Our Anchoring Set Up

Mirrool’s standard anchoring gear:

• 100m nylon rode

• 30m stainless 8mm & 3 ton minimum breaking strain

• Rocna 10 (22lb)

 

Mirrool’s Storm anchoring gear:

• 30m galv. chain for limited use. Reserved for strong wind or special anchoring use.

• Mantus 35lb – stowable.

 

The reasoning behind our Anchoring System

When I was involved in the commercial marine business as a skipper and engineer over the last 20 years, the common anchor rode was all chain. Most usually, attached to a CQR or plough anchor, sized to the vessel. On smaller vessels, this was then attached to the cleats with a nylon rope snubber, using a chain claw to secure one of the chain links.

Aboard Mirrool, we run a different system that is versatile, effective and saves on weight forward.

All anchor systems have to take into account the size of the vessel they are designed to hold. Mirrool is 9.14 meters long with a beam of 2.4 meters and traditionally low free-board. She presents quite a bit less surface area for the wind and sea to act upon than a lot of modern designs, especially catamarans. We also take into account that a large factor in handling the dynamic forces found in strong wind and the resulting sea conditions is how much elasticity is built into the system. The maximum force applied to a boat and its anchor is when there is 100% resistance to a wave’s motion. We need to find a happy medium between no resistance, where the vessel is adrift and full resistance, where things start to break.

Here is a chart, modified a little to account for metric, put out by the American Boat and Yacht Council and, though it has some short-comings due to the range of behaviour of various types of boats in wind and sea, it does give us a starting point to think about vessel anchor loading (The complete table appears in Section H-40 of ABYC’s Standards and Technical Reports for Small Craft available at abycinc.org. )

In all-chain rodes, reliance is placed on the weight of the chain hanging in the water to form a catenary curve and stand as a shock absorber. As waves push the boat back, the force is absorbed by the straightening of the chain. The problem comes, when unusually strong wind sends larger waves that cause all the chain to straighten, with any remaining force transferred directly to the boat and anchor. Either the boats anchoring hardware breaks or the anchor may let go. The situation is worsened in shallow water when the chain may be resting on the seabed and not adding its weight to act as a spring. There are numerous accounts of deck fitting and bow roller failures in shallow, storm swept bays and this may be part of the reason. Anchoring in deeper water is better on this account if waves are present and there is no protective shore immediately to windward.

Often, we hear from cruisers saying that by using stronger chain, they can use a smaller diameter and save weight. As you can infer from above, the weight of the chain is what you are seeking rather than absolute strength. If strength were the only factor, lightweight Dyneema attached to a short length of chain to defeat chafe would be the best solution.

Our answer is an hybrid anchor rode; 100 metres of 3-strand Nylon rope and 30 metres of 8mm or 5/16” chain. The chain length has sufficient weight to contribute a good catenery curve, as well as being proof against abrasion, and Nylon has the properties of great strength and very high elasticity, around 15-20% of its length before dangerously exceeding its working limit as shown below.

If the conditions worsen in our anchorage, more nylon rode can be let out to provide a larger ‘spring’. Usually, boats with all chain rodes will have a short nylon snubber to take out shocks and relieve the capstan of loading but if it is only 1-2 meters long, it doesn’t really have a great deal of stretch to offer in deteriorating conditions. In addition, we aren’t paying a weight penalty for holding 130 meters of chain in the fore-peak of the yacht.

The nylon rode we use is 14mm and has a minimum breaking strain of about 4 tonne. A safety factor of 5 gives a working limit of about 800kg/1760lb, very close to our working chain discussed below. If we look at the chart above for a boat of 30 foot, while violent storm conditions exceed our working load limit, they still don’t approach close to half of the minimum breaking strain of 4000kg/8800lb for our rope and chain used, but would still be a concern. In those conditions, you will find us in the nearest creek!

