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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Seaworthiness - Lloyd register rating?
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Thread: Seaworthiness - Lloyd register rating? Reply to Thread
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Topic Review (Newest First)
09-07-2012 07:04 PM
Jeff_H
Re: Seaworthiness - Lloyd register rating?

Thank you for the kind words....
09-07-2012 02:42 PM
Flybyknight
Re: Seaworthiness - Lloyd register rating?

Quote:
Originally Posted by Jeff_H View Post
Dick (FlyByKnight) asked me to explain my reasoning behind my statement "in and of themselves, the Capsize ratio, Motion Comfort, Displacement to Water Line Length tell you next to nothing about the actual likelihood of capsize, motion comfort or seaworthiness of the vessel in question and can be dangerously misleading." I appologize that this was written by me for an earlier discussion on this topic but it exxplains the basis of my comment.

"This topic comes up a lot. Its seems that as soon as someone posts a question about the seaworthiness of some particular boat, that a well meaning responder sends them to Carl's Sail Calculator to look at the Capsize Screen Formula and the Motion Comfort Index. But as I have explained many times in the past, (and I am about to explain yet again) these surrogate formulas tell almost nothing about how the reality of a boat's likelihood of capsize or its motion comfort. In fact they provide so little indication of a boat's behavior that to rely on them borders on the dangerous.

"To explain, both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. These formulas have limited utility in comparing boats other than those which are very similar in weight and buoyancy distribution to each other. Neither formula contains almost any of the real factors that control motion comfort, the likelihood of capsize, or seaworthiness. Neither formula contains such factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution (of the hull both below and above the waterline), the extent to which the beam of the boat is carried fore and aft, and neither contains any data on dampening, all of which really are the major factors that control motion comfort or the likelihood of capsize.

"I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 500 pound weight at the top of the mast. (Yes, I know that no one would install a 500 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth.

And while this example would clearly appear to be so extreme as to be worthy of dismissal, in reality, if you had two boats, one with a very heavy interior, shoal draft, its beam carried towards the ends of the boat near the deck line, a heavy deck and cabin, perhaps with traditional teak decks and bulwarks, a very heavy rig, heavy deck hardware, a hard bottomed dingy stored on its cabin top, and the resultant comparatively small ballast ratio made up of low density ballast. And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a single point in the deck so that there was less deck area near the maximum beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....And yet, under the Capsize Screen Formula and the Motion Comfort Index it would appear that the first boat would be less prone to capsize and have a better motion when obviously this would not be the case.

There are some better indicators of a vessel’s likelihood of capsize. The EU developed their own stability index called STIX, 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. The Offshore Committee of US Sailing developed the following simplified formula for estimating the Angle of Vanishing Stability (Sometimes referred to as the ‘AVS’, ‘limit of positive stability’, ‘LPS’, or ‘Latent Stability Angle’ ):
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
Where;
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: Hull Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and Hull Draft in this formula are in meters. These values are entered in feet on the webpage and are converted to meters before SSV calculation.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )

There is a convenient calculator at http://www.sailingusa.info/formula.htm

It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to the angle at which it no longer wants to return.
Now there's a man I can respect.
Dick
09-07-2012 01:40 PM
boatpoker
Re: Seaworthiness - Lloyd register rating?

No one can determine if a vessel is "seaworthy" other than for a specific voyage at a specific point in time .......................

NAVIGATION ACT 1912 - SECT 207
Definition of seaworthy
(1) Subject to subsection (2), a ship is to be treated as seaworthy under this Act if, and only if:

(a) it is in a fit state as to the condition of hull and equipment, boilers and machinery, the stowage of ballast or cargo, the number and qualifications of crew including officers, and in every other respect, to:

(i) encounter the ordinary perils of the voyage then entered upon; and

(ii) not pose a threat to the environment; and

(b) it is not overloaded.


the ship is, for the purposes of this Act, to be treated as meeting the condition in subparagraph (1)(a)(i) in relation to that voyage so far as that condition relates to the condition of the ship and its equipment unless the condition of the ship or of its equipment does not correspond substantially with the particulars of that certificate or of any of those certificates.
09-07-2012 01:17 PM
Jeff_H
Re: Seaworthiness - Lloyd register rating?

Dick (FlyByKnight) asked me to explain my reasoning behind my statement "in and of themselves, the Capsize ratio, Motion Comfort, Displacement to Water Line Length tell you next to nothing about the actual likelihood of capsize, motion comfort or seaworthiness of the vessel in question and can be dangerously misleading." I appologize that this was written by me for an earlier discussion on this topic but it exxplains the basis of my comment.

"This topic comes up a lot. Its seems that as soon as someone posts a question about the seaworthiness of some particular boat, that a well meaning responder sends them to Carl's Sail Calculator to look at the Capsize Screen Formula and the Motion Comfort Index. But as I have explained many times in the past, (and I am about to explain yet again) these surrogate formulas tell almost nothing about how the reality of a boat's likelihood of capsize or its motion comfort. In fact they provide so little indication of a boat's behavior that to rely on them borders on the dangerous.

