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Wooden Lightning Capsize and Stability Study

Scott Graham, Graham Marine, Inc.

Recently the subject of flotation and stability after a capsize was discussed on the Yahoo Wooden Lightning forum. The consensus was that past experience has shown that a wooden Lightning without additional flotation has very limited reserve buoyancy and marginal stability after a capsize, and it is very difficult to get sailing again without outside assistance. So the question became how much flotation is necessary, and where should it be located to allow the boat to get sailing.

I never capsized my boat, but I still have a vivid recollection of sailing back from the beach as a teenager, putting the leeward cockpit edge under in a gust and taking about 9" of water into the boat. I was lucky to get it bailed out and continue sailing. Since I now have the appropriate computer software, I thought it would be worthwhile to analyze a few flotation configurations and provide the results to the group. Hopefully it will convince everyone of the need to have some additional flotation and provide some general guidance on where it should be located.


The basic flotation concepts included installation of watertight bulkheads at stations 3 & 8-1/2, flotation bags in bow, stern and under the seats, & foam flotation under the gunwales. The configurations analyzed are as follows:

Case

Wood
Hull
Bulkheads
Bow & Stern
Air Bags
Bow & Stern
Air Bags
Under Seat

Gunwale Form
1
X




2
X
X



3
X
X


X
4
X

X


5
X

X
X

6
X

X

X
7
X

X
X
X
If we assume a wooden boat weighs 700 pounds but subtract 150 pounds for centerboard, mast, boom, rudder and misc hardware, we get 550 pounds of wood. If the average density of the wood in the boat were 32lbs/ft^3, in saltwater we would have an equal amount of extra buoyancy, or about 1,100 pounds buoyancy total. In order to predict the swamped waterline and the stability, it was necessary to build a 3D computer model of a Lightning which had an accurate representation of the planking thickness, frames, floorboards, seats etc. The 3D model shown left was derived from the lines and scantling dimensions on the plans. When fully submerged without the mast, boom or rudder, this 3D model has a buoyancy of 1075 pounds. I considered this close enough given the variations in boat construction. If your boat is a little heavier, it likely has a bit of extra wood and therefore a little extra buoyancy. I also needed a good estimate the center of gravity (CG) of the boat. By adding up the weights and centers of the pieces in the computer model I came up with the CG 10.3 feet aft of the bow & approx 1.7 feet above the baseline on the plans (approx 1.2 feet above the keel). The CG calc included the mast and boom and assumed that the centerboard was down.

Obviously there are many different ways to capsize, and everyone has their own style! Dive in and swim around to the centerboard, climb over the gunwale etc. To evaluate the flotation configurations I looked at two scenarios. The first assumed assumed the boat needed to support an additional 225 lbs of crew weight, the second assumed that there was no one in the boat. Since people generally stand upright regardless of whether the boat heels, they tend to keep their center of gravity over their feet. Hence I assumed that the additional weight acted at the level of the floor boards slightly aft of the midship seats. This would also be reasonable if someone climbed over the side as the boat capsized.
The 3D model was then used to predict the equilibrium sinkage and trim, and righting moment (a measure of stability) as the boat was rolled from upright through completely turtled, mast pointing down. The righting moment is the torque required to hold the boat at a given heel angle. If the righting moment is positive, the boat is trying to decrease its roll angle; if negative the boat is trying to increase its roll angle. The figure below shows the predicted float position of a Woody without any flotation while supporting the additional 225 lbs of crew. As you can see there is very little freeboard and waves would freely roll over the boat making it virtually impossible to bail out.

The blue line in the graph below shows the stability of the boat in this condition. Since the maximum righting moment is about 200 ft-lbs, if the 225 pounds of people were just a foot off centerline, they would create a heeling moment greater than the righting moment and the boat would continue to roll at least until the mast provided some additional buoyancy and righting moment at around 90-degree roll angle. The computer software had difficulty converging for the no-flotation case, hence the curve was truncated at 40-degree roll angle.

The red curves represent the Bow & Stern air bag configurations, the green curve represents bow, stern and under seat air bags, and the black curves represent watertight bulkheads at stations 3 and 8.5.
The dashed line version of each color indicates the same configuration where foam under the gunwales has also been included. The jump in the righting moment curve at around 85-95 degrees roll angle occurs due to the mast becoming submerged. This is an important factor in keeping the righting moment from becoming negative, and the boat wanting to turtle.

