Dave Gerr - ISO !! Scantling !!

[Salvatore84]

Hi!

I was going to make an excel sheet to compare the ISO scantling and the Dave Gerr rules for a composite 45' top speed 45 kts.

Someone has some advice about that? I'd like to see what is the real difference in % terms.

Have a good day

[Ad Hoc]

Forget D.Gerr's scantlings. These are not suitable for 45 knts vessels.

If you want to try a proper comparison, then use any of the free Class rules, such as DNV:
DNV HSLC & SNC 2012 Rules

[PAR]

Geer's scantling rules will work for a 45', 45 knot craft, but it'll be about 15% to 20% (my assumption, based on familiarity with both) heavier then the DNV rules, making for a costly build.

[Salvatore84]

Thank you for answering me! and thank you for DNV Rules! If you have experience in the field whn you make a hull scantling you use register rules or directly the ISO rules??

thank you !

[Ad Hoc]

Originally Posted by Salvatore84

If you have experience in the field whn you make a hull scantling you use register rules or directly the ISO rules??

All the vessels I design are in the commercial field. Thus I always use Class rules, DNV or LR usually. I have only ever used ISO rules once, for a US Army contract oddly enough.

[Alik]

Originally Posted by PAR

Geer's scantling rules will work for a 45', 45 knot craft, but it'll be about 15% to 20% (my assumption, based on familiarity with both) heavier then the DNV rules, making for a costly build.

No, they are not suitable; Gerr's rule does not take into account Froude number and acceleration levels. Those are ALWAYS included in calculations of high speed craft. Actually forget about those rules; they have no legal status and using them is sole risk of designer.

[PAR]

Geer increase his figures based on speed above a specific point (I think 25 knots). Agreed in that these, other then reference for novices looking for something, they aren't particularly useful.

[Alik]

Originally Posted by PAR

Geer increase his figures based on speed above a specific point (I think 25 knots). Agreed in that these, other then reference for novices looking for something, they aren't particularly useful.

But one should understand that 25kts on 20' and 60' is not the same; for the first boat design acceleration would be about 4-5g and planing mode and for second 2g and semi-planing. For 20' boat 20kts is already planing speed, but no increase in structure is provided in Gerr's method; this is the major weakness of his approach.

[daiquiri]

Originally Posted by Alik

Actually forget about those rules; they have no legal status and using them is sole risk of designer.

That is actually a critical point here. Salvatore, I see you are in Italy, thus your 45' vessel will be subject to CE survey and certification. (rif.: Codice della nautica da diporto - Decreto legislativo 18.07.2005 n° 171 - Articolo 8) The inspector will want to see the calculations done either according to ISO or some Class rules. Gerr's method will not be accepted. It can be used for preliminary calculations, when a quick estimate of structural thickness and weights have to be performed. But at the end of the design stage, your calculation report will have to contain reference to either the ISO or to some Class rule.
Cheers

[APP]

Originally Posted by Salvatore84

Hi!

I was going to make an excel sheet to compare the ISO scantling and the Dave Gerr rules for a composite 45' top speed 45 kts.

Someone has some advice about that? I'd like to see what is the real difference in % terms.

Have a good day

Salvatore,
I would need to see any excel sheet regarding scantlings to see the generalities. I you have one, whatever, can you upload it here?
Grazie, Saluti,
APP

[Salvatore84]

Thank you every one, I'll use Gerr for a fast weight estimate, then I'll try to work with ISO rules.

Thank you very much!