Anchoring in strong current

[MathiasW]

The other day I got asked how my anchor chain length and anchor load formulas need to be modified when anchoring in wind and (strong) current.

Have a look at the diagram enclosed. A strong current pushes the vessel such that there is an angle between wind direction and vessel heading. Let this be the angle beta. Now it is not only the windage area from the front, w||, which matters, but the lateral windage area when viewing the vessel from the side, w_|_, will also come into play (and this one is usually the larger of the two). Both need to be weighted properly to calculate the windage area w of the vessel at this angle beta.

Now we need to add the current. The current is closing the force diagram as seen in the drawing. The best way to characterize the current is to estimate the angle gamma that the current has with respect to the vessel's heading. If this angle is large, the current will work against the wind and help reduce the anchor load. When the angle is small, wind and current will push in the same direction and the anchor load will increase.

Using the sine theorem one eventually gets the windage area f that includes the effect of the current as

f = w sin(beta + gamma)/sin(gamma)

So, this is the effective windage area, which one has to use instead of w|| in Expert Mode in my online calculator Anchor Chain Calculator, or any other calculator that you may prefer, like the one from Bjarne.

Admittedly, this is ever so slightly cumbersome, but hey, we know that force diagrams in navigation are always a bit tricky.

When the current is large and the angles are 'against us', it can well be that f is twice the value of w||, or more, and hence must not be neglected.

Of course, there is also the benign case that the current opposes the wind, in which case f will be smaller than w||, but then we are on the safe side when using w||.

In the case of multihulls there is an additional effect in that the two legs of the bridle are not stretched equally, but this is not included in the analysis above.

Cheers, Mathias

Anchor Chain Calculator

[thomm225]

Nice.

I usually just check the wind and estimate the current then either let out a little more rode or go with the amount I usually do according to the depth and rode angle from boat to water surface.

Then let it set a while........to make sure it's holding.

[MathiasW]

Originally Posted by thomm225

Nice.

I usually just check the wind and estimate the current then either let out a little more rode or go with the amount I usually do according to the depth and rode angle from boat to water surface.

Then let it set a while........to make sure it's holding.

Thanks. And yes, of course, nobody would want to do this calculation every time. But playing around with it a little shows how big the effect can be, and how, in principle, to account for it in the maths.

I have been anchoring in La Paz in Mexico with very awkward angles between vessel and chain... Now, when this happens again, I may simply increase the windage area by two in my tool and see what comes out.

Cheers, Mathias

[MicHughV]

It gets tricky when...for instance...you anchor in a strong wind, with an incoming tide coming from the same direction....and then the tide changes..you'll often find your anchor line alongside the boat...a situation compounded by a tide change at midnight..

You can pretty much throw formula's out the window then..as your boat will dancing a merry jig until the next tide change..

[MathiasW]

Originally Posted by MicHughV

It gets tricky when...for instance...you anchor in a strong wind, with an incoming tide coming from the same direction....and then the tide changes..you'll often find your anchor line alongside the boat...a situation compounded by a tide change at midnight..

You can pretty much throw formula's out the window then..as your boat will dancing a merry jig until the next tide change..

Yes, this is very clear. In such a scenario you can still look for the worst case, though, so when current and tide push more or less in the same direction and use that to determine the amount of rode needed.

Next time I anchor in a strong current or tide I will look for those extreme cases and determine the effective windage area. The result is then stored in my head for later use in similar scenarios...

Cheers,

Mathias

[thomm225]

Originally Posted by MicHughV

It gets tricky when...for instance...you anchor in a strong wind, with an incoming tide coming from the same direction....and then the tide changes..you'll often find your anchor line alongside the boat...a situation compounded by a tide change at midnight..

You can pretty much throw formula's out the window then..as your boat will dancing a merry jig until the next tide change..

Which happens ever six hours or so here especially at one of my favorite anchorages Kiptopeke, VA where the tide is always strong and sometimes the cement ships do not block the wind or it shifts more than expected at night.

Think I'll start using two anchors there in the future so the boat does turn beam too the waves for a couple hours during each tide change.

Just behind the ships.....

[Manateeman]

We wrote a post on two anchors which won’t braid chains.
Set two pre cut lengths (100’) what ever you use most, to the bottom of a huge strong swivel. We use a 3/4”stainless New Bedford scallop gear swivel. On the top a chain to your boat hang in a loose loop with a snubber line too.
Storm...let out more.
Chains won’t braid. Strength is similar to hammerlock in the Knight book.
Review my post on the Kodiak System for anchoring which you’ll enjoy.
Manatee crew

[MathiasW]

I am afraid I need to correct my diagram...

I had not accounted for the fact that the anchor rode will also be at an angle to the vessel's heading, and so there is one more variable to include. New force diagram included.

Apologies for that!

Cheers, Mathias

Anchor Chain Calculator