Cast Iron Keel Material Specification

[Andre Galea]

Hi,
I work with structures on yachts and have come across what seems to me as an inconsistency in the use and specification of Cast Iron Keels. In carrying out checks for the strength of a cast iron fin, I find that a common fin section having a thickness of about 90mm would be far from strong enough using the commonly used EN-GJL-200 lamellar graphite cast iron.

As I said this is not an uncommon section and the material selection is not uncommon either, so i would assume that are used in conjunction.

ISO 12215-9 clearly says:

1. Lamellar Graphite Cast Iron is generally used for production boat keels.
2. Higher Grade of lamellar graphite cast iron (EN-GJL-200) has an ultimate strength of 200MPa which is reduced to 30MPa for strength checks (sigma_DES). This is due to the following factors:
   • x 0.5 to get the limit strength
   • x 0.375 to get include the material safety factor (K_MAT)
   • x 0.8 to include the load case 1 safety factor for 90 degree knock down (K_LC)

My questions is: Am I missing something in the structural checks of keel fins with the EN-GJL-200 cast iron? The K_MAT seems like a massive reduction in strength and I can't see most long keels being made of this material - even for small displacements.

Further, I gathered that the spheroidal grades which would be more suited to my current project are up to 80% more expensive! I need to be able to justify going for the higher grade in a reality where most keels use the EN-GJL-200.

I would appreciate any and all help in understanding this.