Living Aboard: Monohull Sailboats, Displacement

In the interest of full disclosure, my wife and I live on a 1968 Morgan 45 with a cutter rig, full keel, very heavy displacement, 11 foot beam, and a tiny electric motor.


If you choose to live on a sailboat, you will live in a completely different world from powerboaters around you. Space is limited, and the weather is both a safety concern and necessary for navigational power. There are many different types of sailboats, ranging in styles and build quality. 

Sailboats fall into two major categories, monohulls and multihulls, and they are just as different as sailboats are to powerboats. 

Monohull sailboats can be further subdivided into various categories based on displacement:

  • Light displacement
  • Medium displacement
  • Heavy displacement

Displacement is directly related to weight. A heavier boat will displace more water where a light displacement boat will displace less water. As a boat moves through the water, the amount of water displaced by the boat needs to move around the boat. More displacement means that more water needs to move out of the way for the boat to pass through. The categories of Light, Medium, and Heavy Displacement are based on a numerical value of Displacement to Length Ratio. 

(Displacement in pounds / 2,240) / ((0.01 x Length at Water Line)^3)

For example, Wisdom displaces 34,000 pounds and has a waterline length of 32 feet.
(34,000 / 2,240) / ((0.01 x 32)^3)
(15.178) / (0.32)^3
15.178 / 0.032768
Displacement to Length Ratio = 463

Light displacement will have a numerical value of 90 to 180
Medium displacement will have a numerical value of 180 to 270
Heavy displacement will have a numerical value of 270 to 360

These three categories can then be broken down further based on their keel designs, rudder styles, and hull profiles; as you can see, there are a lot of choices!

Light Displacement

Light displacement boats are fast! They displace less water, which means they can move quicker through the water. Since they weigh less, they also sail faster with the same amount of sails up as a heavier displacement boat. This means that you can get to your destination quicker and with less effort. As with everything in life, there is always a trade off.

Light displacement boats move quickly through the water, which means they will pound into the seas harder and at a faster rate than other sailboats. Since they weigh less, they will also be tossed around more than other sailboats. In light conditions, the added speed is exhilarating! When the conditions deteriorate, so will the morale on board. Light displacement sailboats do not weather storms very comfortably as they will be thrown around by the winds and waves. 

The argument that negates a light displacements low storm comfort is they can out sail the storm. When a storm is spotted with enough warning, the crew can sail to avoid the storm entirely! If you can get out of the way of the storm, then you never need to weather a storm. 

While light displacement sailboats can escape storms, there is one issue they can not escape: weight. Liveaboards tend to accumulate stuff. As minimalistic as I think I am, I still have storage lockers filled with junk that I need to sort out and decide if I still need it or not. Light displacement sailboats are very sensitive to where weight is stored on them as they will list and lean if not evenly loaded. Something as simple as where you store your anchor and rode can make a huge impact on the boat. Imagine having a 45 pound anchor on the tip of the bow with 300 feet of chain weighing 480 pounds all sitting up in the bow.  That's 525 pounds at the very front of the boat! This will cause the bow to droop and raise the stern (and the rudders) out of the water a bit. I have seen light displacement boats carry very lightweight aluminum anchors in their bilge (centered in the boat) connected to an all rope rode in an effort to reduce weight on board.

Ground tackle aside, imagine where you want to store your canned goods or your books? All of these items must be carefully considered before they are brought onto the boat and then maintained organized in a location that will balance the boats trim. 

If we plan out where we will keep everything on the boat and make sure the boat is perfectly balanced, the final consideration is still total weight. Light displacement sailboats are very sensitive to how much weight they can carry. They are designed to be very light and if you begin loading all of your belongings into them in a sensible and balanced fashion, you may overload the boat. The easiest way to spot an overloaded light displacement boat is to see how high the owner paints the waterline. 

