How stable is your boat? 

Stability

To state the absolutely blinding obvious, stability is a fairly fundamental factor for anyone who is thinking of buying a yacht. You can’t sail a boat if the masts are in the water, and nor would you dream of buying a yacht which was inherently unstable.

However, when you buy a boat there is a general assumption that whatever you purchase is going to be inherently stable, after all no-one is going to deliberately design an unstable boat.

Naturally this is true to some extent, however what many people don’t realise is that boats do have quite different stability characteristics, and whilst this may not be evident in a force four in flat water, it might be the difference between foundering and survival in a force ten storm.

Stability database

One woman who knows more about stability than most is the RYA’s technical standards coordinator Tricia Grady.

Tricia said: “Whether you are buying a new boat or a second hand one, you want a boat that is suitable for your purposes, and part of that consideration should be stability.

 “Since the introduction of the recreational craft directive (RCD) all vessels have had to undergo stringent assessment of data such as displacement and angle of vanishing stability.

“The RYA has endeavoured to make a database of this information for the use of owners and buyers and this is available on the website. At present there are over 400 boats on the database.

“Although it is extensive there are gaps as some manufacturers have been unwilling to furnish us with that information despite the fact that it should be in the public domain.”

What to look for

Before checking the database, you need to decide how seaworthy your boat needs to be; if you are cruising the Norfolk Broads in a bilge keeler, your stability requirements differ from someone heading round Cape Horn.

Having decided this, there are two basic measurements to look at; the angle of vanishing stability (AVS) and the Stability Index (STIX).

The AVS will tell you what angle your boat will heel to before it inverts instead of righting, whilst the STIX takes a number of factors into account which all contribute to the overall stability of a vessel.

The STIX rating is on a scale of ‘A’ to ‘D’, ‘A’ being the most stable rating. The table below will give you a simple indication of how your boat rates.

	A	B	C	D
	approx 7	4	2	0.5
	up to 10	up to 8	up to 6	up to 4
	28	21	17	13
	32	23	14	5
	130-0.002m	130-0.005m	90	75

 

“This kind of information is absolutely invaluable to any yacht owner or buyer, it can also be used for any owners wishing to have their boat certified under the MCA Small Commercial Vessel Code and also if you wish to race” Tricia added.

“Without this information you would have to undertake an individual stability assessment which could be costly.

“It is also worth bearing in mind that any new vessel sold in the European Economic Area must have its STIX and AVS numbers in the owner’s manual, so if you can’t find it, it’s certainly worth chasing up.”