Proposal toward an Inexpensive Sail Performance Analysis Rig (SPAR)
by Dave Zeiger (http://Triloboats.com)
SPAR is an idea I’ve been kicking around for a while. I’ve come to realize that actual development would benefit from the collaborative efforts of a community of potential users, rather than from my own best guess at the needs involved.
The goal is to be able to perform the kind of rig analysis and comparison that typically requires access to a wind tunnel, or to two or more identical hulls. The former is too expensive, and the latter, while loads o’ fun, involves willing participants, considerable resources and introduces many extraneous variables which cloud results.
How about an apparatus home-built to a standard plan, which can mount scaled rigs and measure the angle and intensity of the thrust they generate?
The sketch shows a base of (framed) half sheet of thick, standard plywood. Struts (pipe?) extend from the base corners, converging on a simple gimbaled mount. A mast is mounted with a set length extending below the pin. Four standardized spring tensioners center the mast heel (at rest) over a grid affixed to the base. Windspeeds are measured by a standardized anemometer. The apparatus is fixed in place, oriented into the wind, while a rig is put through its paces.
So long as the system components are standardized, individual comparisons may be conducted under uniform constraints. Test sails of given area, for example. Likely, though, no constraint will be relevant to all investigations, so, as in any experiment, full documentation of purpose, set-up and conditions will be important for 'peer review'.
While on the 'rough and ready' side, such a platform should generate reasonably accurate data when conventions are observed.
Points for equipment standardization:
- Plan of apparatus
Windvane (for determining wind/orientation)
Corner tensioning devices (spring steel? surgical tubing? Bungee? fishing scales?)
- Conventions for reporting sheet and sail attachments
Categories for comparisons (e.g., area, max spar length, location of CE)
Grid(s) for data readings
Guidelines for use
Guidelines for extending comparisons and/or standards.
I’d think the whole thing could be built and equipped for little financial outlay, making it widely accessible.
Smaller scales are possible and may be preferable, using artificial wind. This would require a standard, variable speed fan, and guidelines for scaling.
My guess is that this larger size is sufficient to somewhat smooth 'transients'... small errors from variation of materials or construction, as well as the vagaries of natural wind.
Richard Dryden has suggested an option to record mast base deflections.
My best initial thought is to use thermal paper for the grids and a high IR laser pointer, mounted at the base of the mast, to 'print' a trace of mast base deflections directly onto the paper. This approach avoids any frictional interference from more typical spring-loaded marking devices.