Swain BS_36 Stability curve

Excellent work Tad.

I've also had to make a few assumptions, from yours and also what is given on various websites of dimensions.

But if you plot the wind righting moment (as a lever - Mw) curve as per ISO Stability 6.3.2 and then find the other values required to obtain area A1 and A2, you get this:



Again despite the assumptions and the fact that many items in the formula can default to a given minimum required, it is clear that area A2 is not greater than area A1.

Ergo, fails.

If you look at 6.3.3 Resistance to Waves

At 30degree the righting moment shall not be less than 25kNm....after converting, this has 19.6kNm...again, fails.

[Mmt = (0.76/3.281)*(19000x9.81/2.205) = 19580Nm = 19.6kNm]

 

It takes quit a stretch of the imagination to believe that adding 5 inches of buoyancy amidships, and raising the buoyancy of the cabin and wheelhouse 5 1/2 inches, will "reduce" AVS
It takes an inch of air to float 1/8th inch plate, the other 4 inches is all buoyancy.
You can measure the three boats in Silva bay and Degnen bay anytime.

Metric or inches ? Which is easiest is whatever you grew up with, and have been using. You can get used to either. The only thing metric doesn't work for is navigation, as degrees am nd minutes translate into nautical miles, tenths of a mile ( cables) 100ths of a mile ( fathoms) etc. It is thus a tens system already, to some degree. Doing it in metric would be horrendously complex.

 

Unfortunately, Tad, not being possessed of psychic powers, can only work with the data available in Brent's book and lines/plans for the 36' design. If you have more up to date data, on paper, with the signature of the designer and a date, please present it.

Until then, I'd say that the analysis stands and I sincerely thank Tad for the effort involved in reconciling the contradictory drawings etc.

True, we always used nautical miles in our navigation software. Funny thing though, when we were coming close to a waypoint to deploy seabed anchored moorings or similar, the target plotter swapped to metres

Then again, I wrote the software....

PDW

 

in layman's terms

 

I have absolutely no knowledge of navigation, but what happens to that conversion when you travel diagonally to the coordinate system?

Lurvio

 

Oh dear, same old same old. BS, your postings are generally full of misdirection and smoke and mirrors, which the acolytes tend to fall for. But in this case, you’ve spin yourself a web so tight, you’re completely meaningless and incoherent.

If you have say a 1m x 1m x 1/8thinch thick steel then its volume is 0.003175m^3

The amount of air to float this bit of steel is simply the ratio of the densities thus:

0.003175 x 7800/1.23 = 20.13m^3.

You need more than 20 cubic metres of air, to float 1/8th inch steel plate.

Exactly.

If you wish to contribute, be coherent…and when sprouting off numbers of your boats, either provide the data for Tad or accept Tad’s work. If you don’t accept Tad’s work, then clearly the book is in error and you must inform the publishers of this fact too. Since everyone is using The book.

 

LOL
I had to check the one inch of air thing myself and Jack/Brent was right.
1 square foot of 1/8" steel sheet is 5 Lbs
1 square foot of air 1 inch thick has 5.17 Lbs buoyancy.
Tom

 

Utter rubbish......honestly.....do you want to talk about stability in an adult conversation.....or reiterate nonsensical foolishness...

1/12th of a cubic foot of air has no buoyancy at all to speak of (see Adhoc's post above) ........1/12th of a cubic foot of salt water = 5.17 pounds of buoyancy (See Archimedes principal, liquid will support a weight equal to the displaced volume...etc) ........how often does 1/8" steel plate go drifting off through the air?

 

The fundamental stability formula is...... BM = I/V

B is the center of bouyancy
M is the Metacenter (about which B is shown to rotate)
I is Inertia of waterplane
V is volume (displacement)

We know G (center of gravity) is somewhere above B.

We know stability is proportional to GM......

We can increase stability by lowering G, or by raising M.......

Adding topside height (Jack's 5.5") will add slightly to V (a bit less than 300 pounds), won't change M much, but will raise G....because the rig, deck, deckhouse, and all items on the deck, have been moved 5.5" higher.......

Thus GM is shorter and stability reduced........

 

A more simple way to calculated it for the benefit of Jack Brent Hickson Swain based on a square meter of 1/8th steel plate...

1 meter square 1/8th inch plate (3mm) weight about 23.55kg (1 x 1 x 0.003 x 7850 whereas 7850 is SG for steel)
The "volume" of 1 meter square 1/8th inch steel plate is 3 liters (1 x 1 x 0.003)

1 liter of buoyancy can float 1 kg in fresh water less weigh of flotation medium which in this case is air and not really an issue here. IOW, Brent, you need at least 23.55 more volume of air than volume of steel to just float the steel ... in this case 70.6 liters of air which is much more than the 3 liter volume of the steel plate.

However, when steel is submerged, the factor is 0.88 of weigh because of the density of water and the 1 meter square 1/8th inch plate will still weigh 20.72kg submerged in water. (just for interest sake, GRP submerged factor for weight is 0.33 and aluminum about 0.63) but your claim is for floating the steel plate.

Again, your argument failed spectacularly when numbers are produced

 

Tad, I think you need to check the relevance of your formula to the value of the AVS. Raising the deck and adding a cabin and wheel house dramatically alters the position of the INVERTED centre of buoyancy.

The slight rise in CoG will adversely influence stability through to the point the deck edge is immersed, but after that I have never found a vessel where the stability is not improved.

Your thinking is the same as we had here in the RNLI 30 years ago. We designed everything to be as low profile as possible. But in pursuit of a AVS of 180 (ie passive self-righting), tall topsides and very tall cabin houses are your friend.

 

Like large Ferry boats, yes too true
i fail to comprehend why you are trying to convince this BS man over and over(not you Craig) is work short in the industry

 

What is your point? Making a "flat" stability curve (sounds paradox)

 

Large angle stability is the result of the couple between the CoG and the CofB.
The influence on the Large Angle Stability of raising the deck edge and coach house will be in two parts: from upright to the angle where the original deck edge was immersed and then from that point to 180°.

For the first part, there is no changed to the geometry of the immersed body, so the only change will be a rise in CoG (plus a small alteration in trim) and it is likely that there will be a decrease in stability.

But from the point the original deck edge would have immersed, there is a significant alteration to the geometry of the immersed body with often a large increase in the couple between the CofG and CofB of the new, immersed structure. In my experience, this always has a more positive influence over this part of the curve than the rise in GofG which remains constant.

 

Those are incorrect assumptions.. First there's more plating on the sides (5" x 80') around the vessel. Ad that additional weight and there's for sure a change of immersed body. That and higher deck will decrease stability and sail carrying ability, and thats a fact. It does, as you say increase stability at heels close and over 90degs. Plot those changes on the stability curve Tad provided and it becomes flatter .