There's a saying among boaters: There are two kinds of boaters - those who have run aground and those who will.
If you're in a boat with a keel, you need to know how much you draw. This is a fancy way of saying 'how far down does the keel go in the water?' If you look back at the last post, you can see from the drawing that some keels hang deeper than others. If you're writing about a sailboat, your characters need to know what their boat draws (how deep the keel goes). Otherwise, they're going to be those boaters who WILL run aground.
Running aground can be no big thing, or it can be utterly devastating. During a cruise last summer, my husband and I listened on the VHF as a big power boat struck an unmarked reef (rocks) at high speed. That boat sank within an hour. Everyone made if off the boat and were rescued, but it was a stark reminder that you either have to know the waters were you cruise, or you have to know how to read your charts. Even then, you may not avoid running aground.
Most groundings in sailboats are at low speed and it's the keel that hits first. Not bad, really. Touching bottom with a piece of solid metal may take a bite out of the keel, but it usually doesn't result in a sinking. It's possible to hit hard enough to rip the keel right off the boat, but that's unusual and it does mean sinking. Most of the time, in the Puget Sound region, sailboats go aground in mud, which doesn't damage the boat. The problem is that if you go aground with the tide going out, your boat will settle over on one side until the tide turns and refloats it again. Not a catastrophe, generally, but uncomfortable.
What does this have to do with multihulls? Plenty. Trimarans have three hulls. The middle hull looks a lot like a monohull (most all the other sailboats out there), but it has two hulls out on either side. These boats have no keel at all as the pontoons on either side provide stability. Tris are faster then monohulls and can carry more sail area than a monohull of the same length. Because there's no keel, these boats can sail in much shallower water. It isn't uncommon for a monohull to draw six feet - meaning if the water depth goes to five feet and eleven inches, that boat will be aground. Multihulls without keels can draw as little as two feet. Some are even beachable. The disadvantage is that a trimaran is hard to dock because of it's beam (how wide it is). It can also be difficult to maneuver in tight spaces because of the width. Trimarans usually have a little less living space than similarly sized monohulls because the middle hull is usually the only living space and it's narrower than most monohulls. This one pictured here mitigates that by building the living space up and across all three hulls. This isn't common.
Catamarans have two hulls. Usually, both hulls contain living quarters with more living space built between the two hulls. Catamarans typically don't have keels, either. Instead, they rely on retractable centerboards or daggerboards. This boat pictured is a thirty-four foot Gemini catamaran that has daggerboards - one in either hull. This boat is beachable, though not all cats are. If you opt to put your characters in a catamaran, do a search and look at the different models out there. Most of the dealer sites will tell you whether a boat is beachable, or whether it had daggerboards as opposed to centerboards. Catamarans have a reputation for turtling. This means that while under sail, one hull comes up out of the water and the boat goes all the way over. The width of the boat is supposed to prevent that, but in enough wind, it's all too possible. Another Gemini in our region did flip while out in a squall that blew up 50mph winds. Everyone was fine, but that's what you call a bad day. If you're going to sail a catamaran, you must pay attention to the wind and to the boat. These boats can be fast and very comfortable - cats don't heel. They provide a very stable sailing platform. But you cannot push them when the wind speed starts to climb. Without a heavy keel underneath, once this boat starts going over, it's going over. It's up to the sailor to know enough to prevent it in the first place. That said, these boats are very safe so long as the sailors know when to reef (reduce sail area) and/or when to get the heck out of the weather if they can.
In a catamaran, accommodations go down either hull. In the Gemini, the galley (kitchen) is in the starboard hull. The master cabin is forward where that first row of windows is. In the port hull is the navigation table and the head (forward). Two double cabins are aft in either hull. The settee and table are up between the hulls and the cockpit is in back. You can explore the boat on the builder's website. Again, search on catamarans and you'll find all kinds of them out there in the world. You should be able to find one that suits your story and your characters.
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