Document 2 – Guidelines for Buoyancy

DOCUMENT 2 – GUIDELINES FOR BUOYANCY
Refers to 05.04.00
The Micro rules give very few rules about buoyancy, and these rules should not be considered as sufficient to make a boat safe.  In the last 15 years, some Micros were lost or badly damaged due to insufficient or poorly made buoyancy.A flooded Micro can usually not recover by own means when she is totally flooded. A yacht floating in an abnormal position can make towing very difficult. The water volume can be much more than 1 cubic metre, the total towing weight is in excess of 1.5 tons and cleats, bolder and eyes can suffer a lot.
This explains strong regulations about cleats in Appendix 2!

When a Micro capsizes and fills, some air pockets can remain, giving the yacht an abnormal position. If this leads systematically to a boat floating in a position where towing is almost impossible, a solution could be to make some small holes to allow air evacuation!

After some hesitation, it was decided to allow centreboard cases opening inside the cabin, as this can allow natural evacuation of water from the cabin. Anyway, in that case, it could be useful to have a system allowing closing the opening, as a permanent opening could prevent efficient bailing.

D.02.01 The TOTAL BUOYANCY in litres shall be no less than the boat weight in measurement trim, in kilograms, increased by 51 kilograms. This is really insufficient, as the boat should float with a freeboard not exceeding 1 centimetre! So there is a great probability of a boat floating in a very uncomfortable position.
Anyway, don’t forget that the hull has its own buoyancy. The hull surface is approximately 13 square metres, and with a skin thickness of just 10 millimetres, the buoyancy of the hull is already 130 litres, yachts using 1 inch sandwich foam can lift no less than 325 kilograms, with a very interesting distribution.
The additional volumes shall be constituted of closed-cell foam, allowing a flooded yacht to hold buoyancy for at least 24 hours. Closed air compartments are not sufficient, as the hull can be punctured at this place, and allow water flooding. Filling these compartments with foam flakes proved to be a bad solution, as the flakes can get loose.
Inflatable volumes are normally not allowed.
The buoyancy volumes shall be secured to the hull or its structure, in order to avoid accidental move when the yacht is flooded.
Only volumes under the sheer line are considered, as it is required that the boat should float with deck completely out of the water.
D.02.02 STATIC BALANCE means a good fore-and-aft and transversal distribution.
In the transversal direction, no symmetry is required, but it helps making calculations easy. A volume of 100 litres at 0,15 metre off the centreline on one side can be balanced by 20 litres at 0,75 metre on the other side. However, if there is a large excess of buoyancy, balancing volumes should be placed at the same level above the waterline.
When you take the lateral distances (Yi) from the centreline as positive to starboard and negative to port, the formula giving the lateral offset is:
YB = (Y1*V1+Y2*V2+ … +Yn*Vn)/(V1+V2+ … +Vn)
where YB should be as close as possible to ZERO.
In the fore-and-aft direction, a slight offset to the stem is accepted as it helps maintaining the foredeck above water during towing. Only when the is a large excess of buoyancy this will NOT lead to a flooded boat floating with stem or stern pointing to the stars!
When you take the fore-and-aft distances (Xi) from the centreline as positive to starboard and negative to port, the formula giving the lateral offset is:
XB = (V1*X1+V2*X2+ … +Vn*Xn)/(V1+V2+ … +Vn)- XG
where XG is the distance from stem to centre of gravity (CG). XB should be as close as possible to ZERO, or very slightly negative.  When some data, like position of the CG, are unknown, take the default values at the end of this Document.
D.02.03 The STABILITY shows the ability of the yacht to stay in upright position when flooded.  This is obtained by weight stability and form stability.
a. WEIGHT STABILITY is obtained by giving the centre of buoyancy a position as high as possible and the centre of gravity a position as low as possible. A yacht passing easily the stability test at 90 degrees has a low CG, and an improved weight stability. Installing buoyancy under the berths LOWERS the centre of buoyancy, and REDUCES the weight stability. No more than 50% of the buoyancy volumes should be installed under the berths surface.
When you take the fore-vertical distances (Zi) from the centreline as positive to starboard and negative to port, the formula giving the lateral offset is:
ZB = (V1*Z1+V2*Z2+ … +Vn*Zn)/(V1+V2+ … +Vn)-ZG
where ZG is the height of CG above the waterline.
ZB should be as high as possible, it is believed that all Micros can achieve a positive number (CB above CG).
b. FORM STABILITYshows additional ability to recover to an upright position when the yacht is accidentally set in another position. Form stability is also working on a capsized boat, and an excess of form stability could lead to a total stability making it almost impossible for the crew to get the flooded boat back in upright position.
A very good form stability in fore-and-aft direction is very useful when towing. The same stability is not so useful in transversal direction, and anyway it is impossible to achieve similar results.
In transversal direction, the formula is:
Y2B = (V1*Y1^2+V2*Y2^2+ … +Vn*Yn^2)/(V1+V2+ … +Vn)^0,5
and it is believed that values of Y2B above 0,5 metres can be achieved and are effective.
In fore-and-aft direction, the formula is:
X2B = (V1*X1^2+V2*X2^2+ … +Vn*Xn^2–VG*XG^2)/(V1+V2+ … +Vn)^0,5
and it is clear that high values can easily be achieved with volumes in the stem and the stern. A normal value should be no less than 1 metre.
Form stability is only effective when there is a large excess of total buoyancy.
D.02.04 For calculations, dividing large volumes into several smaller ones gives more accurate results, especially on form stability. Large volumes across the hull (under aft double berth or stern volume) should be divided in at least two smaller ones, each on one side of the centreline. To make full use of the hull own buoyancy in the calculations, substitute the values for volume 1 in the formulas by the default values in table underneath.
An Excel97 template BuoyancyMicro.xlt makes these nasty calculations for you. Save the file as [your sailnumber].xls. The results should be shown to your national class measurer or an event measurer when you claim to have sufficient buoyancy, but failure to do this may not lead to exclusion. You could be asked to prove the buoyancy by other means. A copy of the completed file should be sent tomeasurement@microclass.org for statistical purposes.
D.02.05 TYPICAL VALUES
XG 2,88 m
YG 0,00 m
ZG 0,28 m for a prototype 450 kg, P = 7,30 m
0,26 m for a prototype 450 kg, P = 8,00 m
0,30 m for a Racer 540 kg passing stability test 90� with 10 kg
0,23 m for a Cruiser 560 kg passing stability test 90� with 15 kg
Add 0,01 m for 15 kg in excess
For hull buoyancy:
Hull surface 12,9 sq.m. hull buoyancy obtained by multiplying hull thickness in mm by 12,9.
Xhull 2,90 m
Vhull*Xhull 37,4*hull thickness in mm
Vhull*Xhull^2 133*hull thickness in mm
Yhull 0,00 m
Vhull*Yhull^2 5,1* hull thickness in mm
Zhull 0,16 m