On the water
Anchor rode and scope calculator
Quick answer
Anchoring in 15 feet at 7 to 1 scope needs about 119 feet of rode, counting a 2 foot bow roller. Add 3 feet of tide still to come and it becomes 140 feet. Leaving the tide out is exactly why an anchor that held at slack water drags at three in the morning.
Scope is the ratio of rode paid out to the vertical distance from the bow roller to the bottom, and that is depth plus bow height plus whatever tide is still coming. Measuring it from the depth sounder reading alone is the mistake, because a sounder reads from the transducer down rather than from the bow roller.
The reason scope matters is angle. An anchor holds by burying itself, and it only buries when the pull on the shank is close to horizontal. Short scope lifts the shank and the anchor skips rather than setting, which is what dragging actually is.
Run your own numbers
In 15 feet with 3 feet of tide to come, 7 to 1 needs 140 feet of rode. A 100 foot rode gives you only 5.0 to 1.
| Figure | Value |
|---|---|
| Vertical distance | 20 ft |
| Rode needed at 7 to 1 | 140 ft |
| Without counting the tide | 119 ft |
| At 5 to 1, watching it | 100 ft |
| At 10 to 1, heavy weather | 200 ft |
| Scope achieved with 100 ft | 5.0 to 1 |
Shown with the example figures above. Change any field to run your own numbers.
The working
Rule of thumbvertical distance = depth + bow height + tide still to come rode = vertical distance x scope ratio 15 + 2 + 3 = 20 ft at 7 to 1 = 140 ft of rode
Scope ratios are seamanship convention. Bottom type, wind, current and the anchor design all matter alongside them. Boating and fishing convention rather than a published standard. Taught everywhere, useful, and not a specification. Nothing enforces it and no body publishes it.
Anchors, rode and holding gear
Ground tackle is the gear you find out about when the wind comes up at night. A few feet of chain at the anchor end is the cheapest improvement to holding on any small boat.
Power-Pole Pro Series II shallow water anchor, 4 ft
$1,649.99The hydraulic Power-Pole that the flats boat world runs on, and considerably quieter into the water than a spike driven by a screw.
Check price on Amazon
Power-Pole Micro Anchor
$649.99The smallest and cheapest genuine shallow water anchor, light enough for a kayak or a skiff and the entry point to the category.
Best for: Kayaks, skiffs and small bay boats in under about eight feet
Check price on Amazon
Lewmar anchor rode, 5 ft chain and 100 ft rope with shackle
$207.95A spliced windlass rode from a ground tackle manufacturer, which is the part of the system nobody should be improvising.
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Power-Pole Micro battery pack and charger
$199.99The Micro needs its own power, and on a kayak that means this pack rather than running a cable to a main battery.
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Better Boat 13 lb anchor kit with 100 ft rope and 8 ft chain
$125.99 Published specsAnchor, chain and rode in one purchase, with enough chain to actually set the fluke rather than lift it.
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Seachoice hot dipped galvanized fluke anchor, 13S
$48.69A galvanised fluke anchor from a marine brand at under fifty dollars, which is the sensible default for sand and mud.
Best for: Sand and mud bottoms, which is most inland and coastal fishing water
Check price on AmazonEvery figure shown is the manufacturer own published specification. The rating on the unit you receive is what governs, and the capacity plate on your own boat governs everything above it.
The scope conventions
Seven to one is the figure for overnight or unattended. All chain can hold at three to one, because the weight of the chain does part of what scope otherwise has to.
Rule of thumb Boating and fishing convention rather than a published standard. Taught everywhere, useful, and not a specification. Nothing enforces it and no body publishes it.
| Scope | When it is used | Rode in 15 ft | With 3 ft of tide |
|---|---|---|---|
| 3 to 1 | All chain, settled conditions, somebody aboard | 51 ft | 60 ft |
| 5 to 1 | Daytime, light wind, watching it | 85 ft | 100 ft |
| 7 to 1 | Overnight or unattended, rope rode | 119 ft | 140 ft |
| 10 to 1 | Heavy weather | 170 ft | 200 ft |
Scope ratios are seamanship convention rather than a published standard. Nothing enforces them, no body publishes them, and bottom type and conditions matter alongside the ratio.
