Capsize Ratio

[Mike OReilly]

Originally Posted by DMF Sailing

It's real close! Two completely different boats, two completely different philosophies

(until you get to comfort)

Thanks. Interesting. I've never even been on a Hanse 37 so can't say much about the comparison. Looks like the scale of the bar graphs might be a bit skewed or off when looking at the SA/D and D/LWL. The bars looks identical, but they are significantly different.

As I said, hours of fun. Not to be taken too seriously. There's so much the raw numbers don't say, but it's a good way to compare similar styles of boats, and maybe find prospects. I'd have never considered a Hanse 37, but now I might.

Originally Posted by Lexi22

These calculations are my new obsession.

New?

[Lexi22]

Originally Posted by Mike OReilly

New?

I don't know... What does it say above?!

[Greg K]

Originally Posted by Jim Cate

what is not discussed in those figures is how often and in how much minimum wind will the two boats reach their similar hull speeds? And by how much will they exceed those theoretical limis?

I think we all know the answer to that one!

Jim

Yes, it's obvious from the numbers, especially the huge difference in displacement, that the Hans will accelerate in light winds like a rocket ship. Great fun for inshore racing and coastal cruising, but for offshore ocean voyaging the Rafiki 37 wins hands down because comfort is certainly a safety factor... a comfortable crew is a rested crew, and a rested crew is less prone to accidents and errors. Also, what's not in the numbers is the ballast/displacement ratio which is also a stunning difference between these two boats. The Rafiki at something like 43% ballast will, in moderate conditions, remain as stiff as the Hanze which with a ballast/displ ratio of only 30% depends much on its hull form for its initial stability. Now, crank up the sea conditions to Force 8 or 10 and add a thousand miles of fetch and some sea mounts, and the Hanze is starting to look like a dangerous boat because, whether it's actively sailed or not, in such conditions once it gets pushed beyond it's hull induced form stability, it's going over real fast and, (with that flat, wide aft section and low ballast keeping it stable while inverted), possibly staying there for a while. In similar conditions, the heavy Rafiki would probably just ride it out safely hove to or, if capsized, roll right through...taking care of itself with nobody on deck to worry about.

Is that light wind acceleration advantage a prudent trade off in the off shore environment? for shorthanded mom and pop crews? I think not.

[Mike OReilly]

Here's an interesting source I found for calculating your AVS (Angle of Vanishing Stability). It's beyond my pay grade to assess if this is accurate, but it seems authoritative.

https://www.yachtdatabase.com/en/encyclavs.jsp

Using this I calculate my boat to having an AVS of 140.4º, although I had to estimate the DCB value (which is the draft of just the hull, NOT including the keel).

Originally Posted by Lexi22

I don't know... What does it say above?!

Sorry Lexi, I missed the "my" part -- My bad . I completely get it. I was obsessed for a while as well. As I say, hours of fun comparing different boats,

[Mike OReilly]

Originally Posted by Greg K

Yes, it's obvious from the numbers, especially the huge difference in displacement, that the Hans will accelerate in light winds like a rocket ship. Great fun for inshore racing and coastal cruising, but for offshore ocean voyaging the Rafiki 37 wins hands down because comfort is certainly a safety factor... a comfortable crew is a rested crew, and a rested crew is less prone to accidents and errors. Also, what's not in the numbers is the ballast/displacement ratio which is also a stunning difference between these two boats. The Rafiki at something like 43% ballast will, in moderate conditions, remain as stiff as the Hanze which with a ballast/displ ratio of only 30% depends much on its hull form for its initial stability. Now, crank up the sea conditions to Force 8 or 10 and add a thousand miles of fetch and some sea mounts, and the Hanze is starting to look like a dangerous boat because, whether it's actively sailed or not, in such conditions once it gets pushed beyond it's hull induced form stability, it's going over real fast and, (with that flat, wide aft section and low ballast keeping it stable while inverted), possibly staying there for a while. In similar conditions, the heavy Rafiki would probably just ride it out safely hove to or, if capsized, roll right through...taking care of itself with nobody on deck to worry about.

Is that light wind acceleration advantage a prudent trade off in the off shore environment? for shorthanded mom and pop crews? I think not.

Indeed, the Rafiki 37 (there is also a Rafiki 35 which is quite different) is pretty kindly to the crew. Of course Jim is correct, it takes more wind to get her moving. With a displacement of nearly twice the Hanze, she's a heavy 37-footer. But the full keel also means a lot more surface friction, so it's even more so.

That said, I don't find it hard to reach hull speed with wind abeam or aft. She's a down-wind sailor, so doesn't take much. Of course beating to wind, well, that's a different story. Let's just say, I'm definitely a 'gentleman sailor' .

