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post #409 of Old 08-06-2013
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Re: Pros and cons of steel sailboats

Originally Posted by Brent Swain View Post
Mild steel is 60,000 PSI tensile and compression strength . That is 11250 per linear inch for 3/16th plate. Multiply that by the 96 inches in the side of one of my twin keels. That is 1.08 million pounds per side, times four keels sides.
How are you going to break that with a boat under 20,000 lbs?
Just saw two navy 100 footers in Heroit bay. 3/16th hull plate on 100 ft navy ships . And you say the same plate thickness is too light for a 36 foot pleasure boat? You say that a boat which could survive 16 days pounding in 8 to 12 ft surf on the west coast of the Baja, or pounding across 300 yards of Fijian coral reef in big surf, or colliding with a freighter, or hitting the sharp corner of a sunken barge at hull speed, all with minimal damage, is not strong enough? Now that's a stretch!
My hulls are all single thickness, 1/8th for the decks cabin, etc, 3/16th for the hull ,1/4 for the keel sides, and half inch for the keel bottom ( with 4500 lbs of lead ballast poured on top)
Layering steel is a big mistake, guaranteeing corrosion between the layers unless totally sealed.

A good whack with a sledge hammer and a centre punch on lower parts of keels, etc, where corrosion is most likely, is a good starting point on buying a steel boat. If it doesn't give, you have enough thickness there.
Structural failures of steel boats under 40 feet are extremely rare. Your "Invisible " fractures have zero chance of ever causing any problems in steel boats under 40 feet in their lifetimes.
How does such "invisibly fractured" steel compare in strength to a copper fastening in red cedar every six inches, or six inches of plastic?
Brent, I hate to tell you but your math is extremely flawed. PSI is pounds per square inch, not linear inch. A36 mild steel, and that is the most common, is 36,000 PSI, linear inches could be twenty miles and it would still be a minimum of 36,000 and a maximum of 80,000 PSI in every single inch of it. The 80K PSI is not likely to happen, it would rarely test out at over 46K PSI and even that would vary greatly from one square inch to the next.

How do you calculate the hull stress loads for your vessels? What are the machinery and static loads that you figure in when you are making your calculations? What is the wave moment that you use for calculations on maximum stress loads? What wave heights? How do you calculate the stiffeners needed in the keel, hull, and deck plating loads? How do you do the hogging and sagging stress calculations on your vessels? Do you understand that your vessels, even at 30' do undergo these stress loads and that the Dynamic Amplification Factor on the steel is not just longitudinal, but it is also a shear load, a buckling load, a bending load and an impact load, even when your hull and keel are not hitting a reef somewhere?

I have an updated copy of a little workbook that you will enjoy here, if the math is too tough on you please consider a tutor, because as a ship designer this little bit of math is a bit important.

The rest of the guys will probably find it a good refresher on some math, and very informative. Especially on page 67 where we find this little paragraph. I think it might be an excellent thing if some people might read this and withdraw claims of absolutely no structural damage having been done to vessel which grind themselves on reefs like strippers on a pole.

Code of Ethics for Engineers
(from National Society of Professional Engineers)
Engineers, in the fulfillment of their professional duties, shall:
1. Hold paramount the safety, health and welfare of the public.
2. Perform services only in areas of their competence.
3. Issue public statements only in an objective and truthful manner.
4. Act for each employer or client as faithful agents or trustees.
5. Avoid deceptive acts.
6. Conduct themselves honorably, responsibly, ethically, and lawfully so as to enhance the honor,
reputation, and usefulness of the profession.

It is good to learn from your mistakes, but much better to learn from the mistakes of others...
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