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Old 28-03-2009, 08:21   #1
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Computer anchor rode model

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Originally Posted by thinwater View Post
A sailor did an excellent job of computer modeling rode reactions to wind. The links are below. While computer models have MANY limitations, this one clearly pointed out one thing; when an abrupt gust hits a boat on an elastic rode, the boat moves backwards and then the rode has to resist both the wind and the energy of stopping the boat. The resulting force is nearly independent on the length of the rope; the longer the rope, the faster the boat moves, but the more rope there is to stop it. The conclusion: an elastic rope doesn't soften the blow. With either chain or rope the answer is the same; the gear will feel about 2x the peak wind force. Odd but sensible. The one condition of the model is that the wind is still blowing when the boat comes up tight, which sees reasonable. A chain rode, with the catenary, reacts differently, but I suspect his model may over state the surges, since in reality we know there is enough damping that the boat does not bounce in the manner his models suggest.

Waves are a different matter because the time period is short; the boat won't begin to move backwards signifigantly before the wave is past. A long elastic rode or a very long chain will damp them almost completely; some effect from the friction of the line through the water. However, if the boat is in relatively shallow water, there is big chop, and the scope is short, a boat on chain can hammer against the chop with no damping. The answer is a snubber long enough for the waves only. This will vary with the setting, but might be as much as 60 feet (4 foot waves) or as little as 15 feet (1 foot waves). Since the chain catenary will do something, perhaps 60% ofthis figure is a minimum chain snubber length. As stated, this is most important where the chop is material.

Not my theories, but the logic is sound and seems to reflect experience, at least in aproximation. It also explains why several schools can all think that they are right... because under specific circumstances, they are.

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Rode - Dynamic Behavior

Rode - Static Behavior

Rode - Dynamic Behavior
This last one has a lot of details in the photos I could find fault with. I'm sure you will spot them.

But all criticisms aside, lets respect the work it took to put the links together and post it. It is thought provoking.

I hope you like math!
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Old 28-03-2009, 09:25   #2
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I didn't get your interpretation of what he wrote. What I see is that he doesn't write that chain and rope are the same. Rope tension goes up to 2x gust force, and chain goes up to 4x gust force, so loads on the anchor are double with a chain rode as opposed to a nylon rode in his model. His model for the chain also shows the catenary is lost in the peak load so the anchor is pulling up at the same angle as the nylon rode, and this is with an 11:1 scope. Downside for the nylon rode is that it is always pulling up.

John
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Old 28-03-2009, 10:51   #3
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You right, but I can explain...

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Originally Posted by cal40john View Post
I didn't get your interpretation of what he wrote. What I see is that he doesn't write that chain and rope are the same. Rope tension goes up to 2x gust force, and chain goes up to 4x gust force, so loads on the anchor are double with a chain rode as opposed to a nylon rode in his model. His model for the chain also shows the catenary is lost in the peak load so the anchor is pulling up at the same angle as the nylon rode, and this is with an 11:1 scope. Downside for the nylon rode is that it is always pulling up.

John
Posts are always too short for a full explanation. I wanted to focus on trends without enough explanation.

I believe his model did not include the effect of the catanary dragging up through the water and other damping effects. For one thing, the model show the boat bouncing, which we know does not happen. So, damping is neglected. Yes, chain would still see somewhat more impact. I am not a chain fan or a rope fan. We cannot tell from the model how much difference there is, IMHO. The accuracy is lacking.

The effect of doubling the force due to surging is really what I took away from it. I used to have a very light boat (1300 pounds x 27 foot catamaran) and the effect of sliding back and bungy cord bouncing was dramatic in thunderstorms. I believe that figure.

Also what is important is the observation that a nylon rode does not damp a gust that lasts longer than the stretching time period. It makes it clear that having a lot of rope out will not reduce the force below a certain point. If the excess allows the boat to horse around a bit and get sideways while it is moving backwards, the wind gets a better shot and gain in force would be even greater.

Intersting.
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