Spacing mode
A 24-foot zone with eight-foot maximum spacing requires four fenders at three evenly spaced intervals.
Boat Fender Quantity & Spacing Planner
Created by: Daniel Hayes
Last updated:
Solve whole fender count or even spacing across an entered protected hull zone and calculate one- or two-side inventory.
Boat Fender Quantity & Spacing Planner
BoatingSolve whole fender count or even spacing across an entered protected hull zone and calculate one- or two-side inventory.
What is a Boat Fender Quantity & Spacing Planner?
A Boat Fender Quantity & Spacing Planner places a whole number of fenders evenly across an entered protected hull zone. It can solve count from maximum desired spacing or solve spacing from an entered count.
The protected zone begins after the bow setback and ends before the stern setback. Even mathematical spacing is a starting layout; hull shape, rubbing strake, topsides flare, pilings, dock height, tide, wake, rafting, and contact points determine actual placement.
Fender diameter and load rating must come from manufacturer guidance appropriate to boat size, displacement, freeboard, expected energy, berth, and conditions. The calculator records an entered diameter but does not recommend one.
Severe weather, exposed docks, locks, rafting, rough pilings, and unattended mooring can require different equipment, boards, covers, vertical adjustment, redundancy, or relocation.
How the Boat Fender Quantity & Spacing Planner Works
Protected length minus bow and stern setbacks creates the contact zone.
In spacing mode, count is rounded upward so even spacing does not exceed the entered target. In count mode, entered count is rounded to a whole number with at least two endpoints.
Positions include both endpoints of the protected zone. Two-side mode doubles inventory without changing positions on each side.
spacing = protected zone ÷ (whole count−1)
Input Guide
- Measure the hull length that can actually contact the dock or neighbour.
- Set setbacks around curved bow/stern areas that cannot retain a fender.
- Enter manufacturer-guided diameter separately from spacing.
- Choose one or two protected sides according to the scenario.
Example Scenarios
Two-side inventory
Four fenders per side becomes eight total for a two-side or raft-up inventory scenario.
Changed contact zone
Moving setbacks inward shortens the zone and changes positions even when count stays constant.
How to Read the Results
- Whole count is reported per protected side.
- Even centre-to-centre spacing is calculated across the zone.
- Position table measures from the entered bow reference.
- Total inventory doubles only when two-side mode is selected.
Common Applications
- Dockside placement
- Raft-up inventory
- Fender-board planning inputs
- Transient marina preparation
- Coverage-gap review
Practical Tips
- Adjust fender height for dock and tide.
- Place extra protection at likely contact points.
- Inspect lines, eyes, covers, and inflation.
- Reposition after wind, current, tide, or neighbouring-vessel changes.
Limitations and Assumptions
- No diameter, rating, pressure, attachment strength, or severe-weather protection is recommended.
- Hull curvature and dock geometry are simplified to one line.
- Spacing does not guarantee continuous protection.
Frequently Asked Questions
How many fenders does my boat need?
Use manufacturer and seamanship guidance. This calculator only solves the geometry from the count or spacing you enter.
Why use at least two fenders?
The even-spacing model needs endpoints, but real berths often require more protection.
Should positions be measured from the bow?
Use one documented reference consistently, then adjust for actual contact geometry.
Does tide change fender count?
It may change height, orientation, and contact point more than count. Model and monitor the berth.
Can I use this for rafting?
It can double inventory, but displacement differences, motion, freeboard, spreader bars, and crew coordination require separate planning.
Are larger fenders always better?
No. Fit, pressure, storage, attachment, hull contact, and manufacturer rating all matter.
Sources and References
- Use the current product label, safety data sheet, vessel and engine manuals, marina instructions, and written supplier or yard quotation.
- U.S. Environmental Protection Agency marina and recreational boating pollution guidance, accessed July 16, 2026; https://www.epa.gov/vessels-marinas-and-ports.
- American Boat & Yacht Council standards information, accessed July 16, 2026; https://abycinc.org/.
- U.S. Coast Guard Boating Safety resources, accessed July 16, 2026; https://www.uscgboating.org/.