Is it really “vastly” bigger? Seems to me it is trading width for length and while the arrangement seems better as a living space and probably more square footage I wonder just how much extra you are getting (and what is the cost per square foot on each?).
Faster is part of it but I think hull length has more to do with it. The leverage needed to pitchpole a long monohull has to be substantially more than that need to do the same to a shorter hulled catamaran. Maybe they are close with small, two person sailboats but not so close as they get bigger.
The problem pitch-polling is that you dig a hull in, and the forward speed drives it in further and the momentum of the rig sends the whole mess over. If both bows catch it pitch poles. Being able to dig the bows in is where things start. Wind direction obviously plays a big part. But the time of greatest risk is on a reach when the boat is fastest, not when running, when the wind is behind (with a cat the speeds are so fast that we never let apparent wind never get behind.)
I have only pitch-polled twice, and another near miss. Every time the speed has been at the top end of the boats capability - close to 20 knots. The most violent pitch pole - and the one that is the worry for ocean going boats is when you hit the back of a wave. We call that going “down the mine”. Sometimes the bows pop out, other times you go right over. (We have seen the boat go in all the way to the front beam and manage to pop out.) Happens so fast you barely get time to register it. And the water is hard when you hit it. Digging one bow in will also send you over, but the boat won’t pitch-pole. It sort of screws around and falls over sideways. Forward buoyancy has become much larger in recent design for this reason.
Clearly you need to compare like with like, and that gets hard, as a monohull versus a cat typically: weighs more for the same length, is slower, thus different apparent wind, and has a different mass distribution. How you get to directly comparable boats isn’t clear. But a keeled monohull will at least self right. Getting an inverted beach launched cat upright is a pain. (One safety consideration is whether the crew has enough mass to manage it. Many cat classes impose minimum crew weights for this reason.) Getting an ocean going one upright is essentially impossible without outside assistance.
To add, the symmetry and wide platform of a cat makes a pitch-pole more likely than a simple capsize. If both bows dig in you have a nice stable pivot to fall over. A single bow dug in and the boat will slew and fall over sideways.
I’m mostly going by the relative merit of bare-boat charter boats. For the same price you get a heck of a lot more living space on the cat.
The tradeoffs on design are complex. Cats don’t need a keel, and the amount of material needed to built a boat of a given living area is going to be a complex set of interactions. Gaining walking head-room is a big thing. There is a very real break-point in the attractiveness of a design when you have full height inside.
The big trick with mid-size cruising cats is to put berths in each hull, and a cross hull living space. You can get 4 cabins with double beds in the hulls, and a massive cross deck living space with galley and room for 8 people to hang out and socialise. Plus a similarly huge open deck behind. You just can’t do this on a monohull until you reach a boat very significantly longer and much more expense.
That argument has persisted for 20 years now, with both sides claiming victory.
Cat’s do exceptionally well in heavy seas. I’d take a cat in a heavy sea any day. (we’re talking 50+ foot, if you want heavy seas.)
Cats can broach, but every cat sailor knows not to overpower the boat with the sails, That’s pretty elementary really.
Cats are also faster and sail flatter.
Cats provide more living space with very large cockpits and salons.
Avoid overpowering a sail boat ? I’ve been on a yacht with the deck turning almost vertical.
Sure, we were out in near gale conditions, and near the ocean, and the gust was sort of gale straight.
But its great to know that the lead in the keel is acting as a great ballast and makes the yacht self righting, even if a wave came along and pushed the boat even further than the wind would, it will still come back up … its not going to turn over and trap you on the davey jone’s locker side. thats a bit too close to the locker for anyone , right ?
You might like the catamaran’s speed given relatively small sail area, but it just feels relaxed, and you barely rock with gusts. Its just boring until you up the sail area. You can use a kite down wind, but its dangerous to use it for reaching across the wind, as you might get tipped over.
catamarans are good in shallow water bays and lakes where sand bars cause annoyances… you can get away with the board(s) up on the reach.
The roll on roll off catamaran ferries enjoy the stability of the floatation occuring at the sides, if the load is a bit unbalanced, its really just a tiny thing, and the ocean swell doesn’t cause it to rock much either. But also the ferry doesn’t need large spaces under water, it doesn’t need humungous fuel tanks down there, so it avoids wasting space ,weight, expenside, manouverability issues, speed limits , with a convention hull which has ballast down deep and horrible hull speed. (hint, the non-planing hull has horrible performance above its hull speed. drag really balloons over the hull speed. The incat can go faster because its not too horribly ineffficient on fuel to go faster. You need to calculate what power a convention hull needs to go as fast as the incat ? 8 times the power of the incat … too much.
