WaterWings
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Analysis of a capsizing and technical discussion on small crusier stability!
This is a very good article on the stability of smaller cruisers. Sorry its too long to paste all of the article and pictures here but I encourage
everyone with a cruiser to read it.
I see a lot of cruisers on Cumberland (taller Carvers, Silvertons, etc) that look like they are top heavy and could capsize without being overloaded.
NO expert here... just appears that way to me.
SPECIAL REPORT: Analysis of a capsizing
Posted on 26 July 2012 Soundings Trade Only Today.com
Written by Eric Sorensen
EDITOR’S NOTE: As the formal inquiries into the July Fourth Long Island Sound tragedy continue, Soundings asked Eric Sorensen, a regular contributor,
to discuss boat design principles, especially as they relate to seaworthiness and the physics of buoyancy and stability. Sorensen is the author of
“Sorensen’s Guide to Powerboats”; consults for boatbuilders, the Navy and boat owners; and was the founding director of the J.D. Power and Associates
Marine Practice.
It will be some time before all the facts are known about the Fourth of July capsizing of the 34-foot 1984 Silverton Kandi Won with 27 aboard, but
speculation as to the causes — overloading, a sudden weight shift, a course change, passing wakes, even mechanical failure or a through-hull letting
go — continues. However, I think it would be most constructive to look at the tragedy holistically, including the physics of stability.
The No. 1 factor governing a boat’s carrying capacity is stability. To remain upright and floating, a listing vessel relies on buoyancy pushing up on
one side and gravity pushing down on the other to restore the hull to equilibrium on an even keel. The force of buoyancy is created by the weight of
the water displaced by the hull, and when the boat is on an even keel the center of buoyancy (CB) is on the centerline. When the boat heels (from a
dynamic force such as wind, wave, wake or momentum in a turn) or lists (because of a weight shift to one side) CB shifts in the same direction as that
side of the hull immerses more deeply. This offset buoyant force creates a righting arm, working with gravity to create a couple (a rotation caused by
an offset force times distance) that restores the hull to equilibrium on an even keel. The lower the boat’s center of gravity with all passengers and
gear on board, the more stable it will be.
An inshore planing hull such as the Silverton’s has little depth below the waterline, and it has a correspondingly high center of gravity (CG), with
most of the boat’s mass well above the waterline. Because of the planing hull’s hard chines and wide, flat bottom, it feels very stable at initial
angles of heel, but this is deceiving. However, the deep-draft displacement boat, with its much deeper and often ballasted hull — and commensurately
lower CG — may feel more tender when rolling underfoot but is ultimately far more stable, with positive stability through perhaps 100 or 120 degrees.
This illustration — kindly created, as all of these drawings were, by naval architect Dave Gerr — shows a boat similar to the Silverton 34 in profile
and arrangement. The profile shows a shallow hull depth below the waterline, which, combined with a large area and height above, indicates a boat
designed for and suitable for use in inshore and coastal waters. The plan view shows the deck arrangement. The cockpit, foredeck and flybridge levels
are all above the boat’s unloaded center of gravity.
A planing convertible such as the Silverton would likely capsize at 60 or 70 degrees of list, but it would also start downflooding into the engine
room through-hull air intakes and capsize well before it reaches 60 degrees of heel. Once any vessel reaches its point of vanishing stability there is
no remaining righting energy, and the boat will teeter between capsizing and righting, the result depending solely on the vagaries of the next wave or
gust of wind.
The design of a planing boat such as the Silverton is such that, no matter where you are standing on board, your own body’s CG (roughly at your
bellybutton) will be above the boat’s CG (see chart showing VCG with and without 27 passengers aboard). This is never a positive thing because a
boat’s CG will move in the direction of a weight addition and away from a weight removal. And, as mentioned, a higher CG makes a boat less stable
since the righting arm and the resulting restoring couple are diminished.
Vertical center of gravity is right at deck level with a normal load (left) and rises considerably with 27 passengers on board. M is the metacenter,
the imaginary point about which the boat rolls as if on a pendulum. The distance from G to M (GM) is an indicator of initial stability. With 27
passengers on board, M goes down and G goes up, reducing the restoring moment available to stabilize the hull.
The cockpit on a local Silverton 34 sister ship I measured is on the high side — 17 inches off the waterline, which on this cruising-oriented boat
permits a single-level deck to the cabin bulkhead and a flush deck above the gasoline inboards. With the cockpit deck — let alone the foredeck and
flybridge
Sorry REST OF STORY here:
http://www.tradeonlytoday.com/home/521163-special-report-ana...
WHATEVER floats your boat Dude!
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NearlySatisfied
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I would think Hillman and Patton both on the foredeck of a Bay____er at the same time could be cause for concern...
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quickiemonster
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Ride one of the original waverunners and you will learn about stability. (Especially if you're a lard @ss like me).
If you tell the truth, you don\'t have to remember anything.
Mark Twain
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Watt Money
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lol @ Nearly 
We never really grow up; we only learn how to act in public.
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JabezBoy
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Hilarious! I was there when I read the title of the post and then the first comment I see is yours. Perfect!
\"Boats - Vessels of Freedom, Harbors of Healing - Boats\"
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E_HILLMAN
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lol, I go one week with limited internet use because of a PMP bootcamp and Nearly takes advantage of it. lol However this is ONE of the reasons I
got a wider beam version of the smaller cruisers. JPatton and I shut the boat down on a holiday weekend out in the main lake outside of the State/JRM
harbor just to see how it did with wakes etc and it was VERY stable. I too have thought some of the taller boats appear a little unsteady.
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Buy Made in the USA, the job you save just might be your own.
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NearlySatisfied
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Quote: Originally posted by JabezBoy  |
Hilarious! I was there when I read the title of the post and then the first comment I see is yours. Perfect! |
It's so easy....
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