Improving Capsize Ratings?

[David M]

Originally Posted by Cheechako

Yea, that would be the best advice, but NA's tend to use rules of thumb also... after all.... his boat was likely designed by one! 400 additional foot pounds distributed over approx... 12 ft of 1" thick glass adds what... about 3lbs of stress per inch?

As I already said, 400 pounds a foot or two away below the LCB will do virtually nothing for stability and make the boat weigh an additional 400 pounds. By doing this you will have lost more than you have gained.

[Rvrratt]

Cheechako,
like the idea of a bulb on the bottom...certainly doable. I believe this boat was also available with a 6ft keel (I would want to verify that first) to make sure the bolts can handle the load. and of course, a thorough inspection of the bolts-and surrounding fiberglass. Perhaps a letter to Moody to verify this as not a hazardous modification?

[Paul L]

I assume that you are referring to the Capsize Screening Formula
CSF = Beam / (Displacement/64.2)1/3
as you indicate it is just above 2 and that is desirable to be under 2. Look what this formula actually uses for input data. The beam and the displacement. It has nothing to do with the righting moment or areas under the curve. To get the CSF below 2 on your boat you could add 1000lbs to the top of the mast. The CSF would go down below 2. This shows you how useless this formula is.
Also, is your boat really 10 tons. Seems like it is much less. If it was 10 tons then it would not have a CSF of 2.1

Paul L

[Cheechako]

They are going to run from approving any modification. "liability" Sail your boat and decide how you feel about it, someone writing an article may have exagerrated etc....

[David M]

I assume that you are referring to the Capsize Screening Formula
CSF = Beam / (Displacement/64.2)1/3
as you indicate it is just above 2 and that is desirable to be under 2. Look what this formula actually uses for input data. The beam and the displacement. It has nothing to do with the righting moment or areas under the curve. To get the CSF below 2 on your boat you could add 1000lbs to the top of the mast. The CSF would go down below 2. This shows you how useless this formula is.
Also, is your boat really 10 tons. Seems like it is much less. If it was 10 tons then it would not have a CSF of 2.1

Paul L[/quote]

Thank you Paul.

[Rvrratt]

David M,
"Bash" has corrected that earlier mis-statement. to 5.5 tons.

[barnakiel]

Coming late, I might repeat on what has been said. You have touched upon a number of issues that might suggest that before making any modifications you might like to reach for some good literature related to stability/seakindness/seaworthiness issues. In fact, you do not have to buy any books as most of the concepts and consequences are explained on the Internet.

Anyway, briefly:
- CR is NOT related to balast (yes, displacement includes ballast but they are not same),
- CR is not related to stiffness of the boat nor does it influence the boat's motion/seakindness,
- you cannot realistically add balast ...
- because if you do, then PLS remember you will add water, food, fuel, spares and extra gear (some of which quite heavy - anchors, chain, etc) for your Pacific adventure,
- which would make the boat way too heavy, which decreases the safety of your passages,
- but you CAN sail the boat within her limits,

Really not very much can be done to improve boat's 'design' stability after one has been designed and built. This is why it makes so much dense sense to start with a design and built that have plenty (excess) of stability (if one's planned adventure are ocean crossings or long offshore/inshore stunts). One, but not the only, indication of the boat's design stability is expressed by CR. You must look at other factors too - GZ curve, (point of) vanishing stability, load carrying capacity, etc., and only then you will see the boat as a whole. Beyond the pond, they have invented something called STIX - look it up, calculate it for your boat and then have a look against other designs - is it still bad? It is a long trip thru numbers and ideas but well worth the effort.

And let's not forget the other half of the equation is how well you can sail this specific design - if you are a very good sailor and if you know your boat and her limitations then you will be sfafe in all but the extreme conditions.


Cheers,
b.

[cantxsailor]

The boat has "already been across an ocean or 2" as is. Why mess with a proven boat design. If heeling makes you uncomfortable I suggest you get out and sail the boat more and you'll get used to it.
The only way to get over the fear is to prove yourself foolish.

I know, it works for me. It has to.

At the risk of being thought a sissy by some of the old(er) salts I admit I hate sailing on my ear for any number of reasons. If I actually get out and spend some time sailing I find the fear of falling off/over seems to disappear.

I always know that IN THEORY(reality?) my boat will not heel far enough over to take on water before it rounds up. In MY BRAIN however, I know I'm going to tip over and sink like a stone which is a real problem because I also swim like a stone.

I KNOW this all in my overwrought imagination but there you have it. After a few hours of allowing the boat to heel more than I am comfortable with it finally works into my feeble cortex that I'm perfectly safe and the boat will look after me and I'm fine again.

I have to do this EVERY trip after a long time ashore.

It's like getting my sea legs back..........only different..............m

[Rvrratt]

barnakeil,
good advice, that...

The book and Author that generated my idea for this thread is Richard Henderson "Single-handed Sailing" In it he recounts the cpasize and later death of the designer Angus Primrose. Sorry I did not mentin that earlier in the thread.

[Paul L]

Originally Posted by cantxsailor

The boat has "already been across an ocean or 2" as is. Why mess with a proven boat design. If heeling makes you uncomfortable I suggest you get out and sail the boat more and you'll get used to it.
The only way to get over the fear is to prove yourself foolish.

