UNITAS vs ABS : slamming pressure

[DUCRUY Jacques]

Hello,

I have some problems for calculating the maximum bottom pressure of a planning craft, for UNITAS and BV rules (I assume Kl and Kar = 1).

If D is the deplacement in Tons et Ac the acceleration in g's,I have :

for UNITAS :
70 (D Ac)/Ar
if Ar : 0.3 Lwl Bwl => 233 (D Ac)/(Lwl Bwl)

for ABS :
(100 D)/(Lwl Bwl) * (1 + Ac)

The result at not at all the same ; what is the explanation ?

Thank you by advance

Best Regards


Jacques

[Ad Hoc]

Originally Posted by DUCRUY Jacques

The result at not at all the same ; what is the explanation ?

They shall never be the same. Each society/administration arrives at values in different ways. Doesn't necessarily mean one is better than the other. But you need to look at the societies rules and then factor in how many of "that type of boat" does that society approve. You also need to plough through the numbers to arrive at the final scantlings. That is the true measure. Since whilst some calculated pressures may be higher, the overall scantlings, via the section modulus and their fudge factors, may yield very similar results. So don't get hung up on minutiae of differences, look at the final result, the minimum required structure to pass.



Here is a typical example. There is an obvious difference between DNV and ABS. You select which rules you are most comfortable with and also which plan approval office is easy to get on with and finally the surveyor. Not all surveyors are easy to get on with too....thus, take your pick.

[DUCRUY Jacques]

Thank you very much.

I begin to think that, in actual "state of art", nobody know the exact load on a slamming hull ... But I assume that the final result is statistically correct (in term of risk of accident).

Good evening


Jacques

[ABoatGuy]

Originally Posted by Ad Hoc

They shall never be the same. Each society/administration arrives at values in different ways. Doesn't necessarily mean one is better than the other. But you need to look at the societies rules and then factor in how many of "that type of boat" does that society approve. You also need to plough through the numbers to arrive at the final scantlings. That is the true measure. Since whilst some calculated pressures may be higher, the overall scantlings, via the section modulus and their fudge factors, may yield very similar results. So don't get hung up on minutiae of differences, look at the final result, the minimum required structure to pass.

Attachment 63968 Attachment 63969

Here is a typical example. There is an obvious difference between DNV and ABS. You select which rules you are most comfortable with and also which plan approval office is easy to get on with and finally the surveyor. Not all surveyors are easy to get on with too....thus, take your pick.

Well said.

[Ad Hoc]

Originally Posted by DUCRUY Jacques

I begin to think that, in actual "state of art", nobody know the exact load on a slamming hull ...

It may appear like that, but many of the Classification societies have actually done real testing of real boats and strain gauged them to established loads.

The difference is of course in the boats being used for the test and hence the extrapolated formula that result.

So, again, don't get hung up on differences, just use a classification/administrations rules you feel comfortable with.

My preferences are DNV then LR in that order, after that, if i asked for another (for whatever reason), i shall use whatever seems easiest for me to satisfy the client...unless the yard has a preference. But i always insist on DNV followed by LR in the first instance.

[DUCRUY Jacques]

Thank you again.