BOAT MOTION
Boat motion & replay
See how the hull moves through the stroke and compare that movement with oar effort.
Fix the phone to the boat
Mount the phone rigidly to the hull or a structure that moves with it. Keep it in the same position for the row. A loose phone, pocket or sliding seat also records movement that does not come from the hull.
Keep phone motion, GPS and oar files from the same session together. The analysis aligns their timing so the curves can be compared.
The hull reference
In the cloud report, the default Local gravity reference measures pitch and set relative to a slowly changing neutral plane. It averages gravity directions over neighbouring complete strokes in a 20-second window. This preserves movement within each stroke while removing slow changes in the baseline.
The phone’s mounting angle is accounted for, but its forward direction must match the boat. Enter the known bow direction and phone-to-pin offsets when available; otherwise direction is estimated from the rowing pattern.
The neutral plane is a rowing reference. Slow trim changes are removed along with slow reference drift. Live app values can differ from final analysis near the beginning, end or gaps because live processing has less neighbouring data.
Change the reference under the server’s boat settings, then re-analyze. The Local gravity window can be adjusted from 5–120 seconds. The iPhone app can also use a saved boat-level reference when configured.
Read the motion curves
| Curve | Meaning |
|---|---|
| Forward acceleration · m/s² | Along-boat acceleration after gravity removal. The default filter is 8 Hz. |
| Fused boat speed · m/s | Ground speed combining the GPS reference with faster changes from phone acceleration. |
| Relative boat speed · m/s | Phone-derived speed variation around the stroke-cycle mean. Zero means the cycle mean. |
| Relative boat distance · cm | Forward or backward position relative to steady motion through the cycle, derived from relative speed. |
| Set / roll · ° | Hull roll relative to the neutral plane. Positive is port up. |
| Pitch · ° | Bow-up or bow-down rotation relative to the neutral plane. Positive is bow up. |
| Heave · cm | Estimated rise and fall around the stroke-cycle mean. |
| Turn · ° | Turn from catch or relative to the chosen course. Positive native turn is port/left. |
Choose these under Boat. Advanced includes the Core choices plus pitch, heave and turning. Relative curves describe movement within the stroke; fused speed retains its absolute ground-speed value.
Cycle-based motion estimates limit drift and can remove slower movements. A wave or other movement that does not repeat with the stroke may be represented less well.
Compare stroke metrics
Set range, pitch range and heave excursion describe movement size. Distance check is the full-cycle range of relative boat distance; Speed check is the range of relative boat speed. Both use phone motion.
Energy-loss check uses the complete fused GPS and phone-motion speed cycle. It compares the integral of squared speed with steady travel at the same mean speed. The percentage is a speed-variation index, not a direct measurement of energy lost.
Piece, group and workout summaries average the valid individual values. Compare strokes at similar rate and effort, and look at both the size of a movement and when it occurs.
Boat metric definitions and units →Course and turning
Drive and recovery turn show how the hull rotates during each phase. Use the turning control to compare native hull turn or turn relative to the selected course.
Straight course uses the piece’s endpoint line. Curved course follows a fitted gradual path. Course offset and estimated extra distance require sufficient GPS coverage.
Steering, wind, course bends and unequal oar action can produce similar movements. Compare the route and oar curves before interpreting a turn.
Replay the boat and oar together
Open Boat & Oar, then choose Averaged curves or Individual strokes. The plots and animation follow the same selection.
Use Layout to place curves beside or above the animation, or show either alone. Click a curve to move the animation with the pointer; click again or press Escape to freeze it.
Play a selected sequence to see how the movement repeats. Adjust the camera and zoom, and use 1× motion exaggeration when judging size. Save the view under Setups to reuse it on another workout.
The hull and blade shapes are models driven by the available motion and geometry. A displayed opposite oar may be mirrored from one recording.
Playback, saved views and downloads →Waterline and blade trails
The replay uses a supplied physical waterline reference when available, or an estimated waterline from the model. Waterline offset lets you explore a different position. A saved squared-float reference and a physical waterline height are separate inputs.
Enable Show blade slip trail to see the outer blade trace and a dashed inner trace. The inner trace follows the submerged blade edge nearest the handle. If the shaft enters the water, it follows the wet shaft to the surface.
The trails depend on oar depth, feathering, hull motion and the model’s waterline. They illustrate the assumed geometry; the water texture does not measure current.
Slip
Slip compares modelled oar movement with boat speed. It depends on synchronized data, oar dimensions, angle placement and assumptions about the water.
Average drive slip is a signed longitudinal speed: positive means rearward, and negative values are retained. Missing intervals are excluded. Treat it as an experimental comparison under the stated model assumptions.
Slip speed is different from catch miss, which measures angular travel before sustained load.