The roll on roll off catamaran ferries enjoy the stability of the floatation occuring at the sides, if the load is a bit unbalanced, its really just a tiny thing, and the ocean swell doesn’t cause it to rock much either. But also the ferry doesn’t need large spaces under water, it doesn’t need humungous fuel tanks down there, so it avoids wasting space ,weight, expenside, manouverability issues, speed limits , with a convention hull which has ballast down deep and horrible hull speed. (hint, the non-planing hull has horrible performance above its hull speed. drag really balloons over the hull speed. The incat can go faster because its not too horribly ineffficient on fuel to go faster. You need to calculate what power a convention hull needs to go as fast as the incat ? 8 times the power of the incat … horribly inefficeint. They put the power in to get the hull to hull speed… Now the incat, thats a planer, it can just get up to the plane and there’s no particular “hull speed” to be reached, naturally drag is still expenential, but its not in ^5 (to the fifth power), ^6, ^7 , which is what happens to drag in non-planers… the rapidly increasing exponent causes there to exist an efficient hull speed for that hull.
To return to my question for a moment, might we say that commercial and military catamarans were not developed sooner mostly due to the natural conservatism of people who build, buy and sail large ships?
It makes sense for sailing boats because a catamaran doesn’t need a weighted keel to counteract the force on the sail. But powered ships don’t have keels to begin with. We only see catamarans used in high-speed boats, because catamarans with very narrow hulls can be much faster than monohulls of the same length (see Isilder’s post). But there is a huge cost to this design, in terms of engine power, fuel consumption, complexity, etc.
For large commercial vessels a cat provides essentially no advantage. Modern ships are so big that their displacement limited speed is just not a problem. They are ridiculously easily driven, and can move enormous amounts of cargo with astounding efficiency. For such ships everything about a cat brings downside with no upside.
For medium sized vessels you may get into the zone where you do want high speed, and you are not designing for maximum cargo mass. Here you might find a cat or tri hull provides useful gains because you can cheat the usual displacement speed limits. Military vessels are a good example - and thus the Freedom Class Littoral ships. Also fast ferries. But as the ships get smaller still the loss of internal volume will start to hurt, and you go back to monohulls, and maybe a planing hull if you can manage it.
A beamy monohull is going to be a nice structure to build. Everything is working in your favour to get a strong and materials efficient hull. You don’t see nasty high stress areas, and the build can be safe and not require heroic engineering. Long thin hulls are not so nice, and the moment you start to try to tie them together you get evil stresses where you try to do so. Catamarans fail at the joins between the hulls and the cross beams, or the cross beams themselves fail. You can have catastrophic failure that results is total loss of the boat when one of these lets go. On the ocean going multihull race boats failures of the ama (the crossbeam - name stolen from the Polynesian) where it joins the outrigger has been a common problem. And these boats are essentially 100% carbon fibre. Building a large vessel with multiple hulls will run into similar problems, with very significant stresses to cope with. I suspect that this sort of problem was outside the range of even relatively modern riveted steel ship construction. You might end up with a mix of ship and bridge building technologies. (I guess Brunel might have felt at home.)
I seem to recall a discussion (which I cannot find now) about the downsides to a catamaran when the first one was entered in the America’s Cup race. The opinion was that the monohulled sailboats would only have a chance of beating the catamaran in heavy seas. Very heavy I guess since those America’s Cup cats are not exactly small and can certainly deal with fairly rough seas with no trouble.
The America’s Cup cats won’t deal with any sort of a sea state at all. The last AC (number 35) used a close to one design 50 foot foiling cat, and AC-34 a 72 foot boat that wasn’t originally intended to foil, but eventually did. The AC-34 boats were seriously fragile. One totally broke up, and another was damaged almost beyond repair just sailing in San Francisco bay. Neither AC-35 or AC-34 boats would be able to sail in much more than 1 metre seas. Worse, they are so fragile that they would probably quickly break up. The AC-33 boats were deed-of-gift boats (90ft waterline length if single masted.) They too were fragile creations that would not survive open sea. Going right back to AC-27 (also a DoG match) Dennis Conner’s defender Stars and Stripes was a much more robust beast. Actually two were built, and both still exist and are sailed. Although the version with the wing mast now has a conventional soft sail.
Perhaps appropriate here to mention the trimaran IDEC Sport and its skipper Francis Joyon, who together have recently set trans-Atlantic and round-the-world sailing records.
The latter was 26,000 miles in just under 41 days, for an average speed of 26.8 kts.
