Kayak stability

[pdx1]

The question "how to read the stability numbers in Kayak Foundry" recurrs frequently on the various fora, so here is a quick rundown that may be of help to people who are new to the Kayak Foundry design.
I will not repeat any information that is found on the Nick Shade Guillemot site ('Info'=>'Article' link)
which explains concepts of stability in more general terms.

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Forget the 'Stability factor' (SF) number. It is a snapshot of stability in 1 place (15 degrees of heel)
only, and only for 1 displacement (weight of paddler+cargo) of 250 lbs. As such, it grossly oversimplifies
and misrepresents the stability that YOUR boat will have for YOUR body and cargo weight.
The only usefulnes of SF is in comparing various boats, but not in providing accurate information about the stability of your boat.

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Instead, look at the stability curves diagram.
- Uncheck all the Stability Curves checkboxes except the 'Design Displacement'.
This is the stability curve your boat will have if loaded with paddler and cargo weight you specified in the
'Design Displacement' section on the 'Design' screen.
- Make sure the 'Standard' is checked.

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How to read the curve you see.
There are 3 key pieces of information the curve tells.
- The height of the curve (Y-axis) 'Righting Moment lb*ft' (in metric would be kg*m)
- The X-axis location of the peak of the curve (how far to the right the peak is on the 'Heel Angle' line)
- The X-axis location where Y=0 (point where the curve intersects the 'Heel Angle' line)

1. The height of the curve (Y-axis)
How strongly the boat 'wants' to return to level position when leaned to the given angle
(read from 'Heel Angle' line)
Typical values:
5 or less = quite unstable boat, good balance and experience needed,
boat will not be suitable for fishing, fotography etc.
10 = stability decent enough for all-purpose sea kayak, though not great.
Complete beginners will not be comfortable initially.
10 to 15 = about average stability. Good enough for most people in most conditions.
20 or more = superb stability, good enough for Grandma first time in a kayak.

2. The X-axis location of the peak of the curve (how far to the right the peak is on the 'Heel Angle' line)
The angle when the boat 'puts up the most resistance' to return to level position.
In other words, how far you can lean the boat when she 'tries' to return to level with the most force (read from the 'Righting Moment' axis.
Typical values:
15 or less = unstable, boat may easily capsize at a moment of inattention.
20 to 25 = typical for most sea kayaks, comfortable for most paddlers.
25 or more = very good, boat can be leaned a lot and still recover easily.

3. The X-axis location where Y=0 (point where the curve intersects the 'Heel Angle' line)
How far the boat can be leaned before capsize is certain, unless paddler braces with the paddle.
Typical values:
25 or less = unstable, the boat will capsize when leaned even moderately. Alert paddling required.
40 to 45 = typical range for an average sea kayak, comfortable for most paddlers.
50 or more = very predictable stability with a lot of reserve.

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Putting it all together.
a. The objective is to make the stability cure as high as possible, and with the peak as far to the right as possible.
b. If you can't make the stability curve high, at least make it as wide as possible (see point 3.)
This can be done by giving the boat a lot of 'flare' (sides becoming wider and wider above the waterline)
Wide stability curve gives an extra safety margin before capsize, even though the boat may not feel stable due to low height of the curve.
c. Wide boats are more stable than narrow boats (this is a truism)
d. Long boats are more stable than short boats.
e. Tall boats are more stable that short boats. Here 'tall/short' means the profile view, not the length.
f. Boats with flat bottom FEEL more stable when sitting upright.
g. Boats with a lot of flare ARE more stable than boats without flare.
h. Boats with round bottom are more stable in waves than boats with flat bottom.

[LMcI]

pdx1,
Thanks for posting this. It's completely in line with the advice you gave me for my design, and was a huge revelation to me vis-a-vis the KayakFoundry software and how to figure out what I was designing. I think us newbies will get lots of miles out of this information.

Appreciate the time/effort,
Larry

[estrabanos]

Very clear and use-full. Thank you.
Maybe will be good put this kind of post together .Something like K.F. tutorial. There is Buddy tutorial and
another post, but for new user can be hard to find it. But, probably its part of learning.
This is one of thinks I'm printing on paper.

[stumpy]

Incredibly interesting and informative, I finally found out how to judge the stability issues that had been perplexing me for some time... when I went back to review some designs that I had built, it made perfect sense, and clarified what I thought was a miscalculation (for the better)
Ross, is it possible to to post this, and Buddy's tutorial as a sticky?

[Ross Leidy]

Incredibly interesting and informative, I finally found out how to judge the stability issues that had been perplexing me for some time... when I went back to review some designs that I had built, it made perfect sense, and clarified what I thought was a miscalculation (for the better)
Ross, is it possible to to post this, and Buddy's tutorial as a sticky?

I've got a plan for "FAQs and Favorites" for posts that are especially helpful. Just need to make the time to set it up. Good nominations - I'll be sure to add them.

[Boubous]

Thanks a lot for this post,

In fact it helps so much to understand stability questions... Wich can be rude for newb

[smiley face]

thankyou so much for this, i thought i was reading the curve correctly and i was, but i did not have any point refrences, and now i feel like my boat might be stable enough .

as for the idea of a faq, i think it would be great, and would like to recommend the exagerate feature and that a concave shape is difficult to build, i have seen many posts about that

[Vir]

Thank you for explaining it all and how it relates to real world responses.

[bahia]

Bonjour. merci pour ce poste

kayak bahia new.yak

(43.34 KiB) Downloaded 173 times

j'ai lu avec attention votre poste sur la stabilité. cependant lorsque je vérifie la courbe en question sur mon modele de kayak je constate que la courbe varie avec la position de la ligne de flottaison . ma question est donc comment déterminer la juste ligne de flottaison ?

[Labrat]

Bonjour Bahia

La ligne de flottaison est correcte lorsque le déplacement de conception est égal au déplacement cible. Ceux-ci sont affichés dans le panneau hydrostatique à gauche de l'écran. Faites glisser le triangle à l'extrémité de la ligne de flottaison (dans le panneau de vue de profil) vers le haut ou vers le bas pour modifier le déplacement de la conception. Si vous trouvez qu'il bouge trop à l'aide de la souris, vous pouvez le déplacer par petites quantités à l'aide des touches fléchées haut et bas du clavier.

[bahia]

ok merci . question le gros point rouge qui est sur la courbe désigne déplacement, correspond à quoi ? doit il être le plus haut sur (Y) et le plus loin sur (X) pour indiquer une meilleur stabilité

[bahia]

ok merci . question le gros point rouge qui est sur la courbe désigne déplacement, correspond à quoi ? doit il être le plus haut sur (Y) et le plus loin sur (X) pour indiquer une meilleur stabilité

[Labrat]

Je ne sais pas quel est le point rouge, mais cela peut être lié au calcul du SF.
Comme le dit PDX1 dans son message, l'important est que le pic de la courbe soit suffisamment haut sur l'axe Y et aussi assez loin à droite sur l'axe X. Je pense qu'une fois que l'angle de talon dépasse 25 ou 30 degrés, vous risquez de chavirer de toute façon, donc je ne sais pas si la forme de la courbe au-delà de cet angle de talon importe beaucoup. Dans tous les cas, il ne fera aucun mal de rendre la courbe plus haute et plus large.