Vulnerable to Capsizing?

[Craig Weis]

Small Craft Adviser had a posting by Alan Mick on 1/20/2002, 3:28 am:
The Capsize Screening Formula

Beam in feet.
Displacement is divided by 64, which is the weight of a cubic foot of water.
From the Annapolis Book of Seamanship: "First divide the boat's total weight in pounds by 64 to find the volume in cubic foot of salt water. Second find the cubic root of that figure. Third divide the result into the boat's beam."

If less than '2', then the boat is relatively safe from capsizing and remaining up side down. If greater than '2', the boat is relatively vulnerable.

I may have done this wrong, since it has been a long time since I cyphered anything and I'm simply a product of public education but my C-P 19 XL might be safe at 1.252198 skip.

[brackish]

 Capsize ratio according to Carl's Calculator

Compac 19/3  -  2.22
Compac 23/3  -  2.17

And the description and disclaimer:

Capsize Ratio: A value less than 2 is considered to be relatively good; the boat should be relatively safe in bad conditions. The higher the number above 2 the more vulnerable the boat. This is just a rough figure of merit and controversial as to its use.

[carry-on]

Skip,
Did you use 2000 pounds or add for gear?
Using 2000, I would get a  different result:
 2000/64= 31.25.
Cube root of 31.25=3.149.
7 ft. beam/ 3.149= 2.22.
I may not understand the formula. I guess less beam and equal weight would be more likely to right itself if capsized. Also a narrow beam seams more likely to capsize, like a log rolling. Seems to me the formula ignores the location of weight and does not address sails. If you go keel to the sky with sail up, it would take quite a moment to recover.
Anyway, with 40% of weight in the keel of the CP 19, capsizing is tough to envision.

[Caboose]

Capsizing is a word that has more than one meaning. According to the Racing Rules of Sailing, a capsize is when her masthead touches the water. To a non-sailboat, a capsize would mean the boat is upside down and we would call that "turned turtle". For a sailboat to recover or resist a capsize, it depends on Righting Motion. This righting motion depends on not only the amout of ballest, but it's location. A shoal keel and a fin keel of the same weight does not have the same righting motion. A fin keel with the addition of a lead bulb at the end has even more righting motion. The lower the Center of Gravity, the more righting motion. A few more things come into play for a boat to resist a capsize. The amount, direction, and location of the Center of Effort of the wind and weight above the center of gravity. The outboard, ice chest, gear, and crew will also have an effect on the righting motion. Yes, the most powerful factor is the wind on your sails and we all know as you heel over that force is less. A wider beam will also help in resisting a capsize. Make it easy for yourself, go sail a dinghy and you'll soon discover that all this math will be second nature to you.

If they say my car can safely stop in X amount of feet at Y speed, I don't test it when I see something ahead. I am sure the most of you start slowing down long before the point of where X meets Y. It's nice to know that my boat will recover from a knockdown or a near knockdown, but I don't want to test it. Play safe, sail safe, and practice a MOB with the normal person in charge being the MOB. If you solo, think about what you would do as well.  

[Craig Weis]

I think I cyphered wrong, something about the cubic root of that square...no matter, you guys figured right. You know what I meant. If the extra weight of say 300 lb for gear was included, I don't know but think it would not make a hill of beans difference.

Dad and I when I was around 12 years old 'survived' this off Belmont Harbor in Chicago one fine Sunday day while futsin around with our Star Boat. Dad's best childhood firend Warren Patterson was lucky enough to grab this shot from his own boat.  It was a great day, we laughed so hard!!

Mom said I was still in diapers when we were up in the North Channel when this picture was taken of the 'P's' boat.

Sitting on the can in Belmont harbor, Chicago.

skip.

[curtisv]

Capsize ratio according to Carl's Calculator

Compac 19/3  -  2.22
Compac 23/3  -  2.17

And the description and disclaimer:

Capsize Ratio: A value less than 2 is considered to be relatively good; the boat should be relatively safe in bad conditions. The higher the number above 2 the more vulnerable the boat. This is just a rough figure of merit and controversial as to its use.

I didn't check the math but just a reminder - capsize ratio is an indication of whether the boat would right itself if turtled in rough seas.  A capsize ratio of less than two is supposed to indicate that with further wave action the boat should right herself in less than two minutes, in which case crew in the cockpit might still be conscious.

Wind doesn't capsize crusing boats with heavy keels, even small ones.  Waves on the ocean in a storm can capsize boats of just about any size.

