CHAPTER 1 ANCHORING Forces Affecting an Anchored Vessel

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1 CHAPTER 1 ANCHORING 1.1. Forces Affecting an Anchored Vessel According to the Ecuadorian Navigator Officer Guidelines, anchorage site selection involves two stages, first to take into account all forces that may make the anchor drag or dislodge, and second to account for the area characteristics to ensure the vessel is anchored. Ring Shank Fluke Crown Stock FIGURE1. PARTS OF A DANFORTH ANCHOR. In general, all modern anchors have four parts in common (figure 1): one or more flukes to contact and dig into the seafloor; a shank, the shaft or stem that is pulled to set the fluke; a 4

2 crown that connects the fluke to the shaft; and the anchor ring, which attaches the anchor to the rode 1 (Poiraud, A., Ginsberg-Klemmt, A. and Ginsberg-Klemmt, E., 2008). The external elements that affect an anchored vessel are: wind, wave, tides and the currents present in the anchorage area. The effect of those elements that usually act in combination are forces transferred to the anchor. The estimation of these factors is not a simple task, but the force due to the pressure of wind and current on the vessel is relatively straightforward to approximate, while it is much more difficult to determine the intermittent load caused by wave action (Poiraud, A., Ginsberg-Klemmt, A. and Ginsberg-Klemmt, E., 2008) Winds Gusts of wind cause a vessel to sheer back and forth on its rode, falling off first one way and then the other. The bow is blown off until the rode comes tight, snubbing the bow back into the wind. The boat them surges forward, responding to the weight and elasticity of the anchor rode, until the next gust blows the bow off once more (Poiraud, A., Ginsberg-Klemmt, A. and Ginsberg-Klemmt, E., 2008). The load induced by the force of the wind on an anchored vessel depends on two factors: the wind speed and the exposed surface area of the vessel. Measuring the wind speed is straightforward; estimating the effective exposed surface area is more difficult. The vessel s length, beam, and height above the waterline, as well as the vessel design influence the estimate of the exposed surface (Poiraud, A., Ginsberg-Klemmt, A. and Ginsberg-Klemmt, E., 2008). Loads caused by wind on vessels can be estimated by conducting a wind-tunnel test. The American Boat & Yacht Council (ABYC) has developed the table 1 of the load caused by wind and takes into account a average surface area of an anchored vessel. These calculations are conservative estimates (Poiraud, A., Ginsberg-Klemmt, A. and Ginsberg- Klemmt, E., 2008). 1 Rode is used to define a cable, chain, rope or a combination of each used to link the anchor to a vessel. 5

3 TABLE 1.- ESTIMATION OF HORIZONTAL LOADS CAUSED BY THE WIND (ABYC calculations adapted by Poiraud, A., Ginsberg-Klemmt, A. and Ginsberg-Klemmt, E., 2008). Length Boat Dimension (ft./m) Bea m Motorboat Beam sailboat Beaufort 7 (30 knots) Wind Loads (N) Beaufort 9 (45 knots) Beaufort 11 (60 knots) 15/4.5 6/1.8 5/1.5 1,100 2,250 4,500 20/6.0 8/2.4 7/2.2 1,650 3,250 6,500 25/7.5 9/2.75 8/2.4 2,250 4,450 8,900 30/9.0 11/3.35 9/2.75 3,200 6,350 12,750 35/ / /3.0 4,100 8,200 16,400 40/ /4.3 11/3.35 5,500 11,000 22,000 50/ /5.0 13/3.95 7,300 14,500 29,100 60/ /5.5 15/4.5 9,100 18,000 49,100 69/ /6.0 17/5.2 1, ,000 49,100 82/ /6.7 19/5.8 1, ,500 64, Waves The kinetic energy accumulated by the vessel caused by waves can generate a peak load on the ground tackle of up to several tons when the anchor rode becomes tight. TABLE 2.- KINEMATIC ENERGY ACHIEVED BY A VESSEL DUE WAVE INTERACTION (Poiraud, A., Ginsberg-Klemmt, A. and Ginsberg-Klemmt, E., 2008). Weight of Boat (Kg) Kinetic Energy (Joules) 0.5 Knt 1 Knt 1.5 Knt 2 Knt 3, , , , , , , , , , , , , , , , , , , , , , ,000, , , , , ,500, , , , , ,000, , , , ,166, Wave action causes the pitch and roll movement of an anchored vessel, this continuous actum results in a backward and upward movement of the vessel s hull, and the anchor rode comes tight and slows down the vessel s surge. This motion can result in broken anchor-chain 6

