When is an Oversized Anchor too Much?

[Snowpetrel]

Originally Posted by DoubleWhisky

Interesting (for me even brilliant) really

I made anchor size calculation accordingly to Your formula (using the 25 as a Bigness factor/Snowpetrel coefficient) for several boats I know well and obtained the same results as using my own method

Just my two cents - I personally think the "Snowpetrel coefficient" of 25 is just right for chosing the modern anchor for average, moderate displacement monohull. So I would personally attribute the 1 as "anchor factor/coefficient" to the new generation (Super High Holding Power) anchors, and 25 as a default "Snowpetrel coefficient" for all round cruising anchor.
I also think that "Snowpetrel coefficient" of 32 is the highest acceptable, appropriate for the less demanding cruising grounds only.

Cheers

Tomasz

Thanks for taking the time to look at the figures Tomasz, I wouldn't give you much arguement on the 1 for a next gen, since thats now pretty much the default for anybody buying. The nice thing about this formula is it pretty much takes the size and scaling factors out so you can directly compare different vessels, much as you would sail area/disp ratio or disp/length ratios, and just like them there are all sorts of other considerations that modify the final result, but they give you a ballpark starting point. The old "what is you anchor factor?" thread had a fair few discussions on all this. But for me the real indication that the formula worked was Andrew Troups observation that the Mirabella V original anchor factor was (246×246)/1320=46 and he dragged ashore in 25 knots of wind. They recalculated everything using rigging windage and reccomended a 2376lb anchor giving (246×246)/2376lbs=25.

I have a few spreadsheets of supporting information. Trying to find the time to document it all! But the upshot was that most of the old manufacturers reccomendations didnt scale correctly. From memory only the spade and the mantus had it right.

Saying that I think the bigger anchors dig in better into marginal substrates so thats why some bigger boats get away with comparitively lighter anchors.

Cheers

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[Factor]

So - at least one manufacturer uses the following approach:

For the last twenty years we have supplied anchors for boats under survey, this is where we base our sizing and have always done so, this method of sizing covers boats under survey, covers insurance on anchor size, if you have a boat that is operated under survey then a survey officer will specify the anchor size by working from a list of variables of a boats specifications.

If its good enough for AMSA and all marine authority bodies it is good enough for us. Of course clients are always welcome to discuss their needs directly with us.

[DoubleWhisky]

Originally Posted by Snowpetrel

But the upshot was that most of the old manufacturers reccomendations didnt scale correctly. From memory only the spade and the mantus had it right.

This is not so simple to asess the sizing charts, as each size of anchor need to cover the range of boat sizes.
I tried to run some numbers and I'am now pretty positive that Mantus, Spade, Rocna and SARCA Excel recommendations are sitting rather comfortably in 25 - 32 zone, with Mantus and Spade being apparently closer to 25.

Originally Posted by Factor

So - at least one manufacturer uses the following approach:

For the last twenty years we have supplied anchors for boats under survey, this is where we base our sizing and have always done so, this method of sizing covers boats under survey, covers insurance on anchor size, if you have a boat that is operated under survey then a survey officer will specify the anchor size by working from a list of variables of a boats specifications.
If its good enough for AMSA and all marine authority bodies it is good enough for us. Of course clients are always welcome to discuss their needs directly with us.

And - interestingly - this manufacturer sizing chart is generally in accordance with the results computed using sizing method proposed by Snowpetrel



I have a Rocna on my boat, and ASC (I suggest to give the "Snowpetrel coefficient" the name of "Anchor Size Coefficient") is 31 and I found it completely adequate for my own needs. It is in accordance with Rocna sizing chart and my cruising ground is East Med - not so demanding area. When discussing the choice with Rocna I provided them with all boat details and they suggested the anchor of 31 ASC for Med/Caribbean cruising (out of hurricane season in Caribbean), but also an anchor of 25 ASC for high latitude cruising.
Quite recently I discussed it with Rex from Anchor Right, and his recommendations were exactly the same

Cheers

Tomasz

[ranger58sb]

Originally Posted by Factor

Saying - ONE SIZE UP irrespective of of the manufacturer is a little illogical. Talk to the bloke who build the anchor, if you can't talk to them - then maybe choose an an anchor manufacturer where you can.

I suspect most folks who say that simply use it as a short-hand or abbreviated way to suggest the manufacturer's sizing chart may not fit all situations. And those maker's charts often include the same kind of short-hand: consider upsizing if your boat is heavier than "normal" or has high windage, etc etc etc.

FWIW, I know all the sizing charts I've looked at (i.e., for the few anchors I've cared about researching over the years), every time our particular boat weight and windage factors have suggested upsizing. I don't think that means the charts are unhelpful, just that boats vary and ours tend toward heavier (those pesky engines) and higher (that pesky flying bridge).

In any case, it certainly makes sense to choose based on manufacturer's recommendations further informed by real-world experience.

-Chris

[DoubleWhisky]

Hi, Snowpetrel
I went through my old spreadsheets and made some calculations based on Your methodology.

Here is my take on the anchor sizing formula based on Your proposals:



LOH^2 x DC x WC
-------------------------- = AW
ASC x HPC


Where:

LOH = Length Of Hull in feet

ASC = Anchor Size Coefficient
22 ≤ ASC ≤ 32
Default ASC = 25 for all round cruising anchor
ASC ≤ 24 → high latitude cruising / hurricane anchor
ASC ≥ 26 → cruising anchor for less demanding cruising grounds

HPC = Holding Power Coefficient
HPC = 1 for modern (SHHP) anchors, as Manson Supreme, Mantus, Rocna, Sarca Excel, Spade, Ultra etc.
HPC = 0.8 for previous generation (HHP) anchors, as Delta, Kobra 2, WASI Bugel etc.
HPC = 0.6 for old generation anchors, as CQR, Bruce etc.

DC = Displacement Coefficient
DC = 0.8 for ULDB (racers)
DC = 0.9 for LDB (cruisers racers)
DC = 1 for MDB (cruisers)
DC = 1.1 for HDB (heavy cruisers)

WC = Windage Coefficient
WC = 0.9 for low profile boats without sprayhood and bimini
WC = 1 for average cruising boats with standard sprayhood and bimini arrangement
WC = 1.1 for cruising boats with oversized sprayhood/bimini/solar panels arrangement
WC = 1.2 for cruising boats with hard dodgers, hardtops etc.
WC = 1.3 for flying bridge boats

AW = calculated anchor weight in pounds

After calculating the AW the anchor of desired make can be chosen on the basis:
AW ≤ actual anchor weight.



I do not dare to propose DC numbers for multihulls, or DC and WC numbers for motorboats - my suggestions are made regarding mainly sailing monohulls



Cheers,

Tomasz

[SVNeko]

Have you looked at the mantus? Pound for pound, these anchors are huge compared to a manson supreme (which some posters seem to be interchanging with the mantus). A 175 lb edition will look like you speared a private jet on your bow.

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