Catamarans behave differently at anchor than monohulls, and that changes how you should set up an anchor alarm for a catamaran. The differences are subtle but they directly affect your watch radius and how often you get woken by false alarms. Here's what matters.
Cats sail around at anchor more
With high windage and little underwater grip, a catamaran tends to range — sailing from side to side across a wide arc — far more than a keelboat that hangs steadily downwind. To an anchor alarm, that energetic ranging looks like movement. If your watch radius is too tight, a cat will trip the alarm on a gusty night even though the anchor is rock solid.
Set a slightly more generous radius
The practical answer is to size your radius for the full width of that range, not just your resting position. Work it out from your rode, your boat length and a margin — the method is the same as for any boat in our guide to setting an anchor alarm — but lean toward the wider end of sensible for a multihull. The goal is the tightest radius that still never cries wolf during normal ranging.
GPS accuracy matters even more on a multihull
Because a cat already moves around so much, you can't afford to add 5–15 meters of phone GPS wobble on top. A dedicated multi-GNSS receiver with 1–2 meter accuracy lets you tell real drag apart from normal ranging — the difference is explained in how a GPS anchor drag alarm works. On a multihull, accurate positioning is the difference between a useful alarm and one you mute in frustration.
Where you put the device
On a cat the helm and your sleeping cabins can be a long way apart — and a chartplotter alarm at the helm is easy to sleep through in a hull. A standalone device with its own siren that you keep in the cabin solves that. For the full comparison, see dedicated anchor alarm vs chartplotter and the complete anchor alarm guide.
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