Sorry for being back late, but had not much time to go through my spredsheets and anchor sizing charts
The following is just a bit of my thinking loud…
Let’s start from anchor.
Knox sizing chart show a 18 kg anchor as suitable for Bavaria
40, but with a disclaimer: “This Knox anchor is suitable for use in winds up to force 8 when there are only moderate non-breaking waves. For more severe conditions choose a heavier Knox Anchor. ”
So 18 kg Knox anchor is recommended up to 40 knots of wind and above the 27 kg anchor is appropriate, but we do not know what is the limit. Next step up in production line is 36 kg anchor.
Rocna sizing chart advice
is 25 kg anchor. In their knowledge base You can find an info, as the sizing recommendations are valid up to 50 knots of wind with associated surge, and for heavier conditions the 33 kg Rocna is recommended. Again – we do not know what is the upper limit. Next step up in production line is 40 kg anchor.
Let’s see what is the approximate load on the anchor tackle for different given speeds of wind.
There are different suggestions around, how to calculate the load.
It is well worth remember, that quite popular ABYC tables are actually not the real calculations of the load on the rode, but rather a recommendations for anchor cable sizing, with in-built safety
So I prefer – for my own calculations – use the method suggested by Professor John Knox, well renowned authority in all things anchoring related and designer
of Knox anchor.
For a boat of Bavaria
40 size Professor Knox gives us the following numbers:
40 kts 460 kg
50 kts 720 kg
60 kts 1040 kg
70 kts 1410 kg
80 kts 1840 kg
90 kts 2330 kg
100 kts 2880 kg
So increase of the wind by 10 knots, from 40 knots to 50 knots is resulting in about 50 % increase of the load on the rode. Again, increase of the wind by successive 10 knots, from 50 to 60 knots is resulting in about 50 % increase of the load on the rode.
You did inform us about experiencing the winds up to 50 knots while anchored. I think that a factor of next 10 knots for a safety
will be not an overestimation, so Your rode should be sized for the winds up to 60 knots. Knox anchor of 27 kg will be probably good for the winds of some 50+ knots, Knox anchor of 36 kg will be probably good for the winds of some 60+ knots, Rocna 25 kg should be good up to 50 knots and Rocna 33 should be good up to some 60 knots.
Of course, in close to ideal conditions (well holding ground, excellent set, strong, but steady wind, no windshifts, no tide, long scope
, all chain rode, good snubber, no breakers) each of them can hold in much more of the wind, but it is difficult to rely on close to ideal conditions.
So – just after examining the sizing charts
my choice would be Knox (which is Your preferred anchor) of 36 kg, or Rocna of 33 kg.
Using “Snowpetrel coefficient” discussed in other thread here, the all-round cruising, new gen anchor for Your boat should be over 29 kg, so 36 kg Knox and 33 kg Rocna are good, some safety factor included.
The anchor suitable for Med mooring
and occasional anchoring out in more settled weather
should be not less than 25 kg.
So, for such a limited use Knox 27 kg or Rocna 25 kg should do. On the other hand – it is a mistake I did myself. I chosen an anchor suitable for limited scope of conditions and situations and now I’m in a need of change. Expensive error.
Sooner or later You will find that bigger is better (and cheaper in a long run) I’m quite sure.
Let’s look at the chain.
Here are the Safe Working Loads and Minimum Breaking Loads for different grades of chain:
BBB 8mm SWL 750 kg MBL 3000 kg
G 40 8 mm SWL 1000 kg MBL 4000 kg
G 43 HT 8mm SWL 1300 kg MBL 4000 kg
G 70 8 mm SWL 1400 kg MBL 7000 kg
So, if You are sizing the anchor for the winds up to 60 knots, You need G 43 HT or G 70 chain (to remain with 8 mm).
But there are other solutions also.
You can go for a stainless steel
G 50 chain.
G 50 SS 8 mm SWL 1700 kg MBL 5100 kg
You can also find the high tested German made G 50 SS chain (produced purposefully as an anchor chain) with the SWL of 2500 kg.
Next You can go for a G 60 Duplex, rated for the SWL of about 50 % of MBL (also German made as an anchor chain).
G 60 DUPLEX 8 mm SWL 3200 kg MBL 6400 kg
German CROMOX NAUTIC anchor chains with only 8 mm size will give You SWL good for 90 – 100 knots of wind (in the case of Your boat). It is good enough safety factor, more than galvanized, even high tensile chains can provide.
Next thing – with Your chain locker (or in Your case rather chain well) SS or Duplex chain will allow You to retain all chain rode, as a piling and blocking is not the case for these chains, and You can fit more of it to the locker. I think the all chain rode is far superior to the mixed one and retaining it should be of importance.
Pricing for CROMOX NAUTIC chains is reasonable, and when You take into account costs of regalvanisation (high for G 43 HT and rather prohibitive for G 70) it can appear even attractive
My personal choices would be:
Rocna 33 kg or Knox 36 kg as all around cruising anchors;
Rocna 25 kg or Knox 27 kg as the anchors for Med mooring
and occasional anchoring out in settled conditions (if I ever again would be tempted to own such an anchor. On the other hand I readily admit it is sufficient for most of the time);
All chain rode.
CROMOX NAUTIC G 60 Duplex Steel
8 mm chain (as first choice);
CROMOX NAUTIC G 50 Stainless Steel
8 mm chain (as second choice);
G 70 high tensile galvanized chain (as third choice).
Of course – it is only my thinking loud…