Swain BS_36 Stability curve

The weight study (listing all weights and their centers) was published in post #104 of this thread.......That's a free gift from me to you......which you can now sell to those who purchase your designs.......

Thanks again to SailDesign for a template which fit nicely.......

 

My calculations were in inches and lbs but they roughly agree if I use your numbers for weight of wire etc.The submerged mast has a positive effect (always vertical upward)A wild guess is about 850ft/lbs .
True,its usefulness strictly as a lever declines as the inversion progresses. so that a 180 degree boat is only getting a bit of bouyancy,but the boat is more unstable inverted because of the mast buoyancy.The mast helps,as does the cabin.

 

I know I'm not the only one pointing this out. But when we cut through the noise and the fogbank, it appears there have been multiple genuine offers to do an inclining test on one of Brent's boats. And he's ignored them all, although I think the tests would be more useful (and certainly more relevant) than taping foam to a piece of plate and throwing it in the water.

I don't remember any accounts of transoceanic travel on a plate with foam taped to it. Matter of fact, I've never met anyone who even keeps such a plate anchored as a live-aboard....

 

Realistically you'd need to do a few, but even one would be great. I say do a few because the designer has no control over the as-built boat unless he also built it. This is a problem for all designers not just Brent. I once saw a Colvin Gazelle drawing over 5' because it was so grossly overweight. It should have had a 4' draft. I've seen another one with a steel cabin top even though Tom is adamant that they should have plywood to keep the weight & vertical COG down.

Tad's numbers are great, some solid information to go on. What I'd like to know is, since the boat isn't compliant as designed, what could be done to make it compliant? Preferably without increasing the draft.

I still like the basic concept and am therefore interested in a hull that meets the requirements. I may never build one but I'd certainly buy a copy of the plans for something around the 12m LOD size just in case I decided to build another boat. 12m LOD, 4m beam is as big as I can build inside my shed.

PDW

 

I built 32 yachts and other assorted craft Mostly one off custom The designer would come out near the end, we would weigh her, get the lcg and then he would note the placement of all weight in the boat and from there do the stabilty
One (designer) mentioned in this thread had a launching of a 45 footer , was 12 inches low on her marks. I am not going to use names in here. Build what is pretty and handsome and proven, just as easy and costs same as building a pig

 

Name the designer and the design and base the allegation or leave the ‘‘mentioned in this thread’’ part out. Now you make all designers mentioned in this thread suspected while there is but one in the case you mentioned.

Also in this manner of making suspicions nobody will know and can't check if it was the designers fault or the builder not following the specs or the client wishing to much and the builder doesn't stand firm against making a bad boat.

Where was the extra weight found..?

In this case you failed in first part and the outcome must have been the latter...

And nothing wrong with building something the first time for those who want that. The industry needs early adopters to go on with development* as the early adopters need the development so they can buy their wana haves or improve their business in case of working ships. In fact everything is built the first time once. To avoid trial and error where possible is why calculations are common nowadays...

* The alternative is that boats still only would be available as dugouts and even that is something someone has ever built for the first time . .

Cheers,
Angel

 

Sorry for another newbie question. How did we come to the conclusion the boat is non compliant? Is there a minimum AVS to get compliance?

 

My guess would be making the plates thinner and keep the strenght up by using frames and put the saved weight in the keel. I think using a lighter mast and rigging would also help especially because of the height of that weight... Jack, no need to responce, Brent has often explained those are crazy ideas . .

Cheers,
Angel

 

No need to apologise, simple question, simple answer.

To be non-compliant, of the course the design needs to be reviewed against existing criteria to establish whether it passes or fails. Despite repeated attempts by many, Brent has never supplied such data for review. Thus, if no details are provided the unspoken is left to assume “it is ok”, rather than actually providing data which establishes it is in fact noncompliant; which doesn’t sit well with many.

Tad has done an extremely admirable job in doing a weight breakdown to arrive at a rough LCG and VCG. He also used existing data that he could find, again non ever supplied by Brent, to arrive at the approximate 3D shape for analysis in a Hydrostatics software. From this, he has produced the first ever GZ curves for the vessel..as so far, to date, non has ever been supplied by Brent.

So, looking at the very good data supplied by Tad here:
http://www.boatdesign.net/forums/stability/swain-bs_36-stability-curve-37070-7.html#post452826

And now comparing just 2, of the many criteria for an Ocean going yacht, as Brent claims this to be, using the ISO 12217-1:2002 stability rules, it fails. It is non-complaint. (Despite the negative hits/comments from anon who does not produce data to the contrary).

ISO. 6.3.3 a)

Where the maximum righting moment occurs at a heel angle of more than 30 degree (which this does), the righting moment at 30 degrees heel shall not be less than 25kNm for design category A ...(Brent says he crosses Oceans with this design…so this falls into Cat A under ISO 7.2.1: “…for example across oceans, or inshore when unsheltered from the wind and waves for several hundred nautical miles”)
So looking at the curve, the GZ at 30 degrees is seen to be approximately 0.76 feet.

