Stability rules of thumb?

If you do a basic sketch of your boat and the estimated C. G. vs. the center of buoyancy, you may find that the boat (26PC) will probably right itself at upwards of 90 degrees of tilt.

 

The fundamental difference here (while noting that comparing these two vessels is like comparing the proverbial chalk and cheese) is that in the unballasted condition the Northern Marine vessel is going to fall over regardless, whereas your Thunderbird is perfectly happy without any ballast.
Please don't 'over think' this stability situation too much - your 'portable ballast' (in the form of crew who can move about) is going to have a much more obvious effect on your boat than simply removing 300 lbs of 'ballast' in way of the engine.

 

@alan454 . . . You know at some point your are going to have to show us some pictures of the boat and the engine rebuild. As you work on this boat, keep in mind that there is a small forest of wood in the structure of the boat and it would be a good time to determine if you have any structural rot. It is a pretty common thing for boats of that vintage and now age.

I had to replace rotting bulkheads on both of my Formula/Thunderbird boats (1991 and 1996 models) . Your boat is even older, and may have some issues unless prior work has been done.

 

There was more to the capsize of Baaden than improper ballasting. A steeply inclined launching ramp(the builders apparently didn't realize this posed a problem), 15 degree heel from one dolly falling off the ramp, no ability to abort the launching, and no poppets all contributed. Here's one piece I wrote after the fact.

Capsizing of the Yacht Baaden

Re-cap following release of NTSB accident report.

By Tad Roberts, Yacht Designer.



On the evening of May 18th 2014 the new but still incomplete 85’ motoryacht Baaden capsized while being launched in Anacortes, Washington. The yacht had an estimated value of $10m U.S.D. and was considered a total loss. Some decisive action by those close by rescued 5 people trapped on the lower deck of the rapidly filling vessel. One year later the NTSB released a report outlining the circumstances of the accident and a finding of probable cause.

The NTSB determined that “probable cause of the capsizing of the yacht Baaden during its initial launch was the vessels low margin of stability”. That seems obvious, far more interesting and educational are the underlying reasons for the low margin of stability. Also worth study are the launching conditions (outside the vessel’s stability characteristics) which lead to the accident. My interest is not in apportioning blame or pointing fingers, my primary interest is in explaining what happened, and then trying to learn from the outcome.

LAUNCHING CONDITIONS

As I discussed in Launching Calculations (WM September 2014) Baaden was launched on an inclined ramp using three multi-wheeled dollies. Two dollies were positioned port and starboard aft under her quarters with the remaining dolly on centerline under the bow. There were no poppets to restrain the hull from heeling as she was launched. The NTSB investigation found the ramp was built in two sections, an upper section 40’ wide, and a lower section only 24’ wide. Investigation determined that as the boat was lowered down the ramp the aft port dolly dropped off the wider ramp outside of the narrower lower section. This could have been foreseen had a diver been in attendance.

The aft port dolly dropping off the paved ramp initiated a 12-15 degree heel to port. The stern was partially afloat by this time though the bow was still completely out of water. Later investigation found that this initial heel was limited by the midships port stabilizer fin grounding on the upper ramp. From this point the boat would no longer roll down the ramp, crew aboard used the main engine in reverse to back into deeper water. The drag trail on the upper ramp shows where the stabilizer tip (supporting the heeled boat) dropped off (in the trail of the port dolly) the upper ramp and Baaden capsized.

It may be that this initial heel angle was critical, 15 degrees being beyond her angle of vanishing stability (AVS). It may have been important only when heeled to port. Or it may not have mattered. We’ll look at this again in the weights section.

VESSEL WEIGHT AND CENTER OF GRAVITY

According to the NTSB report Northern Marine weighed Baaden at least twice during construction. The final weighing was more than 6 weeks prior to launching. An error in recording (and/or communicating with the yard’s NA) resulted in an unnoticed 8000 pound imbalance aft, she was heavy on the port side. How much weight was added in the final 6-8 weeks of construction? Total weight at launching and where the VCG ended up remain unknown. As the boat was never inclined before capsize no real VCG will ever be known, neither will an actual displacement. Upon capsizing and filling with water, considerable lose weight shifted plus water soakage changed the Baaden’s stability again.

