Multihulls versus monohulls

Although prejudice still exists in both the monohull and multihull camps there is wider acceptance that both have their strong and weak points.

Let’s fantasize. An inventor has come up with a new product he is anxious to sell to the world. He assembles a marketing team to help him promote the idea.
"This is it," he announces flashing up the first image on the screen.
"What is it?" someone asks.
"It’s a boat."
Patent disbelief from all. "Really? But it’s only got one hull."
"Ahah! Well spotted. That’s why I’ve called it – wait for it! – a monohull. That’s ‘mono’ meaning single, and ‘hull’ – but, hey, you’re probably ahead of me there. Anyway, it’s your job to advise me how we can exploit its many wonderful advantages."
"What advantages?"
"Well, for starters, it would take up less space in marinas."
"But, with only one hull, won’t it fall over?"
"Now, here’s the clever bit," cried the inventor, his eyes gleaming. "What I’ve done is place a lump of lead low beneath the hull on a kind of fin thing. That holds it upright."
"So marinas would need to be deeper?"
"Yes, but only a little. Well, let's say twice as deep."
The head marketing man was thoughtfully tapping his teeth with a pencil. "I see. But back going to the lead. Wouldn’t that make would your ... er...monohull both heavier and slower?"
"Er ... yes."
"Excellent! In which case we could plug the point that passage times would be longer, giving you more time to relish the experience. You know: 'Get the most from your boat by not getting there quickly'.
The inventor was jubilant. "Brilliant, brilliant! I knew you were the guys for
the job."
"But we’ll need more," opined another. "Something more thrilling. Any ideas
we can work on?"
The inventor looked thoughtful. "There is one thing. My prototype tends to lean over when sailing. I call it ‘heeling’. It’s really great fun, particularly when the cups and pans start flying about. Which brings me to the safety aspect…"
Somebody yawned. "Flying saucers always have fantastic potential, but safety doesn’t sell. Not sexy enough. Do tell us about it anyway."
"Well, you see my monohull can’t be capsized. If it rolls over it just bobs right back upright again. Surely we can make something of that?"
"Hmm. Pretty feeble. Where’s the fun in not capsizing? But hang on a second – what about sinking? If it were holed would it go to the bottom?"
"Like a stone."
"Then – wow! – I think we’re onto a winner here. Don’t mess with the shallows you’ll run aground there, sails slower than a glacier going uphill, the excitement of crashing crocks – toss in a few carving knives, some heavy books and a sprinkling of battery acid there – and then, as a clincher, if your boat sinks you might even get to swim the last miles of your voyage."

You might have guessed that this is just a fable but it serves to illustrate how standpoint can challenge accepted wisdom and practice. If multihulls had been the norm and someone really had proposed a monohull, the scorn would have been just as strong. One person’s logic can be another’s folly and it’s possible for opposing sides in an intellectual conflict never to fully understand the other’s point of view. This was a problem when western designers (as opposed to such as the Pacific islanders) first started to develop ‘modern’ variations on the multihull theme during the late 1950s and into the ‘60s and ‘70s. I was one of those designers and took my share of flak.There was considerable skepticism, some animosity, and a great deal of hot air wasted on explaining how intrinsically unsafe – irresponsible even – were these bizarre new creations. The trimaran below was one of my designs, participating in the Round Britain and Ireland two-handed race of 1970.

Thankfully, for multihulls there’s nothing more to prove. They have become respectable to the point when many conservatively minded sailors might seriously consider buying one.

To some extent this meeting of minds has arisen because the trend in the monohull world has been towards lighter and lighter displacement - mainly for the savings in materials this brings. Whether this is a good or bad development will be addressed later, but for now let's simply agree that the sailing world in general has become more familiar with the concept and that the correlation between heavy (safe) and light (dangerous) carries less conviction than it once did.

To avoid dipping into the distinctions between multihull types (cats, tris, proas) let's compare typical cruising catamarans with typical cruising keelboats and examine some of the myths and arguments. And here I intend generalising in the full knowledge that specific circumstances may not support my contentions. All comparisons are made on length-for-length (LOA) basis.

Catamarans are faster than keelboats:
Not necessarily! The main drag component at slower speed is skin drag, meaning wetted surface area of which catamarans have a lot. In light conditions a well-designed keelboat will often out-sail a cruising cat, particularly if the latter is overloaded.

Catamarans are lighter than keelboats:
Wrong! Don’t think that all multihulls are born light. One contemporary popular cruising cat – a typical representative of modern design – has a displacement of just over 12 tonnes (26,400 lbs) whilst an equally popular monohull of the same LOA
weighs in at just under 10 tonnes (22,000 lbs). Both figures are those quoted for ‘light’ displacement (meaning without crew and stores) so it follows that abusing the catamarans superior stowage capacity could widen that gap still further.

To be fair, in some regards length-for-length comparisons are misleading. In terms of accommodation the catamaran is by far the bigger boat and it’s no surprise that it’s heavier. It has a larger shell structure, two engines (probably) instead of one, and loads of extra furniture and fittings.

Catamarans offer more accommodation than keelboats?
Right! This and their spacious deck area are one of the principal attractions. And, because the available space can so easily be split into separate compartments, the privacy this brings lends itself to families or those who like to sail gregariously.

Catamarans are more comfortable at sea?
A guarded 'right!'. It's true that many like the lack of heeling but some cats can have quite a twitchy motion - most noticeably going to windward - that some might find disconcerting. A lot depends on the individual design.

Catamarans are more expensive?
Right! Constructional complexity makes this almost inevitable but the prices look more attractive when you compare accommodation.

Catamarans won't sail to windward?
Wrong! Though there are reasons why this might be the case. Lots of superstructure and the windage this brings doesn't help windward performance and neither do low aspect ratio fixed fins. However, it is true that fast cats can't take advantage of their speed to windward (because of the effects of apparent wind) but if they are prepared to pretend they are monohulls they can be as close-winded as the best of them.

Catamarans are less maneuverable in harbour?
Wrong! Granted their windage can make them flighty beasts but, in larger boats, is more than outweighed by having an engine in each hull, allowing them to turn in their own length if necessary.

Catamarans are dangerously prone to capsize?
This is too broad a statement to be argued without qualification. The secret ingredient is size. Stability increases by the power of 4. This means that if you were to double the size of any given boat (x2) your stability goes up by 2x2x2x2 = 16! This is a massive increase in stability - and therefore safety - and a powerful argument for having as large a cat as you can afford and handle.

But perhaps the most potent deciding factors are determined by geography. To have a beamy multihull in, say, the Mediterranean would expose you to savage marina charges; whereas to have a deep keeled boat in the shallows of the Bahamas (as I know to my cost) would bar you from places you really would like to visit.

I think horses for courses sums it up.

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