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Author Topic: Capsize Screening Ratio - wanders to solar  (Read 8867 times)

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markr

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Capsize Screening Ratio - wanders to solar
« on: March 03, 2010, 06:32:43 PM »

Hi, does anyone happen to know the "capsize screening ratio" for a 1987 MK1 standard rig, standard fin keel 5'7"draft?
i.e. the number that is considered good if it is less than 2.00 for offshore sailing.
There was a very useful post where a member did an assessment on his boat and arrived at a figure of 1.98. I cannot decipher the formula that he used to work out this number (my math symbol ability is sadly lacking!). He reports that his boat has a tall rig and a fin keel, so I am wondering if this number is different or even better with a fin keel and standard rig.
It is however encouraging to know that if the figure is 1.98 or better, it is below the 2.00 threshold and makes the 34 a blue water capable boat in terms of stability.....
« Last Edit: March 12, 2010, 11:01:41 AM by Stu Jackson »
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Rick Johnson

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Rick Johnson, #1110, 1990, s/v Godspeed, Lake Travis, TX

Albreen

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Re: Capsize Screening Ratio
« Reply #2 on: March 03, 2010, 06:58:05 PM »

You may wish to look at the web site "Sail Calculator Pro" for the ratios. If the C34 dimensions are correct, it lists both versions of the C34 above 2.00.
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Paul Leible
1987 C34 "ALBREEN", SR/FK, M25XP
Sailing Lake Champlain

markr

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Re: Capsize Screening Ratio
« Reply #3 on: March 03, 2010, 08:16:01 PM »

Thanks for that info - there seem to be a few numbers out there: 1.98, 1.96 and 2.08!
Any info on the AVS on a MK1 - and any other capsize/stability information?
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waterdog

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Re: Capsize Screening Ratio
« Reply #4 on: March 04, 2010, 08:17:51 AM »

I sat beside somebody at a pool in Las Hadas and they said "Isn't it great to be doing our PhD's in Blue Water Cruising?".     I said I don't really qualify for candidacy because I had no plans to cross any oceans.  They said, "Are you kidding?  You sailed down from Vancouver off the coast of Oregon, you are a blue water cruiser."   It would probably require several rum drinks to settle this decisively.   

With respect, whether one of the numbers is a 1.9 or a 2.1 doesn't make a damn bit of difference.   What are you planning to do offshore?   Are you going to cross an ocean or run the coast a ways off?    The boat will happily cross oceans if prepared properly.   The question is are you comfortable with the notion of this boat in any weather (the true bluewater test where you have to take what's coming because the forecast at the time you left was only good for a few days)?

This is a lively boat.  It's not some full keeler where you can lash the helm and go below and make a pot of tea in a gale.   It will surf at twelve to thirteen knots in a moderately large sea.   You need to have a strategy for controlling that speed or you will broach.   How will you steer?  For three days? (not your autopilot).   It has a huge cockpit with relatively low freeboard.   How will you get the water out when a sea breaks into it?   How will you stop the water from downflooding through the huge locker lids?   When the wave rips off your anchor locker lid and puts a quarter ton of water in your bow, how will she perform?   When it rolls over (think upside down) what will your batteries, engine, and all the contents of your lockers be doing?   How's your rig?  Your rudder?  Your ground tackle?

So the numbers are a good armchair starting point, but not really the deciding factor in sea readiness for blue water.   You are on the light fast end of blue water boats.   Develop strategy and approach around that.   Most importantly find a day off San Diego when the seas are 15 feet and the wind is blowing about 20.  Go sailing.  Are you comfortable?  This boat can be prepared and is a great boat for cruising.   Like any other boat, it will take some time and money to get it ready.   You'll find a lot of lesser vessels on islands in the middle of the Pacific. 
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Steve Dolling
Former 1988 #804, BlackDragon - Vancouver BC
Now 1999 Manta 40 cat

Rick Johnson

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Re: Capsize Screening Ratio
« Reply #5 on: March 04, 2010, 09:07:26 AM »

Very well said....
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Rick Johnson, #1110, 1990, s/v Godspeed, Lake Travis, TX

Ken Juul

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the math side of things
« Reply #6 on: March 04, 2010, 09:24:25 AM »

I agree totally with what was said above.  Some more details.

