How to size an anchor.

How do you pick an anchor size for a cruising boat? How big is “big enough?” Is there such a thing as “too big”? Anchor recommendations are hard, no question. Boat length? Weight? Power boat? Sail? Monohull? Catamaran? Short-range fishing boat? Ocean-going cruiser? Every anchor manufacturer has a table that lists the recommended size anchor as a function of boat size, usually specifying a boat length, and weight, and conditions in which the anchor is designed to hold.  There is a difficult balancing act that every manufacturer has when they develop these recommendations. Recommending a smaller anchor will result in more sales because you are ahead on the price, but recommending an anchor too small for the job results in unhappy customers and is (potentially) a long term disaster for your business.

How “big” is “big enough”?

A couple of things need to be considered. Will the anchor fit? Different bow arrangements will fit different types and sizes of anchors. There is enough variety out there that you should find one of a suitable size that works. If the boat can’t fit an anchor large enough to comfortably go cruising, you need to seriously consider if it is the right boat.

Nobody ever woke up at 2AM with a squall blowing hard through the anchorage and sweated that their anchor was too big. Never happened. The difference in weight between a marginally acceptable anchor and the biggest practical anchor is not that large, 30 to 80 lbs for most boats. It is small potatoes compared to the weight of the anchor chain. Don’t sweat the weight. If it bothers you that much, add 50lbs to the anchor and cut off 10M of chain. You’ll be better off.

Some assumptions about the boat are made in this discussion:  The boat is an ocean-going cruising boat.  One that is expected to go anywhere, and be self-sufficient on its voyages.  The anchor rode is all, or at least mostly chain.  There is an efficient windlass, either manual or electric. Since wind gusts of 60 or 70 knots can easily be experienced during a violet thunderstorm, you know that your anchor needs to hold under those conditions.  For the purposes of this article, I’ll use the parameters of two different monohull cruising sailboats.  First is 40 feet and 18,000lb, the next is 52 feet and 38,000lb.  With those parameters defined, here is what you need to know:

Lets start by looking at the table of anchor sizes from the website of a popular modern anchor, the Spade.

Boat Length Boat Weight Anchor Model Galvanized & Stainless Weight
21 ft. <2,200 lbs. 40 12 lbs.
24 ft. <4,850 lbs. 60 21 lbs.
34 ft. <9,920 lbs. 80 N/A
41 ft. <14,330 lbs. 80 33 lbs.
52 ft. <26,450 lbs. 100 44 lbs.
59 ft. <35,270 lbs. 120 55 lbs.
65 ft. <44,000 lbs. 140 66 lbs.
75 ft. <50,700 lbs. 160 77 lbs.
75 ft. <57,300 lbs. 180 99 lbs.
82 ft. <66,130 lbs. 200 121 lbs.
98 ft. <88,100 lbs. 240 165 lbs.

There is a lot about this table I do not like. First thing I notice is that boat weights and lengths are out of proportion.  The weights are WAY lighter than the typical cruising boat of the quoted length.  Just as an example, a J122 is a 40 foot racer/cruiser, and as light as any such boat you will ever find. It weighs in at 14,900 lbs.  There are no instructions about how to use the table, length or weight?  which is larger? which is less? Next, is there is no qualification about what kind of conditions these anchors are sized for.  30 knots? 50? 100? It is not stated.

In surface area, these anchors are significantly smaller than other anchors of similar design. I am sure the designer of these anchors would argue that he is able to hold into the bottom just as tight as similar anchors that have 25% more surface area, but color me sceptical. What’s important to understand is not that I am saying the anchor is a bad design, just that for a serious cruising boat the recommended sizes are much too small.

OK, next up… The sizing table for the Rocna anchors.  Today these are probably the best selling anchors in the cruising market. Many, many people use this table as the basis for sizing their anchors.  The boat weights and sizes are well matched for “typical” boats, and it is clear what size is recommended for what combination of boat length and weight.  

The supporting technical data is complete and well presented.  There is really just one important catch, and it is in the footnotes for the table.  The anchor sizes listed are to hold a boat of the recommended size in winds of 50 knots. That might sound like a lot, and it is certainly more wind than anybody would chose to anchor in, but it is not an amount of wind that is rare in the leading edge of a thunderstorm or tropical squall. For a local coastal cruiser, this might be fine, but if you are a full time cruiser, you WILL be anchored in winds this strong—and stronger. Even though Rocna touts the “conservative” nature of their sizing recommendations, it is important to take note of the limits of the table.  I would go at least one size bigger than this table recommends for a long distance cruiser. 

Another table, this time from Mantus.  The wind strengths are explicit in the table, the surface areas are clear, and the numbers all make sense. Although there is not the breakdown for boats of various sizes and lengths, the length/weight ratios listed make sense and are typical for a cruising boat.  If we compare the surface areas of the Mantus anchors weight for weight with the Rocna we see they are quite close, and the recommendations for what size anchor to use  in wind speeds of up to 50 knots is quite similar from the two manufacturers.

With all these data in hand what would the various manufacturers suggest? For our 40 foot 18,000 lb boat, here is the list:

Weight Surface Area Conditions MSRP
Spade 44lbs 155 sq in ?????? $606
Rocna 44lbs 177 sq in 50 knots $531
Mantus 55lbs 217 sq in 50 knots $680
Mantus 65lbs 254 sq in >50 knots

And for our 52 foot, 38,000lb boat:

Weight Surface Area Conditions MSRP
Spade 66lbs 217 sq in ?????? $962
Rocna 73lbs 263 sq in 50 knots $1037
Mantus 85lbs 285 sq in 50 knots $1105
Mantus 105lbs 334 sq in >50 knots

Given the differences and lack of clarity around the Spade anchor, I’d have little confidence that the size Spade recommended would be a suitable choice. Again, no judgement that the anchor is a bad design, just that it is too small. In each case, the Rocna is a good size recommendation, in my opinion, but only within the criteria that Rocna gives: <50 knots of wind loading.

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