Why is boat weight a factor in anchor sizing?

[LittleSister]

Anchor sizing charts usually depend on the boat's LOA and displacement to determine the recommended size.

LOA is, I assume, a proxy for boat windage. What is the significance of displacement?

Is it a further proxy for windage, or something else, or is the weight itself of significance and if so how?

 

[aslabend]

Momentum as the boat swings and snubs the line tight would be my guess.

 

[Poignard]

The weight of the yacht doesn't seem to have been a factor in the recommendations of the manufacturers of my 'genuine' CQR :

 

[LittleSister]

Momentum as the boat swings and snubs the line tight would be my guess.

I had never questioned the inclusion of the weight factor and assumed it was something like that, but having started thinking about it, I am now sceptical, hence my asking the question.

I thought that perhaps the additional force applied to the chain/anchor by the greater weight of boat e.g. coming down the back of a wave, but on further reflection a greater weight boat will dampen such movement and reduce the extent of it, and the acceleration due to gravity is the same for any weight of boat.

The swinging due to wind would also be dampened, not exacerbated, by greater weight, the force of the wind while swinging will be down to its windage, not its weight, I think. Which leads me to wonder whether the weight is included as a proxy for the difference between the windage of a sleek light racer and a voluminous bulky cruiser of the same LOA.

If I deployed a yacht anchor from a ship, say an oil tanker, the movement in the waves, and the ranging about in the wind, will be far less than that of a yacht. It will be the ship's windage that would defeat the anchor, not its movement in the waves.

 

[Tranona]

Displacement is used because it is an indicator of "bulk", although weight per se has little influence on the forces. LOA also a poor indicator. OK 90 years ago when the CQR table was drawn up when boats were more similar in design, but not that relevant for the wide variety of modern designs - high freeboard, no overhangs, multihulls etc. For example a 36' cat has the same windage as a typical modern 46' monohull. Windage is by far the biggest factor but not easy to give a simple way of measuring it.

Some of the more modern anchor manufacturers do give advice on how to relate an individual boat to the sizing charts. Not difficult to suss out that a heavy 36' ketch with a wheelhouse, davits and transom arch and short hull overhangs will exert far greater forces on an anchor than say a J 109 of similar LOA will need different size anchors.

 

[LittleSister]

Displacement is used because it is an indicator of "bulk", although weight per se has little influence on the forces.

That was my suspicion.

Can anybody gainsay that, and provide a reasoned explanation?

 

[noelex]

Windage is more important in my view, but in conditions where there is some wave action, even a small amount of pitching can place much a higher force on the anchor and this force is related to displacement (amongst other factors).

There is also some momentum built up in the horizontal plane as the the boat swings or yaws. However, the momentum in this case typically only has a minor effect on the anchor. The increased force from a higher frontal area and larger drag coefficient is more significant.

Manufacturers’ anchor sizing charts are all over the place. Some use displacement, others ignore this parameter, some suggest increasing the size for catamarans, others ignore this issue, some are assuming a very low maximum windspeed, others are more realistic.

Apart from these anomalies, these tables are heavily influenced by the marketing department. Recommend a smaller size than the opposition and you will sell more anchors.

This has resulted in many anomalies such as the absurd situation where manufacturers of high performing anchors typically recommend larger sizes than poor designs. For example, for my boat Rocna recommend a 33kg model, but Plastimo indicate that a Kobra of less than half this weight, 16kg, is perfect.

 

[LittleSister]

. . . the absurd situation where manufacturers of high performing anchors typically recommend larger sizes than poor designs. For example, for my boat Rocna recommend a 33kg model, but Plastimo indicate that a Kobra of less than half this weight, 16kg, is perfect.

No doubt the two manufacturers are well aware that the potential customers for Rocna generally have much more money to be parted from than those for Kobra.

The Kobra customers are looking for, and being sold, something that 'will do the job', while the Rocna customers are being sold 'the ultimate' anchor. (Everyone can decide for themselves how satisfied they are as to the veracity of such claims.)

