Watch circles in survival conditions

Let's say you have two moored structures, say boats on anchor with watch circle radii of 50 and 30m respectively, and you are figuring how far apart to moor them so their anchor lines will not foul each other in a 100 year storm. Can you assume that a severe wave will push both of them in the same direction, in which case they can be within each others' watch circle, or must you assume that they could be pushed toward each other, in which case they need to be moored 50+30 = 80m apart?
If there is ABS or another standard that addresses this issue, please let me know.

 

With or without anchor dragging, tripping, or reset?

Assuming no anchor movement, then the watch circle is just the length of the vessel + cos (water depth/scope out). This means the watch circle should be calculated on MLLW. In "normal" storm conditions you have to account for storm surge on top of high tide to ensure that adaquate scope is out. These two effects generally means that real watch circles for storm moorings are larger than "normal".

If you really want to get into how to design a fixed mooring for given conditions, there is NAVFAC Design Manual 26.4, Fixed Moorings.

 

Yes, that is a good way to calculate the watch circle for the individual vessels in a storm (you can assume the anchor will hold).
Now what I want to figure out is what happens when you have 2 vessels each with their own watch circle, say 30 meters radius for the first vessel and and 50 meters for the second. Do you have to position them >80 meters apart, which means you're assuming that in a storm they could be pushed *toward* each other? Or can you assume that they will always be pushed in the *same direction* in which case they can be less than 50 meters apart? The goal is to avoid having the two anchor lines foul each other.

 

Do you have to position then 80m apart or should you position them 80m apart are two different questions.

Is this for litigation? or just good seamanship?

 

This is for a documented plan assuming the worst case (100 year storm), so we should dot the i's and cross the t's. If it helps, the vessels will remain on anchor a long time, say 5 years.

 

In that case mooring material, design, and maintainence are more important than any other consideration. If these are permanent moorings then best practice is to ensure that the watch circles, including sparred length, do not overlap. That said, there is a lot more, as you may well know, about planning for a permanent mooring...local and national approval and permitting, Coast Guard approved fairway, buoyage, and lighting...etc.