Aspect ratio again

[Clyde_Wanderer]

Sorry I dident explain correctly, yes the aspect ratio I was reffering to was in fact reffering to the keel, which gave an aspect ratio of 52%, so does anyone know exactly what this means now? I did read somewhere about the A R having something to do with the ability to self right, but dident pass much heed to it at the time, now cant remember where I read it.
Thanks

 

[Rabbie]

Are you perhaps confusing aspect ratio with ballast ratio. The ballast ratio is a measure of the percentage of a boats displacement taken up by ballast. It can give some indication of how stiff or tender a boat may be. Note that it takes no account of the location of the ballast or of the hull shape of the boat. Two boats can have the same ballast ratios with very different righting moments. If the hulls are the same, boat A with all it's ballast in a bulb at the bottom of the keel will be stiffer then boat B with a long shoal draft keel even though they may have the same BR. Racing boats tend to have higher BR's then cruising boats.

 

[Salty John]

I think you mean ballast ratio. Divide the ballast by the displacement and multiply by 100. Over 50% is a high ballast ratio, below 40% is a low ballast ratio. It is a measure of how stiff the boat will be, how well it will stand up to it's sail area.

 

[Phoenix of Hamble]

However, the figures that js48 gives don't tell the whole story...

As Rabbie says above, the shape also plays a significant part..... a flat bottomed modern design will have very high 'form stability'... ie resistance to heeling, or equally importantly won't change underwater shape significantly as the hull heels, but may have a lower ballast ratio (its fairly normal nowadays for ballast ratios to be circa 25%) they will be impacted less by heeling even with a lower ballast ratio (and may even heel less) to a point, and then the effects of additional heel will be dramatic, often resulting in broaching or other unwanted effects, whereas an older more rounded hull will heel more progressively (and probably more degrees), but will maintain a useable underwater shape for much higher degrees of heel.... in reality older designs tend to have higher ballst ratios anyway.....

So horses for courses.... you can't just judge a boat by its ballast ratio...

 

[boatmike]

Thats correct. It's a stability problem. Not so much related to the shape of the keel as the hull itself. A broad beamed modern "flying saucer" is inherently stiff (resists heeling) and needs less keel weight to keep it the right way up ( a catamaran with broad beam needs none at all /forums/images/graemlins/grin.gif) An older style narrower beam boat needs more keel weight.
BUT the "stiffness" of an old style boat with high ballast might not be great but it increases with heel and the angle of vanishing stability can be very high enabling it to recover easily from a knock down or even capsize. A newer flying saucer will remain very stiff up to a point but the AVS will not be so great. When she goes as far as her AVS she will knock down easily and not easily recover from a capsize either. 52% ballast ratio sounds good to me! (Unless its a proper boat with 2 hulls /forums/images/graemlins/grin.gif)

 

[Clyde_Wanderer]

Thanks all for your advice, very interesting.
Cheers, Eamonn.