Shell Hull Friction

© 2001 Atkinsopht (03/24/02)

ROWING calculates shell hull friction resistance in either of two ways:

1. A fixed resistance factor, Kw (Kw= R /Vs^2), is entered in input where R is the resistance of a fully loaded hull in force units and Vs the average shell speed near the value expected in the result. Kw remains constant throughout the calculation and is not subject to variation in displacement, fluid density or viscosity with change in water composition or temperature.

2. Resistance coefficient, Cf, is calculated from the *ITTC-'57 Standard Skin Friction Line for each instant of the shell speed variation using the instantaneous value of Reynolds' number, itself subject to variation in fluid density and viscosity with change in composition and temperature. To Cf is added an allowance, (Cw= 0.0002; Scragg & Nelson), for wave resistance and one, (Cv= 1.0 +(0.0097 *2(1/2 bow angle)), for form resistance.

With correct knowledge of hull form characteristics this method permits automatic adjustment for change in hull displacement (wetted surface) with weight thus making possible comparisons of boats with differing live and dead weight loads.

This, however, depends in turn upon accurate knowledge of the hull block, Cb, and prismatic, Cp, coefficients. I have good values for Cb and Cp only for eights (from Scragg and Nelson); and pretty good values for fours and quads (estimated from Lazauskas); but nothing at all (not even good values of Kw) for doubles, pairs, or singles.

Over time huge efforts have been put into the design of racing shells. The field must now be considered virtually mature in that there will be no significant advances in hull design within the current scope of the rules of rowing. However much of the detail of this research seems not generally available to the rowing world.

You can help modeling efforts if you can steer me to good data in this regard. (Sadly, letters to many shell manufacturers did not even elicit an acknowledgement of the letter's receipt).

* The International Towing Tank Conference.

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