Anchoring at both bow and stern: Bad idea

Sometimes I'll see a boat with two anchors out – one from the bow and one from the stern.

This is (usually) a Bad Idea. Here's why.

Wind & Wave Forces

Consider a boat, anchored properly from the bow, so she is free to swing with the wind and waves.

Now add a stern anchor to that boat so that she is not free to swing with the wind and waves. In some cases, she may end up with both wind and sea on the beam.

A typical boat – either sail or power – presents three to four times as much projected area when she's beam-on compared to bow-on. This is obvious just from looking at the boat; they're much smaller viewed from the front than from the side. So, just by being beam-on, we've already tripled or quadrupled the forces compared to being bow-on.

$F_d = \frac{1}{2} \rho v^2 C_d A$

Projected area isn't the only factor, though. The force exerted by wind and water on an object is proportional to the projected area $A$ and the density $\rho$ of the fluid, but also to the coefficient of drag $C_d$ – a dimensionless parameter that describes how streamlined a shape is, i.e. how easily a fluid can flow around it. Sailboat hulls have a very low $C_d$ when viewed end-on; it's typically on the order of 0.04 to 0.1. Viewed from the side, though, $C_d$ is more like 0.3 to 0.8. The hull is, after all, designed to have a lot of lateral resistance and very low longitudinal resistance, otherwise it wouldn't be able to sail.

The rig and superstructure will have a similar $C_d$ in either direction, typically in the 0.6 to 0.9 range, but – like the hull – they present a greater area from abeam than from ahead. And while windage on the rig is certainly a factor in anchor load calculations, it's relatively small compared to the forces exerted by waves hitting the hull.

What this boils down to is that not only does the boat present three or four times more area when she's anchored with the wind on her beam, but the moving fluids – the wind and the water – will apply four to eight times more force per unit area, because of the flat, un-streamlined shape the sideways boat presents. Where the bow would cut an oncoming wave cleanly and with relatively little force, the same wave beam-on will slam into the hull side and transfer all that energy in a single big jolt.

While it's obviously impossible to give exact numbers for "A Generic Boat", it's quite reasonable to estimate the total forces on the hull as being 10 to 15 times greater when anchored beam-on to the wind and waves as when anchored bow-on. And that's after taking into account a reasonable amount of swing in both cases.


The total force on the boat, though, is less than the total force on the anchor rodes. Say we have 10 kN of sideways force on the boat, and the two anchor rodes form a 120 degree V; basic trigonometry shows that there will be an average of  roughly 10 kN on each anchor rode. Spread the V to 140 degrees, and each rode is taking 15 kN; if we snug the anchors up to make a 170 degree V, we'll have 57 kN on each.* In addition, this is now a sideways load on the pulpit and anchor roller, instead of the fore-and-aft load that these parts are usually designed to handle. And we can't bet that the load will be shared equally; the odds are that it'll surge from one anchor to the other in cycles as the boat is tossed around.

(* We've assumed very long scope here, for example's sake; short scope increases the loads further.)

By dropping anchors from both bow and stern, then, we have:

  • Prevented the boat from swinging to present her slender, streamlined bow to the wind and waves.
  • Increased the potential maximum wind and wave load on the boat by 10 to 15 times, possibly more.
  • Put tremendous side loading on pulpits and anchor-handling gear that are usually not designed for this kind of load.

So there we have it. While there are occasions where you do need to drop both a bow and a stern anchor – backing up close to a beach comes to mind – if there will be any chance of the wind or waves changing, you definitely do not want to be anchored from both ends.


If you're wondering how to anchor a boat properly – so she'll stay where you left her, reliably, as the weather changes – the Anchoring Made Easy series (Vol. 1 - Gear and Vol. 2 - Technique) by John Harries would be a fine place to start.



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