A Question About Hull Characteristics

[thomm225]

Originally Posted by Don C L

This video is widely circulated and touted as a boat in trouble but really they were just trying to sort out some problem below from what I have read elsewhere. This deep, narrow hull is not rolling that wildly considering she is running, and in fact at the end, once she turns and is broad reaching instead of running, you can see how she is more stabilized and the hull allows the swell pass under her without adding much to the heel. Not the best video to demonstrate it, but

This boat, a 12 meter "Anita" built in the 1930's in Germany, had suffered major damage due to a hurricane and a knockdown or two in the North Sea.

Several of the crew were injured.

Video was taken by the rescue helicopter that was unable to assist.

During the autumn of ’97 a hurricane in the North Sea generated such monster waves that Anita was knocked down and partly capsized. Major damage was sustained. The doghouse was smashed, the mizzenmast broken and lots of deck equipment washed away. Some of the crew also suffered severe injuries but under the watchful eye of the German Lifeboat service Anita made it back to Helgoland in relatively one piece.


The History of the 12M Yacht

https://www.youtube.com/watch?v=IV0TbR5YAYQ

[thomm225]

Anita after refit/restore.

https://www.youtube.com/watch?v=GjVWovh6ZZg

[Bowdrie]

^ Thanks for posting that.
My searches have proved fruitless, but some decades ago a French sailor got one of the 12 Meter boats that had been built for Baron Bich, (Bic pens/lighters,) and converted it for cruising.
He then sailed it from France to Tahiti, he described it as a "Great sea boat".
Short on the water, narrow, and LOTS of lead.

[Adelie]

Originally Posted by Lee Jerry

CSF is dimensionless and therefore doesn't scale at all. (That's kind of the point of most of the design ratio thingies; some achieve it better than others.) Which is why you can't just blindly compare different CSF numbers. You need to look at other factors along with it to give context (or scale) to what the measure actually is. Something like length, for instance. That's the point I've been trying to make.

So for me, the answer is pretty easy - I'd pick Boat A because it is the longest of the three. Looking only at CSF you might think they would all perform the same (in waves), but the longer one will clearly ride better and require more energy to capsize it.

The result of CSF is dimensionless but the input values are not so there are scaling issues involved.

The Beneteau 37 is 5' longer and 4500lb heavier than a Contessa 32. But I'd rather be in the Contessa during a hurricane at sea. Length is not the sine qua non of capsize resistance. Nor is the CSF, but it provides a more educated estimate than anything else.

Originally Posted by Lee Jerry

No, I don't believe this is true. Firstly, I doubt it happens very often, if ever, but is certainly not the general case. But secondly, it doesn't matter; what matters is the lateral separation of the centers of gravity and buoyancy - the righting arm. As long as it is positive, the vessel will right itself (in a static sense, or work to right itself in a dynamic sense).

See the left and bottom right boats in the attached graphic. In the frame of reference of the boat, center of floatation has moved to be above the center of gravity. If it had not, the boat would go turtle from 90*. In the frame of reference of the world, it is still below.

Originally Posted by Lee Jerry

I still don't think you can make this generalization based only on the beam. If by "wide boats" you mean low length-to-beam ratio, then that starts to get a little better (because that defines the total shape at least a little more). But the shape of the hull (say, as represented by the midship section) has a large influence too. So what you describe is probably accurate for a modern wide, flat sled (Open classes, VOR, late model Beneteau, etc.), but you can have another boat with the same beam (and length) with a more wine-glass shaped section and it will have a flatter run out of righting arm, i.e. righting arm will decrease less quickly. Beam alone cannot tell you about the shape of the righting arm curve.

See the bottom and top right boats in the attached graphic. You can see that there is a longer distance between CoF and CoG for the wider boat. That means that for the wider boat the lever arm F-G will decrease faster than for the narrower boat. In addition, it means that the AVS will be higher for the narrower boat.

But the affect of beam on the righting curve has nothing to do with what it does to capsize resistance. Beam gives a breaking wave a larger area with a longer moment arm in order to drive a roll.

