailers should be, in my opinion, hybrids among sailing boats and motor boats, having amplier volumes than those of a pure sailing boat, as well as a nice pilothouse conceived to steer and navigate (so not being being only a deck or raised saloon) to make them more livable in all kind of weathers; able to carry a generous cargo; have manegeable sails with a short crew (without the needing of very expensive hardware), but able to easily pull the boat as to develope hull speed under sail alone in a moderate breeze (force 4); have a propeller-engine combination allowing for a fuel efficient motoring and having power enough to at least reach hull speed under engine alone, as well as extra muscle to beat dead to winward in a Force 10 storm in protected waters".
http://www.xente.mundo-r.com/motorsailers/what_s_.htm