The three things people leave out
- Bow height. The rode leaves the boat at the bow roller, not at the waterline, and on most small boats that is 2 to 4 feet. It matters proportionally more in shallow water, where it can be a fifth of the total vertical distance.
- Tide still to come. Anchoring at low water and sitting through a rising tide adds several feet of depth without anybody touching anything, and the scope you set quietly shrinks.
- The sounder offset. A depth sounder reads from the transducer down, not from the surface, so in shallow water it is understating the vertical distance by the draft.
An anchor that held perfectly in the afternoon and dragged at three in the morning has usually not moved. The water has got deeper.
Why chain matters as much as length
A length of chain between the anchor and the rope does two things rope cannot. It keeps the pull on the anchor shank horizontal at the seabed, which is what lets the anchor bury, and it absorbs shock loading as the boat surges rather than snatching the anchor out.
That is why an all chain rode can hold at 3 to 1 where a rope rode wants 7 to 1: the weight of the chain is doing part of what scope does. On a small boat with a mostly rope rode, a few feet of chain at the anchor end is the cheapest improvement to holding you can make.
A shallow water anchor is a different tool
A pole style shallow water anchor spikes the boat to the bottom in seconds and needs no scope calculation at all, which is why they have taken over the shallow fishing world. It also holds the boat in one spot rather than on an arc, which for fishing is usually what you want.
Pole anchors top out somewhere between eight and fifteen feet depending on the model, and neither they nor a GPS anchor lock on a trolling motor replaces real ground tackle for anywhere you intend to leave the boat. Most boats that fish shallow carry both.
Frequently asked questions
How much anchor rode do I need?
Seven times the vertical distance from your bow roller to the bottom for overnight or unattended anchoring. In 15 feet of water with a 2 foot bow roller that is 119 feet. Add whatever tide is still to come before working it out: three feet of rise in 15 feet of water takes 7 to 1 scope down to about 5.7 to 1 without anybody touching the rode.
What is scope and why does it matter?
Scope is the ratio of rode paid out to the vertical distance from the bow roller to the bottom. It matters because an anchor holds by burying itself, and it only buries when the pull on its shank is close to horizontal. Short scope lifts the shank and the anchor skips along the bottom instead of setting, which is what dragging actually is.
Is 7 to 1 really necessary?
For overnight, unattended or any weather, yes, and every seamanship text repeats it. For a lunch hook in settled conditions with somebody aboard watching, 5 to 1 is normal and 3 to 1 works with an all chain rode. None of these is a published standard: they are convention, and bottom type and conditions matter alongside them.
Does chain change how much rode I need?
Substantially. Chain weight holds the pull on the anchor shank horizontal at the seabed and absorbs the shock of the boat surging, which is part of what scope otherwise has to do. That is why an all chain rode can hold at 3 to 1 where rope wants 7 to 1, and why a few feet of chain at the anchor end is the cheapest holding improvement on a small boat.
Why did my anchor hold and then drag later?
Almost always the tide. Anchoring at low water and sitting through a rising tide increases the vertical distance without anybody touching the rode, so the scope you carefully set quietly shrinks. Wind shifting the boat onto a different lie, so the anchor has to reset in a direction it was not pulled into, is the other common cause.
Should I use a shallow water anchor instead?
In shallow water a pole anchor is faster, quieter and holds the boat in one spot rather than on an arc, which for fishing is usually what you want. It does not replace real ground tackle for anywhere you intend to leave the boat, or for depths beyond its reach, which is somewhere between eight and fifteen feet depending on the model.
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Researched, not professional advice. This page is compiled from published engineering and regulatory figures, manufacturer specifications and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are boating convention rather than published standards, and they are labelled that way wherever they appear. Marine electrical work is not house wiring. Use tinned, finely stranded marine cable, size it for voltage drop and for the ABYC E-11 ampacity table rather than one or the other, and put overcurrent protection within seven inches of the battery positive terminal, because everything between the post and the fuse is unprotected. A lithium battery needs a charger with a lithium profile. Never load a boat past its capacity plate, and remember that canoes and kayaks carry no federal capacity plate at all, so their stated capacity is the manufacturer own figure. Wear the life jacket, and treat early season water as the hazard it is: cold water immersion takes your breath and then your hands long before it takes your core.