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Sliding Riggers

© 2009 Atkinsopht (09/06/09)

In the ROWING model I have found a simple way to look at the rowing performance of fixed and sliding riggers. It is extreme in that it merely exchanges the masses of heavy rower and lighter rigger mechanism without considering obvious subtlties which are not possible to take into account by this means.

Rather than to alter the program code to produce a new and modified model it suffices to accomplish a similar result by altering the input data file. One need only redistribute the appropriate masses between rower and boat (live-weight and dead-weight) and to make sure that the results are calculated at equal total rower power output.

In the example offered the rower's weight is 90kg and that of the single scull 14kg, for a total of 104kg not counting the oars (about 3kg). A set of sliding riggers is assumed to add about 11kg live-weight to the system. So, in the case of the sliding rigger model subtract 11kg from the rower's weight (90 -11 =79kg) and add it to the weight of the shell (79 +14 =93kg); and assign 11kg to the weight of the rower.

The oarhandle and slide geometry and excursions are assumed to be about the same.

That this mass exchange cannot be the whole story is that, while the rower's hips now become part of the shell's deadweight, the mass of the rower's torso does not. The live weight contribution from the rower's torso is reduced with sliding riggers, but by an uncertain amount. The weight distribution assumed here is further an over simplification because there is a contribution to the weight of the rigger assembly from the "attached" portion of the rower's feet and lower limbs. These unconsidered effects reduce the sliding rigger advantage.

The ROWING model result is as follows:

                              Fixed riggers:        Sliding riggers:

    Rower weight, kg           90                    11
    Shell weight, kg           14                    93
    Oars weight, kg             3                     3
    Total weight, kg          107                   107 
    Peak oar handle pull, N   774                   774

    Total rower power, W      497                   497

    Average shell speed, m/s    4.43                  4.77
    Time 2000m, s             452                   420
    Rating, 1/s                24.3                  32.2
    Stroke period, s/cycle      2.47                  1.87
    Shell advance/sweep, m     13.4                  13.5
    Speed variation amp., m/s   1.6                   0.9
    System mechanical eff.      0.62                  0.75
    Oar blade efficiency        0.72                  0.78
    Absolute blade slip, m      0.74                  0.57

    Rower momentum work:
        External, J/stroke    135                    27
        Internal, J/stroke    220 (lost)             38 (lost)
The saving lies in the work the rower must do in order to overcome the momenta in the system. One-hundred and eight (135 -27)J/stroke is taken from the work done to advance the shell but that is nullified by the overall momentum saving and the increase in blade efficiency (inefficient blade slip is reduced owing to a shorter immersion time at the higher rating). Note the reduction in the amplitude of the shell speed variation.

VanHolst has made a new estimate based on the same simple mass exchange getting a positive but much smaller benefit; partly because he compares at equal rower oarlock power (which excludes internal momentum losses) rather than at equal rower total power. It seems clear that some benefit accrues to sliding riggers but better estimates will have to await the writing of specific modeling code or of difficult on-the-water evaluations.

Had my comparison been made at equal rating (24.3 1/s) the new shell speed would have been only 4.31m/s (worse) but the rower would have been loafing along but with a power output of only 373W (vs. the comparable 497W), ready to pick it up to 32.2 1/s at the same original energy expenditure. This illustrates the absolute necessity of making all such comparisons at equal rower total power.

Boats with sliding riggers can be found at Virus Boats and at Ron Rantilla's FrontRower site. The FrontRower's arrangement is somewhat different from the pure sliding rigger, but I think it confers a similar advantage.

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