These are planning-level estimates only. Actual performance in Caribbean sand can vary a lot because the bottom may be loose carbonate sand, dense packed sand, shell hash, hardpan, coral rubble, or thin sand over rock. For a real tension-leg mooring, the anchors should eventually be sized and proof-tested by a marine/geotechnical engineer.
| Anchor | Depth | Approx. Turns | Likely Installation Torque | Estimated Dinghy Driving Time | Comment |
|---|---|---|---|---|---|
| 6 inch single helix | 7 ft into sand | About 30 to 40 turns | Roughly 300 to 1,000 ft-lb; peaks maybe 1,500 ft-lb | About 20 to 60 minutes of actual turning | A 10 hp dinghy with a 10 ft lever should usually be enough in ordinary sand. |
| 12 inch single helix | 11 ft into sand | About 50 to 70 turns | Roughly 1,500 to 4,000 ft-lb; dense sand/hard layers can exceed 6,000 ft-lb | About 45 to 120 minutes of actual turning if it keeps advancing | A 10 ft lever may be marginal. A 15 to 20 ft lever or powered hydraulic drive would be better. |
Including setup, aligning the anchor, attaching the lever, checking verticality, moving the dinghy, and proof-loading afterward, a realistic field time might be:
At low rpm, the required mechanical horsepower is surprisingly small. For example:
The limiting factors are not raw horsepower. They are:
Torque is approximately:
Torque = Dinghy thrust × lever length
| Dinghy Pull | 10 ft Lever | 15 ft Lever | 20 ft Lever |
|---|---|---|---|
| 100 lbf | 1,000 ft-lb | 1,500 ft-lb | 2,000 ft-lb |
| 150 lbf | 1,500 ft-lb | 2,250 ft-lb | 3,000 ft-lb |
| 200 lbf | 2,000 ft-lb | 3,000 ft-lb | 4,000 ft-lb |
| 250 lbf | 2,500 ft-lb | 3,750 ft-lb | 5,000 ft-lb |
For the 6 inch helix, a 10 ft lever is probably reasonable. For the 12 inch helix, a 10 ft lever may work in loose-to-medium sand, but I would expect it to be unreliable in denser sand. A 15 to 20 ft lever would be much more comfortable.
The best practical lever is a strong tubular beam with a reinforced drive head at the anchor end. Aluminum square tube is attractive because it is much lighter than steel and easier to handle from a dinghy.
| Use Case | Suggested Lever | Approx. Bare Tube Weight | Approx. Finished Weight With Reinforced Head |
|---|---|---|---|
| 6 inch helix, 7 ft embedment | 10 to 12 ft of 6061-T6 aluminum square tube, about 3 in × 3 in × 1/4 in wall | About 32 to 39 lb | About 45 to 60 lb |
| 6 inch helix, steel alternative | 10 to 12 ft of galvanized steel pipe, roughly 2.5 inch Schedule 80 | About 77 to 92 lb | About 90 to 115 lb |
| 12 inch helix, preferred manual/dinghy lever | 15 to 20 ft of 6061-T6 aluminum square tube, about 4 in × 4 in × 1/4 in wall | About 66 to 88 lb | About 90 to 125 lb |
| 12 inch helix, steel alternative | 15 to 20 ft of galvanized steel pipe, roughly 3 inch Schedule 80 | About 155 to 205 lb | About 180 to 240 lb |
For field handling, the aluminum version is much more attractive. A 20 ft steel bar is heavy and awkward in a small boat.
Yes. The bending moment is highest at the anchor end, so the first few feet of the lever should be reinforced. A good design would use:
Using the anchor eye directly as the torque point is not ideal unless the eye was specifically designed for installation torque. Many eyes are intended for mooring load, not repeated torsional installation load. A purpose-built drive head that grabs the anchor shaft or drive lugs is better.
A good larger-anchor lever could be built like this:
If you make it in two pieces for transport, avoid putting the joint right near the anchor. The joint should be far enough outboard that the bending moment is lower, or it should be heavily sleeved.
A tension-leg system can create very high cyclic vertical loads in waves. A single 6 inch or 12 inch helix in shallow sand may or may not provide enough uplift capacity, especially after repeated loading. For prototype testing in sheltered shallow water, this may be acceptable if loads are low and proof-tested. For an inhabited seastead, it needs proper anchor sizing, redundancy, fatigue checks, and environmental permitting.