SailNet Community - View Single Post - Pros and cons of steel sailboats
View Single Post
post #3328 of Old 01-24-2014
Senior Member
Join Date: Apr 2008
Location: Hobart
Posts: 134
Thanks: 6
Thanked 11 Times in 11 Posts
Rep Power: 10
Re: Pros and cons of steel sailboats

Originally Posted by Brent Swain View Post
Tad Roberts calculated the ultimate stability of my 36 at 165 degrees, which he posted on Jim, the Russian computer whizz, calculated it on his computer, and came up with 175 degrees, which he posed on the origamiboats site. …..
You keep saying this, you even said it to Tad himself and his reply is here: Swain BS_36 Stability curve - Page 18 - Boat Design Forums (where you are posting as Jack Hickson).

Tad says 131 degrees based on the lines you provide and his most thorough estimate of weights and moments which he gave you for free to comment on and modify if required.
That was his final figure, anything else was preliminary and was altered as the study progressed and that included a 3d computer model of the hull.

I even posted about this on this thread earlier

I'll repeat this again…..
For a higher figure you are relying on an intact and buoyant mast, neither of which is figured into inversion calculations for any sailboat for several reasons. So “Jim” apparently factored an intact buoyant mast to get 175 degrees. Presuming he used the same center of gravity as Tad he must have.

But importantly you don’t understand why we don’t use mast volume in ultimate stability, masts seldom stay intact in an inversion.
But significantly it isn’t really a real situation, the smooth water GZ curve is just a good rule of thumb for comparison of craft in waves and their relative safety.
If you count the a buoyant mast in the GZ then we need to do that for every design you compare it with, not include a buoyant mast in your stability curves and exclude it in other boats it’s being compared with.

Originally Posted by Brent Swain View Post
The buoyancy of deck structures has a major role in that , and how can that be determined by slightly inclining a boat at the dock, without getting them wet? ........... Which means no one should believe anything you post, from this point on!.
Unfortunately this really shows how little you are prepared to learn if the result doesn’t suit you. It’s really concerning that a boat designer can be so willfully ignorant, this was all explained to you before several times over the last few years. Especially since stability is constantly being misrepresented and misunderstood by you.

So again this is being explained to you here and now:
The inclining test is not a test of the GZ curve to the angle of vanishing stability (AVS, ultimate stability angle, LPS etc). The inclining test establishes the location of the center of gravity (CG). The GZ curve is then generated with a numerical model and the CG using a computer.

So unless you get that inclining test, and it disproves Tad Roberts best estimate of the center of gravity from his weights and moments study, then you should only quote 131 degrees. To tell anyone anything else is misrepresentation. Especially now it’s been explained to you clearly several times.
The design should not be promoted as a good offshore design for a 36 foot offshore boat that’s a very poor stability figure . In severe weather these designs are likely to be rolled, and are well below the sensible offshore requirements.

Last edited by MikeJohns; 01-24-2014 at 10:10 PM.
MikeJohns is offline  
For the best viewing experience please update your browser to Google Chrome