March 20, 2011

Sea anchors versus drogues

TWO PEOPLE have asked me recently: “What is the difference between a drogue and a sea anchor?”

Well now, because two is a significant percentage of my readership, I feel compelled to offer an explanation.

A sea anchor is anything designed to hold a boat almost stationary in the water, anything that is not attached to the bottom. It usually takes the form of a heavy-duty parachute or a large conical canvas bucket with a wide mouth and a small hole in the bottom, such as the one Captain Voss made popular.

In really heavy weather, a sea anchor streamed from the bow will keep a shallow-draft powerboat or a sailing multihull pointing into the waves and making slow sternway, which is probably the safest way to lie.

But sea anchors don’t work very well on most deep-keel sailboats. The windage on the mast, which lies a long way forward, causes the bow to blow off and jerk the line constantly. The result is that most of the time the boat will lie almost broadside on to the waves, which is not always the best position, depending on the boat’s underwater shape.

Then there is the difficulty of streaming a sea anchor, with its mess of canopy lines, in a gale of wind. The makers of sea anchors don’t list this among the selling points.

A drogue on the other hand, is anything dragged in the water behind the boat to slow it down, rather than to bring it to a halt. A drogue is useful when a boat is running before the wind and starts to get out of control. In combination with a scrap of sail up forward, or even under bare poles, a drogue over the stern will help prevent a boat from slewing broadside on to the swells and being laid over on her side, vulnerable to complete capsize on the next wave.

The time to stream a drogue is when your boat starts to get out of control as you race along almost at the same speed as the overtaking waves and the helm goes slack while you wallow in foam-filled water. The drogue slows you down so that the wave will pass under your transom quickly, and restore control to the rudder once more.

Some deep-sea sailors use car tires as drogues. Others use fancier drogues made for the purpose, including one with scores of small conical pockets on a long line that evens out the strain. But the principle is the same. The drogue just slows the boat down. It’s also very useful when running an inlet with a shallow bar, to keep the boat lined up at right angles to the surf.

Today’s Thought
He that mounts him on the swiftest hope
Shall often run his courser to a stand.
— Colley Cibber, Richard III

Boaters’ Rules of Thumb, #174
To estimate the sail area your boat needs, take three quarters of the square of her waterline length in feet. That is, multiply the waterline length in feet by itself, and take 75 percent of the result. The answer is in square feet.

Tailpiece
Two pink elephants, a mauve spider, and two yellow snakes strolled into a local bar.
“Sorry, guys,” said the bartender. “You’re a bit early. John Vigor isn’t here yet.”

(Drop by every Monday, Wednesday, Friday, for a new Mainly about Boats column.)

8 comments:

Micky-T said...

I never had to use them but I've always carried 5 or 6 heavy duty milk crates figuring if I lashed them all together with a few good size fenders I'd have a good way to slow the boat down.

Dan Owen said...

G'day John,
Just read your book Small Boiat to Freedom. Good stuff! I'm just down the Stait from you in Port Townsend. Drop me a message if you get this way. I have a Cpae Dory 27 and would enjoy a chin wag with you. Cheersw Dan O. dowen@q.com

jscottw said...

Hi John, Great post, thanks!

I'm having trouble picturing, "It’s also very useful when running an inlet with a shallow bar, to keep the boat lined up at right angles to the surf."

Anyway you could describe further / maybe make a diagram??

Many thanks!!

Scott

Anonymous said...

Keeping in mind that the guy was selling a product, Don Jordan did a ton of research in to drogues. Scale models, USCG research facilities, and years of experimenting. His results, regarding sailing yachts:
1. The drogue must be attached at the stern.
2. It must be constructed of multiple elements strung out in a line.
3. The number of elements and their characteristics (size and drag) must be matched to the yacht's displacement.
4. The elements must be well submerged.

Those interested in further reading should check out the Drag Device Database.

John Vigor said...

Scott, when you're running into a shallow inlet through breaking surf, the last thing you want is to broach; that is, to have the surf break on your stern and swing you around broadside on.

You want to remain lined up fore-and-aft at right angles to the line of breaking surf. A drogue streamed from the stern pulls the stern aft in a straight line. It helps prevent a broach by stopping the stern from swinging around.
A sailboat coming in like this should have no sail aft of the mast. Powerboats sometimes use this technique, too, particularly those with trawler hulls.

John V.

David Browne said...

A comment not very politically correct:

I have read your analysis of sea anchors in Seaworthy Offshore Sailboat etc, and your comments concerning the Pardey's technique, and have read the Pardy's book on storm tactics many times trying to find something to support my skepticism of what sounds like overly simplistic generalizations (i.e., carrying a trysail in 70-plus knots of wind while lying peacefully to a sea anchor) and their statement that a sea anchor off the bow is the last resort needed to survive basically any storm.

I think both drogues and sea anchors have their usefulness but neither is a blanket tool of last resort. Any thoughts?

A Overg said...

Hello John Vigor

Trying to find more about dragging a drogue - going with the waves in a motorboat.
Have a not so big motorboat, and wonder if a drogue would give reduce problems of broaching.
Do you have any experience yourself?

Greetings A Overg

John Vigor said...

A Overg:

I'm afraid I don't have any experience of drogues on powerboats, but I can tell you that low-powered powerboats, and even lifeboats, sometimes use drogues when running onto a beach through surf, or crossing a sandbar to enter a river mouth. The idea is that the drogue keeps the stern lined up properly to oncoming waves. Without a drogue, a low-powered boat tends to slew around sideways, broach, and possibly capsize as the breaking surf hits the transom. The drogue helps keep the whole boat in line.
But you need a high transom and a good watertight cockpit in case the surf floods over the stern. The old advice is to stay out at sea if the surf looks too rough.
Go to Google and check out the Drag Devise Database.

John V.