Just a quick general woodworking question:
 Flush trim router bits and template guides with spiral cut bits seem to do the same thing in terms of routing out shapes using a pattern. Template guides however, cut an offset line, just a bit outside (or inside if you're cutting a cavity) the original pattern's lines. My question is, why would you choose to use a template guide system over a flush trim bit? Are there certain situations where the template guide and bit are better?
Will
I've been using a flush trimming bit for some time now on a new build. It's lapstrake and I cut all the planking with a "trimmer" bit. This is the type of bit that has the bearing on the bottom and it cuts the same diameter as the bearing. If you use the top mounted bearing or a collar, you can use an inside template and you can plunge down if desired. My building method requires I follow some "line off" battens and place the planking stock over them, so the bearing is on the bottom, following the batten, while the cutting edge is above, sawing the planks to the same shape as the line off battens.
Where collars come to shine is with repeatable patterns. A template can be used over and over, with precise results. If the template is precise, you can make parts fit inside each other (like a dove tail) with a precision that's hard to beat.
There are countless ways and reasons to use these various bit types, though the most common will be the trimmers.
Paul,
this post reminded me of your clinker build and the method shaping the planks. I am planning to use something similar and would like to compare my ideas with yours. Can you remind me of the post where you described your method.
This is my batten trimming system. Image 15 shows the planking blank trimmed a little over size from the template, maybe an 1/8" (3 mm). Image 14 shows what happens when I run the flush trim bit along the batten. On this jig I failed to subtract the batten thickness from the molds, so the trimmer bearing hits the station molds every two feet, but you could make the molds so the battens lay on top, rather than being "let in" like shown. This would let the bit run past each mold without having to come back and clean the planks up. Lapstrake need a lot of "dressing" to make them work. All the edges need to be eased, you have to cut gains at each end, scarfs or slices, etc. all need to be done, before you can hang the plank for the last time. The advantage of this is the planking is nearly ready to paint when it does get hung and most of the work is done on the workbench, not the boat, which is really handy.