Sailboat stability

[PaulJacobs]

The March 2021 issue of Practical Sailor has a detailed article on the subject of sailboat stability.  The article presents a few examples of stability curves - namely righting moment vs. angle of heel.  The curves basically look like somewhat slightly distorted sine functions - starting at zero righting moment at zero angle of heel and then having the righting moment increase with heel until a maximum righting moment occurs somewhere between 50 degrees and 70 degrees.  Clearly, NONE of us want to be sailing at 50 degrees angle of heel, but it is reassuring to know that if this were to occur, one could release all sheets and the boat would be restored to near-normal.

The nitty-gritty details depend upon hull shape, ballast, displacement, cockpit size and a few other characteristics unique to a given sailboat design.  Even the C34 - with the closed cockpit MK 1, the open cockpit MK 1.5 and the broader stern MK 2 would have different stability curves.

Does anyone on this forum know if such stability curves already exist for any of the versions of the C34? 

Especially pertinent is the so-called "Angle of Vanishing Stability" or AVS.  This is the angle at which the boat will no longer right itself after a knockdown.  "Especially seaworthy" sailboats (viz. the kind that are often recommended for "blue water" open ocean crossings - typically have AVS values equal to or greater than 120 degrees.  The article goes on to present discussions about how to locate the metacenter, how sailboat manufacturers perform detailed tests to determine their sailboats stability curves.   Finally the article also provide two different techniques where one can "approximately" estimate the AVS value for a given sailboat.  IF these curves already exist - GREAT.    . However, if they DO NOT exist, since I am a quasi-retired physicist and the equations are not especially daunting, I will be happy to generate relevant stability curves as a service to the forum. 

While none of us ever want to flip our beloved C34's over, these curves are the basis for determining if a given design is considered appropriate for an ocean passage.  Aside from such things as beefing up the standing rigging, double checking the rudder and steering system, and having a means to securely close the companionway in a storm, having sailed three TransPacs many years ago, and having surfed a Cal 40 on 15-18 foot waves, I believe the C34 indeed HAS the inherent stability to survive even a very nasty storm at sea.  I just would like to know if that is really true.

[PaulJacobs]

Apparently nobody has available, or has found, any stability curves for the C34.  So .... I will proceed with the calculation of the "Angle of Vanishing Stability", or AVS.  According to the Practical Sailor article, one method for calculating the AVS for a sailboat is the "Wolfson approach", developed by Southampton University in the UK. 

Their basic equation is as follows:     AVS=100 + 400/(SV-10),     where SV is the so-called "Stability Variable", itself determined by the equation:

SV = (BxB)/ (BRxDCBxV^1/3)     

where:   B = beam (feet), BR = ballast ratio = ballast (lbs.)/ displacement (lbs.), DCB = "draft of the canoe body" (feet) and V = [displacement (lbs.) / density of sea water (or fresh if that is where you sail) or 64 lbs. / ft^3.  I will use Pleiades numbers (MK 1.5, TR, FK) as I have access to them. 

Thus, B = 11.75 ft., ballast= 5000 lbs., displacement = 13000 lbs. (12,000 lbs. for the boat plus about 1000 lbs. of food, water & fuel when in "cruising mode", thus BR = 5000 / 13000 = 0.385, DCB = 1.8 ft. = (draft of 5.7 ft. - 3.9 ft. height of the fin keel) and V = (13,000 lbs./ 64 lbs./ft^3)^1/3 = 5.88 ft.  Thus, SV = (11.75 ft. x 1.75 ft.)/(0.385 x 1.8 ft. x 5.88 ft.) = 33.88 (dimensionless).

Inserting this value into the AVS equation we obtain AVS = 100 +400 / (33.88 -10) = 100 +17 = 117 degrees.
Based upon the literature, and the generally accepted value of "AVS greater than or equal to 120 degrees", it seems clear that the C34 MK1.5 is extremely close to the criteria of having sufficient inherent stability for safe open ocean passages, assuming all systems continue to operate properly even through nasty storms and big seas.  As others have said, the boat can probably take more abuse than the crew!

[waughoo]

Very cool to see the "proof" of this "therom".

Alex

[KWKloeber]

I would check with Warren at CTY in Fla.

[Stu Jackson]

I've always thought the significant weakness of our boats, strong as thedy may be elswewhere, are the portlights, especially on the cabin.

Steve Dolling made some excellent points about the C34 in his Mexico trip reports.

https://c34.org/bbs/index.php/topic,5270.0.html

[Noah]

https://www.yachtingmonthly.com/gear/how-to-measure-your-yachts-stability-72953

Back in my IOR racing days the measurer would come to the boat and do an "incline" test with jerry cans of water tied to the end of the spinnaker pole.

[Frasse Pelle]

Hi Paul,

Thanks for sharing... I love those analysis. So much I did some WEB searching on the "Wolfson method" which was new to me. For those interested in more details the "Wolfson method" and its interpretation:

http://www.gerrmarine.com/Articles/StabilityPart2.pdf

I noted that the author suggest to measure the DCB at 1/8 beam off the center line. That according to them reduce the number by approx. 10% from the straight draft minus keel height number. Not sure if this rule of thumb applies to the CAT34, but using DCB at 1.6 vs 1.8 has a relatively significant impact reducing the AVS from 117 to 114. But what does it really mean? According to the article first graph... a knockdown with a vessel with an AVS of 120 would spent on average 2 minutes inverted. At AVS 114, it would take (again on average) near another 30seconds extra. Probably these 2 minutes would be the longest experienced by any sailor!