[Adelie]

Originally Posted by Don C L

....But remember (almost) every boat with a heavy keel is inherently unstable upside down, the sea state that caused the capsize will soon help it recover. ....

Most if not all boats are stable in the inverted position. They remain stable in this position thru a more limited range than in the upright position. The "wider" the boat, the wider the range of stability in the inverted position.

[rslifkin]

Originally Posted by Adelie

Most if not all boats are stable in the inverted position. They remain stable in this position thru a more limited range than in the upright position. The "wider" the boat, the wider the range of stability in the inverted position.

It's not quite that simple though. Ballast weight and location as well as the volume of the deckhouse will have a significant effect. If all else is kept equal, a wider boat will be more stable upside-down than a narrow one. But if the wider boat has a higher volume deckhouse (makes the boat float higher in the water while inverted than it otherwise would) and a bulb keel (more leverage as the boat rolls around a bit while inverted) with much of its weight way out at the end, that'll knock off some stability when inverted and make it easier to get the thing to roll back upright.

[Don C L]

Originally Posted by thomm225

This boat, a 12 meter "Anita" built in the 1930's in Germany, had suffered major damage due to a hurricane and a knockdown or two in the North Sea.

Several of the crew were injured.

Video was taken by the rescue helicopter that was unable to assist.

During the autumn of ’97 a hurricane in the North Sea generated such monster waves that Anita was knocked down and partly capsized. Major damage was sustained. The doghouse was smashed, the mizzenmast broken and lots of deck equipment washed away. Some of the crew also suffered severe injuries but under the watchful eye of the German Lifeboat service Anita made it back to Helgoland in relatively one piece.


The History of the 12M Yacht

Thanks for straightening me out in that Thomm. It came back to me once you posted it. Still boat looks pretty good for having been knocked down and capsized!

Btw, does anyone here know why the GZ is not included in the CSF?

[Don C L]

Originally Posted by Adelie

Most if not all boats are stable in the inverted position. They remain stable in this position thru a more limited range than in the upright position. The "wider" the boat, the wider the range of stability in the inverted position.

Yeah, guess my view is biased toward the older designs.😄

[Lee Jerry]

Originally Posted by Adelie

The result of CSF is dimensionless but the input values are not so there are scaling issues involved.

The Beneteau 37 is 5' longer and 4500lb heavier than a Contessa 32. But I'd rather be in the Contessa during a hurricane at sea. Length is not the sine qua non of capsize resistance. Nor is the CSF, but it provides a more educated estimate than anything else.

I agree with a bit and disagree with a bit. But rather than go into those details I think I'll just say that I think this supports what I've been saying all along, which is that you can't make conclusions based on one parameter (initially that was beam, then switched to CSF).

See the left and bottom right boats in the attached graphic. In the frame of reference of the boat, center of floatation has moved to be above the center of gravity. If it had not, the boat would go turtle from 90*. In the frame of reference of the world, it is still below.

Yeah, I figured out what you were saying. It's just a strange use of "above." Especially for someone who calls it "gravimetric" stability. Doesn't gravity imply earth reference?

If you're doing a handstand, is the floor above you?

See the bottom and top right boats in the attached graphic. You can see that there is a longer distance between CoF and CoG for the wider boat. That means that for the wider boat the lever arm F-G will decrease faster than for the narrower boat. In addition, it means that the AVS will be higher for the narrower boat.

But the affect of beam on the righting curve has nothing to do with what it does to capsize resistance. Beam gives a breaking wave a larger area with a longer moment arm in order to drive a roll.

Your sketches have a bunch of assumptions buried in them (Are the wide and narrow boats equivalent? In what way(s)? Same displacement? Same keel & ballast? Same mast height and sail area, even if reduced during storm?). And of course, they're just a 2d view of a 3d problem. (I know we communicate through a 2d medium.) However, I see how you can come to that conclusion based on them, but...

AVS isn't the only measure or component of stability (or capsize resistance):
Which boat has higher GM?
Which boat has the larger maximum righting arm?
Which boat has the larger maximum righting moment?
Which boat has the most area under the righting arm curve?
How much energy did it take to get each boat into the 90 deg position shown? Is it the same for both?

