MJ is all over it....he speaks the truth! Preach it brother MJ, PREACH IT!! *;D
There are 3 basic types of glass (cloth, not resin) that are most common.
I call them cloth, mat and weave or woven mat.
Cloth: I use for small repairs that are not as structurally important, but appearance is. I also use as a top layer on mat to help smooth things over and make fairing easier. It turns corners well.
Cloth
The next step up is "mat". This is probably the most commonly used for boat repairs. It is heavier than cloth and very good for structurally stressed areas, like what you are doing...transoms, stringers, basic building of fish boxes, etc...It will turns corners, but unless you have rounded things off pretty smoothly will leave an air pocket behind it.
MAC, make sure you use a coarse disc grinder or belt sander to give your transom a nice round contour whee the corners are or it will "air pocket" on the edges.
Mat
The last and most heavy duty is the "woven mat" or "weave" as I call it. At first glance it looks similar to the cloth we first mentioned. It is in pattern, but not at all in weight...its 10x heavier at least.
This stuff is STOUT. It is very thick and takes a TON of resin to fully saturate it. It is for structural areas that are VERY important. Alot of boat building (including Wellcraft) use this type to bond stringers to hulls. It is hard to turn corners with unless it is a very gradual sweep. You also don't want to use this stuff anywhere that aesthetics matter because it takes alot of work to fair it out.
Weave
Hope this helps!! *;D