Fat or Thin Hull for Offshore

[Stumble]

Originally Posted by TeddyDiver

I wouldn't call 130deg AVS high when 120 is considered as a minimum for offshore sailing. A bit better than bad perhaps..

ps. sorry it's closer to 127deg


Not true necessarily, while a a wider boat will have higher max righting lever GZ the AVS will be less. So the areas on the positive side are quite close to each other.


But as you point out there are different kind of narrow boats as there are wide range of wide boats. Knowing just the length and beam has no value at all.

BR Teddy

Pretty much my point. Saying anything like 'all X designs share Y characteristic' is almost nonsense. All boats are engineered systems, and good designers can do all sorts of smart things to optimize one characteristic or another.

IMOCA 60's for example are one of the widest, lightest, lest ballasted boats on the planet. If you run the numbers its 'capsize ratio' of 2.98 indicated the boat is grossly unsutable for open water... Probably shouldn't be taken out of a mill pond. On the other hand there aren't many boats I would rather sail a southern ocean storm in.

The IMOCA's are also one of the few boats that simply don't have an AVS, with the keel canted they will always self right without outside assistance.


And. Don't know a single professional designer that uses, or even cares about the capcize screening ratio. It is a nonsensical number. If you really want some idea what the boats resistance to capsizing is RM (max) is a far better number to look at. You could easily design a structure with a CSR that is very low, that would be manifestly unsafe in any conditions, heck you could design one that will never float upright.

The CSR ONLY matters when discussing boats that are otherwise very, very similar law in hull shape, and ballast draft. Otherwise it is meaningless.

Just as an example you can lower the CSR of a boat by using progressively heavier mast sections. Because it has zero effect on beam, but adds to displacement. Under this formula given the same hull, the best CSR is achieved by using a solid lead mast section. Of course the boat may flip over at the dock, but it would have a really really low capsize stability ratio.

[thomm225]

Originally Posted by Jim Cate

Stumble (Greg),

Very good post, mate, well explained and pretty definitive. It addressed a lot of common misbeliefs that float around the sailing world.

Well done!

Jim

Stumble is a racer which explains why he said what he said. It's nothing special.

The OP was asking about cruising boats not Open 40's.

You should know better being an old cruiser.

Plus his post looks quite similar to this one on Sailnet from 2012

http://www.sailnet.com/forums/861191-post2313.html

[Cheechako]

Well, a narrow boat will likely: be faster on a given waterline, but need more ballast and may sail more heeled than a wider boat. Older designs were narrower. Doesn't mean they are worse or better... just different.

[KISS]

Originally Posted by Stumble

Don't know a single professional designer that uses, or even cares about the capcize screening ratio.

Ted Brewer:

http://www.tedbrewer.com/yachtdesign.html

CAPSIZE SCREENING FORMULA (CSF): Some years ago the technical committe of the Cruising Club of America came up with a simple formula to determine if a boat had blue water capability. The CSF compares beam with displacement since excess beam contributes to capsize and heavy displacement reduces capsize vulnerability. The formula is the maximum beam divided by the cube root of the displacement in cubic feet; B/Displ.333. The displacement in cubic feet can be found by dividing the displacement in pounds by 64, of course.

The boat is acceptable if the result of the calculation is 2.0 or less but, of course, the lower the better. For example, a 12 meter yacht of 60,000 lbs displacement and 12 foot beam will have a CSF Number of 1.23, so would be considered very safe from capsize. A contemporary light displacement yacht, such as a Beneteau 311 (7716 lbs, 10'7" beam) has a CSF number of 2.14. Based on the formula, while a fine coastal cruiser, such a yacht may not be the best choice for ocean passages.

In this article, Dave Gerr cites it as one of several key criteria for an offshore boat.

Dudley Dix includes CSN in the datasheet for all his designs.

It was developed in the first place by experts in the field.

It's not the be all and end all, but neither is it meaningless as you contend. A boat meeting the requirement is not necessarily stable. But a boat not meeting the requirement is suspect, pending some further explanation. That you have to cite the exotic Open 40 and the IMOCO 60 as counterexamples only proves my point that wider beam means less stability for most boats.

[Stumble]

Originally Posted by thomm225

Stumble is a racer which explains why he said what he said. It's nothing special.

The OP was asking about cruising boats not Open 40's.

You should know better being an old cruiser.

Plus his post looks quite similar to this one on Sailnet from 2012

SailNet Community - View Single Post - Interesting Sailboats

The numbers don't change just because you are a cruiser or a racer. Hull design as a composit of all the engineering decisions is what matters not one specific part of the equation taken in isolation. There is absolutely nothing wrong with a narrow cruising boat, but equally there is nothing wrong with a wide boat. They are solutions to a problem, that is all.

