Given (each leg/float):
r = 1.95 ftDisplaced volume per leg (cylinder):
V_leg = π r² L_immersed = π (1.95 ft)² (16 ft)
r² = 3.8025 ft², so
V_leg ≈ π * 3.8025 * 16 ≈ 191.12 ft³
Total displaced volume (3 legs):
V_total ≈ 3 * 191.12 = 573.36 ft³
| Quantity | Result |
|---|---|
| Total displaced volume |
573.36 ft³
= 16.24 m³ (using 1 ft³ = 0.0283168 m³) |
| Displaced water mass (seawater) |
Using ρ ≈ 1025 kg/m³:
m ≈ 1025 * 16.24 ≈ 16,640 kg = 36,700 lbm (approx) |
| Displaced water weight (seawater) |
Using 64.0 lb/ft³ (seawater rule-of-thumb):
W ≈ 573.36 * 64.0 ≈ 36,695 lbf |
Note: “Mass displaced” is reported above in kg (and lbm). “Buoyant force” equals the weight of displaced water (lbf or N).
Froude scaling basics (same gravity, same fluid):
λ = L_model / L_full = 1/6λλ³ = (1/6)³ = 1/216√λ| Item | Full scale | 1/6 scale (inches) |
|---|---|---|
| Triangle side length (frame) | 60 ft | 120 in |
| Leg cylinder diameter | 3.9 ft = 46.8 in | 7.8 in |
| Leg cylinder radius | 1.95 ft = 23.4 in | 3.9 in |
| Leg total length | 24 ft = 288 in | 48 in |
| Leg immersed length (along the leg) | 16 ft = 192 in | 32 in |
| Leg angle | 45° | 45° |
V_model = V_full / 216 ≈ 573.36 / 216 ≈ 2.654 ft³
| Quantity | 1/6 scale target |
|---|---|
| Displaced volume | 2.654 ft³ (≈ 0.0751 m³) |
| Target all-up model weight (for same 2/3 immersion) |
In seawater (64.0 lb/ft³): W ≈ 2.654 * 64.0 ≈ 169.9 lb (So target model mass ≈ 77 kg.) |
Important: This assumes the model floats at the same immersed fraction (2/3 of each cylinder) and that the model’s geometry (including leg angle) is the scaled version of the full-scale design.
You described: (a) Two cables from the bottom of each leg to the adjacent corners, plus (b) a redundant loop connecting the bottoms of all legs.
Assumptions used to compute lengths (state these explicitly in your build):
Horizontal offset of a leg bottom from its corner (because 24 ft at 45°):
h = 24 cos 45° ≈ 16.97 ft
Vertical drop of leg bottom below the corner:
v = 24 sin 45° ≈ 16.97 ft
| Cable | Full scale length | 1/6 scale length |
|---|---|---|
| Bottom of one leg to an adjacent corner (each of the 2 cables per leg) | ≈ 77.07 ft | ≈ 12.85 ft = 154.2 in |
| Bottom-to-bottom distance between adjacent legs (one side of bottom loop) | ≈ 89.40 ft | ≈ 14.90 ft = 178.8 in |
| Total loop length around all three leg bottoms (no extra for splices/knots) | ≈ 268.2 ft | ≈ 44.7 ft = 536.4 in |
Practical note: add extra length for terminations, knots/splices, and any desired slack/pretension. For a small-scale test, +5% to +15% extra is common depending on your hardware.
Under Froude similarity, wave height scales linearly with λ.
With λ = 1/6:
| Full-scale wave height | 1/6 scale wave height |
|---|---|
| 3 ft | 0.5 ft = 6 in |
| 5 ft | 0.833 ft = 10 in |
| 8 ft | 1.333 ft = 16 in |
There are two separate goals:
H ≈ 0.78 d, so to reduce “forced” breaking:
d > H / 0.78 ≈ 1.28 H.
d > 1.28 * 16 in ≈ 20.5 in
d > L/2 (depth greater than half the wavelength).
To use this you need a wave period T (not just height). In deep water:
L ≈ 1.56 T² (meters) or L ≈ 5.12 T² (feet).
T ≈ 8 s, then
L_full ≈ 5.12 * 8² ≈ 328 ft,
so the model wavelength is L_model ≈ 328/6 ≈ 54.7 ft.
Deep-water would then require d_model > ~27 ft (which many bays cannot provide).
Practical recommendation for Sandy Hill Bay tests: use the deepest available area with the most “clean” incoming swell (least refracted, least affected by bottom), and record the local depth. If you tell me your expected dominant wave period(s) at the test location (or the forecast buoy period), I can compute a depth target more specifically.
Tips: set a known sample rate (e.g., 50–200 Hz), log timestamps, and keep the phone rigidly mounted to the structure. Also note that phones measure “proper acceleration” (includes gravity), so you may want both raw and linear-acceleration channels if available.
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