Well, looks like I was just showing off last week. When I got home a the weekend I looked in my big book (details ina moment) and found, after all the various general and particular kinds of yawl, this definition:

The yawl boat was carried in davits from the stern of coasting vessels. It was an open, rowed boat, later supplanted in fishing vessels by the dory.

So I stand corrected. If anyone is reaaly interested in knowing more about sea-craft and their rigging, I can reccommend this book as having everything you'll need to know, without the bother. It's written (in handwriting!) and illustrated by the author:
Published by John Murray Ltd, 50 Albermarle St., London W1X 4BD; ISBN 0 7195 3615 4
The illustrated encyclopedia of ships boats vessels and other water-borne craft "comprising an alphabetical directory of all types of craft past and present containing much discussion of the development of hulls and rigging together with mention of some of the most outstanding warships and commercial vessels suplemented by a nautical glossary, bibliography and index"

Steve