T&T: Anchor Chain Size

[Rudy and Jill]

> I am in discussions with my builder about galvanized anchor
> chain link size. The steel RPH trawler is 50 x 14.4
> x5.1  draft. It displaces 40+tins and will have a
> steadying sail.
> A review of various web sites indicates that 12 mm or 1/2"
> chain is appropriate. The builder is suggesting 10 mm (just
> over 3/8") chain (We still have not gotten into whether it
> will be BBB or Proof coil)


I guess we look at sizing ground tackle a little different than most. We prefer to size our anchor gear based on wind speed, since it is the wind speed that develops both the wind's force against the boat, and the surge forces against the boat.

A typical fifty foot trawler, in 15 knots of wind will have approximately 400 lbs of force on the ground tackle; at 30 knots of wind, 1600 lbs of force; and at 60 knots, 6400 lbs of force. (This is with some protection from the seas and freedom to oscillate.)

As far as the chain goes, we do not think that the load on the chain should exceed the chain's working load limit (WLL), and with HT chain, less may be indicated. (Every chain manufacturer that we've talked to agrees, some vehemently.)

This allows a safety factor of 2 till the chain reaches its proof load. 

To reach the chain's break load, HT chain has a factor of 3, while proof coil chain has a factor of 4. This additional ability to resist breaking (the difference between the proof load and break load) is built into the chain to deal with shock loads, something that can triple the static load on chain. (These figures are assuming that they chain is manufacturered to an accepted standard.)

Since HT chain only has a factor of 3, till it reaches its break load, the same factor for shock loads, we feel that it is prudent to keep HT chain below its WLL. (We have found that many chain manufacturers are trying to change this, as they too agree that shock loads can come too close to the breaking limit of HT chain.)

Some folks think that using a snubber decreases the load on the ground tackle, but we submit that it doesn't. A snubber will attenuate the speed of loading and shortens the duration of maximum loading, but does not significantly reduce the amount of loading. 

So to our way of thinking, even with a snubber, every component in the system must be capable of resisting the total load imposed, and this load is based on the wind speed.

Enough of the theory, now all you need to do is pick the maximum wind speed that you will be anchored in and match it to a chain's WLL. 

For cruising boats, we suggest that 60 knot wind speed is chosen, and for those in hurricane prone areas, higher may be better. You may never have to anchor in these conditions, but if you do, your chain will be able to handle it. 

If you never have to anchor in conditions like this, consider it as the cost for peace of mind. We can't count the number of boats that we've run across who outfitted with the "I hope it doesn't happen", or the "it probably won't happen" attitude. 

Whether the anchor can hold in this wind speed is another matter, but we think, that here too, it should be big enough and of a design amiable to the bottom conditions anchored in, to be able to do so.

Rudy and Jill
Briney Bug, Panama City, Fl