Gerr's Roll Period Recommendation

Post #2, this thread...

It's just one example of the demeaning manner in which you tend to treat all things Gerr and Westlawn. Adding a smiley face doesn't detract from the inference of your words
I'm not suggesting that you ought to leave! Nor that you shouldn't point out errors or ommissions (as I said in my last post).
Yes - in his reply Dave Gerr questions the extent of your knowledge here... but I can tell you that as one who can look at this objectively... I'm surprised that he hasn't hurled abuse at you before!!

You've made mention on a number of occaisions to the use by a current student of an inappropriate performance prediction method for his entry in a design competition. Did you ask that student why he used it? He may well have been in error doing so... is that somehow a fault of Westlawn... for all you know, he might be a 1st year student.... it's hardly a crime for one who is in the process of learning to make a mistake. If there is fault here, then it is with the competiton judges, who ought not to have awarded him with an equal 1st place... just as they awarded you with the same....

 

This is Your interpretation. My statement just means they don't need to know what is added inertia to design a good boat. Yes, just leave these subjects to hydrodynamics experts and naval architects would be correct.

I don't have any problem with students using wrong methods - they just study. The issue is that mention of using this method was published in Masthead; there is no comment made so it means that school encourages other students to use it?

 

Maybe they just left it there to piss you off!

 

... into the wind.

 

LOL!!!... now that's funny...

 

Hey, ever heard about the worst-case scenario? A boat rolling up in mid-air is surely an example of that...


Just kidding guys. I think the technical side of this issue was cleared through the discussion by several proven and competent designers and NAs like Dave Gerr, Ad Hoc and Alik.

I do embrace Alik's opinion that every time a mathematical formula is mentioned in an official text, a clear summary of underlying simplifications should be given, to help discriminate cases when the formula could not be accurate. Though I realize that magazine articles very often have to leave some space for advertisement boxes and inserts, so some parts of the text are cut away by the editor prior to printing.

Cheers to all!

 

A question, if I may....
Does the addition of appendages - like bilge keels etc, or hull form alter the roll period? Or does it just effect the roll angle?

 

Appendages, when done correctly damped the roll motion. That is to say the roll angle is less, (amplitude), but the period remains the same. Takes a lot to alter the roll period, when you look at the "inputs" into the basic roll equation.

 

Once appendages are big enough, they will effect roll period. We did tests of sailboat hull model with different appendages (just flat plates) of different elongation and compared with bare hull.

And yes, appendages have significant effect on roll amplitude.

 

But is the difference due to hydrodynamic effects or due to different Jx in various cases?

 

That's why I said - just flat plates. They were made of plywood, sure there was negligible effect on Jx, but strong effect on added inertia Ix and on damping.

Actually the reason for those tests was to study Ix that is effected by area and aspect ratio of plates; finally we gave up with Ix and moved to study mx and my (added masses for longitudinal and transverse acceleration). All this was all done for simulator of sailing craft manoeuvrability.

Amazingly, recently I found those results useful for acceleration of planing powerboats, where inertial component of resistance is involved. This effect is discussed by Blount for instance, they state that mx does not exceed 5% of displacement - common figure for ship hydrodynamics. From our tests, it was found to be 10-20% as it strongly depends on acceleration. But this is off-topic...

 

Might be so, but it could be a nice topic for a new thread, imho.

 

roll motions - equations

I tried to find good and simple reference on equations of motion (finally in English!), and now they are in attached file.

There are equations of undamped motions that include Jx and Ix components; equations of damped motions on still water where damping component is present, and equations of motions on regular waves. This is enough to understand what is involved even not going through all maths; as AdHoc said level of complexity is increasingly high and most of formulations used are based on some assumptions.

In this terms, having roll period written as in Masthead (formula on p.8, explained on p.9) is far from being 'exact' and does not reflect the physics of phenomena due to omitting important components. Looking at equations in reference it can be seen that in simplest case - undamped roll assumption - added inertia is still present.

Important note: p.487 top says about kxx increases 5% due to acceleration level; read carefully - this is variation of added inertia, it does not mean that added inertia is 5% of Jx. On ships/powerboats, added inertia is 20...40% of Jx; on sailboats added inertia can exceed 100% of Jx, where Jx is moment of inertia of boat.

 

I think it would have been far better if the article was written "An approximate" or "A good approximation" of the roll period is thus:-, rather than use the word "exact".

 

I will publish a paper on this subject; in Russian first. Actually it is interesting field for research; we do some measurements with acceleration gauges but mainly with purpose of High Speed Craft assessment to HSC Code (side acceleration), and assessment of comfort level.