All of these questions can/will factor into capsize resistance. And can't be reduced to a single indicator, either beam or CSF.


(Notes on nomenclature - center of flotation is the center of the waterplane area, the center of buoyancy is the center of the immersed volume; F-G is not the righting arm, GZ is, the horizontal distance between weight and buoyancy vectors.)

[sailingharry]

I've read through this thread, with lots of good information. I'll make two comments:


* Our boat, a Saga 43, is 43' long, 12' wide, and draws 7'. She has a deep round bottom, deep enough that we carry 100 gallons of fuel and 25 gallons of waste under the floorboards and still have a deep "bilge" area. She also was born just shy of 20,000 pounds, with 7,800 of that in ballast, for a B/D ratio of .39 (and 500 was added to the bottom of the keel by a PO). Compare that to a Bene 43, with 13.5 feet of beam and a B/D of .32. A combination of our narrow beam and round bottom defeats the B/D, and makes us quite tippy. We are just getting started with 15 degrees of heel, and as the Admiral says, "it's life at 30 degrees." Form stability is initial stability, and I bet that Bene is much flatter on a days sail! (note, Bob Perry who designed the boat, vehemently disagrees with my opinion that a 43' boat with 12 feet of beam is "narrow.").



* I'm not sure that long keels with attached rudders track better. I know they don't turn worth a damn, but is that because they track better, or can't steer? We have a rather narrow fin keel, with a fully detached rudder even with the transom -- and in heavy seas/winds offwind, the autopilot just tweaks a bit back and forth. Maybe she doesn't track well, but she corrects like a dream, which is the same end result. Some of it is hull form -- my last boat, a Sabre 34 with longer keel to support a centerboard and a partial skeg rudder, but an IOR influenced hull design, was a beast off the wind in a seaway, a true aerobic exercise!

[sailorladd]

I beg to differ on at least one of these assumptions, the longer the keel the LESS the yawing on axis action will occur, and I might add too that the heavier the displacement the more sluggish will the overall reaction be to wave motion, making the boat more comfortable on a long sail, which in my view is one of the most important considerations when undertaking a long sail ...

[Bowdrie]

Originally Posted by sailingharry

Form stability is initial stability, and I bet that Bene is much flatter on a days sail! (note, Bob Perry who designed the boat, vehemently disagrees with my opinion that a 43' boat with 12 feet of beam is "narrow.").

an IOR influenced hull design, was a beast off the wind in a seaway, a true aerobic exercise!

Haha, I can see the view that Mr. Perry has, and large beam is certainly not a new idea.
In front of me now are the lines of a 40' that has 14' of beam, but only a little over 11' at the LWL, it was designed in 1909.
The designer understood that in lighter air conditions that produce little heeling, a greater ratio of LWL to waterline beam made for a more easily driven hull, but as more heeling occurred the hull form would resist it.
I think that ever since the first "racing rules" were thought up, the proponents have had pretty much only one thought;
"What attributes that make a boat go faster can we penalize?".
Racing rules are designed for, (drum roll,) racing, and much of racing is done on a more-or-less triangular course in reasonable weather.
As such the greatest challenges for the designer have usually been two factors.
1, Beat the rule to get a better rating.
2, Maximize upwind ability.
Yes, the IOR, thru its iterations, produced some "sketchy" hull shapes, bustles and bumps come to mind in the quest for lower rating.

[sandy stone]

Originally Posted by Don C L

Btw, does anyone here know why the GZ is not included in the CSF?

Because the CSF was created as a quick and dirty, back-of-the-envelope calculation to give a rough comparison between largely similarly shaped hulls, using only widely available, published data.

It is a wonder to me that people still consider a certain value of the CSF to be the definitive qualification for a "blue-water" boat, calculated to three significant figures, when a boat's displacement (for instance) is rarely known to much better than two significant figures.