The Capsize Ratio (C/R) has nothing to do with mast height or type of keel.  It is an empirical value that predicts how resistant a boat is to a knockdown.  It was developed by the technical committee of the Cruising Club of America after the 1979 Fastnet disaster.  It is determined by dividing the beam by the resultant of displacement/64 raised to the 0.33 power.  From the Owners Manuals, the Mk I & II beam/displacement numbers are close enough be the same, 11.75/12000#.  That gives an average design CR of 2.089.  In the design numbers, the shoal draft keel actually gives a slightly better CR because the displacement is 500# heavier to make up for the lack of moment arm.  Looking at the reality side of things, most C34s in cruising configuration probably have a displacement closer to 16000#.  This lowers the CR to 1.89.
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Ken & Vicki Juul
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Lance Jones

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Re: Capsize Screening Ratio
« Reply #7 on: March 04, 2010, 09:25:00 AM »

Very good Waterdog :thumb:
That's true with everything from a Sunfish to a TP52. In many cases, YOUR capsize rating is just as, if not more, important than the boats. I have a couple of sailing friends who I would not want to be on their boat in any level wind as they don't have the comfort/confidence level in themselves even though their boats are capable of so much more.
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Lance Jones
1988  C-34 Kitty's Cat
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Jeff Kaplan

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Re: Capsize Screening Ratio
« Reply #8 on: March 05, 2010, 07:51:46 AM »

just want to share a quick story pertaining to this topic. last season, early may, we were on our final approach to salem harbor. one more tack needed. in the distance, maybe 1/4 mile, i noticed what appeared to be water funnels rising from the ocean about 10' into the air. i instantly called to come about, we were under full sail.  the next thing i remember was being on the sole, holding onto the wheel for dear life. we had been knocked over with what turned out to be a 70mph micro burst. as i lay on the sole looking out, not up at my mast,tall rig, water gushing over the combing into the cockpit,  i thought, holy sh_t, we're going to sink. i couldn't believe a 6 ton boat could be put right on her side, in an instant. one mate was on the high side holding on for dear life and i saw my other mate on the low,port, side. i screamed for him to let the genny go. once free, the boat started slowly to right herself. i have been sailing for 45+ years and have encountered many scary episodes at sea,but this was the most frightening. what it did prove was that the boat stayed together and would right herself if tension on sails were off. capsize ratios or not, take heed, be careful of mother nature, she will always win...jeff
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#219, 1986 tall rig/shallow draft. "sedona sunset" atlantic-salem,ma

horsemel

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Re: Capsize Screening Ratio
« Reply #9 on: March 05, 2010, 05:24:54 PM »

Jeff's story makes a very good point.  All the technical information about CR and other things is of no use to anybody if the captain does not keep his head and do the things necessary to stabilize any situation in which you might find yourself.  I am not nearly so experienced as Jeff, but I try to stay ahead of what might happen in the conditions of the day.  I learned a great lesson many years ago from an old army sergeant in a far away place on a very unpleasant night.  I was getting pretty excited over the circumstances overtaking us and in the middle of it all he stood there and said to me, "Main thing lieutenant is don't get excited!"  The captain of the vessel needs to keep his wits.
Mark Mueller
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Mark & Melinda Mueller
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markr

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Re: Capsize Screening Ratio
« Reply #10 on: March 05, 2010, 10:00:04 PM »

Thanks everyone for your input. Ken gets my award so far for "closest answer to the question posed"! :-)
Background to the question: I am actually British, finding myself in San Diego due to marriage, and in possession of a wonderful 1987 34 MK1. We were out saiking today in fact. (In fact, not being a US Citizen, the Admiral actually owns the boat, so I am just the driver really - as she told me today....)
I am a qualified RYA Yachtmaster Offshore and I do appreciate the comments regarding oufitting the boat for longer passages and abilities of the crew etc....all very spot on....I am simply looking for theory and base facts to consider as part of my decisions about the future of this boat and any plans to go further afield....in terms of how far would I take her and therefore how much to invest in the 34 vs perhaps not investing so much and looking for a "blue water cruiser" (whole other debate, a lot of which I do not agree with..,, being really heavy is not the answer in itself!!!) for the "offshore" part.
So, if anyone has some basic "handbook" type facts about the AVS, capsize angles, sceening etc, that is great. Kens answer was very useful and informatative.
Actually, despite the numbers I would more than likely take this boat as far as I wanted to go....I am not looking for a floating condo like many are, am happy to "cruise in what I have"......for crying out load, I'm English, and I'm amazed that she actually has a shower, of all things....I'm used to bobbing about in the Channel washing out of a bucket of seawater....;-)
P.S. The engine overheated today in the way back into San Diego Bay from a great sail, freshwater heat/exchanger problem most likely, so we put her onto a mooring buoy to let the engine cool a bit while a friend came out to tow us back into the vicinity of our slip, where the engine went on for 5 minutes to dock the boat....some engine repairs to do now, so the "blue water" equipping plans will be delayed by reality for now....
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Stu Jackson

