Setting the anchor

[donradcliffe]

As Andrew points out, if you want to determine which RPM in reverse corresponds to which wind load, just reverse into the wind on a windy day and see what RPM holds you steady.

With the new boat, if I backed down at full power with both engines something would probably break...

[estarzinger]

^^

Despite it sounding so obvious, I am not sure this is correct.

Props function differently (less well) with zero water flow (as in a bollard pull or anchor setting) than they do when they can generate water flow over them.

I don't know enough about it to know how big the difference is . . . Its a specific case, but I know my auto prop hates zero flow because the blade pitch is set by flow.

[Andrew Troup]

Evans

Good point and one that some will not be aware of. However I think both donradcliffe and I did take this into consideration.

In both cases (anchored, and motoring in reverse as a test) the speed through the water is intermittently zero (don spelled it out when he said "See what RPM holds you steady" which I took to me "stopped")

I didn't spell out the stationary bit, but I think I covered that by saying the motor in our case could apply "at least" that much force (given that we were able to manoeuvre after aborting attempts to enter the slip, which means at times we were momentarily stopped)

- - - -
on the broader topic, I just came across this:

in a YM article by Paul McNeill in January 2007, ... he described the use of an Anchorwatch linked to a laptop, with simultaneous input from wind instruments, to determine forces generated in the anchor rode. His boat was a Sigma 33C, for which he found that the force generated at maximum revs astern exceeded that of the wind up to about half way between force 8 and 9.

Anyone with that issue could presumably look it up and find details of engine size, prop style and diameter etc..

[noelex 77]

Originally Posted by donradcliffe

As Andrew points out, if you want to determine which RPM in reverse corresponds to which wind load, just reverse into the wind on a windy day and see what RPM holds you steady.

With the new boat, if I backed down at full power with both engines something would probably break...

I do not think that is correct.

I think the major difference is at anchor the boat is moving. The anchor needs to able to withstand not the average force but the maximium force. At anchor as the boat sheers about there is significant momentum that needs to be opposed by the anchors grip on the bottom. There are other factors that also contribute to peak loads being much higher than the average loads, including gusts, wave action, and the added windage of a boat that is sideways. These elements will at some stage combine.

Some stretch in the snubber and (at lower wind speeds) some catenary from the chain help reduce the peak loads at the anchor, but this load is still significantly higher than the average, which is all a boat in reverse is required to overcome.

[FlyingCloud1937]

As soon as a boat in reverse hits stall we begin to approach a 100% prop slip. At 100% prop slip we have almost no real torque, because we have cavitation, it goes for the same in forward.

At least in forward we have a design factor in the screw, so it betters by more then a marginal degree.

When setting an anchor we depend on scope to set the flukes of our anchor. So do we depend on scope to hold our anchor, in a storm.

3-1 scope is good for about 10 knots, in a good bottom. 4-1 is my minimum, for a day anchor while on-board. 5-1 if I plan on leaving for the day.

6-1 if I'm overnighting and the weather is predicted flat, 7-1 if I'm unsure of the weather.

And 10-1 on anchor watch in storm force...all 600 feet if I've lost power and am blowing on a windward shore.

I have 300 feet of 5/8 chain, backed by 300 feet of one inch rope.

Lloyd

In over 30 years on the hook, I've learned what I need

[noelex 77]

Originally Posted by FlyingCloud1937

3-1 scope is good for about 10 knots, in a good bottom.

Holding does go up significantly as the scope is increased, but that is very pessimistic view.
Some anchorages do force you to adopt shorter than ideal scopes.

If it possible setting the anchor at longer scope (say5:1) often helps considerably. Once the anchored is buried and set deep it has much better holding potential and the scope can be shortened with greater holding power and reliability than would normally be the case.

[Andrew Troup]

Originally Posted by FlyingCloud1937

As soon as a boat in reverse hits stall we begin to approach a 100% prop slip. At 100% prop slip we have almost no real torque, because we have cavitation ....

Hmmm.... Prop slip, it seems to me is a theoretical construct rather than an accurate depiction of reality. The water in the vicinity of the prop is moving past the prop even though the surrounding water is not, (a bit like a jet unit without the housing), so the resulting cavitation is peripheral rather than crippling.

I'm also not sure I can agree with "almost no torque" (by which I presume you mean, bollard pull) If that were true, it would scarcely be possible to drag an anchor which was set, using the engine in reverse.

And we would not have been able to reverse into our slip against 45 knots plus, remembering it took several tries, and each time we started a fresh attempt we had to reverse, not just at 100% slip, but at maybe 120% (being blown powerfully forwards whilst applying maximum reverse thrust)

[FlyingCloud1937]

Originally Posted by noelex 77

Holding does go up significantly as the scope is increased, but that is very pessimistic view.
Some anchorages do force you to adopt shorter than ideal scopes.

If it possible setting the anchor at longer scope (say5:1) often helps considerably. Once the anchored is buried and set deep it has much better holding potential and the scope can be shortened with greater holding power.

First,

I hope setting anchor scope isn't the issue, that's how I choose. Second I'd rather be holding tight while others drag, then to be dragging others with me.

