GZ curve

[ivansalasj]

Hi,

I would just like to post the initial estimation of a GZ curve for an ocean cruiser which i am at the moment designing, my aim is to get as much feedback as i can about the GZ and also to open a discussion in what should one aim for the stability to be of a fast cruising sloop. In my case the length is 32 m, Beam 7.9 m. T=1.2m Displacement = 95 tons. There is more information if you would like to know it just let me know and i will post it. I aim to carry 540 m^2 of sail but again this is only the beggining of a long designing spiral. I should also mention that I am a student of Naval Architecture and this yacht will prob. never be build but its part of the course and quiet an interesting part of it too.

Cheers
Ivan

[ivansalasj]

And the GZ curve is...

[Dutch Peter]

Ivan,

The GZ-curve itself doesn't say it all, although a range of about 140 degrees is a good for such a yacht.
What also should be considered is the angle at which downflooding openings are reached.
You could also include a wind moment line at different speeds, related to your sail area.
Download a copy of the Large Yacht code of the MCA, that way you make a link with relevant requirements (something tutors like!).

Good luck

[MikeJohns]

Ivan


Ultimate stability is probably unnecessarily high at 140 degrees for such a big boat. It is good if you can achieve it without too deep a draft.
You need to match your heeling moment with your Righting moment, the old way was to take 1lb per square foot of sail acting laterally at your centre of total sail area, then determine what heel angle you want at this heeling moment, then change your hull masses ballast etc to achieve the desired RM. ie look up your GZ curve for say 15 degrees heel multiply GZ(m) by Displacement (kg) by 9.81 and that is your RM this should equal the Heeling moment (work in kNm).

Hope this helps, not sure what level you are at.

[JimCooper]

I am no professional but I would like to grasp this.
How does the wind overturning force get to equal the boat mass ? doesn't sound logical.

[Dutch Peter]

Originally Posted by JimCooper

I am no professional but I would like to grasp this.
How does the wind overturning force get to equal the boat mass ? doesn't sound logical.

Jim,

It isn't, read the post of Mike again. He refers to the rightning moment e.g. 'GZ x Deplacement', must be equal to heeling moment e.g. 'windforce x height of center of effort of sails'

[nemo]

From the main data and the GZ curve, it looks like a 1950's cruising yacht...
140° of positive stability are not usual in modern boats, and I would say like MikeJohns it could be even too much.
Is it long-keeled boat? Otherwise, how can you provide enough lateral resistance for 540m^2 of sails, with a depth of 1.2 m?

[ivansalasj]

Hi,
Thanks all for your feedback, mikejohns i will decide in a heel angle that i would like at a certain wind speed and direction and i will follow your procedure. The lateral resistance nemo is a very good question and one that i have not thought in depth so far, i guess the only answer i can give you is that the boat will perform poorly close to the wind but that was mainly my intention, but the large beam and the light weight will make a fun ride when sailing with the wind in other directions. The small draught was the outcome of a large interior and therefore a large requirement for breadth and light weight, if i decided to make the yacht perform better when going close wind i would prob. as you suggest make the draught larger and compromise in my interior in order to achieve the right displacement, am i making sense or heading in a wrong direction here?.

Mikejohns i am using a modern naca profile keel with a large bulb at the end of it, which at the moment i have decided it to carry 25 tons of the 95 tons which i am aiming for, the overestimation of the stability is mainly due to the 5 m lever from the bulb to the cg of the yacht but i am aiming to reduce this and also the weight at the bulb in orther to achieve the right stability. I have also got a home made VPP which works quite well and i will post the results as soon as i have it but for the moment i will try what you have suggested, thanks.

And thanks to you too dutch peter i have the code with me and its going to get me some valuable points at the moment of presenting the report.