An overloaded boat will not perform as well as it was intended to and is less sea worthy. If you have an overloaded light displacement sailboat and plan to out run storms, and then weigh it down with tons (literally) of stuff, you might no longer be able to out run the storm. Now when you get hit by the storm, you will find yourself in a very bad situation where your life is actually at risk. 

As displacement is increased, so are the loads on the rigging. Weighing down the yacht will put un-planned loads on the standing rigging, far greater than what the boat was designed to manage. Keeping weight down in a light displacement boat is as much a safety concern as it is a structural concern.

If you love the look and performance of a light displacement sailboat and decide to live on one, make sure you keep it light and pay attention to the waterline. If the boot stripe disappears, the correct answer is not to raise the bottom paint, but instead to unloaded the unnecessary cargo.

Medium Displacement

Medium displacement boats are a compromise between light and heavy displacement boats. They are better performers than a heavy displacement sailboat, but not as fast as a light displacement sailboat. This means that they can out run a storm, but if they get caught (which will eventually happen), they can weather the storm better than a light displacement sailboat would.

These boats are also less sensitive to weight. The trim of the boat is not as sensitive as on a light displacement sailboat. This means that you can pack more of your junk on board and take it with you! 

This displacement range is quite popular with modern boat builders as they can build the boat with lighter materials but don't have to engineer the boat to an extreme level where they shave every possible extra gram off. This means they can build a sturdy boat without worrying about using the latest in ultra light weight materials while at the same time they don't have the extra expense of overbuilding the hull. Medium displacement yachts seem to be rather economical to buy and own, allowing more people to explore the world of sailing with good performance on a capable boat.

Heavy Displacement

There is no nice way to put it, heavy displacement boats are slow and lumbering. They won't outrun a storm, but they will weather the storm comfortably. When the winds are light, they will need all the sail area in the world to move forward. Most people with heavy displacement sailboats give up on sailing in light winds and motor everywhere. When the winds pick up, they will power through the waves in style! Since these sailboats are so heavy, they will cut and plow their way through the waves. Rough seas will feel like if you are being gently rocked back and forth as the boat powers through the chop towards your windward mark. 

Weight is also not such a touchy matter on board a heavy displacement sailboat, the extra weight of your belongings will not affect the trim of the boat much, nor overload her easily. If you think of cargo as a percentage of the boats total weight, each pound has less of an impact on a heavy displacement boat. Each person who lives aboard will tend to bring 2000 pounds with them while cruising. This number includes water, food, clothes, gear, and random junk that people pickup and bring back to the boat along their journey.


Let us consider a set of boats with a 32 foot waterline and see what effect adding 1 long ton (2,240 pounds) per person will have on it.

  • Light Displacement: 3 long tons and 32 foot waterline = 91.5
  • Medium Displacement: 6 long tons and 32 foot waterline = 183.1
  • Heavy Displacement: 9 long tons and 32 foot waterline = 274.7

If we add 2 people to the yacht, the following will happen

  • Light Displacement: 3 long tons becomes 5 long tons, a 40% increase in the weight of the boat
  • Medium Displacement: 6 long tons becomes 8 long tons, a 25% increase in the weight of a boat
  • Heavy Displacement: 9 long tons becomes 11 long tons, an 18% increase in the weight of the boat

As you can see, the heavier a boats designed displacement is, the less sensitive it is to the addition of weight on the boat. If you increase the weight of a light displacement boat by 40%, you need to be completely certain the weight is evenly distributed. The same load in a heavy displacement sailboat will only be an 18% increase and weight distribution is not as critical. This is why displacement to length ratio is an important category that should be considered at the beginning of your search.

If you want to travel far and fast and promise to live a spartan lifestyle with almost no belongings and only enough food to make the journey until your next port, a light displacement sailboat would be a perfect fit.

If you want to travel far and are not that worried about speed, like to carry along some personal belongings and extra food, a medium displacement sailboat might be your ideal cruising boat.

If you want to travel far, do not care about how long it takes or when you will get there and want to bring along whatever you want because this is your home, a heavy displacement sailboat could be your perfect cruising boat and home. 

 

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