  A recent Maxey--Riley theory for \sarg raft motion, which models a raft as a
network of elastically interacting finite-size, buoyant particles, predicts the
carrying flow velocity to be given by the weighted sum of the water and air
velocities $(1-\alpha)\mathbf{v} + \alpha \mathbf w$. The theory provides a
closed formula for parameter $\alpha$, referred to as \emph{windage}, depending
on water-to-particle-density ratio or buoyancy ($\delta$). From a series of
laboratory experiments in an air--water stream flume facility under controlled
conditions, we estimate $\alpha$ ranging from 0.02 to 0.96\pct. On average, our
windage estimates can be up to 9 times smaller than considered in conventional
\emph{Sargassum} raft transport modeling, wherein it is customary to add a
fraction of $\mathbf w$ to $\mathbf{v}$ chosen in an ad-hoc piecemeal manner.
Using the formula provided by the Maxey--Riley theory, we estimate $\delta$
ranging from 1.00 to 1.49. This is consistent with direct $\delta$
measurements, ranging from 0.9 to 1.25, which provide support for our $\alpha$
estimation.
