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Old 01-09-2016, 08:15 PM   #58
waddenkruiser
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City: Southern Baltic
Vessel Name: Sømarken
Vessel Model: AMS 40'
Join Date: Jul 2012
Posts: 188
Anchor Rode Weight Bias

[QUOTE=Paul Swanson;402855]
Quote:
Originally Posted by manyboats View Post
Paul,
Very very good of you to prove my theory. Are you sure you're right and that there's no mistake?

Eric,
Yes, the results of the program have been checked and agree with other peoples calculations so I am about 95 % certain that it gives correct results. i notice the table in the second section isn't come through very readable. It looked OK in the preview but got scrunched up in the final version.

I have attached a word file that shows the table more clearly. I can easily run any other example that you want.

Paul
I‘ve finished my homework now. It was a nice engineers exercise and I will share the results although I realized that Paul has already given the answer. (Hope you will find additional input helpful.)
Recalculating Pauls data I’ve got exactly the same figures – thank you Paul, it gives me confidence in my calculation.

In the following tables you will find for 5 / 10 / 15 m water depth the maximum holding force and the potential energy stored in the rode, both for ultimate catenary situation (last chain link laying on the ground i.e. slope at anchor still horizontal) and for stiff i.e. straight rode situation (should be avoided).
It might be of interest to compare the data and the trends indicated:

100 m / 330 ft all chain 8 mm diameter, 0,9 lb/ft: High holding force and energy. Reasonable force margin between ultimate catenary and stiff rode, poor energy margin.
Ability to store energy is important if it comes to dynamic load situations! Ability to store energy increases with increasing water depth!

Chain 8 mm + 24 mm nylon rope combos (length 50+50 and 15+85 m): Both with reduced holding forces at same rode length and water depth, especially for the 15+85 m combo. Seems we have to pay penalty if we want to save weight which might not be of interest for typical “recreational trawler” but for sailing yachts.
Ability to store energy might be higher or lower compared to the all chain figures depending on the water depth. But the energy (safety) margin between ultimate catenary and stiff rode is increased significantly - the longer the rope portion the more the elasticity of the nylon is important with regard to energy!
Remark the reverse trend if we have a significant nylon part in our rope: ability to store energy increases in shallow water!

Chain 13 mm, 2,6 lb/ft + nylon, length 30 + 70 m (same weight as all chain above): Outperforming holding force and high energy figures. Reduced holding force margin, but reasonable energy margin.

Summary:
All chain vs. chain of same diameter + rope (reduced rode weight): the combos holding force is reduced at same rode length and water depth.
All chain vs. thicker chain + rope (same weight concentrated near anchor): the combo is outperforming with regard to holding force.
Will I change my all chain setup on our boat? No, …
Attached Files
File Type: pdf TF-all-chain-100m-8mm.pdf (16.6 KB, 34 views)
File Type: pdf TF-Combi-50-50-chain-8.pdf (16.5 KB, 21 views)
File Type: pdf TF-Combi-chain-8-15-85.pdf (16.5 KB, 18 views)
File Type: pdf TF-Combi-30-70-same-weight-as all-chain-8.pdf (16.6 KB, 17 views)
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Wadden
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