So converting this to metric = 0.76/3.281 = 0.23m

The displacement is at (from Tad’s figures) 19,000lbs = 19000/2.205 = 8616kg = 8616x9.81 = 84530N.

So the righting moment at 30 degrees is 0.23m x 8453n = 19580n = 19.6kNm...this is less than the minimum required of 25kNm.

Conclusion it fails ISO 6.3.3 Resistance to Waves.

The same procedure was also done for 6.3.2 Rolling in Beam Waves and Wind. Shown here:
http://www.boatdesign.net/forums/stability/swain-bs_36-stability-curve-37070-8.html#post452845

The value of phi (R), from ISO 6.3.2, = 25 + 20/Vd = 25 + (20/(8.616/1.025)) = 27.4 degrees, this is shown in the sketched out region on the GZ curve.

MW, the heeling moment is calculated in the same way (long winded way) shown on the GZ curve as 0.45ft (when converted back into Imperial units)

The area A2 must be greater than A1. As shown this is not the case, again, this fails.

So, Dennis, this is why on the data presented, the vessel is non-compliant.

Brent and his acolytes can of course dispute these values. But to do so, they can only dispute them by providing their own values which they consider "correct". But to do this would expose them to independent verification of their facts and values….don't hold your breath!

And nothing is simpler than an Inclining Experiment, to establish facts, go on…”throw the boat in”... So despite the nicely thrown about words for deflection, he won’t even comply with his own axiom. An inclining expt is very easy to perform and Tad has offered many times to do this for free….yet deafening silence.

Thus, from the above, draw your own conclusions.

 

Thereby, of course, defeating the purpose... though building from 4mm plate should be feasible, that's what I've used and it's much nicer to work with than 3mm. 4mm plate with 32x6 flat bar longitudinals on 300mm spacing from the deck edge down would be easy to do.

Lighter mast, maybe but you might not save much. As I calculated the steel mast is some 160kg and it could probably be dropped back to 100NB in section without dramas. Of course then it would be negatively buoyant I think (unless you flattened it out & taped it to styrofoam).

Tom Colvin in his book quotes steel as 3.8X the density of solid timber but didn't give the timber species. A wooden mast is going to be what - 40kg? Rigging wire etc is going to mass the same or more as the timber mast is *much* weaker in compression.

How about increasing the beam? Gain in initial stability at the cost of greater inverted stability as well? Questions like this are why I'd pay someone who knew how to do the calcs because it's way, way beyond my competence.

PDW

 

PDW
If your thinking steel mast but want to keep it light here is an option
http://www.metalsailboats.com/STEEL-SAILBOAT-MAST-DESIGN.html
Tom

 

the designer is deceased. He cannot defend himself and even if he was alive there is no way I,m going to bandy names in here. Full stop. There is too much of this from people who perhaps have achieved nothing and who just sit judging
Good day to you and thanks for your concern .
In the case mentioned believe me it was not the builder, it was someone who turned his hand to drawing boats.
I could name an early (superyacht) In Akl which was down 18 inches by the bow upon launch, aft her boot top did not show at all. There were piles of lead here there and most everywhere on deck as they tried to fix her. They never did. I spoke to Laurie Davidsons wife who went down harbour on her, she said -- I thought the thing was going to roll, why should I n ame her designer, what does it achieve?
It is one thing to tell somebody abt such things face to face. To put them into print is quite another
Presumably, you have either designed yachts or built them?
if you have you will know of the sensitivity?
I am not saying for once Inst. that people should not have ago
I am saying, that they should pick a proven thing OR do their own thing
I am not abt to sit and boot BS or Jack, I merely observe .
Peut etre I am considerably more Ancienne that you Ms

 

Thanks. Interesting. I haven't seen oval steel tube here in those sizes, must ask the steel merchants. That would be cool. Square tube in galvanised finish is readily available.

The less than 2mm wall thickness makes me pause. OK structurally it's more than capable of its job but steel does rust and thin sections are a ******* to weld fair. The most you're going to save over an equivalent 13.5m tube of thicker section (say 3mm wall) is about 80kg. Not sure I care enough, the greater inertia damps out the roll period anyway. Slower roll, greater comfort factor IME.

I am thinking of using steel but what I'm building has 2 masts and a low aspect rig. Must re-check but I'm pretty sure Tom had no problems with it, he said I could drop from a 125NB sched 40 aluminium tube to a 100NB sched 40 steel one IIRC.

Time to get back to framing up the cabin sides...

PDW

 

Thanks Ad Hoc. Very educational. Rep given.

 

Ad Hoc,

Its all very well to say that the boat does not meet ISO standards, but is it correct to say that this vessel is "non-compliant" as you have. That is to say, should you be applying iso to a design that is home built if it is not prescribed by legislation or regulation.

In Australia, a homebuilt, non commercial (private) cruising sailing vessel can be pretty much anything you want it to be and is not required to be "compliant" to any particular stability or structural standard.

Pretty sure a lot of other countries will be similar (even Europe has loopholes for home builds if I recall correctly).


So I ask under what jurisdiction would this design be considered "non-compliant".


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