I know, it works for me. It has to.

At the risk of being thought a sissy by some of the old(er) salts I admit I hate sailing on my ear for any number of reasons. If I actually get out and spend some time sailing I find the fear of falling off/over seems to disappear.

I always know that IN THEORY(reality?) my boat will not heel far enough over to take on water before it rounds up. In MY BRAIN however, I know I'm going to tip over and sink like a stone which is a real problem because I also swim like a stone.

I KNOW this all in my overwrought imagination but there you have it. After a few hours of allowing the boat to heel more than I am comfortable with it finally works into my feeble cortex that I'm perfectly safe and the boat will look after me and I'm fine again.

I have to do this EVERY trip after a long time ashore.

It's like getting my sea legs back..........only different..............m

The heeling you are referring to is caused by (excess) wind on your sails. In the case of stability in ocean crossings, it is the waves that knock boats down and capsize them.

Paul L

[Rvrratt]

cntxsailor,
thanks for that...
I guess I just have read to many sailor survival stories and the "what if" when I am out so far and will have no chance to run from a big storm. So let's see if we can get a consensus here.....don't worry, be happy, smart, know what I am doing and be prepared and safe, but feel free to take it anywhere?

[S&S]

Originally Posted by David M

The capsize rating relates to the total area under a vessels stability curve. (righting moment (GZ) versus angle of inclination). In technical terms, when the GM (metacentric height) goes negative, the boat has no ability to right itself without any external forces acting on the boat. The compromise is that the larger the metacentric height, the stiffer the ride is, meaning it would be very uncomfortable from what are called snap rolls. Common to all boats, they are a compromise of many factors. Boats with a positive GM at all angles of inclination can be made, but you would not want to live a boat that is that stiff.

I think you have it backwards. Improved recovery from capsize requires a low metacenter (because you want the center of mass low) which makes for a tender boat. International Rule boats (some of which have an angle of vanishing stability of 180 degrees) are quite tender from modern standards. Initial stability (as in stiffness) is inverse to the ability to recover from a capsize. The '79 Fastnet race demonstrated that.

Rvratt: I'd recommend CJ Marchaj's Seaworthiness, the Forgotten Factor for a full explanation.

[S&S]

Originally Posted by slomotion

There are all kinds of calculations you can make based on the dimensions of sailboat. I believe capsize rating is essentially the same as capsize screening value. Go here:

Sailboat Design and Stability

Many of these formulas either do not take account of other important variables or are more or less accurate only for older classic boat designs. The short answer is that if you are going to go cruising on your Moody, you'll be adding way more weight than a 400 lb. mooring anchor.

Do Not use that calculator for Angle of Vanishing Stability- it gives the wrong answer (says that boat's that really are less stable are more stable) I checked it with some done calculations and it's not giving consistent answers.

[David M]

Originally Posted by S&S

I think you have it backwards. Improved recovery from capsize requires a low metacenter (because you want the center of mass low) which makes for a tender boat. International Rule boats (some of which have an angle of vanishing stability of 180 degrees) are quite tender from modern standards. Initial stability (as in stiffness) is inverse to the ability to recover from a capsize. The '79 Fastnet race demonstrated that.

Rvratt: I'd recommend CJ Marchaj's Seaworthiness, the Forgotten Factor for a full explanation.

I don't have it backwards. The greater the metacentric height, the greater the stability. My information comes from my stability and trim class and my naval architecture class at California Maritime Academy. All of the ships in the world don't have it wrong. How I learned it is how stability and trim is calculated world wide.

There is only one accurate way of determining a vessels stability and this method is very accurate. There is no guess work to it. That is to know a vessels center of gravity and its location of center of buoyancy as the vessel inclines. Knowing this, the righting arm is determined. The other formulas you see here are rough estimates which do not take into account the metacentric height.

Its unfortunate that yachts in general are not provided with their stability curves to their owners. I can understand the manufacturers and naval architects reasons for not providing them, because they are relatively confusing to someone who does not know what they are looking at and in general, yachts do not change their displacement all that much as compared to a cargo ship and thirdly, yachts come with a relatively large stability, unlike a ship, if loaded improperly can even capsize at the dock, its actually happened a few times.

I would recommend any book that show how a boats LCB changes with its angle of inclination and how that works with a boats LCG. There are plenty of these books out there.

Also, initial stability is a different thing than overall stability, which is the area under the Angle of Inclination vs Metacentric Height curve...which is different from "ultimate stability" which is the angle of inclination at which the metacentric height goes negative.

[Rvrratt]

David,
Thank you for the insight....I will look forward to researching my particular boat....very interesting! I think many boatowners...cruiser's, anyway, would appreciate these spec's from the manufacturer's. I know I have a good boat...but how good?...who wouldn't want to know that?
It will be time for new sails before I enter the pacific...I will keep the tips on sail modifications in mind, too. Right now I am stuck in the desert...earning money for my eventual trip....far, far away from my sailboat but trying to learn as much as I can before I get back to it and can get out there. I have been "lurking" on this site for years...so glad I joined and can have access to such knowledgable folks!