Swain BS_36 Stability curve

"HEAR" is not factual, and to assume that it is, is indeed foolish. Assumption is the mother of all f**kups.

As for the curves; off-course will the deck shape influence stability when inverted. For a static stability curve a section through midships is used and that is usually where the cabin top, pilothouse etc are.
All my designs done the pilothouse versions had higher stability than its plain deck sisters. To benefit from this when rolled over, it is wise to keep windows small on a pilothouse in the event of breaking thus flooding the boat.

In my view the more critical part of a stability curve is not so much the vanishing stability point, but rather the ratio from positive to negative and vice versa. Anything much less than 5:1 should be avoided, again my opinion for whats its worth....

Here is a link to a Dix's stability curve - Dix 43 in this case. http://www.dixdesign.com/dix43sta.htm

Edit: My website server had a Houston moment last week and is still offline - my apologies to all contacting me for not being able to log on

 

I have edited my post Dean, the point is steel boats can wrack and destroy a weaker material like glass fastened to them

 

Swain 36 hull, deck, and house.........

 

Here's a preliminary curve of righting arms, this is just to check that the model is working properly, actual displacement and center of gravity are yet to be determined.

Displacement is 23,700 pounds.......in every picture I can find of a Swain 36, the bottom tip of the transom is somewhat immersed, so I sunk the model to a draft of roughly 5'4" which equals the 23,700 pounds.....

VCG is set at .33' (4") above the waterline at 23,700 pounds.....I will guess it's higher than this in reality....but we'll see......weight study to come.....

GMt is 3.27'

Note peak righting arm at 45 degrees when the deck edge submerses....

Note the beginning of a hump in RA at 65 degrees when the deckhouse goes under

Angle of vanishing stability is about 137-138 degrees

Note maximum righting arm is slightly less than one foot, nothing like the one metre plus in the first post in this thread........

 

Similarly to what you are saying but less of a rule of thumb:

There has been some very interesting work performed by Andrew Claughton. He wanted to narrow down just what attributes of the static GZ curve if any could be used as an indicator of inversion resistance.

He found that the knockdown to 90 degrees was very similar for all types of sailing yacht caught beam on in a breaking wave regardless of the shape or area of the GZ curve to that point.

He found that the relative areas under the GZ curve, from 90 degrees of heel to the limit of positive stability, is a good comparison between vessels of the resistance to inversion.

If a pilot house is properly built (and operated) it reliably contributes a lot .

 

I know a guy who shipped his BS 36 from BC to Alberta. The steelwork complete, fully ballasted, she weighed, by the Govt highway scale, 15,000 lbs . 2280 lbs is not unreasonable for interior, rig and engine, altho its easy for some to go far above that , if they are careless.
Another friend had his Colvin Gazelle weighed on a travel lift. Designed displacement 22,000 lbs . actual displacement 32,000 lbs.

 

The preliminary curve of righting arms is quite interesting Tad. Cannot wait until the weight calculations are done for a final analysis.

Looking at it I get a positive to negative ratio of about 3.8 (positive 1.0 divided by negative 0.26) that is not so hot in my books.

However, what is interesting is that the "lack" of higher initial stability to about 50 degrees heeled, is offset by the high righting arm when inverted at about 170 degrees. This boat is very unstable upside down and should right itself easily - a spin off due to relative narrow beam with highish boxlike pilothouse.
That said, I would rather have more initial righting moment to prevent a knock down. Again, I would also prefer the traits this hull shows (preliminary) to recover easily when inverted. In short, this boat is a paradox, much like its designer

 

That hull and that stability curve look exactly like the way they used to build them in the early 80s before someone discovered that the factory edge of the plate makes good looking sheer line. Boats since then had the same freeboard in the ends, but 5 1/2 inches more freeboard amidships.
Go check out the BS 36's in your area. The last time I passed though your area, there were 3 , two in Silva bay and one in Degnen bay ( 5 more in Nanaimo) You will find the measurement from the chine to the sheer, 5 1/2 inches more than your model .Do the calculations on that, and you will find the resulting stability curve much closer to what was posted. ( although the one you arrived at is not considered bad ,by todays standards.)

 

Jack,

Working backward from inconsistent and non-existent data is frustrating, some things must be guessed at......My concern is where the deck and topsides intersect, I guessed a constant 3" bulwark/toerail but I have no idea if this is the case..........I'm working with a construction pattern drawing dated Oct 29, 1988. The sheer is represented as the straight plate edge.......

There are no Swain 36's in Silva Bay, there was one in Degnen, I haven't been over to see if it's still there, and I have to go at low tide to get any measurements.

 

Tad

Excellent work, despite minimal info.

I did a quick check using ISO 12217, again not knowing any of the real values. But the heeling moment due to wind I calcuated as 12,225Nm.

Gggrrrr..hate imperial units, confuses me!! Made a mistake. (correction below...i must wake up before i check my units..hahaha)

This gives a righting lever of some 0.12m or 0.38feet, if you lay this over your curve, the area A2 up to the min angle allowable (50 degrees or downflood) doesn't leave you much resistance.

Keep up the excellent work..

 

Problem is I have one piece of dimensioned data...the "half-pattern", but I can't work from that, I have to work to it.........I can make the hull, flatten the surfaces and compare them with the pattern, but I can't take the pattern and fold it up to make a hull......anyway it is now quite close.....and the sections match the body plan from the "Lines" drawing perfectly....so which should I believe?

The red lines are the pattern and the purple lines are the unrolled surfaces from the hull model.

 

...Lurking!But my compliments,Tad....It's a bit like watching CSI!

 

Hey. I have been reading this thread. A newbie question. Why does the righting arm seem to reach zero at around the 140 deg mark. I thought the only time there would be zero righting moment on a keel boat would be when it is perfectly upright and perfectly upside down at 0 and 180 deg? Or does this not include keel weight? What am I not understanding here? Also what is GM and GM slope?

Thanks.

 

Read this power point presentation:

academic.amc.edu.au/~gthomas/yacht/yacht_lec2_stability.ppt

Or go and buy 'How to design a boat' by John Teale which is a gentle book on NA for Newbies.

 

Thanks. My assumption that most keel boats always have a tendency to self right was incorrect. This is only true for yachts with a vanishing stability of 180 deg, which was claimed by the OP of this thread for the BS36. If the angle of vanishing stability is under 180deg the boat will continue to roll and stay upside down if that point is passed until a wave rocks the boat back up to the point of vanishing stability again. IE the BS36 will capsize if rolled to 137 deg or more. It will then stay upside down until a wave rocks back it to 136 deg.

Sound right?