I'm not sure it is correct to say the Kobra is a poor design. (A Honda Cub, for example, is a brilliant design, in that it hasn't needed to be the fastest, most exciting or glamorous motorcycle to achieve world sales vastly greater than any other bike.) The Kobra seems to me a very successful design for the company and its customers. By most accounts the Kobra gives very good holding and service for the price. Its limitations - e.g. the more easily bendable shank - are something its customers are willing to live with for the price.

 

[aslabend]

On small but heavy boats like mine (26ft & 4.5t) with a lot of keel, windage puts less force on the anchor compared to the antics she gets up to if anchored with the tide at right angles to the wind. She sails up to the anchor and then falls off jerking the chain taught before repeating. If there's no tide all is calm and we just swing behind the anchor.

 

[noelex]

I'm not sure it is correct to say the Kobra is a poor design. (A Honda Cub, for example, is a brilliant design, in that it hasn't needed to be the fastest, most exciting or glamorous motorcycle to achieve world sales vastly greater than any other bike.) The Kobra seems to me a very successful design for the company and its customers.

Yes, high performance cutting edge products rarely achieve high sale numbers (although there are exceptions). Most skippers anchor infrequently and very rarely in adverse situations so an anchor that is inexpensive like the Kobra and Delta are likely to make their manufacturers the most money.

I suspect the highest sales volume is when anchors are sold to boat manufacturers to be installed as standard equipment. Boat manufacturers like anchor companies that recommend small anchors. Not only does this save money on the anchor itself, but the ancillary equipment such as the windlass, bow sprit etc can be smaller. Any complaints made to the boat manufacturer can be deflected by pointing out to the customer that the "recommended anchor size" has been installed.

Unfortunately, this commercial incentive to recommend smaller anchors distorts the sizes recommended by some manufacturers.

 

[MontyMariner]

Windage is more important in my view,

Same here, my anchor when I bought the boat was sized on the manufacturers recommendation for length, but the boat is a bulky pilot saloon ketch with a lot of windage, so I upped the anchor size by one.

 

[Pete7]

Same here, my anchor when I bought the boat was sized on the manufacturers recommendation for length, but the boat is a bulky pilot saloon ketch with a lot of windage, so I upped the anchor size by one.

I don't think you have anything to worry about. The height of the deck on this thing was about the same as our boom.

Pete

 

[MontyMariner]

I don't think you have anything to worry about. The height of the deck on this thing was about the same as our boom.

Pete

I don't worry, that's why I sleep so well at anchor, and I bet his anchor was a bit bigger than mine

 

[B27]

Weight matters.
Try standing in the shallows holding a racing dinghy in a gentle onshore breeze with a few little waves coming in, and feel the difference it makes when the crew gets out.
Weight is displacement, and when anchor size matters, the water displaced will be moving water.

 

[Neeves]

One question might be - how many of you know how heavy your vessel might be. We (forum members), or you, know the weight of your yacht (when new) - its part of the specification. None of you know what is the windage nor weight of your yacht.

But we buy the yacht and we add, and add - stuff - biminis, dodgers, high rise structures accepting solar panels and davits, desalinators (not heavy - but the extra batteries needed to run the unit are), extra sails, over sized chain (it stands to reason bigger chain will be safer ), life raft....

The anchor maker has a tough time - he has to accomodate all of this in his recommendation. The anchor maker also knows that everyone is an anchor experts - just look at the people who supported 'bigger is better' (now largely discredited), Consider those that mocked lightweight anchors, whether aluminium or HT steel - now used without issue. Just look at those who think they are gurus - but are shown to be at fault.

No wonder boat builders supply Delta, or Kobra - they have no idea what the owner will do with the yachts they, the builder, sells. No different to sails. There is no such thing as a standard Beneteau - when you spend that money you want it to be personalised to a great or lesser degree - more windage, more weight.

The anchor maker has similar issues a 40' Beneteau could vary in weight and windage (and the owner has no idea its weight nor difference in windage from standard), but the anchor maker is expected to make a 'professional' decision.


