The design and operation of neutrally buoyant Boats that attempt to tr
ack the three-dimensional motion of water parcels in highly turbulent
regions of the ocean, such as the upper mixed layer, are described. Th
ese floats differ from previous floats by combining high drag, a compr
essibility that nearly matches that of seawater, rapid (1 Hz) sampling
, and short-range, high-precision acoustic tracking. Examples of float
data are shown with the twin goals of demonstrating the utility of th
e Boats and estimating the accuracy to which they are ''Lagrangian.''
The analysis indicates that these Boats follow the motion of the surro
unding water to better than 0.01 m s(-1) under most circumstances. Bot
h the mt buoyancy of the Boat and its finite size contribute to the er
ror. The Boat's buoyancy is controlled by making its compressibility v
ery close to that of seawater, by making its drag large, by reducing a
ir pockets and bubbles on the Boat, and by carefully controlling varia
tions in the float's mass and volume between deployments. The Boat acc
urately follows that part of the velocity field with scales much large
r than its own size (1 m) but does not follow components with scales s
maller than itself. A model of this dependence is presented for turbul
ent flows. Several unique measurements are possible with these Boots.
They measure vertical displacement using pressure and therefore accura
tely filter out the vertical velocity of surface waves, since linear s
urface waves have no pressure fluctuations along Lagrangian trajectori
es. Accurate measurements of vertical velocity in the oceanic mixed la
yer are therefore possible. This, combined with temperature, can be us
ed to measure vertical heat flux. A compass measures the spin rate of
the Boat and thus the vertical vorticity. In fully turbulent Bows with
outer scales much larger than the Boat size, the spectra of both vert
ical velocity and vorticity scale with epsilon (the turbulent kinetic
energy dissipation) over a wide range of epsilon values, thus allowing
epsilon to be measured. The floats can, in principle. therefore measu
re many important properties of turbulent Bows even in the presence of
surface waves.