Birdsall 60 Range of Stability Calculations

Hi ya: For years we have lusted after your boats (Wiroa was one) but couldn’t afford them. Our own boat Gungha (Alan Buchanan design in steel, round slack bilge’s and overhangs) was getting too small. We owned her and lived on board for 21 years.

Last year we were lucky to change boats and boat designs. The new boat is a Birdsall 60. She really is the boat of our dreams. Heaps of space, stable and fast-as. I can’t think of anything about the layout or performance of the boat that I could criticize. We have just made our first offshore passage from NZ to Tonga with heavy weather and she went like the clappers. So no problem there. There is however a problem of lingering doubt vis-à-vis the design.

I have been used to the deep draft and 45% ballast ratio of Gungha (nice and rolly). I always knew that if she was capsized she’d snap back upright in moments. I know you’re probably familiar with the Birdsall designs, as they resemble your own in some aspects. This yacht is 63′ on deck, 60′ at the waterline. She has a 14’10” beam which is carried aft but certainly not forward (very narrow there) and she has a 6′ draft.

The yacht is single chine steel with a long (about 17′ fore and aft) fin keel and a skeg-supported spade rudder. The hull of the yacht draws a little over 2′ with the keel making up the rest of the draft. The yacht is double-bottomed fully with welded in tanks, which hold a total of 800 gallons of fluids. The deck plate and all construction techniques have concentrated all the weight as low as possible. The yacht has a center cockpit and trunk cabin with considerable buoyancy.

My problem is this. The yacht is adequately stiff even in hard conditions but…she only has 4 tons of ballast. Her total shipyard weight is 27 tons. What do you think of that and her ability to be tipped over????? We have sailed over 60,000 miles and prior to that I was a commercial fisherman in Alaska…Am I getting paranoid in my old age?

Thanks and cheers, Mike

Hi Mike: The question you pose about capsize resistance cannot be answered in any simple fashion. However, for steel construction, which means a relatively high vertical center of gravity (assuming decks and structure are steel) your instincts (concerns) may be accurate. Hard to make the numbers work out with shallow draft, high VCG, and low ballast. Here’s what I would do:

1-Check with the designer about his calculated range of stability, and the VCG assumptions on which this is based.

2-For a design as that you’ve described, and for sailing back and forth from New Zealand, I’d like to see at least a 125-degree LPs, and preferably 130 degrees.

3-Verify the VCG with an inclining test or a rocking test (see Skene’s Elements of Yacht Design for how to do the latter).

4-If you are shy, consider adding some lead to the bottom of the fin. This is not going to be difficult in NZ.

5-I’m sure you are already aware of it, but keeping the hull tanks filled will add significantly to the LPs

6-If you really want to check things out, hove the boat down to horizontal with the masthead, and then measure the force required to hold her at 90 degrees. The designer can take this data and give you a very accurate LPs figure.

Good Luck!

Posted by Steve Dashew  (November 30, 1999)

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