Pitch - Roll Order and Axes

Alik, was you study of Asian fishing boats published anywhere? If so I'd like ot obtain a copy if possible.

We're looking at many factors for the shape of the boats we're studying, not just stability. Ease of rowing both loaded and unloaded (resistance), availability of exisiting boats and designs, cultural traditions, prefered building methods, cost and material availability are some other obvious factors as well as personal preference.

Within the round bottom, double-ended "peapods" there are some significant variations of shape. I'm currently trying to estimate how these different shapes compare when used on the water. Another question is why the round bottom, double-ended peapod partly supplemented the flat bottom with flaring sides dory for small boat, in-shore lobstering. One theory is that differences in stability characteristics may have been partly responsible.

 

No, it was not published; we this did study for design purposes as we did few fishing boat designs in FRP for Asian market. So we need to know the consumers mentality, why things are done this way and what can be consequences of material change.

Say, for Thai fishing boat there is 'clean' holy place at bow (25% of length) where amulets and fresh water are stored. Stern overhang is wide but shallow as it is used to fit the propeller of maximum diameter (but they never calculate the propellers, of course).

 

Ok, I see what is going on here...We are sort of talking fish or fowl when what we have is a duck.

Ad Hoc is correct in that stability, as required by the standards, is calculated with static waterplanes and angles and have required acceptance criteria. One of the reasons for this is that it would be impossible to calculate the ALL the possible angle and waterplane/wave surface combinations. Therefor, righting energy and minimum freeboard requirements are specified.

I think that DCockey is actually looking to quantify a specific sub-set of those angle and waterplane combinations. Wether this is useful or not depends on your objective. i.e. calculate acceptable stability or calculate the actual downflooding. In any case he needs to conduct the axis transformation tensors if he is looking to find the actual location of the wave surface relative to the gunwale. This may prove more problematic than he knows right now (I once had the history, then had to calculate it for something similiar, we gave up after burning through a million dollars and 3 PhD's and went back to rules of thumb learned the hard way) but he will still need to work through all the math.

 

Not sure why folks want to read so much into what I'm doing, other than it's outside what they normally do.

Flat water only, no waves. Not studying downflooding, only up to incipient downflooding. Actually I find heel and trim angles where low point on chine is a tiny amount (.01 inches or so) above the surface.

Static analysis only.

Most of the "stability" data has been run with zero trim angle. Arbitrary choice. Roll the boat in 5 degree increments while adjusting draft to keep displacment constant. Record center of buoyance location, center of floatation location, waterplane area, wetted surface, waterline length, waterline beam, etc at each heel angle. When rail goes under surface I back up and find the roll angle with rail a tiny bit above the surface. With those quantities recorded I can calculate any stability quantity (assuming I have a CG location).

Plan to do the same at several non-zero trim angles. Hence the desire to know convention for recording trim and heel angles. Center of buoyancy will move longitudinally as the heel angle changes. I may or may not decide to adjust heel angle to keep it in the same longitudinal position. For the current study either is acceptable.

Skipping the tensor math for calculating the angles. Instead determining the angles graphically in 3D CAD. There's more to it than just measuring angles, also have to do some projections, but it only takes a minute or two.

 

Well, that’s because you started off asking about a convention for ’roll’’ and ‘pitch’ angles when a vessel is heeling or trimming. You then went to motions of a rigid body motion, then clarified with hydrostatic analysis. JEH pointed you to the coordinate system - after realising the simplicity of your question yet wasn’t clearly explained as noted by the various changes in request of information. But you then continued by saying you’re looking at stability of small boats, then it became changes in CoG location. Then it morphed into reasons why designs evolved the way they did..but then say you’re not doing stability only want to do the ‘maths’. (Which already exist in any text book).

Then it was back to stability but this time of small fishing boats under different loading conditions, you then clarify again, that you’re studying small open boats of the 19th and 20th Century.

So, considering your constantly changing tack after each answer given to you by the posters above that have responded to your changing questions, why do you think you feel posters are reading more into what you’re doing?

Hence, as engineers we can only respond to the question being asked. When it constantly changes, can you blame anyone for reading more into it?

I have tried to tell him, but to no avail...he clearly feels he can take the job on, as noted by his replies!

Good luck

 

So what am I doing?

 

Only a non naval architect would say this. Since what you have been requesting and seeking and commenting about (owing to a lack of being able to find what you want) professional naval architects do on a routine basis. I don’t expect you, as a non-naval architect to know about or do what “we” do. But you seem to imply because you can’t find this “method/data” to your satisfaction, it isn’t done. Not sure how/why you come to this conclusion?

The requirements that must be satisfied for commercial vessels is voluminous to say the least. Far more than you realise, but not surprising because you’re not a professional NA. How you navigate them is up to you based upon the SOR.

Confusing everyone by starting off asking about XXX…but when the answer given..saying, ”..perhaps I’m not making myself clear..”, I mean YYY. May I suggest you’re just confusing everyone by changing your question mid-sentence!

 

I deleted the first post Ad Hoc responded to and partially quoted because after posting it I decided it was not particularly helpful.

I am amazed that there is nothing more to study regarding boats and how they interact water than the "standard" analysis done during the design and approval process. Truly amazing.

As far as I can tell you do not know what the analysis is that I'm currently engaged in, and how I intend to use the results.

 

D-

The problem of large angle transformation and inverse transformations is not standardized in boats or any other discipline as far as I know. Using the right handed XYZ order for transform and ZYX for inverse makes good sense and simplifies coding but increases number crunching since you run three transforms even if you only need one. Any additional reference geometry is accessed via transforms to the X₀Y₀Z₀ frame and can be carried along as a dummy vector. Analytic geometry has produced a few useful spaces/operators that permit easy transforms and inverses in multiple dimensions. http://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation

A glimpse at what large angle correction looks like in the missile business-
http://edge.rit.edu/content/P07106/public/Docs/Research/Kong_Thesis.pdf
which is why Bob the boatbuilder doesn't want to go there.

 

Thanks! Compounding the complications is when the coordinate system is fixed in space and when it is fixed to the boat.

I've done analysis and written code for large angle transformations. Not my favourite pastime. Fortunately my immediate needs are met by doing the transformations "graphically" in Rhino. I'll probably need to write some code or at least an Excel spreadsheet in the future.

 

I recommend abandonning the terms pitch a roll entirely when considering general spacial transforms. They may be regarded as an operator defined in one reference frame, but its hard to forget about such ubiquitous terms.

 

Understand, considering how this thread went. I was trying to find out if there were usual reference frames for large angle pitch and roll, with the assumption that the same definitions would carryover to trim and heel. What I found in naval architecure references made use, implicitly or explicitly, of small angle simplifications for pitch/trim which eliminated the cross terms. My aero references considered only problems with planar motion which eliminates the cross terms so they were not of any help.

I can work my way through the math, done it before, though your references are most appreciated.