roll patterns and stabilty

We have this new design and am anxious to ensure it is not too stiff, that is it will have an easy roll
As it is now we put it through hydromax with the VCG at .80 m
The AVS came out , depending where we put the VCG at 80-100 degrees
Downflood is fine
the length is 13.6 wl and the beam wl is 3.8 16381 dept kg dept draft .566m, draft max with box keel .980
does anybody have good knowledge of this type of boat
Primary use is for the Rivers OF Eu,, but some coastal usuage may occur
we need ballast , which will be lead to get her down
disp at dept will be a lot more than arrival, i she has 2000 fuel litres and 1600 water
the fuel is low in the boat
really appreciate some input danke, thanks, merci, grats, spaciba
WE as in myself and another member

 

The roll response is coupled to pitch heave and yaw so it’s not always as clear cut as the basic NA approach would tell you more a soft guide than a hard science when it comes to the full vessel response amplitude operators(RAO) .

It's always worth looking at a silimar vessel for a start.


But anyway here is the best you can do by conventional means:

For approximate stability and roll acceleration you should work out your transverse radius of Gyration and produce a roll period estimation. There is also an allowance for entrained water which is affected by keels chines and hullform. Looking at your construction drawing I’d suggest an additional 20%

We can then calculate roll period. This goes hand in hand with roll acceleration wich relates to GM, I’d aim for an initial GM of around 1m half tankage. Tankage free surface reduces GM somewhat.

To do this by hand you need also to calculate the ‘inertia’ of the free surfaces. I can show you what to do there but probably better to try Hydromax or similar software.

GM reduction from free surface can be significantly reduced by dividing the tanks up if you find it has too much effect (if the range changes more than 200mm) I would consider that its worth doing this.

I like the full stability curve to AVS it does give a useful indication of the energy side of things in the roll response. The same energy is returned to the system well damped. The stab curve initial gradient is important and relates to the above.

You may still end up having to fine tune things in the prototype vessel, then make allowance for some ballast in your scantlings freeboard etc and maybe consider some bilge keels for a greater mass of entrained water if things are still too lively.

Did you try seakeeper for basic RAO’s ?



The Westlawn Alma Mater should have some good material on vessels this size, what's their approach? may be more practical rule of thumb.

 

Stu
It is a topic that I am interested in but not something I have looked at closely with design or test data.

There has been a couple of threads on seasickness and some useful discussion here for example:
http://www.boatdesign.net/forums/stability/roll-acceleration-what-s-best-crossing-oceans-20655.html
The discussion on the speed of motion brings to mind that my brother could travel for hours in a planing hull through any sort of sea without getting sick but as soon as we stopped to fish he would be calling for HERB.

As you know a good skipper can make a big difference as well when under way. So it will be best to get input from someone who has worked different boats. My father was a skipper for Hayles Moreton Bay and River Cruises many moons ago. I have memories from a young age of some holiday weekends on a round chine vessel called Cameo. He did not like taking this boat from the River to Tangalooma. It would roll in a ripple.

I also know a little flat area for and aft can do wonders for pitch damping but it is not something I can evaluate quantitatively. I am sure there are design criteria for roll and pitch periods.

Rick W

 

mike ric, thanks v muchely
I am going to have to study your replies, you all know I have no training in such, but give me time I will figure it
good night

 

according to a NA in NZ, who is also a ships master (real ship),and does most of the accident enquiry over there, seakeeper is not the ideal tool for small boats
in Seakeeper in various conditions, it did not seem to me to be a true simulation of how this boat would behave in 2m seas
2m seas are not small seas , and I have owned vessels in this size range, of narrow round form, that behaved as the simulation showed, rolling the screw clear at times But this one has more form stabilty than those boats had
Anybody esle use seakeeper small vessels?

 

Stuart

Don’t expect too much from these packages, they are useful for basic wave spectra and characteristic motion, but they are not complex enough to give you a detailed prediction for a complex and combined wave spectra. Even the high end CFD packages are only basically indicative and the full RAO is still conducted in a wave tank.

It’s just that Maxsurf or similar will simplify what you otherwise will need to do by hand and with some knowledge.

 

mike I the surveyer bloke with do all the stabilty stuff, I just want to see if I can get the motion how I would like it

you are right I never gave much thought to the damping from the considerable box keel , But I like what Bills done aft with the almost zero deadrise should make a good fishing platform too

 

here is keel again

 

Mike
"...Don’t expect too much from these packages, they are useful for basic wave spectra and characteristic motion, but they are not complex enough to give you a detailed prediction for a complex and combined wave spectra. Even the high end CFD packages are only basically indicative and the full RAO is still conducted in a wave tank..."

Every software program has it limitations, ROAs, as you note are no different. As i noted on another thread, just pointing out a 2m wave, and then trying to related, in this case, to an RAO, is somewhat meaningless. Since what kind of wave is it, and how are the waves from an RAO derived...they are totally different, chalk and cheese.

Since to assist the calculations to make them 'simpler' to compute, the "size of wave", is related to infinite 2D waves, ie what you would draw on a 2D sheet of paper.

The properties of an equivalent regular waves series is also not the same as the normal nomenclature people use to describe the sea, ie significant wave height (or single wave of 2m as noted above). Since a sea state or spectrum is generally described as:
" A collection of a great number of simple, regular waves of different lengths, all of small height and all mixed together with no apparent relation to each other except that they are all there and are all travelling in the same direction."

Again, that is just the 2D spectrum. When looking at "normal" 3D waves, as in those for your boat...it becomes more complex.

So when someone says what will it perform like in a 2m wave....what type of wave, which "area of surface" of operation....so defining the "wave system" is just as complex as ascertaining the true roll period.

Hence whey i said on the original thread...make a model and test it.

 

mike did you see this, this is the man who did the structural analyses

http://www.boatdesign.net/forums/classification/rcd-ce-proof-28859.html

 

For any confusion over what a RAO is see:

http://en.wikipedia.org/wiki/Response_amplitude_operator