Capsize screening formula

[idkwiad]

I understand that a Capsize Screening Formula below 2.0 is considered safer, but to what degree do actual buyers take that figure into account before purchasing a deep water cruiser ? Does the formula hold more significance for boats of a particular size ?

[JoeRobertJr]

It is one of many factors I consider. As I add for-sale boats to my comparison spreadsheet, higher comfort and lower capsize numbers improve the calculated overall rating.

The boat ranked 4th right now is a Beneteau 381, which has the worst comfort (22.2) and capsize (2.1) ratings of the top 10 on my list. Its age, price and location helped boost its overall rating.

[Chris31415]

“A lower value is supposed to indicate a sailboat is less likely to capsize. A value of 2 is taken as a cutoff for acceptable to certain race committees. However this is an arbitrary cutoff based on the performance of boats in the 1979 Fastnet race. The CSF does not consider the hull shape or ballast location.”

https://en.m.wikipedia.org/wiki/Caps...eening_formula

That last bit about not considering hull shape or ballast location is important. Sailboat hull shape and ballast location has changed considerably over the past 40 years.

[Mike OReilly]

Like all these calculated factors, it's interesting to look at, and possibly useful to consider, but I wouldn't place a lot of stock in it.

It might be useful as a means of comparing similar types of boats, but as with the Motion Comfort Ratio, it's a very simplified tool for looking at a complex system.

If you want to play with these kinds of measures there's a great database here: Sail Calculator Pro v3.54 - 3200+ boats

You can compare boats and do searches based on specific criteria, such as Capsize Ratio. Hours of fun ... but don't take it too seriously. All boats are more than just the sum of their raw specs. It's how it all fits together that matters.

[Jacky]

I agree with the link.

https://www.cruisersforum.com/forums...html#post54442

[JoeRobertJr]

Originally Posted by Jacky

I agree with the link.

https://www.cruisersforum.com/forums...html#post54442

Thanks for that. Jeff H. makes a lot of sense in that thread.

[Adelie]

Originally Posted by Chris31415

“A lower value is supposed to indicate a sailboat is less likely to capsize. A value of 2 is taken as a cutoff for acceptable to certain race committees. However this is an arbitrary cutoff based on the performance of boats in the 1979 Fastnet race. The CSF does not consider the hull shape or ballast location.”

https://en.m.wikipedia.org/wiki/Caps...eening_formula

That last bit about not considering hull shape or ballast location is important. Sailboat hull shape and ballast location has changed considerably over the past 40 years.

Beam is what drives the roll in a capsize situation. Exact hull shape may affect that by +/-5%.

If by ballast location you mean the increased use of arrangements such as bulbs which lower the center of gravity of the ballast that depends on whether the goal of such change is to make the boat stiffer using the same ballast which would increase roll inertia or to have the same righting moment using less ballast which would maintain the roll inertia or decrease it.

Generally I think it is the later. Pursuing a lighter boat so it will be faster usually also means that the mast has also been lightened as much as possible, and that’s where the boat will take a big hit in capsize resistance, the mast contributes about 50% of the roll inertia despite being on 2-3% of the boat’s mass. If you remove 30% of the mass of the mast you will lose about 15% of the roll inertia, more if tapering the mast is part of that.

I don’t see that any current design trends have significantly altered the validity of the formula.

[Chris31415]

Originally Posted by Adelie

I don’t see that any current design trends have significantly altered the validity of the formula.

“The Cruising Club of America (CCA) put together a technical committee that analyzed race boat data. They came up with this formula to compare boats based on readily available data. The CCA characterizes the formula as "rough".”

The people that came up with the formula describe it as “rough”. That alone gives me pause.

You point out that the moment of inertia of the mast is an important factor. Basic physics says your right. The formula doesn’t account for it. In a similar fashion, a fin keel has a higher moment of inertia than a full keel and this isn’t accounted for.

[Adelie]

Originally Posted by JoeRobertJr

Thanks for that. Jeff H. makes a lot of sense in that thread.

I read that post. He conflates stability and capsize resistance, which to a certain extent work against each other.

High initial or form stability drives the roll and high initial stability is a function of beam. Even before the breaking portion of a wave impacts the hull, the increasing slope of the wave starts to roll the boat thru form stability. Ballast stability is still trying to right the boat. When the breaking portion of the wave impacts the hull there is an added impetus to the roll potentially enough to capsize the boat. More freeboard provides more area with a longer lever arm for the breaking portion of the wave to act on.
Higher displacement improves ultimate or gravimetric stability which continues to resist overturning until the boat reaches the angle of vanishing stability.

[Jacky]

This makes sense to me.

https://www.cruisersforum.com/forums...html#post63163

Throughout that thread, Jeff H makes great points in a couple of more posts. I'd recommend them all to everyone interested in the subject.

[sandy stone]

The capsize screening ratio is a quick and dirty formula that uses commonly available numbers for different boats, ignoring other factors that only the designer could supply, possibly after significant labor. The roll moment of inertia is a good example.

I also have a problem with people blindly accepting that a boat with a capsize ratio of 1.99 is "blue water", but a boat with a ratio of 2.01 is a "coastal cruiser" only (that is, if you are brave enough to get on one). This despite the fact that the displacement (a major factor in the formula) of any boat is probably not known within to 10%, maybe even 15%. Try varying the displacement entered into the formula by 10% and see what it does to the resulting capsize ratio.

[thomm225]

Originally Posted by sandy stone

The capsize screening ratio is a quick and dirty formula that uses commonly available numbers for different boats, ignoring other factors that only the designer could supply, possibly after significant labor. The roll moment of inertia is a good example.

I also have a problem with people blindly accepting that a boat with a capsize ratio of 1.99 is "blue water", but a boat with a ratio of 2.01 is a "coastal cruiser" only (that is, if you are brave enough to get on one). This despite the fact that the displacement (a major factor in the formula) of any boat is probably not known within to 10%, maybe even 15%. Try varying the displacement entered into the formula by 10% and see what it does to the resulting capsize ratio.

The 2.00 number was strictly a racing requirement which occurred after Fastnet '79.

It had nothing to do with Blue water vs Coastal Cruisers, but just looking at the boats it's pretty obvious that those with a lower Capsize Screening Formula are more seaworthy

https://sailboatdata.com/sailboat/bristol-27

https://sailboatdata.com/sailboat/great-dane-28

https://sailboatdata.com/sailboat/twister-28

https://sailboatdata.com/sailboat/j30

[Knotical]

Boats with narrow beams may have a lower capsize ratio but they are not that comfortable unless you like living at 25 degrees of heel 😊

[Adelie]

Originally Posted by Knotical

Boats with narrow beams may have a lower capsize ratio but they are not that comfortable unless you like living at 25 degrees of heel 😊

That is the nature of boats; trading off various attributes:
Upfront cost
Maintenance cost
Interior volume
Side deck width
Capsize resistance
Speed in light wind
Average speed
….

[Bowdrie]

A competent and successful NA that I knew, (somewhat taciturn,) told me: "Smart money buys lead".
I don't think he would have thought much of "todays" boats.