Are The RYA Anchoring Guidelines Wrong? Real World Anchoring Advice From Seasoned Cruisers.

This article explores three alternatives to the RYA anchor scope calculation. 

The Royal Yachting Association (RYA) anchor scope calculations follow a simple formula. 

Four times depth for all chain and six times depth for chain and rope.

This sounds straightforward. 

But, if you regularly anchor out in a range of conditions and locations, then at some point you may question the RYA advice.

From shallow rivers to deep bays, the ‘one size’ RYA anchoring scope mantra can often leave you wondering if you're doing the right thing. Problems with the RYA advice are more noticeable in shallow or deep water anchoring scenarios. 

So, if you are a cruising sailor on an all chain rode, then the following 3 alternative anchor scope calculations will help you make a more informed decision about how much chain to lay out.

Alternatives to Consider. 

Number 1 

3×Depth +10m 

Paul Farren is editor of the Royal Western Yacht Club's 2016 guide ‘Plymouth anchorages and moorings’. 

After many years of using an all chain rig I have concluded that the RYA formulae is a little simplistic and calculates too little chain at shallower depths. 

Paul goes on to note that..."In my opinion, proportionately more chain is required as the depth of water reduces to take into account the catenary effect which is required to ensure the pull on the anchor is nearly horizontal.

I devised the formulae 3D+10 which means 3 times the depth of water plus 10 meters as an easy to remember aide memoir and the table shows the comparison formula. The choice is yours!"

RYA Recommended Anchor Scope in Meters 

DEPTH IN  METRES	WARP  RYA=6×D	CHAIN  RYA =4×D	CHAIN  PF=3×D+10m
1	6	4	13
2	12	8	16
3	18	12	19
4	24	16	22
5	30	20	25
6	36	24	28
7	42	28	31
8	48	32	34
9	54	36	37
10	60	40	40

Paul Farren's Scope Chart: PF=Paul Farren. 

 

Number 2.

15m+2×Depth (+10m in strong conditions)

      

Richard Macfarlane has a YouTube channel and describes his anchoring scope calculations in the video here.

Richard makes the observation that "Too many yachts drag because they use small, old-style anchors, they are often sized for ‘normal’ conditions, so they drag when the wind picks up. The new generation anchors are much more secure".

Traditional scope calculations are also a problem, giving too little chain in shallow water, and too much in deep water. The RYA recommends a scope of 4 times the water depth, with no allowance for bow height or strong winds. I use 15m (50ft) plus double the depth, plus an extra 10m (30ft) in strong conditions.              

Number 3.

Chain needed (m) = wind speed (knots) + boat length (m)

Christopher Smith discusses his anchoring scope calculations in this article in Yachting Monthly.

 

The scope calculation above, is a wind-only formula for shallow anchorages (4 to 8m). 

                  

If you will not see anything greater than the top end of Force 4 (16 knots) and you are anchoring a 10m yacht in fairly shallow water, by which I mean anything under about 8m depth, you should be fine with 16m + 10m = 26m of chain. But if you think a Force 7 gale (33 knots) is coming, try setting 33m + 10m = 43m of chain.

 

Chris explains... "This rule of thumb works in the majority of anchorages relatively close to the shore where the water is quite shallow, but for deeper anchorages (of around 10-15m) you obviously need more chain."

The answer to this is simple: you just need to use a factor of 1.5 times the wind speed, which will deliver a better result.

Tide can also be taken into consideration, as Chris notes, by motoring into varying headwinds and checking the engine rpm and then seeing what boat speed the same revs provide in calm conditions can provide a surprisingly accurate wind speed to tidal flow conversion. 

So a wind and tide formula for shallow anchorages looks like this:

Chain needed (m) = wind speed (knots) + 4 x water- speed + boat length (m).

A fuller explanation can be found in the original article here.

Pros and cons 

Paul Farren's formula is both simple and offers a sensible solution for shallow water anchoring.  

Gaining catenary is  difficult in shoal water as the RYA guidance lacks an allowance for freeboard. This can be as much as 2 metres on larger boats. 

One Sailng Anarchy contributor noted that 

Many anchorages are deep - you're not setting 5:1 scope in 25+ metres of water. There's also scaling effects - pulling 25m of chain bar tight happens much sooner than 80m, so you maintain that catenary curve much longer in deeper water, even if the scope is lower on paper. 

Richard Macfarlane adds an additional 10m in 'salty' weather.  

Chris  Smith suggests it would be wise to add roughly one additional boat length to our calculations to ensure there is enough slack to counter the yawing at anchor without a direct upward pull on the anchor.  Noting, "I like to think I would lay enough scope for a sudden summer gale, but anchorages can often be crowded at the height of the season". 

 

Conclusion 

Having an easy to remember formula that suits the type of anchoring situations you regularly encounter makes sense.

All three of the examples above both question our conventional wisdom, and often unquestioned dogma, as well as ask us to make a real world assessment based the conditions we find ourselves in. 

It makes sense to factor in wind and tide as a variable that affects any anchor scope decision.

Which in itself isn't a bad thing.

And finally, if you don't want to do the maths, you can use an anchor scope calculator. Sv Amanda has one on their website here. 

Just type in you boat length, ground tackle details and what you expect the wind to do and the calculator does the rest!