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Re: Capsize Screening Ratio
« Reply #11 on: March 06, 2010, 02:41:19 AM »

Mark, you might be interested in steve's 1500 mile report and then some: http://c34.org/bbs/index.php/topic,5270.0.html
« Last Edit: March 08, 2010, 09:36:06 AM by Stu Jackson »
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Stu Jackson, C34 IA Secretary, #224 1986, "Aquavite"  Cowichan Bay, BC  Maple Bay Marina  SR/FK, M25, Rocna 10 (22#) (NZ model)

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Albreen

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Re: Capsize Screening Ratio
« Reply #12 on: March 06, 2010, 05:39:45 AM »

I think markr may be attempting to determine the blue water readiness of his C34 and whether to invest $'s into the boat if he intends to travel long passages. IMHO, it is fair to answer Markr with saying the C34 was intended to be a solid, comfortable, well performing coastal cruiser and not a blue water boat. Modifications to components may make it more blue water ready, but it was neither designed, constructed nor intended for such use (see Practical Sailor C34 boat review). Waterdog infers correctly with
"What are you planning to do offshore?   Are you going to cross an ocean or run the coast a ways off?    The boat will happily cross oceans if prepared properly.   The question is are you comfortable with the notion of this boat in any weather "
Readiness, crew abilities and weather windows are all considerations for trip planning in addition to the capabilities of the vessel and the the conditions you choose to sail in.
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Paul Leible
1987 C34 "ALBREEN", SR/FK, M25XP
Sailing Lake Champlain

Albreen

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Re: Capsize Screening Ratio
« Reply #13 on: March 06, 2010, 05:56:57 AM »

This is interesting - the CE designations for Catalina models......see this link.
http://www.catalinayachts.com/certif.cfm
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Paul Leible
1987 C34 "ALBREEN", SR/FK, M25XP
Sailing Lake Champlain

waterdog

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Re: Capsize Screening Ratio
« Reply #14 on: March 08, 2010, 08:26:22 AM »

Very interesting.  So the 34 is A rated but not designed for ocean crossing.   It would be interesting to see the detailed spec on what an A or "Ocean" rating means.

Mark, if you are looking for a handbook, Chapter 3 of Beth Leonard's "The Voyagers Handbook" is an excellent piece explaining the various numerical criteria and qualitative design aspects for A Bluewater-Capable Yacht.  It's really worth a read.   In fact she includes a Catalina 34 in a comparison of stability meaasures for similar sized boats.   In the around 35 feet class she includes a Crealock 34, Tartan 34, Bristol 355, Contessa 35 all of which exceed all criteria.   The Catalina, Hunter, Baltic & J34 miss the mark.   

She lists the Catalina 34 as:

DLR 252
Righting Moment at 1 degree of heel 885
IMS Stability Index   113.9 (recommended > 120)
Stability Ratio 2.1 (recommended >2)
CSV 1.95 (recommended <2)

So the boat is right in the middle of this pack and just barely misses the mark for Bluewater on the numbers.   Reading the rest of the chapter will give you way more value than further numerical analysis.

We were in exactly the same place as you a year and half ago.   Tracey wouldn't let me sell the house so we opted to sail what we had and limit ourselves to a small little coastal cruise from Vancouver down to Zihuatanejo.   We just turned over 4000 miles on this trip coming into Banderas Bay the other night.   The boat is beautifully suited for this type of trip (prepared of course).  It seems we have the comfort performance and space of most of the 40 foot "bluewater" boats we run into.   We are very happy with the choice.   
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Steve Dolling
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