[wingssail]

Originally Posted by sailingharry

...Our boat, a Saga 43, is 43' long, 12' wide, and draws 7'. She has a deep round bottom, deep enough that we carry 100 gallons of fuel and 25 gallons of waste under the floorboards and still have a deep "bilge" area. She also was born just shy of 20,000 pounds, with 7,800 of that in ballast, for a B/D ratio of .39 (and 500 was added to the bottom of the keel by a PO). Compare that to a Bene 43, with 13.5 feet of beam and a B/D of .32. A combination of our narrow beam and round bottom defeats the B/D, and makes us quite tippy. We are just getting started with 15 degrees of heel, and as the Admiral says, "it's life at 30 degrees." Form stability is initial stability, and I bet that Bene is much flatter on a days sail! (note, Bob Perry who designed the boat, vehemently disagrees with my opinion that a 43' boat with 12 feet of beam is "narrow.")...

- my last boat, a Sabre 34 with longer keel to support a centerboard and a partial skeg rudder, but an IOR influenced hull design, was a beast off the wind in a seaway, a true aerobic exercise!

I don't agree that your boat is narrow (and I do agree with Bob Perry about that and most other things). My boat is only .9' beamier and is considered "beamy".

When I compare your boat to mine I see that you have more sail area for the displacement and less displacement for the length (so you are lighter and more powered up). Maybe your problem with being tippy is due to carrying too much sail for the conditions.

And lets get rid of the obsession with numbers like Capsize Screening Formula and Comfort Ratio. Deciding what is a "blue water boat" by looking at a bunch of numbers is a waste of time. By most of these kind of measures my boat is no way a blue water boat. Yet is has been happily cruising in blue water for 30+ years. It has never capsized or come near to that, but my capsize screen formula, at 1.99, at the verge of being too dangerous to even go to sea. We are not "tippy" because we reduce sail. Even though it is a classic IOR design, it IS NOT a hand full or an aerobic exercize off the wind even in a sea way. In fact the monitor windvane steers the boat in ALL conditons. We just relax and ride. Up wind we don't find that it pounds but it does make miles going to weather when our friends with more "cruising" boats are nearly stopped dead trying to motor upwind in waves.

Does our fin keel boat track well? Absolutely! Because the rudder keeps it on course.

Did IOR result in "sketchy" hulls shapes? Wierd maybe, but they do fine.

We've cruised around the world. Our boat is fast and comfortable and easy to sail. We typically cover 150-175 miles a day while sitting on our butts. We get plenty of sleep and take showers and arrive clean, well rested, and first.

This winter we took a little trip, 150 miles each way and it was windy and rough. Down wind we had breeze in the mid 20's and it was quite rough. But we were hitting 9's and 10's and saw 11knots a couple of times. The windvane steered and the half full coffee pot on the stove never spilled. Coming back we had true wind speed of 25-29 knots and waves as big as cars. The last leg was 147 miles direct upwind. We sailed (beating) the whole way in 30 hours. Our track covered 175 miles. It was rough and wet but the pounding was not terrible. When we got home we really didn't have to do anything except put our wet clothes in the laundry. Then we took off the dodger and wind vane and entered a major 4 day regatta and got second place. Tomorrow we race again and expect to win.

All of this in a IOR boat that, by the numbers, should never be considered to be a good all around sea boat or a blue water boat.

Get your head out of the numbers and do more sailing. You can find a well designed boat and it will do you well

[Bowdrie]

Originally Posted by wingssail

Did IOR result in "sketchy" hulls shapes? Wierd maybe, but they do fine.
All of this in a IOR boat that, by the numbers, should never be considered to be a good all around sea boat or a blue water boat.

Wingsail, in no way was I intending to disparage a boat like yours.
I've always thought that the early S&S Swans, (~'68>'72ish,) were exceptional sea boats and they were designed to "fit into" the IOR rule.
The later ones by Farr and Holland were far more "racier" and started exhibiting the "bustles and bumps" that I referenced, but changes in the rule pushed the designs.
Perhaps "sketchy" was a poor choice of words. , but I do think they lost much of the "easy on the eyes" aesthetics of the S&S boats.
Of course, the underbodies were much different.
The later IOR boats with the bustles and bumps did exhibit easier downwind handling when being pushed hard, and on the S&S Swan forum that is acknowledged.
When I say, "Pushed hard", I mean that a racing crew is likely to keep more sail up for longer in conditions than a cruiser might.