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Old 02 August 2012, 07:02   #11
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Something to think about and i always say you can never have good enough anchor or ground gear and length of rope ,, but. ! Correct me if I am wrong
When many of the recommended anchoring calculations were made they were done many years ago when most leisure boats were wood heavy displacement boats /yachts calculated using old type admiralty pattern or fishermans type anchors using natural fibre rope in sheltered severe storm conditions ,
Modern planing hull shapes using high hold anchors (bruce )with nylon rope perhaps can get away with less .
I am Not saying that the 3x depth using chain & 5x using just rope doesn't still stand in most cases but when the original Bruce anchors were first designed they recommended using just a short length of chain for chaffing/ wear purposes on the sea bed .
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Old 02 August 2012, 18:23   #12
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[QUOTE=
like the breaking strain of 12mm Anchorplait is already 3.3 tonnes, and I suspect a bow eye will let go at much less than that in very rough conditions if the boat is really jerking on the line, anybody care to hazard a guess at what a typical bow eye (usually about 8mm stainless loop) would fail at?



Ta [/QUOTE]

The tensile strength of SS316 is 515MPa, that is, a bolt with a cross-section of 1 square meter will hold 515000000N force. The effective cross-section of a 8mm bolt is 36 square millimeter.
Multiply these, and we get approx 19000N or 1,9Tons. A u-bolt has two legs, so it should hold nearly 4 tons, provided the pull is at the right angle. I have not made any allowance for distortion of the arch; on one hand, the bending will cause it to harden so it may not deform, on the other, any distortion may weaken the bolt
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Old 02 August 2012, 18:50   #13
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Quote:
Originally Posted by geir View Post
The tensile strength of SS316 is 515MPa, that is, a bolt with a cross-section of 1 square meter will hold 515000000N force. The effective cross-section of a 8mm bolt is 36 square millimeter.
Multiply these, and we get approx 19000N or 1,9Tons. A u-bolt has two legs, so it should hold nearly 4 tons, provided the pull is at the right angle. I have not made any allowance for distortion of the arch; on one hand, the bending will cause it to harden so it may not deform, on the other, any distortion may weaken the bolt
Thanks for this, I assumed the line was weaker than the U bolt too, just didn't have any data or calculations to support that.
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Old 02 August 2012, 20:34   #14
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Its been a long time since I did calcualtions of breaking loads, but I reckon you would pull the metal fixnig out of the GRP of the boat before it failed ....
I figured that too ... sorry I should have been clearer, when I said breaking strength of the eye I meant breaking strength of the whole thing fitted into a GRP hull, not specific to the failure of the metalwork. However the metal bit is going to snap off at somewhere between 3 and 4 tons so that's the maximum and it's probably less.

It's all pointing towards anything more than 12mm being a waste of time anyway, which answers the question of which to buy.

There was an interesting anchor test in some magazine a few years back where they tested breakout force in a range of anchors and also estimated the peak loads on the anchor line on something like a 40' yacht in storm force winds. I wonder if anybody has ever done a similar thing with light planing hulls.
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Old 03 August 2012, 06:40   #15
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Haha, you've obviously never weighed your VM Will, mine weighs 1600kgs......:-)

I was surprised at how heavily built they are.

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Which I've always thought is an anomaly.

Ours is 1300

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For a 7m vessel it is recommended that you have:
9kg with 10m of 8mm chain and 20m of 12mm warp (30m of overall length). This may be reduced by up to 20% (but not the length) ( I would recommend the reduced diameter as its only a light RIB)
Usually people recommend 1m chain/1m vessel + 1m. I think we have 10mm chain though. And then lots more than 20m warp!
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Old 03 August 2012, 08:07   #16
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[QUOTE="gotchiguy"]

Which I've always thought is an anomaly.

Ours is 1300
/QUOTE]
Likewise, we weigh in at 1100 so Jigsaw does seem bit lardy.
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Old 03 August 2012, 14:04   #17
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Which I've always thought is an anomaly.

Ours is 1300

Usually people recommend 1m chain/1m vessel + 1m. I think we have 10mm chain though. And then lots more than 20m warp!
I've quoted the MCA guide. Doesn't mean it is right though!
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Old 03 August 2012, 16:12   #18
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Anchors-Rocna-demos-and-testing!
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