Tuesday, January 6, 2009

From Down Under....

FYI - a most excellent email exchange between a former student, one of our instructors, and myself - concerning a knock-down test way down under....a good read on stability can be found in David Gerr's - The Nature of Boats

Re: Need a little bit of help - stability and 'knock down test'.

Hi John,

Happy New Year from Down Under. We are currently in Sydney Australia and managed to get here just prior to the start of the Sydney Hobart race which was a hoot to see. We are currently docked at the Cruising Yacht Club of Australia which are the sponsors of the race. We have signed up to join the club on a cruise to Hobart, but given the club's experience of the 1998 race (http://news.bbc.co.uk/onthisday/hi/dates/stories/december/29/newsid_4034000/4034603.stm) there are all sorts of regulations that boats must meet in order to just sail with them. Sigh.

Anyway, they want each boat to have a Certificate of Stability. Given that our boat was a custom boat built in New Zealand we do not have this certificate and will have to meet with the audit committee in the next couple of days. We have reviewed the Australian rules as promulgated by the government (http://www.yachting.org.au/ and specifically Appendix D to part 1).

I am trying to figure out if there is a way to come up with Righting Moment Index without having to do the "test" which includes pulling the mast in a horizontal athwartship position with a large mass suspended from the top of the mast. I really don't need to try and wreck our home.... there has got to be another way (we understand the club's issues as I bet there were a number of lawsuits after the 1998 disaster, but come on... ). Anyway can you think of any other way of coming up with the Right Moment Index? I remember in class dealing with this a little, but....
Anyway, Happy New Year again and all the best.

Hi -

My last email got away before I finished my thoughts - is there's a provision in their rules which could allow the builder and or designer to provide this information - do you know who designed your boat? If so - can you contact that person or the yard to see if they have any information.

I can help you calculate your metacentric height (GM = (Beam X .44/Roll Period in seconds) Squared) which requires knowledge of your roll period (which we can establish) - to determine roll period measure the beam (in inches) and divide by 8. Take this number in inches and measure down from the sheer amidships - using this number - place a piece of black tape at this distance on the topsides - do the same on the other beam. At docksides (make sure you have the tanks pressed full to reduce free surface effect and stow and secure all gear to stabilize G) - have a bunch of folks jump on and off until you get you boat to roll back and forth until these marks just break the water plane. Keep her rolling and measure time (in seconds) for the vessel to do 6 complete roll cycles - ie., from the starboard mark back to the starboard mark again. Determine this time in seconds and divide by 24 - this should give you the vessel's roll period.

I have cc'd our stability guru, is there someway we can use this data to determine RMI or does the vessel need to be pulled down?


John (who I work with on maritime education) cc'd me on your issue about 'knock down test' and what could be done to avoid the test.There is a formula that is used to screen vessels for off-shore racing, or for any other extensive cruising work. The formula is based on some vessel dimensions and imperical testing and is often, but not always, accepted by race organizers.You could do the calculations yourself PROVIDED you know your displacement accurately. That may require your vessel to behoisted out of the water by a Travel Lift that has 'calibrated' strain gauges in its lift mechanism; many TL do, but they must be accurate and shown to have been verified by some independent weight testing agency.The formula will produce a number. This number should be less than 2 for offshore vessels. The lower the number below two the betteryour position with the race organizers to avoid a 'knock down test'.

The formula:

The calculation is - Beam dimension in feet divided by the cube root of Displacement expressed in cubic feet of sea water (64 pounds per cubic foot).

Capsize Ratio = Beam / (Displacement / 64)0.333

Having said that I would encourage you to subject your vessel to the 'knock down test' - it is not as bad as it sounds - and will reveal strengths and weakness in your vessel that will increase your confidence in the vessel and its ability to successfully accomplish the voyages you wish to complete. The test can be stopped at any time if an unsafe or vessel damage situation arises. Stowing all loose gear inside, ballast, and movable stuff is something prudent mariners take as a common task. What is particularly revealing with the test is where the 'DECK EDGE' and hatch combings are in relation to the water surface. I have seen vessels fail the test because the cockpit filled over the combing and the water would have found its way into the interior of the vessel. Some boats ride high on their side, others don't - and it is not that intuitive to tell just by looking at the vessel floating normally. I wish you 'bon voyage' and contact me or John if there is anything else we can help you with.


Thank you for your help. Just want to make sure I was doing the calculation correctly. My values are:
Beam 14'-5".... so the Beam is 14.41667
Displacement (weight of the boat last time we hauled out - fully tanked, oil, water, food, etc) 52,700 lbs..... so 53,500 (to be ultra conservative) divided by 64 = 835.93. The cube root of this is approximately 9.4155 (I don't have a calculator that can calculate a cube root, but I am within 1 point...so I think I am close enough)... so
Capsize ratio = 14.41667/9.4155
Capsize ratio = 1.541
BUT... in some respects this seems a little odd.... the heavier the boat is the "better" the value. If we are 60,000 lbs, the capsize ratio would be 1.448 and therefore lower than the 1.5 and supposedly better...
Before we loaded on some new equipment and a complete load of fuel and water and provisions for 3 years (I did over do just a little bit, what with a heating system and new batteries, etc), the boat weighed 48,000 pounds... and the capsize ratio value would be 1.59.... but the boat would ride higher and most of the weight is below the waterline. Our lead ballast is 11,800 pounds and is located in the keel from 5 feet below to 8 feet below. It would seem that this is an important number (location and amount).... as well as the placement of the rest of the heavy equipment which is primarily at the water line.

HOWEVER I am thrilled to have a value that I can take to the audit committee but I sure want to make sure that it is a good number. Thank you again for your assistance and any other suggestions that you may have. Would it be helpful if I sent any photo's? Maybe of the dining salon set with the ship's china and crystal (which I would want to take off if we did the 118 degree horizontal loading as I am sure that there little holders would keep everything in place beyond more than a 75 degree roll)... We have an enormous respect for the sea and quite frankly never want to be in any condition greater than 40 knots... well, maybe not more than 30 knots but there are times that can't be helped. Our worse wind (70 knots) has been at the dock and that was hard enough, just dealing with mooring lines.
All the best.

Hello Captain:

Your calculations are OK.Your observation is also good, that the heavier the boat the lower the capsizing screening number.The reason this is so is because when the vessel is laid on its side the fulcrum is some place by the deck edge. Hence the importance, in this case, of knowing the angle of heel at which the deck edge immerses.When the vessel is at an extreme angel of heel the total weight of the vessel is on one side of the deck edge and the external capsizing wight is on the mast side.At very large angles of heel, as in a knock down, the center of buoyancy has also moved considerably. The immersed shape of the vessel, which determines the location of B, has changed considerably from that of a vessel floating and rolling in a normal manner. The center of gravity has remained, we hope, where it was when the vessel was upright. The meta center has also moved under the influence of the shift of B (center of buoyancy) and that of the new under water shape of the heeled vessel.Attached is a spreadsheet that compares characteristic values between several types of cruising yachts. The capsizing screening value is shown. You might have fun plugging in your own vessels values if you or a college have a PC with Excel.

Bon Voyage

No comments: