# Freeship Discussion Regarding Cross Curves Calculation

Discussion in 'Software' started by EAP, Apr 3, 2007.

1. Joined: Aug 2006
Posts: 9
Likes: 0, Points: 0, Legacy Rep: 10
Location: MIAMI, FL

### EAPJunior Member

The Freeship Crosscurves Calculation Shows As The Righting Lever: Kn*sin(theta). Two Questions: First, What Is 'n'? Second, If The Intent Is To Express The Righting Lever As Seen Passing Through The Keel (since There Is No Kg) Shouldnt The Expression Read Km*sin(theta)???

2. Joined: Apr 2005
Posts: 254
Likes: 24, Points: 0, Legacy Rep: 401
Location: Netherlands

### Martijn_vEMarine software developer

KM is only valid at small angles of heel. At larger angles of heel the metacenter shifts and at those angles it is denoted KN rather than KM.

3. Joined: Apr 2005
Posts: 4,127
Likes: 149, Points: 63, Legacy Rep: 2043
Location: Ontario

### marshmatSenior Member

On a related note, although I realize this thread is a couple months old....
In converting Delftship/Freeship cross curves from KN*sin (phi) to the actual righting arm GZ, is it correct to interpret the Delftship/Freeship output as follows: Divide the output value KN*sin(phi) by the sine of the heel angle to get KN, then subtract KG*sin(heel angle) to get GZ? Or am I misreading something....

4. Joined: Apr 2005
Posts: 254
Likes: 24, Points: 0, Legacy Rep: 401
Location: Netherlands

### Martijn_vEMarine software developer

Stability issues

Marshmat,

I hope the attached image clarifies things a bit.

Martijn

#### Attached Files:

• ###### stability.jpg
File size:
49.5 KB
Views:
7,415
5. Joined: Feb 2007
Posts: 127
Likes: 6, Points: 0, Legacy Rep: 58
Location: Ukraine

### Victor TSenior Member

Hi!
That is right only for small angles (< 5 degrees )
The cross curves (pantacarenes) build for big angles theta.
Formula would be this: l(theta)=Y_theta*cos(theta)+(Zc_theta-X_g)*sin(theta).
Because X_g unknown -> X_g=0 -> l_conditional=Y_theta*cos(theta)+Zc_theta*sin(theta).

6. Joined: Apr 2005
Posts: 4,127
Likes: 149, Points: 63, Legacy Rep: 2043
Location: Ontario

### marshmatSenior Member

7. Joined: Apr 2005
Posts: 254
Likes: 24, Points: 0, Legacy Rep: 401
Location: Netherlands

### Martijn_vEMarine software developer

If you look carefully you can see that I'm talking about KN, not KM which is indeed only valid for smaller angles. N is the virtual metacenter at larger angles of heel.

8. Joined: Jun 2010
Posts: 1
Likes: 0, Points: 0, Legacy Rep: 10
Location: uk

### despatchesNew Member

blagger/gaff

hi,

i'd very much like to hear from you if the project is still alive , , ,

even if the project has stalled temporarily or otherwise, i'd love to catch up and find out how far you progressed, perhaps sneak a preview at your research,

i am now looking to commence restoration with an original hull, very little to go on rig wise,, but hull good basis as a pattern.

if you were looking to make a fibreclass hull , or a light cold molded hull, maybe we should talk.

all the best edward

9. Joined: Nov 2009
Posts: 203
Likes: 7, Points: 18, Legacy Rep: 97
Location: Houston

### conceptiaNaval Architect

Marshmat you are correct. KN-KGsin(theta) will give you the GZ..

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.