Boat stability

Discussion in 'Stability' started by Alexanov, Oct 23, 2019.

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AlexanovSenior Member

Is it any practical way to quickly measure initial stability ( KMt ) of the small boat???
Boat is on the water and we know only main dimensions.

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AlikSenior Member

Apply ISO12217-1 or -3 standard. The testing methods described there are intended for on-water measurements. You don't need KMt for such boat; use offset load test and, residual heel angle and freeboard instead.

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AlexanovSenior Member

Thank you. I will study it.

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bajansailorMarine Surveyor

You could try the method described in the attached sheet - if you measure the roll period this can give you a good idea as to what the stability is like.
I have tried it with 'small' (ie less than about 22' / 7 metres long), and it seems to work.

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AlikSenior Member

The problem with this method is that 'K' is dimensional, and its value strongly depends on many factors, say, size and inertia of boat, hull shape. etc. This method works for small boats ONLY if You have 'anchored' it to similar type of boats before, so You identified 'K' from other boats of same type.

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The actual correct formula for roll period, has - 2.Pi.kxx - on the numerator. Which is where the 'confusion' perhaps comes from.

The "kxx" is merely the transverse radius of gyration of the vessel's mass.

It is not that difficult to calculate if you have all the weights and centres, but just a bit tedious, and for this purpose, one doesn't have said data to begin with. Thus, IMO and ITTC suggest a value between 0.40 and 0.45B , and as such does cover all types. Although other work attempting to cover all types also suggested 0.35 - 0.45B.

So roll period can be rewritten as = 2.Pi.(0.35 ~ 0.45B)/SRT(g.GM)

When doing model testing, the radius of pitch gyration is also required. In the absence of this value, most test tanks default to 0.25L. In a similar way default to 0.4B for roll.

The MCA has a slightly more methodical approach for rolling period, but essentially the same thing here. Where they suggest 15 rolls, rather than 5.

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AlexanovSenior Member

Yes, It work. I saw something similar in another book. Very practical.

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AlexanovSenior Member

Found something more interesting.

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• The+Application+of+a+Smartphone+in+Ship+Stability+Experiment (1).pdf
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AlikSenior Member

It is not only boat's own gyradius, but also added inertia of water and roll damping. Thus, K factor will strongly depend on submersed shapes. Small boats have varieties of hull shapes and underwater appendages, and using above IMO recommendations for them would be inappropriate simplification. Say, we defined this K coefficient for number of small craft, and the range is 0.8 to 1.4.

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Yes, that's correct, Kxx = Sum Mass Moment Inertia + Add Mass (A44 )

By the 'K' I assume you're referring to the added mass aspect of 'K'.
Added mass for most vessels are in the 0.80 range...and could be less, if more slender. If you go into odd shapes that have plan forms which are greater at their base tn their waterplane, then this value can increase above 0.80. Swaths for example, can be anything from 1.0 - 1.40 at their extremes. But these are extreme shapes. It can also be affected by beam to draft ratios, in the cases of simple shapes like catamarans.

Typical catamaran A44 is something like this:

Whereas a Swath is more like this:

The more you investigate, the more complex it becomes and the more factors seem to have an influence....yet while the values may fluctuate wildly, the overall net effect on roll is often not as much as one would image, only when looking at extreme hull shapes:

Hence the IMO's experiments to establish a "rough" baseline norm, in the absence of hard data. And it does, for most 'typical' vessels have a relatively narrow range for K, that noted as roughly 0.35 ~ 0.45B.

Is that from your own experiments?

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AlikSenior Member

Yes, it is.
Note that 'K' factor (in roll period - GM formula) also includes non-inertial roll damping.

Say, many small craft have developed skegs, chines. Also developed superstructure and mast. Those are not 'odd shapes', those are normal shapes - for small boats...

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That is a wide variance 0.8 - 1.40m.

Im not familiar with superstructures and mast having an influence on the A44 added mass of a vessel.

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AlikSenior Member

But they do have influence on inertia of a vessel.

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Just as does: WTBs, Eng girders, a crane or a winch or windows or seats etc. etc. when calculating the mass moment of inertia.
Everything has an influence, some minor some major...that is the point of the calculation to see what, if any, effect it has on the overall characteristics of the vessel.

Removing a deckhouse to change the inertia doesn't help those in their seats when it rains though!

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