Seaworthy

Hey i'm no big boat expert just wanted to know how seaworthy are those catamarans out there , something like the lagoon 470 or aything that size , are they good enough to go around the world ? , can they hold trough storms ?

[tspeer]

Catamarans can be very seaworthy. John Shuttleworth wrote the most widely quoted article on the subject - see http://www.steamradio.com/JSYD/Articles/NESTalk.html.

There are a number of books out there on mulithulls that you might want to look into - Chris White's "The Cruising Mulithull" (http://chriswhitedesigns.com/books.php) and Charles Kanter's "Cruising on More Than One Hull" are two good ones. Kanter is a respected multihull surveyor and his book has info on a large number of models with his assessment as to their pro's and con's for cruising.

Chris White's Atlantic designs are also widely respected as good sea boats (http://chriswhitedesigns.com/). They have the key attibutes of an open (as opposed to solid) foredeck, adequate bridge deck clearance and a sheltered helm station. Beware of low bridge deck clearance, say much less than 3', as low decks will pound badly in a seaway.

After reading the article and doing some further research on the internet , i have the opposite question
what advantage do monohull have over multihull since the later are just as seaworthy and faster/comfotrable oceaangoing vessels beside the price ?

[tspeer]

I'd say the pro's of a monohull over a multihull are:
- a monohull is more forgiving in that its maximum performance lies inside its operating envelope, while a multihull's maximum performance is limited by its operating envelope. For example, if a monohull is overcanvassed for the breeze, it will lie on its ear and sail slower, but typically nothing catastrophic happens. The boat actually sails faster if it's reefed - that's what I mean by its maximum performance lying inside the envelope. A multihull, on the other hand, goes faster and faster as it's pressed. It's fastest when just flying the windward hull(s). But that's also on the very edge of disaster. So you have to depend more on judgement as to when to back off - the boat won't necessarily tell you.

- a monohull has the most interior volume and load carrying capacity for a given length. Overloading a mulitull results in a significant loss of performance compared to a monohull. A multihull with a flared out cabin can seem to have a living space comparable to a mono, but under the seats, where you'd have storage on a mono, you have hull and fewer places to put things. The sensitivity to weight also means it's better to forego a lot of complex systems and appliances. So a multihull requires a more Spartan, disciplined lifestyle compared to a monohull of comparable size. Not a seaworthiness issue, but definitely a consideration.

- It's easier and cheaper to berth a monohull due to smaller beam. Again not a seaworthiness factor, but one that extends to haulouts and many other practical aspects of ownership.

- If a monohull experiences a wave-induced capsize, it will likely right itself. Once capsized a mulithull will stay that way, since most multi's are actually more stable upside down than right-side up and there's a huge range of negative stability to get through before getting to the stable-upright regime. So while a catamaran is probably less susceptible to a wave-induced capsize, once it happens the consequences may be more severe.

The flip side of the capsize issue is that the monohull can sink while the multihull becomes its own liferaft. So in terms of hazard levels the monohull and mulithull are more alike than most people give them credit. The monohull has a low probability of a catastrophic event (sinking), while the multihull has a somewhat higher probability of a less severe event (capsize). This distinction is voiciferously debated between the two camps, with each one using it to point fingers at the other. But as I see it, it comes down to which risks you're prepared to trade off for other attributes and what steps you take to mitgate the risks you choose to accept.

[DJB]

Tom,

> the monohull can sink while the multihull becomes its own liferaft

Why is a monohull inherently more likely to sink than a multihull?

Dan

[Polarity]

Hi Dan

1) Ballast - all that foam/grp/wood will float when full of water - unless you attach a large lump of lead to it!

2) 2 hulls - if you manage to hole both hulls you are probably aground so she won't sink very far anyway.

[tspeer]

3) The materials in a modern multihull (sandwich construction) float. You can cut it into pieces and the pieces still float. So even if it's extensively holed, it stays up.

4) In addition to the hulls, you have the cross beams, which themselves will often have enough buoyancy to support the boat (especially trimarans).

And, let us not forget that monos, being more forgiving of weight, they can be built more strongly without much performance penalty.

Tim Dunn

Catamaran if build properly can be very sturdy for offshore voyages. The two hulls will be bridge together by at least one tier of superstucture. Without the superstucture bridge, one can adopt a strong deep beam to connect the hulls, but this method will restrict the operating condition to coastwise trips in the partially protected waters.

No doubt a mono hull can be very seaworthy. Regarding sinking of either the mono hull or the catamaran. The rule when considering an extent of side damage, related each hull beam at the water. So the catamaran is assumed to have about half the depth of penetration for comparing mono hull to cataman of the same immersed beam, when comparing similar disiplacement status. e.g. damage extent B/5 or B/6.

Side damage of a catamaran hull is automatically protected by the inner hull skin of the damaged hull body. The inner hull skin acts like a longitudinal bulkhead.

Each hull type have different damage stability criteria. The mono hull is the basic criteria and the catamaran assumes less damage as listed in current stability rules. The initial stability and damage stability response or compliance are better in multiple hulls.

Peter

I mean multiple deep beams in lieu of the preferred superstucture bridging the two hulls.

Peter

[Tim Dunn]

Apparently, my information was out of date. From www.gepolymershapes.com: (Which you have to register with to access)

LEXAN sheet can be drilled easily using a standard twist drill design. High speed steel or carbide tipped twist drills will give the best results with the following conditions.

Hole
Diameter Speed
(rpm) Feed
(mtls./rev) Time
(sec.)
1/8 1750 1-1/2-3 25-30
1/4 1000-1500 1-1/2-3 30
3/8 500-1000 1-1/2-3 30
1/2 325-650 3 45-50
3/4 350 3 50-60

LEXAN sheet can be drilled easily using a standard twist drill design. High speed steel or carbide tipped twist drills will give the best results with the following conditions.

Hole
Diameter Speed
(rpm) Feed
(mtls./rev) Time
(sec.)
1/8 1750 1-1/2-3 25-30
1/4 1000-1500 1-1/2-3 30
3/8 500-1000 1-1/2-3 30
1/2 325-650 3 45-50
3/4 350 3 50-60

To minimize the drill's tendency to pull into the material, modify the standard steel twist design by grinding a small flat on the cutting edge.
NOTE: Do not use drills that have been ground for use on acrylic sheet. They will overheat the material, and induce unwanted stresses........

For further information and technical literature, please call 1-800-451-3147.


Unquote

[eryka]

Hi guys,
A very interesting discussion. Don't if anyone is still around. But just a quick question. Judging from the comments here (and the articles referred to) it seems that in heavy water, you'd be better off in a large multihull than running for it in, say, a largish RIB? Correct?

eryka.