Join Date: Oct 2009
Location: Gloucester, MA
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Re: My Brain is Anchored
It appears that your concern is over breaking the chain. Unfortunately, one of the hardest things to predict is the load placed on an anchoring system. It has been shown that the ABYC guidelines overpredicts the load placed on your anchor by 2-3X in benign conditions and underpredicts the load in really bad conditions. This is because of dynamics where waves will throw your boat several feet backwards. To keep the loads down, it is necessary to have a shock absorber in the system which you do with the line. ABYC predicts a load of <2100lbs for your boat in storm conditions which is within the safe working load of the chain so you should be fine provided that the chain is in good shape. You would have to be anchored in a reasonably exposed location in extreme conditions to ever see this load so we are talking about an unlikely scenario. It might be a bit different if you were planning on this setup being your hurricane plan but anything short of that, you should be fine.
Both chain and line deteriorate over time which must be considered. You can prolong chain life somewhat by using a slightly larger size such as 5/16 G4 but that is a call that you need to make on how much weight you want to haul up versus how long you want it to last. Regardless, once it shows significant degradation, it needs to be replaced.
For reference, on my 30' 10000 lb boat, I use a manson supreme 35, 30' of 3/8" BBB and 250' of 5/8" three strand nylon. I would have used 5/16 G4 but I already had the other chain lying around so I am using it until it wears out. I do not have a windlass so this is all hauled up by hand in depths to 50'. It has worked through several severe thunderstorms and tropical storm Irene without a problem.
It would be great if someone would do an experiment with a couple of mooring fields of boats in storms and provide data on mooring loads but I doubt that this will happen anytime soon. Until then, I think that your proposed system is fine.