Observation and a question about how tides affect a boat at anchor.

[Bosun Higgs]

I am fairly sure it is nothing to do with wind: have had this often whilst anchored in total glassy calm waiting out a tide. As I said earlier, I've always believed it to be because the drag of the tide/current on the anchor warp/chain to be greater than the drag of a hull bow-on to the same current. If you anchored using a wire with minimal water resistance it wouldn't happen.

Simple basic Newtonian mechanics ( Newtons First law of Motion) would tell you that this cannot be true. Leaving aside the implausibility of the drag on the ropes to be greater than that on two hulls, the drag on both is in the same direction . The weight of the rope or more likely chain could cause a catenary but at no point would this move the boat forward over the anchor .

If the boat is going forward into the tide then there simply has to be a force up the chuff pushing it forward and that can only be wind.

 

[scrambledegg]

If the boat is going forward into the tide then there simply has to be a force up the chuff pushing it forward and that can only be wind.

Agree with that. However the harder thing to explain is this comment from the OP, which nobody has yet picked up on:

"The strange thing is that as the stream begins to slow down nearer HW, the boat slowly moves backwards until the snubber bridle is hanging vertically, even though we still have the breeze."

So, taking the wind-pushing-the-boat-against-the-tide argument to its logical conclusion, you'd expect the boat to be straining even harder against its aft-pointing bridle as tide slackens.

 

[JohnGC]

Agree with that. However the harder thing to explain is this comment from the OP, which nobody has yet picked up on:

"The strange thing is that as the stream begins to slow down nearer HW, the boat slowly moves backwards until the snubber bridle is hanging vertically, even though we still have the breeze."

So, taking the wind-pushing-the-boat-against-the-tide argument to its logical conclusion, you'd expect the boat to be straining even harder against its aft-pointing bridle as tide slackens.

But it does stack up in my model. Slower current = less lift = drop back.

 

[jwilson]

Simple basic Newtonian mechanics ( Newtons First law of Motion) would tell you that this cannot be true. Leaving aside the implausibility of the drag on the ropes to be greater than that on two hulls, the drag on both is in the same direction . The weight of the rope or more likely chain could cause a catenary but at no point would this move the boat forward over the anchor .

If the boat is going forward into the tide then there simply has to be a force up the chuff pushing it forward and that can only be wind.

I am not saying the boat is uptide of the anchor: that would be impossible. When I've had this it has been with VERY long warps in deepish water with substantial tide - usually in a flat calm. Everything about it is counter-intuitive, but it has happened to me and others.

The first time it happened (anchor warp heading from bow back under the boat) I thought it meant I could lift the anchor and make way. Lifted the anchor and it was immediately clear from a shore bearing that the anchor had been holding, and that un-anchored I was going the way I didn't want to at 3-4 knots.

There MAY be different currents at different depths but in some places where it has happened to me (eg off the Culver Sands) I don't think there will be massive differences - on the flood zillions of gallons of Bristol Channel is all heading upchannel at once.

Logically I suppose I could have experimented by adding drag to the hull by towing a bucket or two (and I suspect it would have made the warp lie normally), but when you've let out lots of warp to get an anchor to hold in a strong tide you really want just to stay where you are, and if that is happening all is well.

 

[RichardS]

Agree with that. However the harder thing to explain is this comment from the OP, which nobody has yet picked up on:

"The strange thing is that as the stream begins to slow down nearer HW, the boat slowly moves backwards until the snubber bridle is hanging vertically, even though we still have the breeze."

So, taking the wind-pushing-the-boat-against-the-tide argument to its logical conclusion, you'd expect the boat to be straining even harder against its aft-pointing bridle as tide slackens.

But it does stack up in my model. Slower current = less lift = drop back.

Absolutely .... which is why I liked John's theory even though it all sounds so counter-intuitive. Apart from that very good point, if it was the simpler explanantion that the wind is stronger than the current that just doesn't seem feasible as surely the boat would flip around as the wind continued to push it upstream. The snubber line was streaming tightly afterwards and I could hardly pull it out. For the boat to remain in that kind of unstable equilibrium for two hours would be very suprising. You could argue that a cat with a bridle snubber does have an innate stability which might hold it head to current and overcome the apparent instability of the situation but I don't see how this could apply to a monohull with the line drawn tightly aftwards down one side and yet lying bows to stream. If this as simply the wind being stronger than the current then surely the stern of the boat would flip around until the bows were pointing into the wind as the tight snubber line down one side is presumably highly unstable under normal circumstances?

Richard