Anchor Chain Stoppers and Chain Locks: Sizing to Your Chain

Quick Answer

Work out the job first, then the chain diameter. A deck-mounted stopper fixes the chain and holds it where you want it. A chain lock uses a locking ring to grip the chain and take the pull off the windlass. A bow roller only guides the chain and does not hold load. All three are sized by the chain link diameter, and the published stainless ranges cover 6–8 mm and 10–12 mm, while DIN766 anchor chain in the same catalogue runs from 3 mm to 9 mm and vertical windlasses list chain sizes from 6/7 mm upward. Match the device to the chain you actually run, then to the windlass gypsy.

Real-World Explanation

Chain is the one part of the anchoring system where the numbers are unusually well published, and that makes it easier to get the interface right than it is with most deck fittings. The catalogue chain table gives a diameter, an inner link length, an inner width and a published breaking load for each size: 3 mm chain is listed at 4.1 kN and 9 mm chain at 40 kN, with 6 mm and 8 mm at 16 kN and 32 kN. Those figures are the manufacturer’s published values for the chain, and they are the reference point when a stopper, a lock or a gypsy has to be matched to it.

The reason the chain diameter matters so much is that a stopper does not grip by friction over a length. It holds at one link, so the part has to sit around a link of the right size. That is why the published stainless stoppers are offered as 6–8 mm and 10–12 mm versions rather than as a single adjustable model. Those are ranges that cover two common link sizes, and within the range the fit is a compromise rather than a precise match. If your chain is 8 mm and the device is listed as 6–8 mm, ask which link size the drawing was cut for. A device that sits loosely on the chain will still stop it, but it will wear at the contact point and can allow the link to sit at an angle.

Stopper, Lock and Roller Are Three Different Jobs

The stopper’s job is to take the chain and hold it. The published deck stopper is described as mounting to the deck or hull, and it is a fixed fitting, so the load path runs from the chain into the deck structure. That makes the structure under the fitting the deciding factor: a stopper concentrates load at one point, and a deck that is sound enough for a cleat may not be sound enough for a stopper without a backing plate or a local reinforcement.

The chain lock is a different device. The published version uses a locking ring and is described as lightweight and portable, placed where it is needed without additional adjustment, and intended to protect the chain from damage at the bottom. It grips the chain rather than being bolted down, so it does not spread load into the deck in the same way; instead it changes where the chain is held. For a boat where the windlass is the only thing between the anchor and the seabed, that matters: a windlass is a pulling device, and the published tables separate a maximum load figure from a maximum pull figure, which are not the same number. Leaving the boat riding on the windlass motor puts the boat’s motion into a gearbox that was specified for retrieving chain.

The bow roller is the third part in the chain, and it is the one most often confused with the other two. A roller guides the chain over the stem and reduces wear at the bow; it does not stop the chain and it does not hold the boat. The published roller range is worth checking for groove width against your chain, but a roller does not replace a stopper in the system.

The Gypsy Is Cut for One Chain Size

A vertical windlass is specified by the chain it will handle, and the published tables list chain sizes such as 6/7, 8/10, 8/10/12 and 10/12/13 mm against motor power, working current, maximum load and maximum pull. A gypsy listed for 8/10 mm is cut for those two sizes, so a 9 mm chain in the same catalogue is not a match for it, even though 9 mm sits inside a 6–8 mm stopper’s upper range and inside a 10–12 mm stopper’s lower edge. This is where buyers get caught: the chain, the gypsy and the stopper are each sized individually, and a system assembled from three separate orders can end up with no common number.

Where the Stopper Sits Changes What It Does

A stopper mounted well aft of the bow roller holds the chain on deck but leaves a length of chain between the stopper and the roller that can jump the groove in a seaway. A stopper mounted close to the roller holds the chain nearer the lead point, but it needs structure underneath at a place where decks are often thin and curved. Neither position is wrong; they do different things, and the one that suits your boat depends on the deck build and on where the chain is when the anchor is stowed. Decide that on the boat, with the anchor in its stowed position, rather than from a catalogue drawing.

Practical Insights from Marine Hardware Supply

1. Ask for the link size the drawing was cut for

A 6–8 mm stopper covers two chain sizes, and the drawing is made for one of them. Suppliers can usually confirm which, and that answer is worth more than the range on the listing.

2. Keep the chain’s published figures with the boat’s papers

The chain table gives breaking load per size. Recording which chain is fitted, with its diameter and published breaking load, means a later replacement stopper or gypsy can be checked against a known part rather than against a guess.

3. Buy the chain, the gypsy and the stopper as one set

Three separate orders, each correct on its own, is the most common way to end up with a mismatch. Give the supplier the chain diameter you run and ask for the stopper and the windlass gypsy that match it.

Buying Checklist

- Chain diameter: the actual link diameter, measured with a caliper on a clean link.

- Chain standard: the designation on the chain, which sets the link proportions the fittings are made for.

- Stopper range: the chain sizes the stopper model covers, and which one the drawing was cut for.

- Mounting position: the point on deck where the chain sits when the anchor is stowed.

- Structure below: solid laminate, cored panel or reinforcement, and the backing the stopper needs.

- Lead to the roller: the run of chain between the stopper and the bow roller, and whether it stays in the groove.

- Windlass gypsy: the chain size the gypsy is cut for, checked against the chain.

- Windlass duty: the published maximum load and maximum pull figures, read as different numbers.

- Load at rest: what holds the chain when the boat is anchored and the windlass is off.

- Records: chain size, published breaking load and part codes for the fitted set.

Decision Guide

Choose a Deck-Mounted Stopper If

- The chain needs to be held at a fixed point on deck, close to the lead over the bow.

- There is sound structure under the fitting, or room to add a backing plate.

Choose a Chain Lock If

- The chain has to be gripped where the windlass should not take the load.

- You want a device that can be placed where it is needed rather than bolted down.

Choose Both If

- The boat has a windlass and the chain is left under load for long periods.

- The stopper holds the chain on deck and the lock takes the pull off the windlass.

Review the Windlass Setup If

- The gypsy is cut for a different chain size from the one fitted.

- The published maximum load and maximum pull have been treated as the same figure.

Defer Ordering If

- The chain diameter has not been measured, or the chain designation is unknown.

- The deck structure under the intended stopper position has not been checked.

The rule that matters most: size every part of the system to the chain link diameter, and confirm which link size the stopper’s drawing was made for.

Need Help Choosing or Sourcing?

Send the chain diameter and designation, the windlass model and the position you intend to fit the stopper, and contact Alastin Marine for a match against the anchor chain range, a sample or bulk supply. The published stainless options include the chain stopper in 6–8 mm and 10–12 mm versions and the chain lock stopper with locking ring, with DIN766 anchor chain from 3 mm to 9 mm and vertical windlasses listed with their chain sizes. For a stopper cut to a specific chain or a matched chain-and-gypsy set, use the OEM/ODM enquiry and send the chain data with the drawing.