Helical Mooring Screw – Installation Time & Lever‑Bar Design

This page gives a quick‑reference estimate for installing a single‑helix mooring screw with a dinghy‑pulled lever bar driven by a 10 hp outboard. It covers:

Torque needed to screw the anchor into Caribbean sand
Required pulling force and resulting rotation speed
Approximate installation times for a 6‑in and a 12‑in helix
Recommended lever‑bar material, geometry, weight, and tip reinforcement

1. Assumptions & Basic Parameters

Parameter	Value	Comments
Outboard power	10 hp ≈ 7.5 kW	Typical small‑boat outboard
Thrust available (low‑speed)	≈ 800 N (≈180 lbf)	Conservative for a 10 hp outboard moving a dinghy in a circle
Pull‑rope length (lever arm)	10 ft (3.05 m) for 6‑in helix 12 ft (3.66 m) for 12‑in helix	Long enough to keep the dinghy clear of the anchor
Helix pitch (single‑start)	0.33 ft (4 in) for 6‑in dia. 0.50 ft (6 in) for 12‑in dia.	Typical for these sizes
Soil (Caribbean sand)	γ ≈ 18 kN m⁻³, φ ≈ 35°, c ≈ 0	Medium dense to dense sand
Efficiency of rope‑lever system	≈ 70 %	Accounts for friction, rope stretch, & motion losses

2. Required Installation Torque

For a single‑helix anchor in cohesionless sand the ultimate torsional resistance is often approximated by an empirical “torque‑capacity” factor:

T_req ≈ k · D · S
where
D = helix diameter (ft)
S = embedment depth (ft)
k ≈ 150 lb·ft / ft² for dense sand (conservative 200 lb·ft / ft² can be used)

Applying the formula gives:

Helix	D (ft)	S (ft)	T_req (ft·lb)	T_req (Nm)
6‑in (0.5 ft) dia.	0.5	7	≈ 1 100 ft·lb	≈ 1 490 Nm
12‑in (1 ft) dia.	1.0	11	≈ 2 500 ft·lb	≈ 3 390 Nm

3. Pulling Force Needed at the Lever End

The effective torque after losses is

T_eff = η·F·L → F = T_eff / (η·L)

With η ≈ 0.70 and L = lever arm length:

Helix	Lever arm L	Required pulling force F
6‑in	10 ft (3.05 m)	≈ 340 N (≈ 77 lbf)
12‑in	12 ft (3.66 m)	≈ 680 N (≈ 153 lbf)

Both forces are well within the ≈ 800 N thrust the outboard can develop at low speed, so the system is mechanically feasible.

4. Rotation Speed & Installation Time

The dinghy’s forward speed translates into a tangential speed at the lever end:

v_dinghy ≈ 2.5 m s⁻¹ (≈ 5 knots) → ω = v / L

Thus the theoretical revolutions per minute (rpm) are:

Helix	L (m)	ω (rad s⁻¹)	RPM (≈ v/L)
6‑in	3.05	≈ 0.82 rad s⁻¹	≈ 7.8 rpm
12‑in	3.66	≈ 0.68 rad s⁻¹	≈ 6.5 rpm

Number of revolutions needed = depth / pitch:

Helix	Depth (ft)	Pitch (ft)	Revolutions
6‑in	7	0.33	≈ 21
12‑in	11	0.50	≈ 22

Combining rpm and revolutions gives the minimum theoretical time:

Helix	Theoretical time @ 7.8 rpm (6‑in) / @ 6.5 rpm (12‑in)	Realistic time (allowing for higher soil resistance, rope slippage, & start‑up)
6‑in	≈ 2.7 min	3 – 5 min
12‑in	≈ 3.4 min	4 – 6 min

Practical tip: If the dinghy can maintain a higher cruising speed (up to 3 m s⁻¹) the times drop to ≈ 2 min for the 6‑in and ≈ 3 min for the 12‑in. In rough seas or loose sand, add 30 % to the estimates.

5. Lever‑Bar Design

5.1 Material

High‑strength alloy steel (AISI 4140 or 4340) heat‑treated to ≥ 900 MPa yield strength.
If corrosion resistance is a concern, use 316  stainless steel (though heavier for the same strength).

