# calculating dynamic stability

Discussion in 'Stability' started by Boston, Sep 10, 2009.

1. ### BostonPrevious Member

Im trying to better understand the parameters that go into improving dynamic stability and Im not finding much literature concerning boats

some things are obvious like mass and buoyancy distribution beam to length ratio and the like but

what is the magic formula

I cant find it anywhere and its not in my wiley engineers desk reference

thanks
B

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Define improving?..and what does 'dynamic stability' mean to you?

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### stavrou2000Junior Member

Please Explain Better...dynamic Stability..about What? What Is The Force...is It Wind?gust?beam Waves?...if You Dont Know The External Force How Can You Define The Dynamic Stability Equation?

4. ### BostonPrevious Member

ya that was a poorly phrased question

Im considering a 60' motor yacht semi displacement hull with a 12' beam, 42"~48" draft, displacement of about 21 short tons, engines of about 2400lbs ttl, typical fuel load of about 3 tons

Im most interested in playing with weight distribution, beam and draft in order to gain some advantages in stability in various sea states

wind I will factor in at some point but one thing at a time

there is something referred to as the "Dynamic Stability Factor" I keep bumping into as I look into seaworthiness issues and Im dam curious as to how this is calculated ( rumor is I want this number to be about 40 for a blue water cruiser )

that way I can possibly tweak a few things still get the look Im after and a safer boat all around

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Dynamic Stability Factor (FDS) is from the ISO stability standard and is used in the calculation of the STIX (Stability Index) number. You'll find much discussion of STIX on this forum.

For sailing yachts using metric units (See Principals OF Yacht Design)
FDS = Agz / (15.81 * sq rt Lh)

Agz is the area under the righting arm curve up to the downflooding point.
Lh is the hull length as defined by ISO standard.

FDS will be something between .5 and 1.5 (again metric units)

STIX will be something between 20 and 60-70 perhaps.

6. ### BostonPrevious Member

Dam
I have got to order that book
I get another nice draw tonight so Ill order that soonest
B

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### AlikSenior Member

In classic ship theory dynamic stability is characterized by positive area under stability curve (RM curve). This area is a measure of work required to capsize the boat. Requirements to area under RM curve is fundamental for IMO criteria for commecrial ships and ISO standards for small craft, namely for 'wind and wave' factor. Yes, area is also used in FDS as part of STIX, but this factor is applied to sailboats only.

Only problem with this method - it purely theoretical and does not consider roll damping that can be significant for sharp chine boats and boats with skegs, bilge or fin keels. But this is different story...

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