  Weather routing methods are essential for planning routes for commercial
shipping and recreational craft. This paper provides a methodology for
quantifying the significance of numerical error and performance model
uncertainty on the predictions returned from a weather routing algorithm. The
numerical error of the routing algorithm is estimated by solving the optimum
path over different discretizations of the environment. The uncertainty
associated with the performance model is linearly varied in order to quantify
its significance. The methodology is applied to a sailing craft routing
problem: the prediction of the voyaging time for an ethnographic voyaging canoe
across long distance voyages in Polynesia. We find that the average numerical
error is $0.396\%$, corresponding to $1.05$ hours for an average voyage length
of $266.40$ hours. An uncertainty level of $2.5 \%$ in the performance model is
seen to correspond to a standard deviation of $\pm 2.41-3.08\%$ of the voyaging
time. These results illustrate the significance of considering the influence of
numerical error and performance uncertainty when performing a weather routing
study.
