roll and pitch Gyradii?

[gunship]

in seakeeper it asks me to input (Roll and Pitch) gyradii. does seakeeper's default values have anything to do with the design in question? i wonder because the default values seem awfully high to me (in comparision to the manuals 'normal' values). is there any way to, with not that hard numbercrunching get decent values?

thanks

[gunship]

is there really no one who knows about this?

[JRMacGregor]

What is the type, beam and length of your boat - and what are the default values and "normal" values of gyradii that we are talking about ?

There are two basic options for calculating the gyradii;
1) add up the 2nd moment of every different part of the boat to get a total
2) make an estimate based on a percentage of the length (c. 25% L for pitch gyradii) and the beam (maybe 35% B for roll gyradii)

Like most people, I guess you dont want to do 1) and are going for option 2)

I would have thought the software default and "normal" values would be based on the same simple algorithm - using percentages

[gunship]

thanks for the long awaited answer, MacGregor,

the software asks for input for Roll and Pitch Gyradii in percentage of LoA, and defaults are set to 25% Pitch and 40% for roll, but after measuring hull and such of the model, it enters a roll gyradius of 50-60%.

i think maybe i should start with getting the hang of what this actually is? so far i only know that the software calculates it by taking the sqare root out of the Intertia/mass.

What does these numbers do when calulating on a ship's stability? i would have thought that keels and such would affect it, but this is just a wild guess!

[capt vimes]

the 2 numbers define how much and how violently the boat will pitch/roll... the bigger the number - the more it will do so...

keels usually have a great deal of impact on the roll - not so much on the pitching though because they are mounted near the long.CG around which the ship pitches...
but the mass of the keel pretty far away from the vert.CG and the lateral area gives resistance to roll... the biggest impact on roll resistance has nevertheless the rig... at least on sailing boats...

oh - and on pitching as well...

[JRMacGregor]

Hi gunship

Are you trying to do seakeeping calculations or stability calculations ?

The gyradii dont have any effect on calculations of STATIC STABILITY (GZ curves etc). You can use any value, and the output STABILITY results should be OK.

The gyradii are a necessary part of SEAKEEPING calculations, where you are going to produce RAOs of heave, pitch, roll etc.

The gyradii is just a number used to ESTIMATE the mass inertia of the ship.

For example, pitch mass inertia = ship mass x (pitch gyradi)^2

In fact there is the pitch inertia of the added fluid mass to consider - but that is going further than the subject of this discussion.

[gunship]

well, im trying to make some kind of seakeeping analysis on my theory yacht, to see wether a few details would get wet and such.

so, if im estimating the Gyradi, i guess there are things influencing the estimations - a sail boat and a motor yacht would have pretty different characteristics, right?

this little theoretic boat is a two masted schooner, with a long keel and a little slim in the B:L ratio, for example. how would this effect the estimations?

thanks for all the answers!

[JRMacGregor]

Hi Gunship

The type of vessel does indeed affect the gyradii

Some examples

1) Sailboat with no mast - and only light keel
- pitch and roll gyradii probably very similar to the default values seen in text books for "ships"

2) Sailboat with tall mast amidships and heavy ballast keel
- roll gyradii will increase - because of significant vertical distance from CoG of heavy mass items
- pitch gyradii will also increase, but not so much because longitudinal distance from CoG of heavy mass items is not great

3) Sailboat (schooner) with two heavy masts at ends of ship
- pitch gyradii will increase compared to 2) because the mast masses have a substantial distance from the CoG

4) Double sided oil tanker with wing ballast tanks full, cargo tanks empty
- has large roll inertia because of mass in the wing tanks and no mass in the centre

5) Double sided oil tanker with wing ballast tanks empty, cargo tanks full
- lower roll inertia than 4) because mass is closer to centre and so gyradii is less

I am not a yacht designer so I don't have data on typical mass gyradii for yachts of different types - but I expect many yacht designers don't have VERIFIED gyradii either. Very difficult to measure in real life !!!!

As I said in my first post there IS another way to get the mass inertias (and hence gyraddi)

Make up a table listing (for each component) the following;
- mass
- inertia I about its own axis
- distance K from the centre of mass to the overall CoG
- mass x K^2

Then overall inertia - sum of all the I plus sum of all MK^2

You can do this calculation in the same general style as you calculate the structural section modulus of a structural cross section.

You can choose how detailed (how many components) you want to have in your table.

Even if you made a simple calculation using only 4 or 5 major items (main hull, mast 1, mast 2, ballast keel) you would see the effect (on pitch at least) of altering the distances between the different masses.

[gunship]

interesting, i have dug up formulas for iinertia, but is this method you described, what does it output? how do i get it to % of LoA?

also, how is the distance to CoG measured? is it to the roll/pitch axis, or is it the direct path to the point itself?

[JRMacGregor]

Gunship

the methods I described will give you the roll and pitch INERTIAS - in units of [tonne metres squared]

Divide the pitch inertia value by the mass of the boat in tonnes - and get a value in metres squared = [Kp^2]

Take the square root of [Kp^2] and you get the pitch gyradii Kp in metres

Divide the pitch gyradii by L and you will get the gyradii as a % of the boat L.

The above is just the reverse process of what the computer is doing to calculate inertia when you input a roll or pitch gyradii as a percent of boat B or L.

But please don't expend too much energy over this. These seakeeping computer programmes are not some perfect analysis tool. Different computer programmes will give different results for the same hull. And they are all based on approximations.

One of the biggest approximations is that most of them don't need to know anything about the shape of the hull above the waterline. But in real life we KNOW that the shape of the hull above the waterline DOES influence the seakeeping performance.

Another big assumption in SOME of them is that the hull is long and slender (L/B = 6-10) like big ships. This does not describe most small boats (L/B = 3 or 4).

These seakeeping programs are ATTEMPTS to get close to the reality - but they are mostly imperfect attempts. Should only be used with knowledge of their limitations.

I would not base the design of a "conventional" small craft solely on the output from a seakeeping programme. Following good practice in the basics like L/B ratio, L/D ratio, freeboard/length. shape of sections etc, metacentric height, waterplane area/displacement ratio will be a good check.

[JRMacGregor]

Hi Gunship,

Some of the subjects that interest you are discussed in post 152 on this thread on this site;

Favorite rough weather technique

Mike Johns sounds like a designer who DOES have data on yacht gyradii - AND has experience with Seakeeper.

He is giving a very clear explanation of what is going on with a vessel/mass moving in the sea.

[gunship]

ok, thanks. the thing is i'm just theorycrafting here, so the simulations is as close to reality this boat will come, at least. thanks for your help, much appreciated.

[formsys]

Just to clarify the answer to the orginal question, no Seakeepers default values have no relation to the actual design, they are just commonly used values. You do need to make an estimate of the gyradius based on the actual distribution of mass in your boat as described above.
As always, Seakeeper and other analysis tools are best used for comparative analysis rather than absolute values.

[gunship]

thanks! always nice to get a specific in depth answer. do you happen to know if seakeeper calculates based on hull shape above waterline and such? and to what extent (all axis etc.)?

[formsys]

No, as a strip theory formulation, Seakeeper just uses the shape up to the waterline. It computes pitch, heave and roll motions.