[DMF Sailing]

Originally Posted by Greg K

Yes, it's obvious from the numbers, especially the huge difference in displacement, that the Hans will accelerate in light winds like a rocket ship. Great fun for inshore racing and coastal cruising, but for offshore ocean voyaging the Rafiki 37 wins hands down because comfort is certainly a safety factor... a comfortable crew is a rested crew, and a rested crew is less prone to accidents and errors. Also, what's not in the numbers is the ballast/displacement ratio which is also a stunning difference between these two boats. The Rafiki at something like 43% ballast will, in moderate conditions, remain as stiff as the Hanze which with a ballast/displ ratio of only 30% depends much on its hull form for its initial stability. Now, crank up the sea conditions to Force 8 or 10 and add a thousand miles of fetch and some sea mounts, and the Hanze is starting to look like a dangerous boat because, whether it's actively sailed or not, in such conditions once it gets pushed beyond it's hull induced form stability, it's going over real fast and, (with that flat, wide aft section and low ballast keeping it stable while inverted), possibly staying there for a while. In similar conditions, the heavy Rafiki would probably just ride it out safely hove to or, if capsized, roll right through...taking care of itself with nobody on deck to worry about.

Is that light wind acceleration advantage a prudent trade off in the off shore environment? for shorthanded mom and pop crews? I think not.

All true (as far as the bits I can confirm from actual experience on the boat I've sailed is concerned), and thanks for posting!

Please note that my comparison was somewhat facetious. The only things that made me compare these two boats were 1) the existence of that online calculator and 2) the fact that Mike's boat is a 37 built for offshore passage.

Having been on that Hanse in a Force 8, I wouldn't take her outside sheltered waters in Force 6.

That said, in a Force 5 with a single reef, she can be sailed by an 8-year-old. And when it's time to maneuver through a mooring field, that Hanse behaves like a Porsche on greased rails. I could dock her all day.

[sandy stone]

While we're having fun comparing numbers, consider this - one of the inputs to most of these formulas is the least accurately known statistic on a boat, the displacement. People calculate the capsize ratio of a particular boat, then say 1.98 is good and 2.03 is bad - but the displacement used in the calculation is only accurate to 5 or more likely 10%. The same thing applies to motion comfort ratio, SA/D, D/L, and even ballast ratio. It is very informative to repeat the calculations for a particular boat, only increasing the displacement by 10% (probably a pretty good real-world estimate) and see what happens.

Also, most of the measurements used in the formulas are based on a bare, factory stock boat. Adding a simple thing like a jib furler can have a surprising effect on righting moment, to say nothing of an in-mast roller furling main, or a second furling jib on a solent stay. Then, as mentioned above, you have big beefy stern arches, dinghies on davits, and everything else.

So we should just keep in mind that when comparing all these interesting numbers calculated out to two decimal places, they probably only have a very rough relationship to the actual qualities we are interested in.

[sandy stone]

Originally Posted by Mike OReilly

Here's an interesting source I found for calculating your AVS (Angle of Vanishing Stability). It's beyond my pay grade to assess if this is accurate, but it seems authoritative.

https://www.yachtdatabase.com/en/encyclavs.jsp

Using this I calculate my boat to having an AVS of 140.4º, although I had to estimate the DCB value (which is the draft of just the hull, NOT including the keel).

Thanks Mike, this is a very interesting resource. I feel compelled to quote a couple of sections:

"Note: Many authors use - incorrectly - the hull draft (HD) as in the formula in stead of DCB. DCB is always less than HD. HD is only equal to DCB if the boat has a flat bottom, which never happens. If the hull shape is round, using the HD will introduce a 6-10% error. If the bottom has V-shape then the error may be considerable larger; dependent of the V-shape/deadrise of course."

"Note: The formula is only an approximation based on the actual results of inclining tests and calculations on a number of typical sailing yachts. The calculated estimate may over/under estimate up to 10-15 degrees in certain cases."

and:

"There are some better indicators of a vessel’s likelihood of capsize.
STIX: The EU developed their own stability index called STIX[3], a series of formulas which considered a wide range of factors and provides a reasonable sense of how a boat might perform in extreme conditions. Unfortunately meaningful results require a lot more information than most folks have access to for any specific design."

I think the last quote is the most telling - the simplistic formulas generally used as a basis for internet discussions can only be rough guides because they must ignore important data (for instance, the volume of the above-deck cabin house) that is just not generally available.

[Mike OReilly]

Originally Posted by sandy stone

I think the last quote is the most telling - the simplistic formulas generally used as a basis for internet discussions can only be rough guides because they must ignore important data (for instance, the volume of the above-deck cabin house) that is just not generally available.

Yes, exactly. That's why I keep referring to all this as just playing with data. It's really not to be taken too seriously. Or rather, it is just one piece of a much larger picture when it comes to selecting or comparing boats.

As I said, a boat is more than just the sum of its statistics.

