For those considering a straight stitch machine for marine canvas, some information on types of machines might be helpful. Almost all folks who sew marine canvas for a living use a rotaty-hook, compound walking-foot machines (aka, compound-feed, unison-feed, triple-feed).
A compound walking-foot has both drop-feed (bottom-feed) and needle-feed (top-feed) with an alternating presser-foot (walking-foot).
The following links might be helpful, and do click on "show more" on the YouTube video.
Sewing machine - Wikipedia, the free encyclopedia
The Sailrite LS and LSZ machines are drop-feed, walking-foot machines.
Canvas can be sewn with what are sometimes called semi-industrial home sewing machines and are drop-feed machines. Another limitation of these machines is clearance under the foot, and the number of layers of material can mount-up quickly. Just try hemming the inside flat-felled seam of a pair of Levis to get the idea which is fewer layers of material than is usually encountered sewing marine canvas.