Sunday evening physics question - prop thrust

[NormanS]

Popcorn at the ready!

 

[RichardS]

Seems to me there is some confusion here. A snubber just absorbs the shock in the chain as all the scope is pulled tight. The total loading is the same because the other end of the snubber is connected to the anchor. It just applies it to the anchor more gently but can still pull the anchor out. In some case it makes it easier to dislodge because you get a kinetic action taking place. In effect you end up catapulting the anchor out of the sea bed.
Of cause non of this would happen if we anchored properly in the first place. Its the chain that should hold you in position, the anchor on the end is incidental.

Do you know a chap called KellysEye by any chance?

Richard

 

[jdc]

...you write about motoring very slowly. You may well know, but you shouldn’t run your engine at low power settings continually, or better still not at all. ... bore glazing and high wear.

Thank you for your kind remarks. I know I 'shouldn't' - according to some scuttlebutt - run my engine at low revs for a long time, but I do, and have done so for >1000 hrs at a steady 1400 rpm without apparent problems.

 

[Neeves]

Seems to me there is some confusion here. A snubber just absorbs the shock in the chain as all the scope is pulled tight. The total loading is the same because the other end of the snubber is connected to the anchor. It just applies it to the anchor more gently but can still pull the anchor out. In some case it makes it easier to dislodge because you get a kinetic action taking place. In effect you end up catapulting the anchor out of the sea bed.
Of cause non of this would happen if we anchored properly in the first place. Its the chain that should hold you in position, the anchor on the end is incidental.

I am sure you are correct and consequently please take your anchor off - its a waste of space and money. Give your anchor to an unbeliever - of which there are thousands. Please any time you anchor without your anchor raise a red anchor flag, it seems the most appropriate - and then other people will keep clear.

Now you can also set yourself up as a consultant and make an absolute fortune as your practice would save a lot of effort (and weight and money) in the commercial marine sector. Sadly Bruce anchor and Vryhof will need to find a new business model - as there will be no need for anchors for oil rigs. How silly we have all been.


It all beggars belief.

Weeps quietly in a corner.

Jonathan

 

[Neeves]

Of cause non of this would happen if we anchored properly in the first place. Its the chain that should hold you in position, the anchor on the end is incidental.

You, Kelly' Eye and.........

Yachting Monthly!

https://www.yachtingmonthly.com/gea...31.1129475317.1571998607-845354258.1571998607

Specifically I quote

"but when talking about anchoring it is always worth bearing in mind that it is the catenary weight that holds the vessel in position."


You are not alone

Don't these people like anchor makers?

Beggars belief

Is there enough popcorn to go round?

Jonathan

 

[thinwater]

Seems to me there is some confusion here. A snubber just absorbs the shock in the chain as all the scope is pulled tight. The total loading is the same because the other end of the snubber is connected to the anchor. It just applies it to the anchor more gently but can still pull the anchor out. In some case it makes it easier to dislodge because you get a kinetic action taking place. In effect you end up catapulting the anchor out of the sea bed.
Of cause non of this would happen if we anchored properly in the first place. Its the chain that should hold you in position, the anchor on the end is incidental.

None of that makes any sense as written. Perhaps the energy is the same. A nylon rode can increase movement, but not a snubber of up to 50 feet (I've tested at diffeernt lengths to test this hypothesis). A nylon rode will reduce peak tension by several times, and yawing can be addresssed by other means (chafe is still an issue).

The chain provides catenary and some reduction in swing at low wind strength, but in no sense does it "hold you in position" in a storm. If you believe that, just once, take the anchor off your chain in 30 knots. I dare you. As a tester, I mean dares sincerely. I call it testing. If you will not take a dare, in the interests of group knowledge, we will presume you are not serious. BTW, I have anchored with only-chain to measure the drag force. Obviously, it is trivial, something you can pull with one hand.

I've dragged many hundreds of times in testing, but never at night. It was an undersized Delta on a new-to-me boat that really triggered my interest in anchoring, though I had always been interested and had already learned much.

 

[doug748]

..

Yachting Monthly!

https://www.yachtingmonthly.com/gea...31.1129475317.1571998607-845354258.1571998607

Specifically I quote

"but when talking about anchoring it is always worth bearing in mind that it is the catenary weight that holds the vessel in position"

Jonathan

To be fair, I think he means the catenary pulls the boat towards her anchor and in that sense it does hold the vessel in position. Lacking weight in the rode is why boats lying mainly to rope tend to roam over an anchorage in light weather.

 

[thinwater]

To be fair, I think he means the catenary pulls the boat towards her anchor and in that sense it does hold the vessel in position. Lacking weight in the rode is why boats lying mainly to rope tend to roam over an anchorage in light weather.

"... in light weather...." I think this thread was about storms.

a. A kellet will solve that in a jiffy. I attach a loop of chain with a prusik sling. comes right over the rollers (my trimaran). A non-problem, although many never figure it out.

b. In a storm the chain is off the bottom, as is the kellet, and the only things that will stop yawing are proper balance or two anchors. I've also had all chain (boat in avitar). I've watched freighters yaw all over. It's just slow.

 

[doug748]

"... in light weather...." I think this thread was about storms.

.

There is no kind way of putting it, you are mistaken

 

[thinwater]

There is no kind way of putting it, you are mistaken

That's a point of view. I inferred, and did most on the thread I believe, that serious anchor testing is what you do if you expect a storm, not 10 knots. That's how most poster's seem to have taken it.

 

[doug748]

Feel free to post in whatever way suits you and I will do the same.

The OP's question was not about types of anchor, anchor testing, rope or chain. It was not about snubbers, kellets, riding sails or bridles. It was not about any of the red herrings that have been trailed, in particular it was not about storms.

Not that I mind - just don't criticise my post because it does not chime with a theme that exists only in your noggin.

 

[Neeves]

To be fair, I think he means the catenary pulls the boat towards her anchor and in that sense it does hold the vessel in position. Lacking weight in the rode is why boats lying mainly to rope tend to roam over an anchorage in light weather.

To be clear - I think he might have rephrased his script.

Jonathan

 

[thinwater]

Feel free to post in whatever way suits you and I will do the same.

I'm sorry I offended. It is a watercooler conversation and everyone gets challenged. No big The OP's question was not about types of anchor, anchor testing, rope or chain. It was not about snubbers, kellets, riding sails or bridles. It was not about any of the red herrings that have been trailed, in particular it was not about storms.

Not that I mind - just don't criticise my post because it does not chime with a theme that exists only in your noggin.

I'm sorry I offended. I'm not offended. It is a water cooler conversation and everyone gets challenged. There is also drift, with dozens of themes in one thread. No big deal.