NTSB also uncovered a 14,300 pound shortage in ballast between the NA’s calculations and the amount actually in the boat at launch.

STABILITY

Due to all the unknown factors it remains impossible to make definitive statements concerning Baaden’s stability at launching. The NTSB report includes stability calculations indicating positive stability up to about 25 degrees heel, even with the 8000 pound portside heel. The launching video doesn’t support this, she rolls through 25 degrees and right over with no hesitation. Did shifting interior weight and downflooding as she rolled contribute to the “low margin of stability”? Certainly, but to what degree cannot be known from existing data.

The actual location of Baaden’s VCG at launch remains elusive. Absent an intact (Before capsize) inclining we’ll never know. There is a procedure for doing an inclining in air, but it would be really difficult with a vessel the size of Baaden. Usual practice would be to calculate GM throughout the launching process and if was found to be less than two feet, add temporary ballast to ensure adequate stability during the launch event.



FINDINGS

All this points out at least two key issues. First; Take weight studies and record keeping seriously during construction. Obviously some projects are less risky than others, and investment in calculations should be commensurate with the risk. I can well recall launching a 125’, $12m yacht when no one knew her total weight, and no one would take seriously my plea to get some load cells and weigh the boat. But that was 16 years ago and I’ve learned quite a bit since then.

Second: Work out a realistic launching procedure and go over it with a qualified Naval Architect. Arrange some backup failsafe protection, ensure the ability to abort the launch, install poppets forward, and have a diver on hand.

REFERENCE

NTSB report is here; Home http://www.ntsb.gov/investigations/AccidentReports/Reports/MAB1514.pdf

 

yep they couldn't be more different no doubt - I am really just noting the small weight changes that apparently caused the capsizing of the Baaden - however, I see that somebody below thinks that there was more involved in the capsizing than just ballast and I will now review that post. Might be overthinking but on the other hand I'd like to measure twice and cut once so to speak. I am assuming the original designers looked at worst case stability with no fuel and maximum payload with all the cast iron engine components and I would now be building a new worst case scenario that the designers didn't look at. Also, I believe that term "ballast" is weight added very low in the vessel and will thus lower the CG. Weight added above the CG (like crew, bait tank, beer ...) would therefore constitute negative-ballast I suppose.

 

Alan, there was a LOT more to it than just 'small weight changes' as Tad notes in his detailed analysis above.

Tad's first paragraph says it all -
"There was more to the capsize of Baaden than improper ballasting. A steeply inclined launching ramp(the builders apparently didn't realize this posed a problem), 15 degree heel from one dolly falling off the ramp, no ability to abort the launching, and no poppets all contributed."

And yes, you ARE overthinking!

Launching a boat down a slipway is much more complicated / difficult compared to launching it with a conventional Travel Lift hoist.

 

@Tad - Nice article. Certainly there seems to be lots of problems with the Baaden including the launch. However, even with a perfect launch procedure I think that you correctly point out that there was actually no region of stability for the Baaden during launch, even though the NTSB report shows a small region between about 12 deg and 25 deg. I needed to go back and review that video more carefully but indeed it doesn't look like there was any event such as a the port fin suddenly dropping off the initial ramp resulting in the development of rotational energy necessary to drive the Baaden through the small region of stability noted in the NTSB report. It does indeed look like there was actually no region of stability for the Baaden. Still as far as I can tell the ballast errors are still the root cause of the loss of the Baadan and had it been properly ballasted even with the centerline ballast error but without the heeling moment error she could have been successfully launched even with all those launching errors and possibly the centerline ballast error would have been caught with an incline test as you point out. The 8000 lb heeling error looked to be the final straw it seems to me just based on the NTSB curves and that purple curve was actually bit lower than they estimated and completely below zero GZ.