Flighty, perfomance yachts are less likely to have added windage but heavy yachts sold as 'cruisers", 'ocean', 'blue water', 'expedition' are likely to have the extra real estate on the stern - so high weight *specification' vs length ratio equates to extra windage.

Its all very crude.


Oddly - anchor makers actually seem to have it right. The spread sheets have been honed over decades and polished as new designs have emerged. In fact the anchor makers have learnt - stories of anchors dragging were common place 20 years ago - today......?

Jonathan

 

[Neeves]

I don't think boat builders 'recommend' specific anchors - they supply an anchor. If you are buying a yacht you can have any anchor you like. The cost of requesting a different anchor instead of the 'standard' is peanuts.

10s of thousands of people have used Deltas and Kobras - they were in their time the best available. They are as good now as then and members here still comment on both designs with favour - as a budget buy. Of course if you want a top of the range yacht you pay extra for everything - if you want a budget yacht you get budget kit. To blame the successful boat builder for minimising costs does not stand up - they are still in business. I can think of one or 2 boat builders who were not cost conscious - who no longer exist.

Jonathan

 

[boomerangben]

I suspect that using length and displacement is a means of selling more bigger anchors. If you have a light 35’ yacht that needs a say 15kg anchor you buy that. If someone else has a heavy 28’ yacht and the length graph says they need a 12kg anchor, but then sees the displacement graph for his yacht says 15kg…. They’ve just sold a bigger anchor.

Displacement would in theory require a bigger anchor since for the same dynamic movement, there is more energy that needs to be transferred to the seabed, hence bigger anchor. But as mentioned before displacement might mean more damping and less motion. It’s too complicated a system to base an anchor size simply in displacement.

 

[ylop]

That was my suspicion.

Can anybody gainsay that, and provide a reasoned explanation?

I think if you do a thought experiment it becomes intuitive then mass does matter as well as size.

Imagine a 10mx2m giant paddle board. It sits only a few inches above the water so has tiny windage and would be relatively easy to secure (to a dock or sea bed).

Now strap 20 empty IBCs (those 1m plastic cubes used for transporting chemicals). On top of it and it weighs very little extra but now has massive windage. In a blow it’s going to be hard to stop.

Now will those IBCs with water. And flip the board over. It will float with the same cross sectional area out the water as the first experiment but it have a mass of 20000kg. Clearly you can’t hold that easily. It will not accelerate quickly in the wind - but with so much more “hull” in the water it will response to waves/current differently.

From a simplistic school physics perspective when it moves around in a wave or wind and the anchor tries to stop it the boat applies a force to the anchor. Assuming everything was in a straight line that Force (f) equals the mass (m) times the acceleration (a). F=ma. You may think of the acceleration as being a deceleration - it’s going from moving at maybe ~1 knot (0.5 m/s) to zero in the time it takes the chain and snubber to pick up slack.

Which B27 illustrates with real life experience:

Weight matters.
Try standing in the shallows holding a racing dinghy in a gentle onshore breeze with a few little waves coming in, and feel the difference it makes when the crew gets out.
Weight is displacement, and when anchor size matters, the water displaced will be moving water.

One question might be - how many of you know how heavy your vessel might be. We (forum members), or you, know the weight of your yacht (when new) - its part of the specification. None of you know what is the windage nor weight of your yacht.

Do most people who get lifted not know their weight?

 

[Tranona]

Do most people who get lifted not know their weight?

Only if by a crane with an accurate gauge. Travel hoists and sleds don't normally have any way of measuring weight. However, not that difficult to get a good estimate, at least on modern boats by taking the design displacement and adding the maximum payload on the CE plate.

The problems from an anchoring point of view come from adding windage by way of gantries, wheel shelters, biminis etc on boats that were not designed to have them. Then add on in mast to boats that were not designed for it, inner forestays and furling sails, radar and very quickly boats in the 33-40' range can have sufficiently reduced stability to take them below Cat A.

Just as an example from the lower end of the Bavaria range mjambo.de I doubt that Michael's boat would pass the stability requirements for Cat A in its cruising form because of the extra weight above deck and you can also see why he now has a 20kg Rocna when the standard boat is well within the 15kg range.