Curtis

[newt]

I would back up what Curtis said. I have layed my 23 over, but never had the mast in the water. Curtis, how does the 23 stand up in the Gulf of Mexico?

[curtisv]

I would back up what Curtis said. I have layed my 23 over, but never had the mast in the water. Curtis, how does the 23 stand up in the Gulf of Mexico?

Newt,

I wouldn't know how the CP23 stands up in the Gulf of Mexico.  Orleans Massachusetts is part of Cape Cod (on the outer cape) and its a long way to the Gulf of Mexico.  I can't say that I've sailed nearly that far south.  Never sailed beyond about 200 miles from my mooring.  Not even 1/3 the way to Bob23's home waters.  Maine is a lot closer than New Jersey where Bob sails and that obscure town with a similar name, New Orleans, is very far away.

Curtis

PS - resisting the temptation to make a BP oil spill joke out of this, so we don't stray into political waters.

[newt]

Thank you Curtis:) I just saw Orleans on your sig and I assumed....

[curtisv]

Thank you Curtis:) I just saw Orleans on your sig and I assumed....

Like I said, there is some obscure town with a similar name somewhere. 

I spend a lot of time on the west coast and I used to get asked about Katrina if I wore anything with "Orleans Yacht Club" on it.  I just say "missed us by 3,000 miles" and wait for the puzzled look, then explain.

Curtis

[rsahlender]

I could not help wonder how this ratio might work out for a CP16 so here it is rounding everything to 2 decimal places if anyone was wondering...

1100/64 = 17.19
cube root of 17.19 = 2.58
6 foot beam / 2.58 = 2.33

Rich

[curtisv]

I could not help wonder how this ratio might work out for a CP16 so here it is rounding everything to 2 decimal places if anyone was wondering...

1100/64 = 17.19
cube root of 17.19 = 2.58
6 foot beam / 2.58 = 2.33

Rich

Here is what Gregg Nestor writes in his excellent book "Twenty Affordable Sailboats to Take You Anywhere".

    The CFS was developed by the Cruising Club of America and estimates a boats resistance to capsizing.  It compares beam with displacement since excess beam contributes to capsizing and heavy displacement reduces the vulnerability to capsizing.  In general heavy boats with narrow beams are more stable.

Actually that is not right.  It is still a great book.

A beamy boat is more stable than a narrow boat but is also more stable upside down.  For example, catamarans are beamy and very stable, about equally stable right side up or upside down.  A heavy displacement boat has a greater tendency to flip upright as long as the weight is down low (or up high when upside down).

The section goes on to say:

   The CSF ratio serves only as a guide.  A boat is an acceptable blue water candidate if the CSF ratio is 2.0 or less.  The lower the better.

That is consistent with what I've read elsewhere.

Reading further:

  The CSF ratio does not take into account the vertical position of the boat's center of gravity (VCG).  [...]

This is important.  Other books I've read also mention that a high narrow cabin top contributes to righting moment.  Buoyance up high, like a mast that doesn't fill with water (for example is filled with foam or keel stepped with very few or only very small leaks in it) helps a lot.

Anyway, good advice is don't turtle your CP16, or your CP-anything as it isn't going to be the fastest boat to flip back upright.

This is really not a consideration unless taking the boat out on the open ocean (or Great Lakes).  If so, watch the weather very closely and if you want to be very safe don't attempt more than a day or two passage in a small CP, maybe 2-3 days at most in a CP27 or CP35.  Taking a longer passage means relying on the weather report being right for more than 2-3 days out, and there is some risk in that.

Curtis

[brackish]

I would back up what Curtis said. I have layed my 23 over, but never had the mast in the water. Curtis, how does the 23 stand up in the Gulf of Mexico?

Very well Newt.  Like any other body of water, the Gulf can have high sea states and wind, however, prevailing conditions are relatively mild in my view.  I had my 23 on the Gulf late spring, early summer for three weeks this year and it was delightful.  I found it a lot easier and less challenging to sail there than the small lake I'm on the rest of the year.

Most of my life I've sailed the open Gulf and the Sound on a year round basis.  I would imagine that my average conditions would be less variable and challenging than Curtis's home sailing ground.

[newt]

Cool. Can't wait to do the Great Circle route. Just got to retire. So far my partners are holding out on me....

[Salty19]

I can't imagine the 19 capsizing..the darn thing is so stable you hardly heel even in stronger winds.  You would need larger ocean/Great lake waves I imagine.

Speaking of the Great Circle, I know a guy that did this two years singlehanded.  He has more stones than I'll ever have.