Join Date: Oct 2009
Location: Gloucester, MA
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Re: recommended "storm" anchor setup for small boat
Everyone has their own definition of "storm" and all anchorages provide different protection but the way that I define storm, it means anything short of a hurricane. The best estimates of loads is still the ABYC recommendations which would put your maximum load at 1500lbs. Obviously, if you were in an area with small enough fetch so that there were no waves, the loads would be lower and if you were in a wide open area with large breakers, the loads could be much higher but it is a starting point. Keep in mind that the forces seen on an anchor rode are approximately proportional to the square of the wind speed so if the definition of storm is only 35 knots, designing for 1500lbs is overkill but that never hurts.
Nylon line is great because of its shock absorption but it can degrade significantly without being noticed. Over time, UV will break it down and there has been some testing that shows this can decrease strength by close to 50%. Also, under repeated cycling, the line can get hot and chafe internally and they have been known to break at way less than their rated strength. For this reason, I recommend using a 5X safety factor with the line meaning that it must have a breaking strength of 5*1500 =7500lbs. This equates to a 1/2" 3 strand high quality line.
For chain, you do not need to introduce a safety factor as the chain manufacturer will provide a safe working load. Grade 30 1/4" chain has a safe working load of 1250 lbs which is slightly lower than the stated 1500 lb design load. It is up to you as to whether you need that extra bit of strength and either need to go up a grade or up a size.
Make sure that the entire system is up to the design load. Obviously, the anchor must reliably hold the stated load in the type of bottoms that you are likely to encounter. Just because it held that much once in an anchor test in an ideal bottom doesn't mean that it will hold the same amount in your situation. Also, many people use undersized, low quality shackles which are weaker than the rest of the system. Chafe is important because it is the most likely thing to weaken the line so make sure that you understand that.
In my opinion, the west marine recommendations only apply to day boaters who won't be relying on their ground tackle when a squall comes through. I may be overly conservative but I know that I end up anchored out in nasty weather a few times a year and I don't want to break anything when it is blowing 50.