"To explain, both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. These formulas have limited utility in comparing boats other than those which are very similar in weight and buoyancy distribution to each other. Neither formula contains almost any of the real factors that control motion comfort, the likelihood of capsize, or seaworthiness. Neither formula contains such factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution (of the hull both below and above the waterline), the extent to which the beam of the boat is carried fore and aft, and neither contains any data on dampening, all of which really are the major factors that control motion comfort or the likelihood of capsize.

"I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 500 pound weight at the top of the mast. (Yes, I know that no one would install a 500 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth.

And while this example would clearly appear to be so extreme as to be worthy of dismissal, in reality, if you had two boats, one with a very heavy interior, shoal draft, its beam carried towards the ends of the boat near the deck line, a heavy deck and cabin, perhaps with traditional teak decks and bulwarks, a very heavy rig, heavy deck hardware, a hard bottomed dingy stored on its cabin top, and the resultant comparatively small ballast ratio made up of low density ballast. And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a single point in the deck so that there was less deck area near the maximum beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....And yet, under the Capsize Screen Formula and the Motion Comfort Index it would appear that the first boat would be less prone to capsize and have a better motion when obviously this would not be the case.

There are some better indicators of a vessel’s likelihood of capsize. The EU developed their own stability index called STIX, 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. The Offshore Committee of US Sailing developed the following simplified formula for estimating the Angle of Vanishing Stability (Sometimes referred to as the ‘AVS’, ‘limit of positive stability’, ‘LPS’, or ‘Latent Stability Angle’ ):
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
Where;
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: Hull Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and Hull Draft in this formula are in meters. These values are entered in feet on the webpage and are converted to meters before SSV calculation.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )

There is a convenient calculator at http://www.sailingusa.info/formula.htm

It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to the angle at which it no longer wants to return.
09-07-2012 08:06 AM
jimjazzdad
Re: Seaworthiness - Lloyd register rating?

Quote:
Originally Posted by Supern0va View Post
Yes. And that is exactly the thing I wanted to know Ihese CE ratings for, since those do take into account the builder's practices.

As for the steel boats, I'm sure there are plenty but I'd rather stick with polyester. Mostly based on prejudice
I think that the structural integrity of a professionally built steel boat will always be greater than that of an equivalent design in polyester (though maintenance may be an issue). YMMV
09-07-2012 07:34 AM
Supern0va
Re: Seaworthiness - Lloyd register rating?

Quote:
Originally Posted by jimjazzdad View Post
However, it takes no account of the builder's practices.
Yes. And that is exactly the thing I wanted to know these CE ratings for, since those do take into account the builder's practices.

As for the steel boats, I'm sure there are plenty but I'd rather stick with polyester. Mostly based on prejudice
09-07-2012 07:30 AM
jimjazzdad
Re: Seaworthiness - Lloyd register rating?

I second Dick's recommendation of Sail Calc. Pro - a very useful tool for comparing boat designs. However, it takes no account of the builder's practices. Having owned an Albin Vega and having worked on a friend's Ballad, I can attest that Albin boats are VERY well built. I believe the Ballad was more of a racing design (IOR 1/2 ton?) and may not be as suitable a bluewater boat as some of the others in the Albin line. The Nimbus is a nice cruiser, though perhaps larger and more $$$ than you want.

If you are in Holland, there should be lots of great steel boats for sale, no?
09-07-2012 04:22 AM
Supern0va
Re: Seaworthiness - Lloyd register rating?

Quote:
Originally Posted by VK540 View Post
It started in Europe (...)
By now I've taken the effort to set my country of residence to the Netherlands on my sailnet profile

Anyhow, thanks for all your replies. I'll take in to account capsize / comfort ratio as an extra parameter, separate from a CE rating.

Unfortunately the best of the three second hand Ballads that are for sale in the NL has just been sold... Oh, well. I'll be on the lookout
09-06-2012 04:38 PM
Jeff_H
Re: Seaworthiness - Lloyd register rating?

Quote:
Originally Posted by Flybyknight View Post
Here are some serious boat comparisons.
Look at Capsize ratio, Motion Comfort, Displacement to Water Line Length,
etc. and that should address your concerns.
Dick

Sail Calculator Pro v3.53 - 2500+ boats
Of course, in and of themselves, the Capsize ratio, Motion Comfort, Displacement to Water Line Length tell you next to nothing about the actual likelihood of capsize, motion comfort or seaworthiness of the vessel in question and can be dangerously misleading.

Jeff
09-06-2012 03:24 PM
Flybyknight
Re: Seaworthiness - Lloyd register rating?

Here are some serious boat comparisons.
Look at Capsize ratio, Motion Comfort, Displacement to Water Line Length,
etc. and that should address your concerns.
Dick

Sail Calculator Pro v3.53 - 2500+ boats
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