But the Capsize Ratio thing, ya that's crap. If you want a single number that actually tells you anything about how resistant to a capsize a boat actually is, the right number is RM(max). At least it will tell you how much force it takes to roll the boat over.

[Stumble]

Originally Posted by KISS

Ted Brewer:

Ted Brewer Yacht Design

In this article, Dave Gerr cites it as one of several key criteria for an offshore boat.

Dudley Dix includes CSN in the datasheet for all his designs.

It was developed in the first place by experts in the field.

It's not the be all and end all, but neither is it meaningless as you contend. A boat meeting the requirement is not necessarily stable. But a boat not meeting the requirement is suspect, pending some further explanation. That you have to cite the exotic Open 40 and the IMOCO 60 as counterexamples only proves my point that wider beam means less stability for most boats.

As Gerr pointed out in the article you listed...

Keep in mind; however, that the capsize screening number doesn’t mean much by itself. It must be used in conjunction with other characteristics discussed above when evaluating reserve stability.

So long as you restrict the use of the index to boats that were designed in the 70's sure it works fine. But all it really tells you is what generations of boat designers have knows, IOR resulted in dangerous boats that had dimensions that were all out of whack with good offshore boats. The ratio simply took the two major components of IOR design (lots of midship girth and light ballast) and designed a formula that 'proved' the boats were unsafe.

Fundamentally if a theory can't explain the data then it's a poor theory. If the stability index cant explain why ultra wide and very light boats are incredibly resistant to capsizing and prone to quickly self right when capsized then it simply doesn't explain the data and should be rejected as informative. Yes for old IOR designs the index does a good job of showing the boats were dangerous, but if it also can't explain why modern boats with even worse ratings are not dangerous then it doesn't work.

And you don't have to go to IMOCA's or Open 40's to show how it breaks down, any modern vessel with a bulb keel will do poorly. By moving the ballast deeper you can get away with building a lighter boat wife same RM. but your formula doesn't take that into account.

[KISS]

Originally Posted by Stumble

As Gerr pointed out in the article you listed...

Keep in mind; however, that the capsize screening number doesn’t mean much by itself. It must be used in conjunction with other characteristics discussed above when evaluating reserve stability.

Needing to be used in conjunction with other characteristics =/= "worthless bit of drivel."

So long as you restrict the use of the index to boats that were designed in the 70's sure it works fine.

Here are the stability curves for some modern designs.



The boats can be ranked as follows by area of positive stability (low to high):

IMS 33
LM 27
Dufour 385
IMS 41
Southerly 110 (keel down)
Malo 41

If you take the listed D and B and calculate CSN, they rank as follows (high to low):

IMS 33
LM 27
Dufour 385
IMS 41
Southerly 110 (keel down)
Malo 41

Well, how about that, there's a direct correlation!

If you look at the listed STIX number, you'll see that also correlates with CSN, which the exception of the Malo and Southerly (Malo has slightly lower CSN and slightly lower STIX number).

If CSN were meaningless, this would have to be a coincidence.

[Stumble]

You keep trying to justify a ratio that simply isn't predictive of the real world. Let's take your own example for a moment. The CSN for the Southerly 110 is exactly the same if the keel is up or down. It makes zero difference because the ratio simply doesn't account for the depth of the ballast. In fact it doesn't account for if the boats displacement is ballast or gear stored on deck.

Your ordering of the CSN should have read...

iMS 33
LMS 27
Dufor 385
IMS 41
Southerly 110 (keel down)
Southerly 110 (keel up)
Malo 41

So did you leave it off the chart because doing one more calculation was to difficult, or because you didn't realize it was there?

Note that while the CSN is exactly the same regardless of where the keel is the AVS changes substantially, as well as the RM(max), and the STIX, and the area under the positive curve. I don't know about you, but I think in heavy weather I will lower the keel, even though by your argument it doesn't make the boat any safer.

[KISS]

Originally Posted by Stumble

You keep trying to justify a ratio that simply isn't predictive of the real world.

If I had looked at those boats only knowing their CSN, and predicted that the ones with the lower CSN would have greater areas of positive stability, higher AVS, higher ratios of positive to negative stability, and higher STIX numbers, I would have predicted correctly, wouldn't I have?

Let's take your own example for a moment. The CSN for the Southerly 110 is exactly the same if the keel is up or down. It makes zero difference because the ratio simply doesn't account for the depth of the ballast. In fact it doesn't account for if the boats displacement is ballast or gear stored on deck.

