Here are two yagis for 6m, one 1 wavelength long with a gain of 9.5dBd, and the other a 2.1wl model with a gain of 11.8dBd.
The design rules follow the proven method used for the 70cm OPT yagis. Solutions for element lengths and distances have to satisfy a criterion based on an optimum current slope and constant phase delay in the array of parasitic elements. If a solution satisfies this criterion, high gain per boomlength together with an excellent pattern can be expected.
Another constraint in the development of these Yagis was the requirement for 50 ohm input impedance. This allows simple feed structures like a folded dipole, which is electrically efficient and mechanically rugged.
During the final verification with NECII software it was found that these antennas have excellent stacking capability, with high stacking gains in excess of 5.8dB (2x) and minimum mutual detuning.
If properly built, these Yagis are excellent performers. They are cheap to build, and can be made from easily available materials.
Model, elements

Length  Gain [dBd] 
Front/ 
Beamwidth 
Stacking 2x2  
[m] 
[wl] 
E [deg] 
H [deg] 
E [m] 
H [m] 
Gain [dB] 

BV61.0 
5.850  1.0  9.40  >19  45.6  54.0  7.7  6.6  5.9 
BV62.1 10 
12.90  2.1  11.90  >25  36  40  9.4  8.6  5.9 
These lengths are correct.
Element 
Position [mm] 
Boom diam [mm] 
Length [mm] 
R  0  30  2966 
DE  922  30  2850 
D1  1232  30  2734 
D2  2411  30  2734 
D3  4329  30  2702 
D4  5850  30  2670 
These lengths are correct.
Element 
Position [mm] 
Boom diam [mm] 
Length [mm] 
R  0  30  2924 
DE  1209  30  2844 
D1  1706  30  2724 
D2  2712  35  2705 
D3  4012  40  2686 
D4  5536  40  2656 
D5  7222  40  2636 
D6  9040  40  2616 
D7  10970  35  2595 
D8  12900  30  2584 