Anchors need to be pulled laterally along the sea floor to set and hold well on the bottom. This requires considerable weight of the anchor rode to produce the catenary curve within the specified scope.
When using all rope rodes, more scope is needed to achieve the same lateral pull. A common rule of thumb is 5:1 scope for all chain, 7:1 scope for all rope. Since the goal is to have more weight in the system, the idea of adding weight along the rode came to be. This was the birth of the anchor kettle.
An anchor kettle is nothing more than a weight tied into the rope rode. It hangs on the rope and pulls it downward. This in turn causes the rode leading to the anchor to pull at a lower angle and would nessecitate a shorter scope!
In theory, this works well, but in practice, it leaves much to be desired.
The first problem with an anchor kettle is that it causes additional wear in the line. When the weather is calm, the kettle will fall to the bottom and the rope that runs between the anchor and kettle will lay on the sea floor and chafe away as it moves around.
When the wind begins to blow, the yacht will be pushed back and the kettle will begin to do its job. It will hold the line down in the water and keep the rode from it to the anchor at a low angle. The line from it to the boat will proceed up at a steep angle. This is precisely what they sell it to you as, and this is the only time it will work like this.
When the wind really begins to blow, the rode will become tight and the kettle will be lifted up. All of a sudden, anchoring with a shorter scope becomes a problem as the anchor will not b able to hold as well. The kettle is hanging on the line, but the line is pulled taught as the yacht is pushed back with the surge of the wind and waves.
In addition to the problem with the scope being too short and the kettle merely hanging lifeless on the rode, you have the issue with the weight of the kettle in the line. When the yacht relaxes, the kettle will sink down a bit. When the yacht is pushed back with a wave, the kettle will be flung upwards and its mass will continue to yank upwards as the rode becomes taught. This upward jerking motion will actually work to unset the anchor and cause it to drag!
Anchor kettles are a cool concept, but they do not provide the benefits that they claim and actually harm your anchors ability to hold when the weather deteriorates, which is when you need to count on your anchor the most! If you are looking at an anchor kettle to allow you to anchor with all rope rode, consider investing the cost of the rope and kettle into a good chain rode that will provide you with the needed weight and catenary curve to hold well in all conditions.