how to make a drogue or sea anchor

"sufficient forward motion as a wave passes underneath to encourage flotation."

Huh? are you saying the boat will float better if it moves in the direction of the wave?
Howso, if the whitecap water is moving at wave speed? Are you assuming that the hull is sucked down by the oncoming water; "hydrodynamic sinkage"?

"the water supporting this tethered boat becomes less dense from centrifugal forces and therefore more of it will have to be displaced in order to equal the weight of the boat, and thus float it."

Nah, I don't believe you. How can centrifugal forces make anything less dense.
The water in the trough moves against the wave direction, and on the crest it moves in the wave direction (but not as fast as the wave, except the whitecap water) so on a surfboard there would be a loss of apparent water velocity near the crest - which might lead to sinkage. Or what?

"encourage it to meet an oncoming breaking wave near the crest rather than in a trough,"

Well, the crest has to be met at some point, and I don't see how you can tune a drogue so that the wave stops breaking before it meets you? Is that possible? Or what do you mean, can you perhaps explain it differently?

 

Hi Sigurd,

I assume you are asking me, so let me try to answer your question. You are right in noting that the terminology is more than a little vague in my post above. And I want to apologize in advance because I’m about to get a little long-winded here.

No, I’m not referring to the “whitecap” where air is mixed with the water, although that would certainly make the problem worse. Let’s go back to basics for a moment, and hopefully I can clear things up.

The apparent density of our flotation medium is very important. Fresh water has a density of roughly 1000 Kg per cubic meter, but salt water has a density of roughly 1035 Kg per cubic meter. So, if your boat weighs 1000 Kg it will displace one cubic meter of fresh water to float to a particular water line, but only about 0.966 cubic meters of salt water to float somewhat higher. In other words, the weight of the boat is exactly balanced by the weight of the water it displaces when floating, and the weight of the water is determined by its inherent mass and by the forces acting on it, as is the weight of the boat. As the density increases our boat floats higher, and as the density goes down so does our boat. Under the usual circumstances the only force acting on the water and determining effective density we are concerned with is pretty exclusively gravity.

Now consider a thought experiment. You have a pool of water of any density you like in orbit around the earth and a boat to float in it. Ignoring the minor detail that the water wouldn’t stay in that pool very long, the fact that there is no effective gravity acting on it (it is “weightless” because the attraction from the earth’s gravity is equaled and effectively cancelled by the centrifugal forces of the circular orbit) means that this water has no “apparent density” because it has no apparent weight. Density is defined as the mass of the material divided by it’s volume, and for the purpose of calculating flotation the mass is equivalent to the weight. So, you can’t float your boat in an orbiting pool at all. Your boat has no weight for the same reason the water has no weight while in orbit, and you would be able to place it anywhere you wanted on or in the water with no problem at all under those conditions. If your boat did have weight though, and the water didn’t, you would sink immediately no matter what your displacement was!

Getting back down to Earth, water can also “hang,” apparently weightless, when the centrifugal force exerted from it’s rotation in a wave exactly equals the gravity trying to bring it down to the surface of the sea. It is the same as a pool of water in orbit, that water at the top of the curling wave has zero effective weight, and does not simply fall back into the sea because of this. Thus the “tube” or “barrel” that surfers love is formed.

But it is the effective weight of the water that gives us flotation. If that weight is cancelled out, then there is no flotation at all!

If a boat floating on a curling wave crest is traveling at the same velocity as the water in the wave surface the same centrifugal force cancelling the effective weight of the water will cancel the effective weight of the boat . . . and with a lot of luck the boat will maintain it’s position on the wave as opposed to in the wave. On the other hand, if the boat is tethered such that it cannot travel with the surface of the wave following it’s circular path, thus generating centrifugal forces of it’s own, then it’s effective weight is unchanged and remains high, but the effective weight of the water it displaces is relatively reduced . . . perhaps to the point of none at all.

Which brings up the interesting question of how much of this effectively weightless water your boat must displace in order to float. And the answer is a bad one: you can’t displace enough! And the corollary to that fact is that given that you can’t displace enough water weight to equal the mass of your boat and thus support it on the surface . . . your boat will automagically head for the nearest firm land, never more than seven miles distant, straight down. (It’s a design feature.)

Of course in most cases waves passing under our boats are not so vigorous that they are forming “tubes” under our keels. (Yikes! What a scary thought!) But the point remains that water that is passing under us is “humped up” by the centrifugal forces acting on it, and thus is less dense than the water in the trough. I have personally seen waves “humped up” sufficiently to roll down the flight deck of an aircraft carrier that was normally 90 feet above the water line. That is a lot of centrifugal force at work!

So in consideration of the physics of this situation it seems to me that to completely tether the boat to the point where it cannot do anything but bob up and down, thus eliminating all centrifugal forces, is an exceedingly dangerous thing to do! But just going along with the wave as in lying ahull is also a dangerous technique as you risk getting broached, or out of sequence with the waves and pooped (been there, done that . . . and didn’t like it one bit), or dropped off the crest of one wave into the trough of the preceding one with a big crash that tends to “shiver your timbers” but good.

The serial drogue gives you the opportunity to “tune” things to a compromise between an absolute tether and nothing at all, which seems to me to be the best possible way to deal with this situation.

Bill

 

Isn't the centrifugal force of water particles acting upwards? So in the crest the boat has lowered bouyancy but a force upwards caused by water particles flying up. For me, it seems, that the boat floats as before, but loses a part of her stability, while the bouyant force is decreased.

 

Hi terhohalme,

As I understand it, the centrifugal force at the upward "hump" of a wave is, indeed, directed upward. But then the effect of gravity at the same location is directed downward, so the two add algebraically. The end result is no significant particle flow either upward or downward but, rather, in a circular motion parallel to the wave surface and in the direction of wave travel if we are talking about the portion of the wave raised above the mean sea level which I have been referring to as the “hump.” In most normal cases where we experience this motion from the deck of a boat, the boat is moving more or less with the wave so it experiences the same forces as the wave it floats in. That is, if centrifugal force in the water flow of a wave “lightens” the wave it also “lightens” the boat floating in it and the effects cancel each other from a practical point of view, thus making any effect hard to discern. The boat will, as you say, float as before.

It’s only when conditions become extreme that this is even an issue. Conditions where we might consider a serial drogue or some other aid that can have a drastic effect on the play of forces we are dealing with.

Bill

 

A simple and effective Drogue is your Sturdy Bucket tied to some rope and run out from the back of the boat It will do a wonderful job of keeping the boat straight in a following Sea
A length of chain on a rope run out from the back of the boat makes a Perfect sea anchor to keep your boat straight in Heavy winds
Seamanship and expiriance will help you to know when to use them.
All the tools are on your Boat
regards Alex

 

I would suggest taking a look at "Storm Tactics Handbook" written by the Pardeys

Storm Tactics Handbooks. Paperback / 192 pages / 1995 ... Author, Pardey, Lin/ Pardey, Larry.

This book is specially geared toward sea-anchors for Heaving to. The book recommends parachute anchor sizes for different types of boats.

 

With an anchor,it isnt just the weight that makes it stop,its the shape.

 

I have a friend who sailed from here to Europe. Along the way the boat started surfing or getting squirrely some which way so he tied a 1/2 qallon whiskey bottle to a long line and dragged that astern. Apparently it worked perfect and he was able to fine tune it by partly filling it with water so it didn't skip along the surface. Sam