If a rode with greater strength is selected, it will be less elastic relative to forces applied to your boat and may be a less effective shock absorber. In this case, bigger, may not be better. The 100 metre length allows us to anchor in 20 meters of water with a scope of better than 6:1 when the chain part of the rode is included. We wouldn’t normally anchor in this depth but the ability to do so in case of engine failure or of other emergency with a hazard to our lee is reassuring.

Next we have 30 meters of 8mm chain. That length covers 99% of our anchoring situations (5-8 meters depth) by just letting out all the chain and then whatever amount of rope to act as a shock absorber for the wind and waves present and to give us the correct scope for the highest tide we expect to see during our stay. For usual weather conditions and in depths less than 5 meters, we use 5:1 scope and in water deeper than 5 meters we use 4:1 scope.

If scope is a new term to you, it is the ratio of sea level depth relative to amount of anchor rode let out. 5:1 scope means for every meter between the seabed and the ships bow roller, we let out 5 meters of rode. If your boat’s bow roller is a meter off the water and at high tide you will have 4 meters of water depth, you need to put out 25 meters of anchor rode. Scope is important because it dictates the angle your anchor rode makes on the anchor. If it is too vertical, you will pull out the very best of anchors. We can reduce scope in deeper water because a greater amount of chain will be hanging down from the bow, forming a better shock absorber. If the wind picks up to over 25 knots, we will increase our scope to 6:1. Beyond that, we will let out more nylon rode to act as a greater shock absorber rather than have concerns about the angle of pull on the anchor.

We use 316 stainless proof coil chain with a WLL (Working Load Limit; about 1/5 the breaking strain of the chain) of 865kg. That is strong enough for the stresses we expect to encounter in winds up to 40 knots with our vessel. One concern other cruisers voice with stainless chain is that it may become ‘work hardened’ but at our typical loading, the chain isn’t subjected to enough force to cause repeated elastic movement of the metal, which is the cause of stress fracturing and elastic deformation. We do cast an eye over it every couple of months though for signs of pitting and corrosion cracking. As a benefit, stainless chain stows in the locker smoothly without piling and never flakes rust onto our deck or into the locker.

If conditions are expected to be above 30 knots or we will be caught in a particularly exposed position for an extended period, we also carry 30 meters of galvanised high test chain with about double the WLL (1770kg/ 3990lb) of our stainless working rode. This chain is coupled with our Mantus storm anchor. Galvanized chain is easier to inspect for failures, mainly in the form of elongation before failure and this chain is held in a compartment and only used for storm conditions or the odd occasion when we may need two anchors. This preserves the galvanising on the chain and we know it hasn’t been damaged from day-to-day use. We have to carry a spare rode and anchor so this is a good solution for us.

For years, I have spliced the nylon rode directly to the chain and contrary to appearances, the splice is very reliable. In over 5 years of constant use, I have never suffered the slightest chafe of the rope or a failure of any kind. Our capstan configuration prevents the use of a metal thimble or eye in the rope’s working end, but I haven’t felt the rode has been compromised.

Our working, or, bower anchor is a Rocna 10kg/ 22lb model. We think that these new generation anchors are amazing in their ability to hold in difficult conditions and set very quickly. I used to swear by plough style anchors but must say that I am a full convert now.

Our storm anchor is a Mantus 16kg/35lb model, which also has superior setting and holding qualities, with the added virtue of coming apart into 3 separate components with the undoing of 6 bolts. This means we can stow our Mantus in an awkward bilge compartment rather than have to find space on our already busy pushpit. Assembling it is very quick and the 4 bolts to secure the spade to the shaft is pleasing over-kill in terms of strength.

Once the anchor is set, we still use a snubber of sorts on our rode. Nylon’s ability to stretch can lead to two problems. As it bends over the roller on its way to the water, the stretching fibres can chafe on even the smoothest roller. Also, in times of heavy wave action, it has been found that Nylon rope has heated up due to friction in the inner most fibres and melted, leading to unexpected failure. Our solution is to use durable, low-stretch double braid rope in either a rolling hitch or Prussic loop as a sacrificial line to join the Nylon to the ships bow cleat. There are several ways around the problem but this way is low-tech, easily inspected and easily replaceable.