Your ordering of the CSN should have read...

iMS 33
LMS 27
Dufor 385
IMS 41
Southerly 110 (keel down)
Southerly 110 (keel up)
Malo 41

So did you leave it off the chart because doing one more calculation was to difficult, or because you didn't realize it was there?

Note that while the CSN is exactly the same regardless of where the keel is the AVS changes substantially, as well as the RM(max), and the STIX, and the area under the positive curve. I don't know about you, but I think in heavy weather I will lower the keel, even though by your argument it doesn't make the boat any safer.

Again, nowhere have I said that CSN is the sole determinant of stability.

Obviously other factors (like having a keel or not!) play a role.

I'm saying that CSN is a meaningful indication of stability.

...contrary to your claim that it is "a worthless bit of drivel."

[robert sailor]

Dashew preferred skinny boats for serious offshore sailing, easily driven with smaller rigs that could knock off 300 mile days. Modern pizza cruising designs are also a choice but they are not all that quick, certainly not compared with their racing cousins. . but as others have said they are very roomy. There is something to be said about both of these choices, they are after all just different options for doing the same thing. I personally like many of Dashews ideas but you do end up with a much smaller boat than the beamy alternative. So go fat or go skinny..both will do the job.

[vtomanov]

There is a lot of good information here - generaly old narrow is safer...

Understand your boat and her statistics


brief comparison :

https://keyassets.timeincuk.net/insp.../LRGZCurve.jpg

[Snowpetrel]

Interesting fairly recent research found that the chance of a complete capsize can be reasonably correlated with the area above the RM curve between 90 deg and AVS.

Another intereting thing is that a lot of the old RM curves dont include the deck and cabin in the calculations. Most modern ones do. Be careful when comparing them, as cabin structure and deck camber significantly reduce inverted stability, and often increase AVS.

Thats how a lot of these modern designs get to cat A, big high volume cabins.

I prefer narrower boats. They just seem more efficient at sea and comfortable. Offshore its hard to beat length. Inshore its harder to pay for length...

A wise man once said moderation in all things!

Sent from my SM-G930F using Cruisers Sailing Forum mobile app

[Don C L]

Intuitively it just seems that skinny is asking less of the ocean to get out of the way than fat is, unless fat is planing, but I am always ready to accept challenges to my intuitions.

[dennismenace111]

IMHO....there is absolutely nothing worse than beating into a sea and have the boat pounding every time it comes down on a wave... I ll take a thin boat anyday.. Recall years ago, Hunter made a 54' foot model.. it was a bleach bottle for sure, but it was really comfortable and really fast.. It was a Racer/Cruiser in the true sense..

"Here's to Fast Women and Tall Boats... or was that Tall Women and Fast Boats?"

[stevensuf]

Originally Posted by KISS

The wider the beam, the greater the initial stability. Initial stability is resistance to initial heeling. A boat with higher initial stability will heel less initially for a given force applied abeam (like wind or a wave). This makes them faster (can carry more sail), but also less likely to self-right if they capsize. Think of a flat raft (wide beam) versus a kayak (narrow beam). The kayak is tippier, but easy to recover if you flip. The raft is less tippy, but if you flip it, you're pretty much cooked (this is why you occasionally find cat owners talking about putting access hatches on the underside of their hulls).

Wider beamed boats tend to have shallower hulls and higher centers of gravity (they don't have to, but this is the norm), which means they have less reserve stability. Reserve stability refers to the angle of heel a boat can reach before it capsizes. Narrower, deeper, better ballasted boats will have greater reserve stability, and be able to survive more extreme heeling angles without capsizing.

So, in general, I'd say a narrower boat is better for extreme conditions.

It will be more tender, but it's less likely to actually capsize, and better able to recover if it does.


The reason wider boats are more popular is speed and living space.

Consider the Capsize Screening Number, one metric of offshore seaworthiness:

Beam / cubic root of Displacement

Lower is better, 2.0 being the maximum (1.7 for high latitude sailing).

So the smaller the beam for a given displacement, the better.

The above is partly incorrect , the beamier boat is less likely to capsize, but less likely to self correct quickly, breaking waves capsize boats, the greater the beam, the bigger the breaking part of the wave has to be to capsize the boat.

This applies to boats that will self right, ie AVS greater than 90.

Say we have 2 boats of 45 ft, one with 10ft beam the other 15, all things being equal the boat with 10ft beam will have a much higher avs and will self right much quicker, but willr equire a much smaller breaking wave to capsize, the boat with 15ft beam will need a much larger wave to self right, but again a much higher wave to capsize in the first place.