5.2 Geometry

Helix size	Recommended bar length	Bar diameter / tube spec	Tip reinforcement
6‑in helix	10 ft (3.05 m)	2 in (50 mm) solid or 2.5 in OD × 0.25 in wall tube	Solid 6‑in long forged tip that fits the mooring‑eye bore
12‑in helix	12 ft (3.66 m)	2.5 in OD × 0.25 in wall tube (≈ 33 kg) or 2.5 in solid rod (≈ 75 kg)	Solid 8‑in long tip, larger bore, and optional hardened sleeve for the eye

5.3 Weight

Bar configuration	Volume (m³)	Mass (kg)	Mass (lb)
2 in solid, 10 ft	≈ 0.0062 m³	≈ 48 kg	≈ 106 lb
2.5 in OD × 0.25 in wall tube, 10 ft	≈ 0.0035 m³	≈ 27 kg	≈ 60 lb
2.5 in solid, 12 ft	≈ 0.0116 m³	≈ 91 kg	≈ 200 lb
2.5 in OD × 0.25 in wall tube, 12 ft	≈ 0.0042 m³	≈ 33 kg	≈ 73 lb

Recommendation: For the 6‑in helix a 10‑ft tube (≈ 27 kg) is light enough for two people to lift and maneuver, while still providing the needed torque. For the 12‑in helix a 12‑ft tube (≈ 33 kg) gives the best strength‑to‑weight ratio; a solid rod is unnecessarily heavy.

5.4 Tip‑Reinforcement (Eye‑End) Details

The tip that goes into the mooring‑eye should be solid and at least 1½ × the bar diameter for the first 6–8 in.
Option: weld a hard‑faced sleeve (e.g., 410 stainless or a wear‑resistant alloy) over the tip to protect against galling and wear.
If you want a quick‑change system, use a bushing‑style hitch pin that slides into the eye and is secured with a cotter pin. The pin can be made from the same alloy and heat‑treated.

5.5 Handle & Quick‑Disconnect

Opposite the tip, add a rubber‑coated grip (≈ 12 in long) for the operator on the dinghy.
A simple quick‑release shackle (rated ≥ 1 ton) can be used to attach the rope to the bar, allowing fast removal after installation.

6. Quick‑Connect / Re‑Use Tips

Mark the bar with a depth‑stop collar (e.g., a welded ring) that hits the deck when the helix is fully driven – this prevents over‑insertion.
Store the bars in a protected rack on the seastead. A lightweight PVC sleeve can keep the tip free from sand and corrosion.

7. Summary of Results

Helix Diameter	Depth Required	Estimated Time (ideal)	Estimated Time (practical)	Suggested Lever Bar	Approx. Bar Weight
6 in (0.5 ft)	7 ft	≈ 2.7 min	3 – 5 min	10 ft, 2.5 in OD tube, solid tip	≈ 27 kg (≈ 60 lb)
12 in (1 ft)	11 ft	≈ 3.4 min	4 – 6 min	12 ft, 2.5 in OD tube, solid tip	≈ 33 kg (≈ 73 lb)

8. Putting It All Together – Example HTML Snippet

You can copy the table below directly into a web page:

<table>
  <tr>
    <th>Helix</th>
    <th>Depth</th>
    <th>Est. Time (ideal)</th>
    <th>Est. Time (realistic)</th>
    <th>Lever Bar</th>
    <th>Weight</th>
  </tr>
  <tr>
    <td>6‑in</td>
    <td>7 ft</td>
    <td>≈ 2.7 min</td>
    <td>3 – 5 min</td>
    <td>10 ft, 2.5 in OD tube, solid tip</td>
    <td>≈ 27 kg (≈ 60 lb)</td>
  </tr>
  <tr>
    <td>12‑in</td>
    <td>11 ft</td>
    <td>≈ 3.4 min</td>
    <td>4 – 6 min</td>
    <td>12 ft, 2.5 in OD tube, solid tip</td>
    <td>≈ 33 kg (≈ 73 lb)</td>
  </tr>
</table>

9. Safety & Operational Notes

Always use a Safety line from the dinghy to a fixed point on the seastead in case the lever bar slips.
Check the eye of the mooring screw for wear or deformation before each use; a damaged eye will dramatically increase the required torque.
If the outboard shows signs of propeller overload ( RPM drop, excessive vibration), pause for a minute to let the motor cool – repeated over‑loading can shorten its life.
For repeated installations, consider a small electric winch or a torque‑multiplier gearbox to reduce manual effort, but for the prototype the lever‑bar method is perfectly adequate.

Good luck with the prototype – the method described above should allow a small crew to embed a 6‑in helix in under five minutes and a 12‑in helix in under six minutes, using only a standard 10 hp outboard and a modestly heavy lever bar.