EvC Forum: About that Boat

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Topic: About that Boat - Noah's Ark

Hmmm

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Message 231 of 296 (169137)
12-16-2004 7:01 PM

Reply to: Message 229 by AdminJar
12-16-2004 6:08 PM

Re: Getting WAY off topic

Hooray!
Did you want to shift those posts elsewhere? I came here to talk HULL, but when they bring up other points it's not fair to let it go.

I'm happy to ignore non-NOAH'S ARK specific questions from now on.

Thanks
Hmmm

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Hmmm

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Message 236 of 296 (169282)
12-17-2004 6:36 AM

Reply to: Message 234 by contracycle
12-17-2004 4:24 AM

Re: Too Big or not too Big...

Hi contracycle;
Nice to be back on the ark again...

You appear to be saying a ship in a perfectly random sea will always experience no net primary wave loadings, such as wave bending moment. Or did you mean to accuse only the Hong study of saying this?

In either case, how do you explain the inclusion of the wave bending moment (Mw) in the elementary beam theory equation (9)?
Missing Link | Answers in Genesis

Or this derived relationship between wood thickness and wave height?

(BTW: This graph shows the effect of wave loading only - the static (still water) loads are not shown, which one of those naval architecture things - in case you were wondering.)

Hmmm?

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Hmmm

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Message 240 of 296 (170590)
12-21-2004 7:57 PM

Reply to: Message 233 by tsig
12-17-2004 1:44 AM

Re: Strength of Materials

Hi Flying Hawk;

I've been away a few days.

Thanks for you little example of Galileo's square/cube law. (Already nicely covered in Missing Link | Answers in Genesis )

While you are on the right track regarding scale-up, using a ship hull as a simply supported beam is several orders of magnitude more severe than the American Bureau of Shipping rules regarding the maximum design wave bending moment. This is because waves are a distributed load.

So with this in mind, let's run the numbers for fun...

Wood: Douglas Fir: Density approx 500 kg/m3
At Ark scale (scale=1), and using cubit of 0.5m,
A solid timber lump weighs 28125 tonnes, giving simply supported bending moment of 5.168e9 Nm. With a section modulus of b*d^2/6, you get Stress = 5.168e9/937.5 = 5.5e6Pa = 5.5MPa (800psi)

Being well short of the 85MPa MOR (maximum failure), we could take it further (without safety factor applied) like this...

To get bending stress of 85PMa, scale=15.4, Length of block 2313m, breadth 385m, height 231m, mass 103 million tonnes .... This is an absurd scale - far bigger than anything afloat today, and equivalent to a 2.3km (1.4 mile) long bridge!

Obviously these numbers are a bit silly since the solid block has no carrying capacity (and the exaggerated stress loading is without a safety factor) but it does show that your argument isn't going to arrive at the popularly quoted 300ft limit doesn't it?

To do it properly you would use something like the ABS bending moment rule to get a realistic applied bending moment, and a more reasonable hull wall thickness.

If it works, it would show the Biblical Ark is quite a reasonable scale after all - at least Genesis doesn't say 3000 cubits. That would really make Gen 6:14 a problem for YECers!

Regards
Hmmm

This message has been edited by Hmmm, 12-21-2004 08:00 PM

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	Message 233 by tsig, posted 12-17-2004 1:44 AM		tsig has replied

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	Message 241 by tsig, posted 12-28-2004 12:40 AM	Hmmm has replied
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Hmmm

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Message 242 of 296 (171990)
12-28-2004 8:20 PM

Reply to: Message 241 by tsig
12-28-2004 12:40 AM

Re: Strength of Materials

Thanks Flying Hawke;

It is good to talk about hull bending moments on this thread. Very ON TOPIC!
Just let me clarify the beam comparison a little;

Engineers generally start with a worst case analysis. At some point the ship will be simply supported.

The ABS rule is exactly that - the worst case analysis. You're focus on comparing to a simply supported beam (bridge) is arbitrary, and turns out to be approx twice the span that the ABS rules would suggest.

Using conservative values of 20.6" cubit and block coefficient of 0.98, the ABS wave bending moment is around 111000 tf.m (from Missing Link | Answers in Genesis )

The span of an equivalent simply supported beam will depend on the hull mass. At spec gravity of 0.4, the ABS bending moment would matched by;
Sagging; Simply supported uniformly dist load, span = 74m (47%)
Hogging; Cantilevered uniformly dist load, span = 37m (23%)

But BM=wL^2/8, i.e. Bending moment is proportional to square of span. So Flying Hawke just arbitrarily increased the American Bureau of Shipping worst case rules by a factor of 4.5 (BM = 505021 tf.m)

So your'e saying you could suspend a 300' section of douglas fir between two points?

I just did an example calc in the last post. Are you asking a different question? If so, what sort of "section" do you mean?

If you are saying Noah's Ark could not sustain a 300' span, you may or may not be right. But who says this is any sort of test - 4.5 times higher than the already very conservative ABS rules designed for ships with a working life measured in decades?

However, if a hull could be designed to handle this sort of extreme test then the argument against the hull strength wouldn't hold water.

The first question is whether the hull could span 74m / 243'(sagging) or 37m / 121' (hogging).

Hmmm

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