Online tutorial tool for calculating anchor loads and required minimal chain lengths

[MathiasW]

Originally Posted by carlosproa

I did not understand the table, but you are saying for shallow depths better to have a combination on chain and rope? When I was a kid back in late 60’s in Venezuela’s Morrocoy keys, we had a stilt house back then and my Dad did used that combination for up to 5 meters depth and changed to all chain for deeper anchoring. He just used what the local fisherman did back then

The table was meant to show that the rope's ability to absorb energy does not depend much on anchor depth and that the more elastic the rope is, the more energy can be absorbed and the smaller the shock load at the anchor will be.

But when anchoring in deeper water with a mix of rope and chain, the pulling angle at the anchor shaft becomes and issue and so it is advisable to use this combination only in shallow water, just like your father did.

Obviously, when using more rope, one can go into deeper water, so it is all relative. Chain loves deep water, so again, I would agree with your father's choice.

Cheers, Mathias

[MathiasW]

After lots of discussions on this topic I would like to stress one point here:

The effects of dynamic anchoring, which I have tried to make very clear in previous posts, are most relevant for lighter vessels that tend to move a lot at anchorage. So, smaller boats that get washed away by every wave or gust, and multihulls with their high windage area and little weight in particular. When you have a heavy displacement vessel, which stays put even when exposed to very severe gusts and swell, then it is not so much affected by dynamic anchoring. Consequently, my guidance to anchor in deeper water, if you are chain only to reduce the impact of swell and gusts, is less relevant for heavy displacement vessels.

This is also the reason why big ships are not affected by this...

Hope this clarification helps and that it takes the heat out of some discussions...

Cheers, Mathias

[rondelais]

As Mathias has pointed out, with waves (or wind gusts) the important thing is to absorb the wave or gust energy. This can be done by lifting chain, or by stretching the snubber. So, in deeper water (for me, say 30-40 feet) lifting the chain does most of the work, and the snubber mostly just takes the load off the chain stopper. I think that unless the waves are huge, the snubber does little. In shallow water, there is just not enough chain off the bottom to lift, so the snubber is really the whole game. Of course, if the waves are big, anchoring shallow is a bad idea for lots of reasons - you could bottom out, the waves may break, and it is hard on the gear.

I kind of wonder if the the ideal rode is a mix of chain and nylon (for a built-in long snubber, and motivated in part by minimizing total weight). If you generally anchor in a fairly narrow range of depths, as I tend to do, you could “tune” the chain length so that you put out all the chain and say at least 30 feet of rope - more if in deeper water or if wave action is high or expected. If the target depth is, say, 20 feet with 5 feet of freeboard, 100 feet of chain might be about right. It would be best to use the heaviest chain the windlass can handle. As an example, if in 20 feet and low wind and little wave action is anticipated, let out say 75 feet of chain, put on the chain hook with a fairly short snubber to just take the load off the chain stopper. If more wind and/or waves are expected or come on, let out the rest of the chain and say 30 feet of nylon line. If it really gets nasty, let out even more line.

I personally like the idea of having just a few anchoring plans - fairly shallow (10-20 under the keel at low tide), moderate to deep (20 - 40 feet), and nasty weather (any depth). Then, there are no calculations required except for “nasty” - you just do the “standard” for your situation.