Log size, taper, sweep: why small and crooked logs lose yield

The small-log penalty

Yield depends strongly on log size. In a simulation of Swedish spruce and pine logs (Fredriksson et al. 2015; Europe), the reference cant-sawing process gave a planed-board yield of 43.7 percent of log volume for all logs but only 35.8 percent for the 2,214 logs with top diameter under 185 mm. The difference is 43.7 - 35.8 = 7.9 points, about eight points.

This matters for benchmarking: a mill that buys smaller logs will show lower yield with identical sawing quality.

Taper

Taper is the decrease in diameter along the log. A cylinder yields boards of full log length from one cut, but a tapered log loses wood because the boards must fit inside the narrow end. Steele (1984, USDA Forest Products Laboratory, North America) notes that taper lowers recovery and argues that log length has no direct effect; the apparent length effect comes from the greater taper of longer logs.

A BOKU conference presentation on Austrian coniferous sawlogs (Blinzer and Treml 2022; Europe) gives a worked case. The product is a glued-laminated timber lamella 43 x 146 x 4,000 mm, sawn by a three-stem pattern:

Notice that the sawn volume stays constant at 0.075 m3 while the log gets bigger. The log volume grows with the mid diameter, but the product cannot, because it must fit inside the narrow end. Yield falls purely from the denominator.

Sweep and crook

Sweep is a gradual bend; crook a sharp local bend. Straight saw lines cut across a curved log, giving shorter pieces, more edging waste and lower grade. Open evidence is mainly North American and mainly hardwood:

• Hamner and colleagues (2006, United States, 134 small-diameter mixed hardwood logs) compared straight and curve sawing. Overall yield was 62 percent for curve sawing and 58 percent for straight. With slight sweep (0 to 15 percent deduction) both gave 62 percent. For the 16 to 30 percent group the figures were 54 (straight) and 64 (curve); above 30 percent, 48 and 60. The authors conclude yield can be as much as 10 to 12 percent higher with curve sawing sweepy hardwood logs.
• Maness and Donald (1994, 834 spruce-pine-fir logs, interior British Columbia) found that sweep and cross-section eccentricity both reduced value recovery, and that rotating each log into its best position gave significant benefit, more strongly linked to eccentricity than to sweep.

For European spruce and pine, no open source quantifies sweep loss in the same way. A Skogforsk report (Sweden) points out that assuming a circular cross section would overestimate lumber recovery, which is why stem models include ellipticity.

Worked example: reading the sweep table

For the group with 16 to 30 percent sweep deduction: curve minus straight = 64 - 54 = 10 points. For the group above 30 percent: 60 - 48 = 12 points. For slight sweep: 62 - 62 = 0. The 10 to 12 range quoted by the authors comes from the two sweepy groups, and the benefit is zero for straight logs. The curve-sawing investment therefore pays only if a meaningful share of logs is sweepy.

Checklist: log sorting for yield

• Sort by top diameter class and report yield per class, never as one pooled number.
• Record taper or at least log length and mid versus top diameter.
• Record sweep class for hardwood or crooked logs.
• Compare patterns within a class, not across classes.
• Keep a note on how the log was measured (see lesson 1).

Key points

• Small logs recover less: 43.7 percent (all logs) against 35.8 percent (top under 185 mm) in a Swedish simulation, a gap of about eight points.
• Taper reduces yield because boards must fit the narrow end; in the Austrian lamella case yield fell from 49 to 35 percent as taper grew from 5 to 25 mm.
• Curve sawing helped sweepy small hardwood logs (10 to 12 points in the US sample) and made no difference for slightly swept logs.
• Rotation into the best position raises value more for eccentric than for swept logs (British Columbia study).
• European spruce and pine sweep losses have no open quantification in the articles; do not assume the hardwood numbers transfer.