In fact all of the configurations with flotation maintain a positive righting moment out to around 130-140 degree roll angle, assuming the mast is buoyant. However, it is also important to note that the crew weight being assumed to act at the level of the floorboards in the boat also contributes to stability through most of the range of roll angles. With the boat on its side, this is similar to standing on the centerboard up near the keel of the boat. The next set of curves will look at just the boat without the influence of crew weight. At approximately 160-170 degrees, the maximum negative righting moment occurs. This is the torque necessary to right the boat after it has turtled. For instance if the 225 pounds of crew weight were standing or hanging off the chine, they would generate about 675 ft-lbs (3 ft x 225 pounds = 675 ft-lbs) of righting moment . This should be enough to get the boat to roll past the 160 degree point and continue to roll upright.
The Bow & Stern air bag configuration (solid red curve) reaches its maximum stability of approx 600 ft-lbs at about a 30 deg roll angle. Our 225-pound crew on the gunwale of the boat would cause it to roll past the 40 degrees, and the boat would continue to roll to around 90 degrees if the crew didn’t fall or submerge into the water. Adding the foam under the gunwale (red dashed curve) increases the max righting moment to approximately 870 pounds. For the other configurations, the foam under the gunwale increases the maximum righting moment by a little over 200 ft-lbs, providing additional margin against capsize. However, it should be noted that it also tends to make the maximum negative righting moment larger, making the boat more difficult to right if turtled. The total area under these curves out to where the righting moment becomes negative represents the total amount of work or energy required to capsize the swamped boat with crew.

Obviously another important consideration is the capsized freeboard. The figure below illustrates the floating equilibrium with Bow & Stern air bags and the 225 pounds crew weight. This configuration barely keeps the centerboard trunk above the water, but this could be helped a little by moving the crew weight a little farther aft from where it was assumed to be located. In this configuration there is around 1 feet of freeboard, but also a lot of water in the boat, and as discussed above, the boat is not very stable. Adding gunwale foam doesn’t help the boat float any higher since the foam is not normally submerged, but it significantly improves stability since it adds buoyancy as the boat rolls.


Adding under seat air bags helps the boat float higher as shown below, reduces the water on-board, and also improves stability up to around a 60 deg roll angle, after that they reduce stability slightly. Also, since the flotation under the seats is aft of the CG of the boat, it floats down by the bow a little. Again this could be corrected by moving the crew weight aft a little more.

Adding watertight bulkheads at stations 3 & 8.5 also adds a substantial amount of flotation. This configuration is show below. Surprisingly this configuration floats a bit lower than the above configuration with the air bags under the seats, however this configuration also has substantial additional reserve buoyancy that the above configuration does not. Obviously adding the under seat air bags would also help this configuration float higher. Adding gunwale foam also increases the reserve buoyancy for all configurations.
The plot below illustrates the effect a buoyant/watertight mast has on the resistance of the boat to turtle. With a buoyant mast the boat maintains a positive righting moment to approximately 135-dereeg roll angle. With a free-flooding mast the righting moment becomes negative at about 93 degrees. Up until the mast contacts the water, the righting moment curves are the same. Note that the boat with the floodable mast will be more difficult to roll back up-to 90 degrees, and of course will be extremely difficult to get upright with the additional weight of water in the mast.
The photo below was posted by Corky Gray illustrating a capsize test he did years ago on a boat with the watertight bulkheads. The computer image is the prediction of how the boat with bulkheads would float. The computer prediction appears to be a pretty good estimate.


Obviously, everyone’s situation and boat is a bit different, so the above results should only be considered general guidance. When considering how much and what type of flotation you should have installed in your wooden Lightning, you need to consider where you sail, is there outside assistance readily available?, how cold is the water?, how good a swimmer are you and how comfortable are you in this type of situation? How about your crew? How much do you and your crew weigh? Will you be able to right the boat easily? If sailing in fresh water you need a bit more flotation.

Volume
cubic feet
LCB
feet
VCB
feet
Bow Bag
6.22
3.95
1.70
Stern Bag
5.00
17.65
1.60
Seat Bags (each)
2.40
12.57
1.07
Gunwale Foam
3.40
10.93
2.32
It is pretty clear that a capsize in a wooden Lightning with no flotation leaves the boat dangerously low in the water and with marginal stability. Flotation bags weigh very little and appear to help significantly. Complete bulkheads fore and aft require approximately 15 square feet of plywood which weighs about 9.4 pounds plus a bit of framing. Using 3 pounds per cuft foam under the gunwales will add about 10 pounds. The table left lists the volume of the buoyancy bags and gunwale foam used for the predictions. I hope the results have been of interest and will help you in deciding the flotation solution best for your circumstances.



Sponsor Number URL address
Sponsor 1 https://www.marksetbot.com/
Sponsor 2 http://www.onedesign.com/Default.aspx
Sponsor 3 https://www.thesailinginc.com/lightning
Sponsor 4 http://nickelsboats.com/
Sponsor 5 http://awardandsports.securesites.net/
Sponsor 6 http://www.capitallogoinc.com/shop/pc/home.asp
Sponsor 7 http://www.rookesails.com/
Sponsor 8 http://www.sailorstailor.com
Sponsor 9 https://www.harken.com/
Sponsor 10 http://www.sailorstailor.com

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