Twin Keel Inverse Stability

[jwnsc]

Hello, I've been lurking for quite awhile and thought I'd take a stab at posting a question. I am reviewing possible twin-keel metal designs and have done a moderate amount of internet research in the last year of so. While I am very familiar with the pros and cons of twin keels, I have yet to run across any data on whether the inverse stability of a twin-keeled hull is increased, decreased or is about the same when compared to a mono-keel in all it's variations, e.g. full, shoal, short fin, centerboard etc. My intuition tells me that everything else being equal, a twin keel boat turned turtle would be slower to right itself than if the same boat was a mono-keel. A year or so ago, someone on this forum asked the same question with many others about twin keels but the inverse stability inquiry didn't seem to get answered. I guess I'm hoping to be directed to some sort credible tank testing data, reliable mathmatical formula or first-person anecdotes that would suggest that a twin-keeled boat is not dependent on the next huge wave to turn it back over after it is capsized. Thanks.

Jim Williams
Charleston, SC
jwnsc@msn.com

[Guillermo]

In my opinion ALL other things being equal, including total keel(s) area and volume, and asuming a uniform distribution of the same amount of ballast weight through keel(s) body(ies), inverted stability of a twin keeler with keels of normal proportions should present a higher area under the negative side of the GZ curve than in the case of a fin keel of also normal proportions, because of the relative higher position of the ballast's COG. But this is only a theoretical 'divertimento', as not ALL things are equal.

Cheers.

[dodo74]

My opinion is, that stability (righting arm) depends on location od COG and COB in specified angle of heeling

...nice explanation you can see at

http://uk.geocities.com/faymarine@btinternet.com/Righting_curve.htm

...so if you will compare two identical boats - one with twin-keel and one with mono-keel the stability will be the same if COG of mono-keel and COG of twin-keel version will be in the same location - see the picture


http://img509.imageshack.us/img509/7900/tezistemh9uu7.jpg

so, my conclusion is that the stability doesn't depend on if it is a mono or twin keel but on the construction of each type

what do you think ?

[jehardiman]

If BG and GM and hull form are the same, then the only difference would be in Imxx which controls the rate of roll, not the stability. However, as Guillermo pointed out, in the actual practice of just swaping twin keels for a fin keel, BG and GM and Imxx are are all likely to be less for for a twin keel and therefore you would expect less area under the positive GZ curve and more negative GZ area. This would mean that the twin keeler would be more stable inverted....not that it really would matter as a well designed hull form should keep that area low with either keel form.

Now if it's a twin keel scow.........;)

[Brent Swain]

For a given draft the inverted stability of the twin keeler would be exactly the same as that of a single keeler of the same draft. Shapes above the decks have a great influence on ultimate stability. If you consider a beach ball, a tiny ballast ratio gives 100% self righting ability. With a raft shape even a 75% ballast ratio wont bring her back once inverted. So compare the midship section with that of a beach ball. A trunk cabin with a high camber adds greatly to ultimate stability. A flush deck with wide beam, resembling a raft when inverted, has poor ultimate stability. The bouyancy of my wheelhouse full of air has the equivalent effect on ultimate stability of adding another 3,000 lbs of ballast.
Thus you can get at a glance a good idea of the chances of ultimate stabilty problems on cruising boat
Brent

[jwnsc]

As designed, does the BS 36 twin-keel have the same draft of a BS 36 mono- keel?

[Brent Swain]

No . With the deadrise the twin keel 36 footer has 4 ft draft and the sigle keel ,being on the centreline, draws 5ft10 inches empty.
Brent

[jwnsc]

Thanks Brent, PM sent.