forestay fitting and chainplate design

[Steve W]

When designing something like, for example a simple ss strap chainplate what sort of factor of safety is typically designed for and do you use uts or yield to achieve this? What sort of fos is usually used in rig design?

 

[Ad Hoc]

You would never use UTS for a design stress. Always yield.

I have used 5 as FoS in the past, but sometimes higher. It depends on the arrangement and expected build quality too. But I design more commercial than pleasure boats.

 

[Steve W]

Thanks Ad Hoc, that's what I expected but wanted to be sure.

 

[TANSL]

I enclose a file with some pages extracted from the regulation for SSC of the Lloyd's Register. It does not speak exactly of the chain plates but it will give you an idea of the FoS used by Lloyd's. It should be noted that the FoS vary according to the structural element in question.
The second file is a draft that was prepared for ISO 12215-9, which finally did not come to light, but which is indicative. <ISO document removed from post due to ISO copyright>
You may be interested in looking at the regulations applicable to your boat to make sure you do not use an excessive FoS. A huge FoS does not improve your boat, at all, but it does make it much more expensive.

 

[Steve W]

Thanks TANSL, I will wade through that when I get the time. The reason I am interested in this is I have a production 31ft catamaran that used standard off the shelf Scheaffer ss strap chain plates that have a specified maximum wire size of 7/32" with a pin size of 7/16" but they used the same size for everything including the cap shrouds which are 9/32" and the 1/4" intermediates and fwd lowers so they drilled out the pin holes to 1/2" apparently thinking a bigger pin gives a higher fos where in actuality while the pin does have a higher shear the already undersized chainplate is reduced in tensile strength by about 500 lbs ( sorry about the imperial dimensions). The boat manufacturer should have gone up a size in chainplate imho. I see this sort of thing quite a lot around the marina where I work where a customer will want to go up a size in rigging for no good reason and finds the pin size is larger and then drills out the chainplate hole without any thought of the implications so now they have reduced the strength of the chainplate and are carrying around more weight aloft, all negatives.