Scope is everything, only defined by angle of the fluke to stock, then compensated by the length of chain, then by the size/weight.

3-1 being driven by reverse thrust can/and does lead many to spend to much time/embarrassment trying to set the hook in a crowded anchorage.

When stretching 4.1 with low power, then backing off can make you look like a seasoned vet. Then slowly let out 5-1.

Lloyd

[FlyingCloud1937]

Originally Posted by Andrew Troup

Hmmm.... Prop slip, it seems to me is a theoretical construct rather than an accurate depiction of reality. The water in the vicinity of the prop is moving past the prop even though the surrounding water is not, (a bit like a jet unit without the housing), so the resulting cavitation is peripheral rather than crippling.

I'm also not sure I can agree with "almost no torque" (by which I presume you mean, bollard pull) If that were true, it would scarcely be possible to drag an anchor which was set, using the engine in reverse.

And we would not have been able to reverse into our slip against 45 knots plus, remembering it took several tries, and each time we started a fresh attempt we had to reverse, not just at 100% slip, but at maybe 120% (being blown powerfully forwards whilst applying maximum reverse thrust)

Andrew,

Go tie yoursef to a dock, first at 1500 rpm, then at 2000, then at 2600 rpm, watch your pyro-meter(s), log each. Now go out and run your boat in free clear water at each speed and report back.

At 100% prop slip you have so much oxygen in the water that it will erode the bronze.

Lloyd

[Andrew Troup]

Originally Posted by noelex 77

I do not think that is correct.

I think the major difference is at anchor the boat is moving. The anchor needs to able to withstand not the average force but the maximium force. At anchor as the boat sheers about there is significant momentum that needs to be opposed by the anchors grip on the bottom. There are other factors that also contribute to peak loads being much higher than the average loads, including gusts, wave action, and the added windage of a boat that is sideways. These elements will at some stage combine.

Some stretch in the snubber and (at lower wind speeds) some catenary from the chain help reduce the peak loads at the anchor, but this load is still significantly higher than the average, which is all a boat in reverse is required to overcome.

Hmmm, Noelex ... by that line of reasoning, I could make a plausible case for heavy chain.

The "chain of the Light Brigade" is light because (they claim), under steady high tension, even a heavy chain will be virtually straight.

It is implicit from their reasoning, that it's only once the chain is straight (and a steady state load is established) that the anchor starts to respond by dragging. Because up to that instant, the rate of change of force is very different indeed for a heavy chain from a light chain.

(The slope and the more particularly the curvature [second derivative] of force versus time)

The heavy chain will spread the application of the same inertial forces out over a considerably longer period, and the loading graph will be a progressive curve rather than the hockey stick of a light chain.

Ergo if the rate of application of force mattered, then heavy chain would help considerably.

Whereas it seems to follow from what you are saying that the rate of application of force does matter, in fact that it's the main thing. Which seems inconsistent with the claims of the light chain brigade.

Which side are you on, mate?

- - - - -

This is a side issue, if the OP's question was capable of being answered. I just wanted to open a chink in the revised conventional theory of catenary, a sort of opening sighting shot preparatory to a few broadsides (not on this thread, I hasten to add)

I interpret the OP as asking "How much do I have to crank up the revs to simulate a constant wind at anchor, coming directly bow on, in flat water, of x knots"

Because otherwise, if the vessel in question was ranging around to an indeterminate extent, snubbing, dipping her bows into waves, and tugging, the question has no answer.

[noelex 77]

Originally Posted by Andrew Troup

It is implicit from their reasoning, that it's only once the chain is straight (and a steady state load is established) that the anchor starts to respond by dragging. .

I would be in trouble if this was the case

The "little chain" theory only proposes that the effect of catanery disappears in strong wind. The chain remains tight even in the lulls so chain weight plays little role in averaging out the forces. Stretch in snubber takes over this role, but the anchor is still subject to a variable load.

However a good anchor does not drag it just buries until its holding power matches the peak forces. Once in this equilibrium the anchor holds steady.

[MarkJ]

Originally Posted by FlyingCloud1937

3-1 scope is good for about 10 knots, in a good bottom. 4-1 is my minimum, for a day anchor while on-board. 5-1 if I plan on leaving for the day.

6-1 if I'm overnighting and the weather is predicted flat, 7-1 if I'm unsure of the weather.

And 10-1 on anchor watch in storm force...

Would love to find those unpopulated bays you must find.

Those scopes are really quite unreasonable for most anchorages I have been in, worldwide.

Might be fine if you are anchored furtherest out in the Bahamas in 7 feet of water, but in the Med, BVIs, Caribbean, Pacific where anchoring is between 10 and 25 meters depth, 30 to 70 feet, and lots of other boats, then you are putting out too much and the last half of yours is rope so your boat would float around the anchorage like an unguided missile.

The naval ships in sailing days had a standard of 5:1 on all rope rode. I don't know why it went up to 7:1 for rope. I certainly don't know why it needs to be 5:1 or 7:1 for chain, unless very windy. And then when the wind dies in the butt one should pull in their extra scope.

Mark