[MicHughV]

I would think that the Vendee boats with their water ballast...canting keels...and foils do not fit a capsize ratio...

My first boat had hard chines...don't know how this plays into the capsize ratio, but I can tell you from experience, it would heel to a certain point, and heel no more...

In the same breath, I've sailed on boats, that when sailing hard to weather, the leerail is touching the water...

Back in my dinghy racing days.....you, the crew, were the ballast....moveable ballast to boot.......have experienced capsizing first hand..

Even cats and tri's have their "upside down" moments...once you go over, you stay over...no ratio would appear to apply to them...

And finally...we get to the AC75's foiling boats..neither boat nor plane...once they go over.....they stay over....

Have to agree with the above comment....a " number" does not tell the story......

[DMF Sailing]

Originally Posted by sandy stone

While we're having fun comparing numbers, consider this - one of the inputs to most of these formulas is the least accurately known statistic on a boat, the displacement. People calculate the capsize ratio of a particular boat, then say 1.98 is good and 2.03 is bad - but the displacement used in the calculation is only accurate to 5 or more likely 10%. The same thing applies to motion comfort ratio, SA/D, D/L, and even ballast ratio. It is very informative to repeat the calculations for a particular boat, only increasing the displacement by 10% (probably a pretty good real-world estimate) and see what happens.

Also, most of the measurements used in the formulas are based on a bare, factory stock boat. Adding a simple thing like a jib furler can have a surprising effect on righting moment, to say nothing of an in-mast roller furling main, or a second furling jib on a solent stay. Then, as mentioned above, you have big beefy stern arches, dinghies on davits, and everything else.

So we should just keep in mind that when comparing all these interesting numbers calculated out to two decimal places, they probably only have a very rough relationship to the actual qualities we are interested in.

To say nothing of replacing 200 feet of rode with chain. See what that does to that Hanse's numbers with that plumb bow in a any kind of seaway...

[DMF Sailing]

Originally Posted by MicHughV

My first boat had hard chines...don't know how this plays into the capsize ratio, but I can tell you from experience, it would heel to a certain point, and heel no more...

Oh boy, I have an occasional admiral who would love that boat...

You're referring to the Roberts 38, right?

Come to think of it, I'd love to get on one. I've sailed past a lot of ketches, never been on one.

Maybe I'll start a "Anyone know of a Roberts I can sail?" thread

[barnakiel]

It is one of those stupid ratios that are not completely useless. I remember reading somewhere Titanic had CR about 1.12.


A capsize is not the worst nightmare that can happen to you on the water.


And now to be more specific : in my opinion, it is not so much in whether she will capsize or not, but rather so much more in how she will recover.


Many racing boats get wiped out (beyond 90 degrees) on a regular basis. They come back and keep on racing.


99% cruising boats are built so that they are less likely to get wiped out. When they do, they often get flooded and sink. Or else come back and are useless.


So I say capsize, but do it in style.

[MicHughV]

I had no particular reason to build the Roberts 38 ketch, other than it had two masts. Back in the day, I didn't know a ketch from a stick in ground, but a boat with not one, but two masts, just rung a bell in my head. It looked "cool".

It was a surprisingly good sailing boat. I often raced it, on handicap off course, and it was a breeze to handle. I built it as a center cockpit, but it really should have been an aft cockpit.

The Roberts had long overhangs both on the bow and stern, so when heeled over, this also put more boat in the water. It was extremely stable. I've had that boat in some seriously rough weather, but never once had to worry about capsizing or other drama.

Now, after almost 40 years, it is still around !! The new owner was in the process of converting it into an aft cockpit, but I think that project came to a standstill.

I've attached a pic for funsies.

[DMF Sailing]

Originally Posted by MicHughV

I had no particular reason to build the Roberts 38 ketch, other than it had two masts. Back in the day, I didn't know a ketch from a stick in ground, but a boat with not one, but two masts, just rung a bell in my head. It looked "cool".

It was a surprisingly good sailing boat. I often raced it, on handicap off course, and it was a breeze to handle. I built it as a center cockpit, but it really should have been an aft cockpit.

The Roberts had long overhangs both on the bow and stern, so when heeled over, this also put more boat in the water. It was extremely stable. I've had that boat in some seriously rough weather, but never once had to worry about capsizing or other drama.

Now, after almost 40 years, it is still around !! The new owner was in the process of converting it into an aft cockpit, but I think that project came to a standstill.

I've attached a pic for funsies.

Looks great. Without any experience on ketches, just looking at it as a solo/shorthand sailor, I'd want the center cockpit because it gives me easy access to both main and mizzen...but I guess that's thread drift. I'll go search ketch threads to find out why an aft cockpit. Maybe access to cabin/storage are the problems...or docking

Also, I'll google why "ketch with mizzen aft of cockpit" is not called "yawl"