The following is a short compilation of my thoughts on the attributes of two different strongback setups. One is an external strongback (Ext) and the other is an internal strongback (Int).
I have a slight bias for the external as this is the setup that I am most comfortable with.
I would be interested in hearing others thoughts about these comparisons and the reasoning behind their thoughts.
The external strongback here is a 6"x6"x16' boxbeam built from 1/2" ply.
The internal strongback is a 2"x4"x slightly-shorter-than-kayak built from 1/2" ply.

1. Time to build: Ext - 1.2x, Int - 1.0x

The external strongback requires only slightly more time to construct - they both use the same number of pieces of plywood (although different sized pieces) to build - this translates to the same number of rips made on the table saw. They both use about the same amount of glue and nails/screws to put together.

2. Weight: Ext - Light, Int - Very light

The external (24" circumference) will weigh about three times that of the internal (12" circumference) when taking into account the additional weight of the plywood risers and blocking required for the external.

3. Expense to build: Ext - $25, Int - $10

4. Ease of construction: Ext - Easy, Int - Easy

5. Mounted forms: Ext 1-piece, Int - 1-piece

6. Direct Alignment: Ext - Yes, Int - Yes

External and internal alignment is direct with no need for a level or flat floor.

7. Floor space required: Ext - Kayaks footprint, Int - Kayaks footprint

Floor space required is identical - they both need supports with or without wheels.

8. Integral workbench: Ext - Yes - 6"x16', Int - Yes*

External acts as a 16' x 6" workbench on which to place misc. items such as glue bottles, stapler, block plane, etc. and with simple outriggers supports all of the strips. *Internal needs platform/s built in order to act as bench.

9. Ease of storage when not in use: Ext - Easy, Int - Easy

Both are easily stored in the rafters or on the wall when not in use

10. Portability: Ext - easy, Int - easy

Both can be easily fitted with wheels and moved around the shop as needed

11. Strength: Ext - Very strong, Int - Moderate

External has significantly more strength

12. Stiffness: Ext - Very stiff, Int - Moderate

External has significantly more stiffness

13. Ease of form setup: Ext - Very easy, Int - Easy

External references off of plywood risers for initial setup. Internal references off of end forms.

14. Ease of form removal: Ext - Very easy, Int - Easy

External allows easy removal of forms, especially if any hot glue has been used to hold strips to the forms - by accessing through the cockpit cutout you can simply tap with a stick and remove each form one at a time. Internal requires prying to remove deck/hull

15. Misc.: Ext - Requires 1/2" plywood risers and 2"x2" blocks to secure forms to strongback, Int - Requires holes to be cut in forms. Requires blocking and wedges for securing forms to strongback.

Thanks,
Joe Greenley