Sailing – Monohull Vs Multihull

A few weeks ago I had the opportunity to hold an American Sailing Association (ASA) clinic to certify some students for the ASA 114 level which is the Cruising Catamaran level. This advanced cruising standard is for individuals with cruising experience. Successful completion of the clinic qualifies the student to act as skipper and crew of a 30-50 foot multihull sailboat by day in coastal waters. The standard includes those skills unique to a 30-50 foot multihull.

For years, although I have sailed on a few catamarans, I was always somewhat leery of them. My motto was always “give me a good sturdy monohull and everything will be okay.” I was always concerned about the stresses placed on the two hulls which were separated by several feet. I thought they might twist or otherwise behave poorly and fail. I’m afraid that I am now reconsidering my position.

The clinic was held aboard a 40′ Manta. Man…what a beauty she was. The cockpit itself could comfortably seat 10 including the helmsperson at a separate station. There were deck chairs anchored just outboard of the cockpit on each pontoon. On the bow, there were more anchored deck chairs and a trampoline area for sunning. Aft of the cockpit was a bank of cooler boxes at least 8 feet wide which could store enough drinks and snacks for more people than you could carry. There was a built in propane grill (all stainless) that I would die for. I’m not sure how many square inches of heated surface there was but I’m sure you could cook steaks and potatoes for 12 or more.

Below decks was even more impressive. The combination main saloon, galley and computerized navigation station were huge, rivalling the size of my shoreside digs. Oh, by the way, notice the keyboard in the picture but no computer screen. Just push the button in the middle and the screen raises from the flat chart table. (excuuuse me!) How about two queen size staterooms, each with private head and shower, hanging closets and plenty of drawer space. Add to this the “under power” mobility of twin diesel inboards and one starts to think, “I could get used to this.”

But…could she sail you ask? As Rowan and Martin used to say on Laugh In, “you bet your bippy.” With kevlar sails, every halyard, sheet and reefing line run back to the cockpit to a two speed electric winch, raising sails was a matter of releasing a couple of cam cleats, taking a couple of turns on the winch and pushing a button. Lock the halyards down and you’re ready to rap the sheets around the electric winch to trim the sails. Even the jib was self tending having its own “bow boom.” In light winds we were able to sail 50 degrees off the wind at 6-8 knots. As the wind picked up we scooted along at 10-12.

Just how stable are those things ?

Multihulls have gained popularity over the years because of their stability. Their wide stance provides little to no heeling. Monohulls use heavy, weighted keels suspended under water to keep the boat upright. In theory, no matter how much you heel, gravity will not let the keel come out of the water (in theory). A multihull substitutes its breadth or beam for the monohull’s heavy keel.

This graph shows the difference between
mono and multi hulls heeling angles.

One of the major advantages of the multihull over monohull is its speed. This quickness comes from their power-to-weight ratio. They have a lot of sail power and little weight. Also, because of the breadth that lends to stability they will normally have much less wetted surface than a monohull. A clever twist of history is pointed out in “Cruising Multihull Fundamentals” by Rick White. He poses the question:

“What if America had been discovered by the Polynesians, rather than Scandinavians? Well, we’d all be sailing multihulls. Then Someone would come along and say, “Hey, I have a great idea. I’m going to design a boat with just one hull.”

Everyone would ask how it would keep upright and Mr. monohull would reply, “weights and a deep keel. I’ll put a lot of weight down deep in the water.”

Then everyone would ask if that wouldn’t make the boat go very slow, and he would enthusiastically reply, “Yes!, Yes!, YES!”. You can bet slow and heavy would soon have advocates and a new novelty would be born.”

What about capsizing ?

Most people who have never experienced a cruising multihull get their fears of capsizing from watching those little Hobie Cats zipping along the beach on one hull. Eventually (inevitably?) the sailor loses control and over it goes.

A cruising cat is much different. The power-to-weight ratio is much lower than its smaller Hobie counterparts. For instance, an Elan 7.7 carries only 310 feet of sail on a boat that weighs 1300 pounds and is over 19′ wide. This is a lot different from a beach cat that may have 400 feet of sail on a 400 pound boat that is only 8′ wide.

The same comparison can be made on monohulls. Sailing dinghies such as Sunfish and Lasers have a very high sail to weight ratio and often capsize. Whereas on a cruising size keel boat that ratio, once again, is much lower.

Most multihulls will not capsize even under intense conditions. But, if the unlikely should occur “Stay with the boat!” Multihulls don’t have ballasted keels and most have additional flotation built in so they stay afloat. A capsized and “holed” monohull will sink like a rock. However, you can have comfort in the fact that it will hit the bottom with the mast pointing up.

What about the sailing characteristics of multi vs. monohulls ?

Basically, the operations of sailing a multihull and monohull are the same. A few differences will surface. One shows up in tacking. Because of the wide beam and the lack of weight, tacking can be challenging especially in light winds. Where in a monohull you probably ignore the main when tacking, in a multihull you would sheet it in to allow it to act like a wind vane to help steer the bow around. You also may need to backwind the jib to get the bow through the wind. Remember, because of the light weight, the multihull will slow very quickly when tacking.

Because of the speed of your multihull compared to that of the wind on a run, the main will not react as violently on a jibe as it does on a sluggish monohull. You should, however, tend the main by bringing it to center prior to jibing.

My concerns about the stresses of having two hulls several feet apart are still there, although I must say that after carefully inspecting the Manta, it is obvious that the engineering of multihulls has come a long way since the early years. Unfortunately, it took the breaking up of several multi’s to get the improvements adopted. At this point in time, it is my opinion that multi’s are engineered to safely stand up to adverse conditions.

All in all, it is obvious that the beauty of, and choice of, multi vs monohull is in the eye of the beholder. However, although multi’s may look a little strange at first glance, you should investigate their inner beauty and space.