Dear Tabby,
I have heard of a double bobbin boat shuttle, but why would I want to use that instead of just winding 2 threads together on a bobbin?
—Not Sure Nori
Dear Not Sure,
You can certainly wind 2 ends together on a bobbin, say if you want to blend colors together or double 2 fine threads for a thicker weft yarn. If you have done this, then you know that even though these ends have been wound together, they never unwind quite right. Often you get a little loop at the selvedge because the yarns are not feeding off the bobbin evenly. Not what you want.
When you use a double bobbin boat shuttle, you wind each yarn on separate bobbins that unwind separately and feed off more smoothly as single yarns (rather than a doubled yarn). The Schacht Double Bobbin Boat Shuttle has 2 separate bobbin rods so that each bobbin can be changed out separately. Schacht Double Bobbin Boat Shuttles come in slim and regular widths and use 4ʺ bobbins.
–Tabby