There are a number of worrying things about the account and I think MarkJ is right on target. I'll just add that a 998 bar and down two recently in that area along with a distinct cloud line is almost always a sign of serious weather. I don't know what the weather charts
were showing but WYSIWYG. So, I've given up on the unpredictable microbust as a contributing factor in this case.
Originally Posted by estarzinger
Yes, that was the old theory/debate . . . but that's just simply not true if they will/could flip in 30-40 something true. We have been caught multiple times in 'sudden/unexpected' 2am squalls with +40kts true . . . just guessing but perhaps once every two years or perhaps a bit more frequent than that. We have actually not sunk (yet)
Didn't Beth research
this for a book that included a section on multihulls?
One of the most accessible papers on the subject is Multihull Design Considerations for Seaworthiness
There are generally two different categories of stability (static and dynamic) and within them a number of distinct considerations. It's too much for me to cover all of that while I drink my morning coffee. But my attempt at a brief and shallow summary is:
The reality is that multihulls have a good safety record
. Giant multihulls that can be sailed over in less than 10 knots of wind have been single
handed across oceans and even non-stop around the world. Okay, Joyon is a superman. But consider the time and effort it takes to reef or even trim sheets
on a boat of that size and power. And consider the stability needed to stay upright while hooked into powerful lows deep in the Southern Ocean.
How is it that a boat that can be flipped in a mild breeze can also navigate the Southern Ocean's storms with a single
man as crew and still make extraordinarily fast passages? Certainly, a large part of the answer is the "tiller nut". But there are design considerations too. I think the key to understanding this and answering your concern about flipping in some particular amount of wind (pick a number any number) is to understand that a designer can always design a rig that will use all of the available stability at any particular wind speed independently of the actual value of stability. IDEC, for instance, has a huge amount of stability, more than many ships and vastly more than any conventional monohull
of a similar length. She also has an enormous rig that can develop enough heeling force in a mild breeze to capsize the boat. The designer could easily have put a smaller, shorter rig on her that would not need to be reefed in 40 knots of wind or 50 or 60... Of course, within limits, the same thing can be accomplished by reefing the large rig.
I've used Joyon and IDEC as an example because the boat is huge and powerful and it was sailed by one person suspected of being human. But the same design considerations apply for mortals sailing lesser craft. A designer can put as much rig on any boat as he or she feels is appropriate to the service
independent of the amount of stability the boat has. Smaller rigs are largely equivalent to reefed larger rigs. When a designer chooses a rig size to some extent he is making a judgment about the expected abilities and tastes of the crew rather than determining the ultimate seaworthiness of the craft. Or, put another way, a boat that comes from the builder with a rig that doesn't need a reefed at 40 knots may not be any more stable than a boat that needs to be reefed at 15 but is clearly designed for a different user. Equally, a boat that needs to be reefed at 15 but is sailed with reefs
for 40 will perform similarly to a similar boat that has a cut down rig designed for 40 knots. In short, designed reefing points tell you who the boat was designed for but not how stable it is. A fast boat can be sailed slowly but a slow boat is always slow. A fast boat may or may not have lots of stability and the same is true of a slow one. All else being equal a more powerful boat needs to be reefed earlier than a less powerful one.
So, IMO, the answer to is a multihull that can flip in X amount of wind safe for crusining depends both on who's driving and how much stability the boat has. But, just knowing how much wind will capsize the boat doesn't tell us how much stability it has much less how seaworthy
it might be. There is a danger
in assuming that a multihull that will not capsize with full sail at 40 knots is inherently more stable or seaworthy
than one that needs to reefed earlier. One may reasonably infer that the more conservatively rigged boat is intended for a more conservative audience that does not want to be bothered with reefing as often and doesn't want to pay extra for the "privilege" of working harder. However, one must not assume that the less conservative boat is less stable. When considering seaworthiness I believe it is important to remember that the sea is the same size for all boats. More so than in other design questions ratios used without regard for absolute size can be misleading. Static stability for a multihull is fairly easy to calculate with reasonable accuracy. Roughly speaking the static stability will be a bit less than 0.5* the distance between the centers of buoyancy of the hull times the displacement
. For what it is worth heavy "cruising" style cats generally have less effective beam because their centers of buoyancy are more inboard and they also tend to have less beam overall. They do tend to have good static stability because of their greater displacement. However, dynamic stability and seaworthiness are more complicated. For similar reasons that a monohull
with its rig intact may be more resistant to capsize in waves than one without a rig a wider multihull is more resistant to capsize in waves than a narrower one. Also related a multihull with is weight distributed farther outboard
(transversely) will have more resistance to wave capsize than one with a more central distribution. So, larger, wider multis with narrower hulls are expected to be more seaworthy than smaller, less beamy mulits with fat hulls. I recommend the Shuttleworth
paper for details.
IMO, then, if considering a multihull for cruising one should consider the question of power (at what point will she capsize) and seaworthiness independently. The fist is all about how often one wants to reef and how much one wants to pay for rigs and sails and the latter is largely a function of beam and weight. Assuming, of course, competent design -- in the same way it is possible but stupid to put a 400 horse power engine
into a minivan with standard breaks it is possible to make an overpowered difficult to control multi that would be dangerous in any but the most skilled hands.
I am just trying to recalibrate my thinking here to these new facts.
Just for kicks:
LOA 66 57
Beam 22 28
Mast height 87 78
Just looking at those numbers I would have thought the A57 was rather more 'squall resistant' than the Gunboat (Which has less beam and more mast)! I don't know what the real displacement for the A57 would be, as the number on their website is obviously hyperbolic. Perhaps the Gunboat is a lot heavier, but you would not think so from their marketing or pricing.
The Gunboat is not heavier (all else being equal). But the question is meaninless without a knowledge of how much sail each is carrying rather than the potential sail they might carry. The gunboat is a much more powered up craft when full sail is set and it has less static and dynamic stability. But as we all know it is still quite possible to sail to sail the A57 over. Keep in mind that reefed down to the point that it was in no real danger of capsize in a 40 knot
gust either of these boats would still be pretty quick by most standards. The power is there but there is no requirement to use it all.
My coffee is calling must run!