But, before being concerned about recovering from a knockdown, one should worried about not capsizing. Here there is the well known Cap Size ration [Beam/(disp/64)1/3]... and the Catalina 34 is doing ok at 2.06 where blue water vessels normally hit below the 2.0 mark. Many Beneteau's in the 35ft range are above 2.10 while at the other end of the spectrum Island Packets can hit near 1.8. This formula is again an approximation and getting the actual "stability curve" for the Catalina 34 would be more telling.

All in all, I agree with you that the Catalina 34 has great abilities in open ocean. Better than most Coastal Cruisers.

Francois
S/V Whitecaps
MK1.5 Hull 1292
San Diego

[Ron Hill]

Paul : I like your calculations, but I've always questioned the 12000 lb displacement!!!  The wing keel draft was published at 3.5? inches and suddenly Catalina said it became 4' 3" ? !!! (Sorry, I gave all my data sheets to Stu!). 

Personally, my wing keel C34 has a draft of 4' 5 inches and I've always suspected that the displacement was closer to 15,000+ lbs !!  I had some measurements from a "new" travel lift that confirmed it weighed more than 12,000 lbs !!

Long ago I bottom painted over the narrow white hull stripe, over the thin 1" colored strip and half way into the wide 3" colored stripe.

The only thing that makes a hull sink into the water deeper it's weight!!   

A few thoughts

[Frasse Pelle]

Ron,

Good point. My boat was weighted at 16000lbs at the Shelter Island Boat yard during the survey. This was full loaded (gas, water, dinghy engine, radar, tools, sail inventory, etc...). Assuming this is a bit more than 1000lbs... I can assume the total displacement near 15,000lbs. That would lower the capsize ratio below 2 but also lower the AVS.

Report from Shelter Island Boatyard during survey inspection:
Hailing Port: San Diego, CA
Beam: 11' 9" *
Weight: 16,000 lb. (travel lift's scale)
* listing specifications
Displacement: 11,950 lb. *

Francois
S/V Whitecaps
MK1.5 Hull 1292
San Diego

[Ron Hill]

All : When that draft increase in the wing keel C34 first came out, I hit up Gerry Douglas.  His reply was simply "The 3+ft draft was a design Goal"!!
I immediately went to the marina as my boat was in winter lay up and measured!!  That's when I confirmed the 4'3"+ and fully loaded to
4' 5" reality!! 

A thought
 

[Noah]

Science (and logic) aside, my wife always thinks we are going to "tip over", no matter what I say

[Jon W]

I have similar experience to Francois. My 1987 fin keel weighed 15,500 on Shelter Island Boatyard's new travel lift. That changes the AVS to 114.2.

Confusing the AVS reduces at 15,500, but the capsize ratio improves from 2.06 to 1.89 (at least according to the attached).

[PaulJacobs]

I am pleasantly surprised at the level of interest in this somewhat esoteric subject; esoteric until your sailboat is hit with a massive wave!  I agree that in evaluating the stability ratio    SR=B/(Disp. / 64)^1/3      INCREASING the displacement actually helps reduce SR , which is good.  However, increasing the displacement actually REDUCES AOV, which is not good.

Curiously, I think this makes sense.  Most of us are concerned with excessive heel and avoiding getting "knocked down".  Here, more displacement, especially if it is "down low" to lower the center of gravity will resist heeling.  I also agree with Ron & Francois and others that while the listed displacements - for both the FK and the WK - are around 12,000 lbs., it is likely that "very few" C-34's leave the dock at 12,000 lbs.  By the time you add in crew weight, full or nearly full water tanks, a full or nearly full fuel tank, spare parts, nuts, bolts, screws, washers, spare lines, spare blocks, extra batteries (Pleiades has four group 31 AGM batteries (3 house, 1 engine), pots, pans, dishes, glasses and three fire extinguishers I suspect she displaces at least 13,000 lbs., and if we are going cruising for two weeks, all the extra food and beverages are likely pushing her displacement close to 14,000 lbs. and perhaps even more.  Somehow, 16,000 seems a bit high- but some of you may also count a dinghy on davits, a larger crew, and possibly a small but wonderfully varied wine cellar!

Here are the values of the stability ratio (SR) as a function of displacement, assuming sea water @ 64 lbs./ft^3.
Displacement:           SR
12,000 lbs.             2.05
13,000 lbs.             1.99
14,000 lbs.             1.95
15,000 lbs.             1.91
16,000 lbs.             1.87

Clearly, since hardly ANY C34's leave the dock at 12,000 lbs. displacement we can safely say that virtually ALL of them have stability ratios UNDER 2.00!     Thus, we are all sailing C34 sailboats that are intrinsically quite stable and are extremely unlikely to capsize.  That said, however, if the conditions were sufficiently horrible (massive, 50 ft. breaking waves, hurricane force winds, ubiquitous semi-submerged cargo containers, and random sea monsters), the values of AVS - the angle of vanishing stability - would be below 120 degrees, and issues with water ingress through broken ports, the companionway, and even the Dorade boxes, would not be pleasant.  Then again, under such conditions I don't think the conditions on ANY vessel - including an ocean liner - would be very pleasant.