Let's say I'm anchoring
a sailboat (LOA = 28 foot) using a 15 kg Rocna
on an all-chain rode
in a bottom noted on the chart notes as a sand bottom.
A. My questions are:
(1) for how many minutes should I run the engine
in astern so as to be sure that the Rocna
embeds in the bottom and gives me reasonable confidence of staying put in, let's say, a 40 knot
(2) at what engine rpm
should I run the engine (further details below at C) when setting the anchor
(3) if the chart noted the bottom to be mud, would your answers to #1 and #2 differ?
B. Theory and Guidelines (if CF or any other group of expert practitioners could so agree!)
The Vryhof Anchor
Manual (2010 edition), hereafter VAM2010, presents what I take to be robust guidelines.
i. "Drag embedment anchors are generally installed by applying a load equal to the maximum intact load" (p. 41). The maximum intact for a temporary anchorage is defined (on p. 40) as 1.0 times the quasi-static load and/or 0.8 times the total load. Quasi-static load is defined (on p. 39) as "the load due to the swell, wind
and the frequency of the system" and total load is defined (also on p. 39) as the quasi-static load plus "the individual wave forces causing a high frequency motion". And adds "Generally the quasi-static loads will be equal to 50% to 90% of the total dynamic load". VAM2010 backs this up with a graph showing an anchor load curve with quasi-static load peaking at 2342 kiloNewton and total dynamic load peaking at 3895 kN.
ii. For a permanent anchorage (not a temporary anchorage, which is not further discussed), "the installation
load should be held for the period specified by the classificiation societies" (p. 41). The specified periods are listed in a table (also on p. 41): Lloyd's specifies maintaining installation
tension for 20 minutes; the American Bureau of Shipping
specifies 30 minutes; and Det Norske Veritas specifies 15 minutes.
iii. In mud/clayey soils, "the penetrating anchor disturbs the soil and the soil temporarily loses strength. With time, the disturbed clay reconsolidates to its initial shear strength, this takes from a few hours up to 1 month. ... On reloading, the parallel soil resistance gains strength, it takes a larger load to move the anchor again. ... Observation on anchors for drilling rigs and theoretical considerations for a 3 to 4 week consolidation time demonstrate a typical set-up effect factor = 1.5" (p. 41)
Professor John H. Knox reckoned, from his practical experience measuring rode
load with a strain gauge, a formula for estimating rode lode for a sailing vessel of a given LOA
and a given wind
speed: Load (rode) = 0.002 * LOA^2 * windspeed^2 with units of metres for LOA
, knots for windspeed, and kilograms.force for load.
For a LOA 28 ft boat
(8.53 m) and 40 knot
wind gust, Prof. Knox would estimate a rode load of 233 kg.f (about 513.7 pound.force, about 2.29 kN). Probably about right.
C. Practical considerations
Led Myne, my 28 footer, has a Yanmar
3GM30F spinning a fixed 3-blade prop. My best estimates for bollard pull astern are: Maximum (3600 rpm) 173 kg.f (381 lb.f, 1.7 kN); and about half of that at 3000 rpm
(which I regard as full throttle, while considering 3600 rpm as emergency
throttle). Led Myne carries a 5/16" BBB chain rode, with a Working Load Limit around 861 kg.f (1900 lb.f). So my ability to install an anchor is no more than 74% of the likely maximum rode load/maximum intact load - I cannot reach 80% of total dynamic load, nor 100% of quasi-static load.
Can I compensate for that inadequacy by longer time of applying installation load? Anyone know of any real evidence and research
Over to you as expert practitioners, especially the vendors and anchor makers among you. I know that one or more of you have designed and used devices to test anchors much more like my 15 kg Rocna than a Vryhof. I know that more than one of you have done considerable testing of anchors and have published your results.
Given the limited bollard pull of a sailboat auxiliary:
(1) for how many minutes should I apply astern thrust to set the anchor so I am confident of riding out a 40 knot squall?
(2) what engine rpm should I use (given that Yanmar
admits that the cooling
system of a 3GM30F cannot sustain more than 60 minutes at 3600 rpm)? and
(3) were I anchoring
in mud/clay/silt, how would your answers to #1 and #2 vary?
Al (my first post! Thanks for being gentle)