In short
- BoatUS sizes storm lines by boat length: 1/2 in to 25 ft, 5/8 in for 25 to 34 ft, 3/4 to 1 in above that.
- Storm lines should be at least as long as the boat, and longer is better, so the boat can rise on the surge.
- Tie high on the piling, put chafe guards on every contact point, and double lines where you can.
- Line counts, eye allowances and surge are planning assumptions, not engineering. Check pilings and cleats too.
On this page
This hurricane dock line calculator is for Cape Coral boat owners who tie up at a canal dock. Enter your boat length, the distance to your pilings and the surge you want to plan for, and it returns a line size range, the length to cut each line, how many lines to set and how many chafe guards to bring. It uses lengths and BoatUS sizing only, with no prices. It supports our Cape Coral hurricane and storm surge history.
En español: calculadora de amarras para huracán en un muelle de Cape Coral
Example: a 24 ft boat, pilings 25 ft away, 8 ft of surge, tie-off point halfway up the surge range (4 ft), 2 bow, 2 stern and 2 spring lines.
How the numbers are worked out
- Pick the diameter
Use the BoatUS size for your boat length as the low end. The high end is one size up, our planning assumption for exposed docks.
- Find the run
Take the straight distance to the piling and the height of the tie-off point. The line has to reach both at normal water and when the boat has risen with the surge.
- Add allowances
Add 4 ft for eyes and cleat wraps (our assumption), then compare with the boat length. Use whichever is longer.
- Round up and multiply
Round up to the next 5 ft, then set 6, 8 or 10 lines depending on boat length, with 2 chafe guards each.
BoatUS line sizes by boat length
| Boat length | BoatUS size | Calculator range |
|---|---|---|
| Up to 25 ft | 1/2 in | 1/2 to 5/8 in |
| 25 to 34 ft | 5/8 in | 5/8 to 3/4 in |
| Over 34 ft | 3/4 to 1 in | 3/4 to 1 in |
The BoatUS figures are the published sizes. The calculator adds one size up as the high end because BoatUS says larger diameter resists chafe and stretch, and West Marine says to size up for exposed locations with surge. Boats over about 50 ft or very heavy for their length need a rigger or marine surveyor, not a rule of thumb.
Assumptions behind the line count
| Boat length | Lines | Split |
|---|---|---|
| Up to 25 ft | 6 | 2 bow, 2 stern, 2 spring |
| 26 to 40 ft | 8 | 3 bow, 3 stern, 2 spring |
| Over 40 ft | 10 | 4 bow, 4 stern, 2 spring |
These counts are our planning assumption. BoatUS describes the idea (a boat held in a web of lines, doubled up where possible) but not a count. A canal boat is usually tied to both sides. Other assumptions: 4 ft of line for eyes and cleat wraps, 2 chafe guards per line, and lengths rounded up to the next 5 ft.
Line length each for a 24 ft boat at 8 ft of surge
| Run to piling | Length each | Set by |
|---|---|---|
| 5 ft | 25 ft | the boat-length rule |
| 10 ft | 25 ft | the boat-length rule |
| 25 ft | 30 ft | the run to the piling |
| 40 ft | 45 ft | the run to the piling |
Short runs hit the boat-length rule: storm lines at least as long as the boat. Only long runs make the distance the deciding number.
The lines are only as good as what they tie to. BoatUS warns that old or badly deteriorated pilings should not be relied on and that cleats need real backing. Pilings must also stand above the surge. Have a licensed marine contractor check pilings, cap and cleats before the season. See how many pilings a dock needs.
Hurricane line checklist
- Chafe guards on every contact point. Chocks, pilings and rails wear through line fast under storm loads.
- Tie high on the piling. Lines tied low can slide up or chafe as the water rises.
- Eye on the piling for outer pilings. You can adjust from the boat. For the others put the eye on the boat.
- Double lines where possible and spread them evenly, bow toward the canal entrance.
- Remove windage. Take off canvas, biminis and anything loose on deck.
- Do it before the watch. Boats are hard to move once the storm is close. Lifts do not make a boat safe in surge.
Questions to ask a marine contractor
- Are my pilings tall enough? Ask how high they stand above normal high tide and against the surge you plan for.
- What will the cleats and cap hold? Ask whether they are through-bolted and backed.
- Can you add pilings or taller ones? See seawall and dock permits in Cape Coral.
- What does the lift or the dock add to the load? See the boat lift capacity calculator.
- How do I choose a contractor? See choosing a seawall contractor.
Next step
This is a planning estimate, not an engineering design or an insurance requirement. We are a referral service, not a contractor; we connect visitors with independent local providers. Use the form below to ask independent marine and seawall contractors to check your pilings, cap and cleats.
Sources
- BoatUS Hurricane Preparation Guide: storm lines at least as long as the boat, 1/2 in to 25 ft, 5/8 in to 34 ft, 3/4 to 1 in larger, chafe protectors, tying high, the spider-web layout, piling condition.
- UF/IFAS Extension and Florida Sea Grant, Secure Your Boat for a Hurricane: the same line sizes and longer lines for surge.
- West Marine, How to Choose the Right Dock Lines: the 1/8 in per 9 ft rule and sizing up for surge.
Frequently asked questions
What size dock lines should I use for a hurricane?
BoatUS suggests 1/2 inch line for boats up to 25 feet, 5/8 inch for 25 to 34 feet, and 3/4 to 1 inch for larger boats, with the lines doubled up where possible. This calculator shows that size as the low end and one size up as the high end, because larger line resists chafe and stretch better. Check the line maker's strength chart and your marina or insurer's rules.
How long should hurricane dock lines be?
As long as you can make them. BoatUS gives a rule of thumb that storm lines should be at least as long as the boat, and longer lines let the boat rise on the surge without hitting the dock or pilings. The calculator takes the longer of the boat-length rule and the run to your piling plus allowances.
Where should I tie storm lines on a piling?
High enough that the line does not slide off the top or get chafed as the water rises, and with the eye on the piling for hard-to-reach outer pilings so you can adjust from the boat. Pilings have to stand taller than the surge you plan for, and badly deteriorated pilings should not be trusted.
How many dock lines do I need in a hurricane?
BoatUS describes a boat held like a spider in the center of a web of lines. How many lines is a judgment call; this tool uses 6 for boats to 25 feet, 8 to 40 feet and 10 above that, as our planning assumption. A canal boat is usually tied to both sides.
Do I need chafe guards on hurricane lines?
Yes. BoatUS says chafe protectors must be on any part of the line that could rub on chocks, pulpits or pilings. The calculator counts two guards per line, one at the boat and one at the piling, as a planning assumption.
How much storm surge should I plan for in Cape Coral?
There is no single number. Surge depends on the storm's track, size and strength. BoatUS notes that even a Category 2 can bring 6 to 8 feet or more, and that surge of 10 feet or more is possible. Pick a figure from current National Hurricane Center forecasts and your own risk level.