How To Interpret A 'Stability Category Allocation'

[Deleted member 36384]

Hi,

My Rival 41C has a 'Stability Category Allocation' which states that 'The above vessel is suitable for Stability Category : 1'. I assume this means 'Area Category 1 - Up to 150 miles from a safe haven' as defined in MGN 280(M), but maybe it doesn't and 'Stability Category 1' is something else, any comments on this?

The actual data is as follows: -

Listed under the title 'Stability Estimates'

RR1 131
RR2 125
RR3 118

FVA1 0.83
FVA2 0.96
FVA3 1.14

STOPS 1 44
STOPS2 50
STOPS3 59

Listed Under the Title 'Categories'

Wolfson 3
STOPS 1

The RYA issued the calculation and she also has a Commercial Coding Certificate for voyages up to 60 miles from a safe haven, also issued by the RYA.

What do the various numbers in the stability estimates mean?

Thanks,

BlowingOldBoots

 

[andyp]

I will give it a try...


MGN437(M) at this link https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/447607/MGN_437_-_Gov.pdf provides UK guidance on stability requirements for vessels coded under the Small Commercial Vessel Code (SCV). Stability is covered in Annex 5 which repeats Section 11 of an earlier document.
ISO 12217-2 is the standard referred to and will result in a Stability Category Allocation which in your case is 1. This does not equate to an Area of Operation since Stability Category 1 vessels could be allocated Areas of Operation 0 through 5 depending on their Range of Stability.

In your case your vessel has been assessed as fit to operate in Cat 2 waters, up to 60 miles from a safe haven.

The STOPS numeral, which your vessel may have been assessed against, in the absence of any inclining test, was developed by the RYA. For operation in Cat 2 waters a STOPS number of 30 or above is required

To be coded for operation in Cat 0 or 1 waters a STOPS calculation is not sufficient and would need to be replaced by a stability curve or inclining test.

If coded, your vessel may have a Stability Information Booklet as part of her paperwork which should contain more detail.
I imagine the different Stability Estimate numbers refer to different load conditions although the RR and FVA are beyond my knowledge. I would guess RR refers to your Range of Stability in degrees. WOLFSON refers to an assessment technique developed by the Wolfson unit at Southampton University to model/measure stability.

And that is about all I know...!

 

[Deleted member 36384]

.... And that is about all I know...!

Thanks that clears up any misconception that Stability Category was the same as Area Category, she is not coded now but had been, it expired in 2005 and there is not stability booklet in the documentation that came with the boat. It is not my intention to code her, just wondering what the Stability Category Allocation means from a sailing perspective.

Having sailed this class extensively, I know that she heals easily, then digs in and does not really heal much further. I am interested in the stability curve and AVS but I guess that would need the inclination test that you referred to, or for it to be calculated by a competent authority.

 

[Tranona]

This all came about because of the lack of reliable design data before the RCD came in and pressure to improve coding practices. Many designs (particularly racing orientated) from the 70s and 80s had very poor stability, despite their hefty ballast ratios. On modern designs this data is calculated automatically to comply with the RCD.

You are right about the relationship between stability categories and coding. The higher coding categories need additional characteristics such as watertight bulkheads as well as the stability requirement. AVS is about extreme stability and not necessarily angle of heel. So my broad flat Bavaria will heel far less than your boat because of its form stability, but may ultimately have a lower AVS than your "tender" Rival.

 

[KellysEye]

>The higher coding categories need additional characteristics such as watertight bulkheads as well as the stability requirement. AVS is about extreme stability and not necessarily angle of heel. So my broad flat Bavaria will heel far less than your boat because of its form stability, but may ultimately have a lower AVS than your "tender" Rival.

Given the choice I would go with a Rival anytime it's more heavily built and a better handling sea boat, plus it''s pretty not like an AWB.