Always a great question to ask and watch the discussion fire up when you do. Anchor systems work in such a variable environment nobody can give you a direct size guide as such.
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Note: The following is meant as a guide only and is a bit generic. If you are not sure ask us or someone else with more experience than yourself. Actually ask a few then average the difference as everyone has a different view than the next person.
1lb a foot or 1.5kg per metre is a starting point and not too bad a one but only a guide. From there you adjust for the varying factors.
Increase the size if –
While it is not recommended to go much smaller, some smaller boats can drop a small amount if -
Note: The above is a bit generic as there are many differing types and styles of anchors available these days just like the boats and users hanging off them. Each is better in some bottom types than others. Some anchors will hold a lot ore than others as well so that maybe a factor in the calculation.
While most manufacturers have some sort of sizing guides these must be used as a guide only. Some are 'suggested' sizes but only to a certain wind speed i.e. 30kts. In parts of the world that is fine but in others it is not. Some manufacturers guides go a bit the other way and could be a bit on the overkill side for a weekend sheltered water boater.
These charts DO NOT take into account the many and varying types of boats and bottom conditions.
One thing is for sure, there is no standard test or similar so comparing one chart to another is a not a good thing to do unless you have read the small print.
If you throw an anchor off the bow, what happens? You have to buy a new one and the boat drifts away.
Throw some chain and rope off the bow and what happens? You'll stay stopped on a calm day but drag as soon as any weather conditions come up.
So an anchor by itself is quite useless. Not far behind 'quite useless' is using just chain and/or rope.
Hence when picking an anchor or chain or the warp, think 'Anchoring System'. All parts work together to hold your boat. Match these parts well and they will all work as a 'group' to enhance your anchoring systems performance.
The biggest thing to remember –
By this we mean does your anchoring system make you happy when you are anchored.
If you answer No, the chances are you won't sleep well and/or will always be looking over your shoulder to make sure you are where you hoped to be. Obviously that just detracts from any boating enjoyment so change something so you are.
If you answer Yes, good on you.
As strange as it may seem, the 'feel good factor' is something we regard as very important to any anchoring.
Think about it like this – We have a 2mm rope with a 1000kg break load so you could happily hang 100mts off the ground on it, but would you?
No, why not? Your undies filling with a smelly substance, no wonder really. While you know it's strong enough your mind is saying "get me the hell off this now!". Hang off a 12mm rope and your mind will be in a happier place.
The same applies to anchoring, if you're not happy it will just niggle at you so change it so you are and your mind can rest easy.
Note: This article has been written for informational purposes only. In no way does it cover the entire subject in great depth or is meant as being correct for every one in every situation. In no way does this article mean to imply all comments made are 'law' even if some maybe. The intent of this article was to inform a little. Be aware regional differences in standards, laws and procedures will mean things maybe a little different from place to place even if the same basic principals apply.