(Yup, another Practical Sailor project in its infancy)
While a single large anchor of modern design is generally the best answer for every situation, there are times when something more is needed. For a Chesapeake sailor the problem is a soupy bottom that won't hold anything and a fast approach squall with a 50-knot gust front. You don't want to re-anchor elsewhere:
- Is anywhere very nearby better?
- My anchor is already well settled in; moving will very likely mean a weaker set.
- I've got 10-20 minutes to do what ever I'm going to do.
There are many possible ways to rig tandems.Here are just a few I'm playing with.
I've been playing with these on the beach an on my boat, so I'm not guessing, not entirely. But I have a lot more measured testing to go.
Secondary rigged to tripping hole. Terrible idea. If the tripping hole does not rip out (it has been reported--it is not made for high load), it will make the primary trip when the wind shifts and prevent re-setting; I've tested this, don't even try it.
Secondary rigged to tandem hole. Better. This is what Rocna suggested, and it is doing better in my testing. But
- You need a Rocna
- You would need to lift the primary to add it, which we don't want to do.
- It is a bugger to know if BOTH anchors are set, not fouled with chain, and that the primary set first.
- It's not much good in wind shifts. Better, but not much.
- The second anchor must be smaller or it trips the first.
Secondary rigged near primary shackle. This is what you might do if there is no tandem hole. Not quite as good, same problems. Additionally, if the primary is not buried, the secondary rode can foul, and since we set them on the same rode, we can't be certain that it is (the secondary might have grabbed first and held the primary on its side).
In these first 3 cases the scope must be ~ 2 X normal scope, because the lifting force on the primary is 100%, but the pull is only 50% (the rest is passed to the secondary). If the secondary is Fortress-type, they are all hopeless; the secondary tension lifts the primary out, since the secondary is holding 70-85% of the load--the Fortress generates its holding without dragging (it is stiffer).
Secondary rigged 1-3 boat lengths up the rode. This can be done without lifting the primary, as I have done many times. Simply row the secondary out ~ 20' past the primary, set from the boat, then come up on short scope and connect the 2 rodes with a soft shackle. The secondary rode must terminate here to avoid tangles (we clip a long polyester line to it during setting and recovery to make things easy). The primary will not be fouled by the secondary rode if is set deeply in soft mud, which should be the case. You know that both anchors are set, because you power set each separately.
And what if the secondary is a Fortress? Since this is what we have, what many of us have, and what I have used for years, we need to understand how to use it. It turns out that this is NOT trivial change. In fact, because of its greater stiffness, it changes the math considerably. Since the Fortress has higher holding power in soft mud than the primary, most of the load is on the Fortress. The primary, on the other hand, serves more to keep the load in-line with the Fortress. On the beach, at least, this is by far the most robust rig.
Unlike the first 3, the scope requirement on the latter 2 rigs is not effected because the rodes are sufficiently separated. 7:1 should do fine.
So why is this last rig not the obvious answer? Several reasons:
- Tripping. If the primary is not buried, the rode better not slide over it. Cutting is also a concern with a fiber rode.
- Deploying. Actually, not really. You only handle one at a time. The primary is already down. Setting and retrieval are a matter of either extending the rodes to the boat (easy) or clipping a temporary extension while setting and retrieving (better).
Yes, I have been advised that small anchors do not scale up. I've had the small anchors for 20 years and I really do know how differently they perform compared to big brothers. But I do think the affects of geometry will scale up; bad rigging is bad rigging, and I expect the small anchors will be even more vulnerable to mistakes, which I like.
Fun stuff. I will play on the beach. I will play in the shallows. I will anchor my boat (full scale tandem) over soupy mud and pull these mini-tandems with my primaries, taking force measurements and diving to watch. A lot of playing in the mud this summer!
Comments PLEASE! Do you have a favorite tandem rigging you would like to see tested? I will add it. I will be using Claw, Guardian, and Mantus anchors, since these come in small sizes and have close analogs at full scale. I will test in both firm sand and soup.