Forces on an anchored / moored vessel??

[craigsmith]

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Just had a quick run through this site. Very interesting! Needs more detailed study, but already I'm re-thinking the critical effect of the rode (type and length) in keeping the pull horizontal and minimising shock loads. The principle I already know, but the simulations gave me a numerical feel for it. Many thanks.

[/ QUOTE ]Fraysse's site is excellent, but not the final say in the matter if you are interested in the details. Because the conclusions happen to be "right", I for one am happy to refer people there. However, his models get steadily less and less accurate as you progress through the site.

Without going into detail, the conclusions tend to unduly shun chain, particularly for larger boats. For something like your boat, bluedragon, they are probably spot-on. The only consideration I would look at is the propensity of your boat to sail at anchor, and whether long lengths of nylon or even polyester would exacerbate this.

 

[bluedragon]

Thanks for the answer. That's helped me understand how you go about it.

 

[GMac]

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Note your last assumption is deeply flawed. The resistance from chain on the seabed is trivial. Apply any load to the chain attached to your boat, and the anchor immediately feels it.

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Your assumption about his assumption is wrong sorry Craig.

I say assumption as it is obvious you've never anchored on chain only or dived by the looks. Err.. Should that be chained rather than anchored?

In a good anchor system the anchor does nothing at all for quite a long time.

Put that bloody calculator away.

 

[jimbaerselman]

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The only consideration I would look at is the propensity of your boat to sail at anchor, and whether long lengths of nylon or even polyester would exacerbate this.


[/ QUOTE ] At 12m, fairly heavy displacement (10 ton), fin keel, my sailboat starts to sail at around 20kts wind, and is pretty active at 30kts.

I've tried all chain rodes, and I've tried all rope (nylon) rodes in a variety of depths to see if either makes any difference. There was none noticeable. Why? I assume that damping of the swing is supplied mainly by the drag of the rode as it shifts sideways through the water, rather than by dragging the chain along the bottom. And the jerk with a rope rode was much lighter.

But the 'sailing' problem is best dealt with either by using a forked moor, or on some boats, raising a small steadying sail along the backstay.

 

[bluedragon]

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The only consideration I would look at is the propensity of your boat to sail at anchor, and whether long lengths of nylon or even polyester would exacerbate this.

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Well the most wind I've experienced at anchor is about 25-30kn in this boat and she sits pretty much rock steady (though I've been through tropical waves at anchor in the Caribbean on big Beneteaus 40+ kn....so I know what you mean). In the latter case we usually had two anchors out in a "V". This helped, and that's what I'd probably still do. I really try to avoid anchoring in high wind situations and it's usually possible in my part of the world with the good forecasts we get. So rather than use a big anchor, sized for storm conditions, I prefer the approach of using one that's easy to handle, then if required lay out another. You and others may or may not agree with this, but with no mechanical assistance, my back tells me this is a good solution! Hence my interest in loads and not over-specifying the tackle (rode and anchor) beyond what's needed. Small, light, reliable anchor systems are the holy grail for me...the less iron, Al or whatever I have to lug around the better.

 

[craigsmith]

Sure, sounds fine, although I think a 10Kg anchor is still perfectly able to be handled. You could get away with less though.

On laying out more than one anchor to increase performance.

 

[silver-fox]

Propensity to sail on anchor..

Has anybody tried chucking a bucket (or similar) over the stern on a rope to see if that quieten things down?

 

[GMac]

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Propensity to sail on anchor..

Has anybody tried chucking a bucket (or similar) over the stern on a rope to see if that quieten things down?

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It's better about 1mt below water on the anchor rode itself. A soft parachute type is better than a bucket. Not a monster drouge but one with a 12-18" opening.