Apparent Discrepancies in ABS Rules

[Willallison]

I was looking at the ABS rules - in this case for Motor Pleasure Craft - and there seems to be a consistent discrepancy between the values for SI (metric) and Imperial calculations.

By way of a simple example:
Design Pressures: 8.1 c.
Pd = 9.8(D + 1.22) kN/m^2
or
Pd = 0.44(D + 4) psi

if we assume a simple case where the moulded draft (D) = 1m, or 3.281 ft

Pd = 9.8(1 + 1.22) kN/m^2
= 21.756 kN/m^2
or
Pd = 0.44(3.281 + 4) psi
= 3.204 psi

Now, 1 psi = 6.895 kN/m^2

3.204 x 6.895 = 22.09... not 22.756.....

Every other formula that I've looked is similar.... the formulae given for imperial & metric don't give the same results....

It's hard to imagine that nobody's come across this before.....???

Edit: In this particular instance, the difference is probably not significant... other calcs showed a far greater difference, however...

[conceptia]

well, all i gotta say is the Americans screwed up with the unit conversions..

[jehardiman]

It was intentional to give metric designers a ~1.534% advantage in strength as the limit of draft (D) goes to infinity as everyone knows that it is harder to work with such screwed up units as kiloPascals (Kg force? per meter squared... how screwed up is that?).......

Really now, remember your significant figure rules from the days of slide rules. The equations have a minimim of 2 significant figures so the answer can have at most 2.

So 9.8(1+1.22)= 22 (21.7 rounds up) and 0.44(3.28084+4)*6.894757= 22 (22.08 rounds down)

[MikeJohns]

Its all apples really, if you had 4 1/2 apples to a pound then each apple produces 1 Newton force in earths gravity. Even imperialists can understand that :-)
Then material strength is just apples per square millimeter

Anyone in my workplace uttering the words kilogram force was immediately guillotined. Messy, but necessary.

[conceptia]

okay so jehardim explained it well.. Newton and Pascal are still alien to Americans that seems. I remember an American school joke - a child claiming the first human being on earth is George Washington.. i thought that was a joke....

[jehardiman]

Originally Posted by conceptia

I remember an American school joke - a child claiming the first human being on earth is George Washington.. i thought that was a joke....

Yeah, everyone in India knows that it was Gandhi....