2019 M1x DEN
2016 M8+ St.Paul
2016 LM4- DEN
2016 W8+ Cornell

Experimental evaluation of four types of 2x hulls

Experimental evaluation of four types of 2x hulls

In November, a biomechanical comparison was conducted to determine the hull efficiency of several different boats. The method came in the form of two-boat side-by-side trials, which eliminated the effect of weather conditions, based on the assumption that the wind speed and direction were the same for both boats. Four boats (double sculls) were compared:

·       WinTech new 2019 for 90-105kg (WT new);

·       WinTech older 2012 for 90-105kg (WT old);

·       Empacher 2015 for 75-90kg (R34 C060);

·       Filippi 2015 for 72.5kg (F13CB41Z).

All hulls were of 27kg mass (WT new was 27.5kg); the first three doubles had aluminium stern-mounted riggers, with the Filippi having carbon bow-mounted riggers. The span was set to 1.60m in all boats. Four sets of identical sculling oars were used (Concept2 Skinny Smoothie-vortex blade), all with inboard 0.88m and length 2.885m. Four scullers took part in the study, which allowed two constant crews:


The main purpose of the experiment was to compare the new WinTech hull with three other boats: the experiment consisted of three rounds, each round containing two trials, where the WT new hull was rowed by Crew 1, and another other hull – by Crew 2. The crews then swapped boats over and repeated the trials. In this way, the evaluation of the hull didn’t depend on the weight and rowing technique of the crew. Six 1000m trials were made altogether with two boats running parallel in the following order:


Each 1000m trial was performed with incrementally increasing stroke rate with the following five sections:


The weather conditions were very good for this sort of experiment: practically no wind, with an occasional light cross-head wind and showers, a temperature of 10-11deg C, and flat clean water at the Eton Dorney Olympic rowing course.

All four boats were equipped with BioRowTel systems (32 data channels each, 14 bit, 25 Hz), which measured the following biomechanical variables:

•      2D oar angles in horizontal and vertical planes,

•      Handle force from all 4 oars,

•      Seat movement of each rower,

•      Rowing speed with a 10Hz GPS of each boat,

•      3D hull accelerations from each boat,

•      3D hull rotations with gyroscope.

Data samples were selected based on consistent stroke rate in each of five target sections (S01-S05) in each of the six trials. The data in each sample was averaged and typical patterns representing one stroke cycle were obtained. Additionally, the whole 1000m trial was selected as a data sample, and average patterns were obtained. The following main indicators were calculated from the typical patterns and then compared and analysed:

·       Average rowing (boat) speed Vav (m/s),

·       Stroke length – Total oar angle Atot (deg) swept over the stroke cycle,

·       Average force applied to the handle Fav (N) over the drive phase,

·       Work per stroke WpS (J) over the stroke cycle,

·       Rowing power P (W) over the cycle/sample;

·       Waste power lost in blade slippage through the water Pw, Propulsive power Pprop = P – Pw and Blade efficiency Ebl = Pprp / P;

·       Gross rowing drag factor DFg - a ratio of the power P to the cube of average speed Vav over each sample: DFg = P / Vav3;

·       Net rowing drag factor DFn - a ratio of Pprop to the average of cubes of the instantaneous boat velocity Vi over the stroke cycle: DFn = Prop / average(Vi3) .

The difference between the Drag Factors (RBN 2015/04) is that the net factor DFn doesn’t depend on energy losses in variation of the boat speed and blade slippage, but the gross factor DFg does.

Results

The stroke rate (spm) maintained was very close to the targets in each section (averages of both crews in the two paired trials are shown here and below):


The average rowing speed (m/s) was higher with WT new hull in all samples except the lowest stroke rate (WT old), and three lowest stroke rates with Empacher (marked in red):

 

Rowing power (W) was fairly consistent: 

 

The Gross Drag Factor DFg was lower in the WT new hull in all samples, except stroke rates 25 and 30 in comparison with the Empacher hull. 


The Net Drag Factor DFn was lower in the new WinTech hull in all samples at racing stroke rates 35 and 40spm, except 35spm with WT old: 

 

As the WT new hull was shorter, it was suspected it may have higher pitch amplitude (deg) over the stroke cycle. However, it was found that WT new hull had about 10% less pitch amplitude compared to WT old and only 2-3% more compared to other boats:

To verify the results of the experiment, the rowing indicators were averaged for each crew across the whole 1000m trial across all four boats they rowed in: 

 

As expected, the heavier crew 1 produced 16.7% more power and showed 2.8% higher rowing speed (Table 10). Both DFg and DFn were higher in Crew 1, as expected: DFg was 7.9% higher, but the difference in DFn was only 3.8%, which could be explained by the higher blade efficiency in Crew 2, and slightly higher boat velocity efficiency, i.e. lower variation of the instantaneous boat velocity over the stroke cycle.

It would be interesting to compare the ratio of DFn to rower mass m found in this study with the previous statistical data (RBN 2007/07), which relates it for doubles/pairs as:

DFn = 0.020972m + 2.931142                  (1)

With average crew mass 86.3kg for Crew 1 and 78.0kg for Crew 2 (Table 1), equation 1 gives DFn factors 4.74 and 4.57 respectively. This is slightly lower than in this study (Table 10), which may be related to weather conditions. However, the difference between DFn of the crews, 3.74%, based on equation 1 was very close to DFn 3.76% found in this study. This means every 1 kg of extra rower’s weight makes DFn 0.45% higher, and rowing speed 0.55s slower over 2km at the same rowing power. 

 

Conclusions

The results confirm that the new WinTech hull has a lower average drag factor compared to the older WinTech hull and hulls of other boat brands (Empacher and Filippi). Importantly, this advantage was the highest at the racing stroke rates 35-40spm

  

Based on gross Drag Factors found at 35 spm and the target rowing power for Olympic level rowers (475W (Table 11)), it can be concluded that the new WinTech hull gives about 1% (3.7s over 2k) advantage over the old WinTech hull, 0.11% (0.4s) advantage over Empacher, and about 2,5% (9.3s) advantage over Filippi hull.

Acknowledgements. Thanks to WinTech racing boats, Oarsport Ltd., and Reading university RC for organisation and kind support of this experiment.

©2019 Dr. Valery Kleshnev www.biorow.com