4 connectors and bent anchor shanks. To prevent these failures, seamanship guidelines suggest that the energy related with this type of force be absorbed by catenary in the rode ensuring the presence of loads on the anchor will be decreased. Hence, having plenty of sag in the rode decreases shock loads and helps the anchor remain embedded in the sediment by lessening the angle between the rode and anchor as the loads increases. In severe situations the angle becomes positive as the weight of the rode is overcome by the tension. This positive angle creates a force component acting on the anchor to pull it out increasing until the catenary becomes a straight line, ultimately dislodging the anchor. If the catenary allows vessels to move a some distance before the chain tightens, the associated force will be reduced proportionally to the distance achieved (Poiraud, A., Ginsberg-Klemmt, A. and Ginsberg-Klemmt, E., 2008). For example if we have kinematic energy (table 2) accumulated by a vessel of 5,000 Kg mooring at 1.08 m/sec and if the vessel moves 2 meter until the chain become tight, the final force load to the anchor could be calculated using the formula that follows: F= Kinetic Energy/dddddddddddddddd F= 2,915.53/2 ; F=1,458 N. For practical purposes, the length of the chain used for anchoring varies between 3-8 times the averages depths present in the anchoring site. This extra length, known as scope, provides for this movement and keeps the angle between the anchor and the rode small Currents Loads caused by currents are relatively insignificant in many protected anchorage areas, but deserve consideration when anchoring areas are in river estuaries or areas subject to significant tidal currents. This is the case especially during the tidal changes when the anchored vessel will swing successively in one direction, and then the other (Poiraud, A., Ginsberg- Klemmt, A. and Ginsberg-Klemmt, E., 2008). 7

5 The environmental forces that affect anchored vessels are difficult to quantify, but some tables based on testing are available, and mariners can extrapolate the results to their own vessel and them by make some reasonable estimates for planning purposes Setting of an Anchor While anchoring, the static and dynamic setting characteristic of the anchor should be considered. Static setting refers to the anchor s ability to penetrate the seafloor due its weight. This type of setting is more important if the vessel s anchor is several hundred pounds to several tons; theses anchors are designed to sink into the seafloor from their weight and shape alone. Mushroom anchors are a typical example of this setting mechanism; their special bowl-shaped flukes sink directly into soft mud. Dynamic setting complements static setting, using the load on the anchor rode and the resistance of the seafloor to embed the fluke or blade. In general the ability of an anchor to be set and remain set on the seafloor is called holding power. The anchor should offer sufficient holding power to resist heavy loads from the vessel due to wind, wave and currents. If the load on an anchor exceeds the holding capacity of the sediment in which it is planted, the anchor can drag by moving through the sediment or by coming free of the sediment and riding along on the surface o the seabed often. The anchor will slowly drift through the sediment until the loads exceed the maximum holding power, and the anchor breaks free from the seafloor (Poiraud, A., Ginsberg-Klemmt, A. and Ginsberg-Klemmt, E., 2008). Manufacturers and marine equipment groups estimate the holding power for each anchor type by practical tests, A test conducted by the Sailing Foundation and West Marine in 1995 evaluate anchors that might used by 35-ft to 45-ft cruising sail or powerboats. The anchors were chosen based on manufacturer recommendations, and the results of the test are shown in table 3, blank space in this table means that anchor did not set in all of the several attempts, and the bold number show the highest holding power achieved. 8

6 During this test one of the conclusions was that local bottom condition made a significant difference in the anchor behavior. The sailing Foundation and West Marine concluded that (West Marine, 1995): Selection of Anchorage. The tests indicate that because no anchor performed well in rocky, kelp-infested areas, a selection of suitability of bottom for anchoring may be more important than selection of an anchor. Indications on nautical charts of bottom characteristics are very general. TABLE 3.- RESULTS OBTAINED DURING A ANCHOR TEST (Sailing Foundation, 1995). Maximum Line Tension Before Dragging (lb/n) Anchor Site # 1 Site # 2 Site # 3 Site # 4 Site # 5 Pebbles Sand Rocky Mud (thick) Mixed Sand, Hard Clay Shells Luke 50 (53 lbs) 1 320/ / / / /1927 Bruce 44 (45 lbs.) 628/ / / / /2639 Davis 45 (45 lbs.) 400/1836 CQR 45 (47 lbs.) 1392/ / / / /4650 Delta 35 (35 lbs.) 831/ / / / /3846 MAX 17 (43 lbs.) 2616/ / /3429 Performance 35 (40 lbs.) 2066/ / / /12085 Fortress 37 (24 lbs.) 3185/ / /10979 For illustration purposes related to external forces versus holding power of an anchor, we are considering the following situation: a 35-feet powerboat and the captain has to select the anchorage site from among the anchoring sites listed in the table 3. This boat has onboard a CQR 45 anchor, and the estimated vessel weight is about 5,000 kg. The weather forecast indicates that gusts of wind about 30 knots and currents about 2 knots are expected for the following two days. With the information described above we can estimate first the loads causes by external forces and compare with the holding power obtained in the anchor test, and based on these results 9

7 choose the best site. Base on the table 1, the loads caused by gust can be around 4,100 N; we are going to assume that 50 percent of the loads caused by wave will be absorbed by the catenary, and can be estimated about 729 N. Current loads can be estimated around 1,458 N. The total expected loads due to external forces are about 6287 N and the anchorage site selection will be base on the results shown in the table 4. TABLE 4.- ANCHORING DECISION BASE ON CALCULATION Anchor Maximum Line Tension Before Dragging (lb/n) Site # 1 Site # 2 Site # 3 Site # 4 Site # 5 Sand Rocky Mud (thick) Pebbles Mixed Sand, Shells Hard Clay CQR 45 (47 lbs.) External forces Anchoring site selection Yes Yes* Yes No No * while the anchor may have set and held in this test, rocky areas are generally unsuitable for anchoring because the holding characteristics are not predictable and this is a high likelihood that the anchor may become stuck in the rocks and not dislodge when attempt to retrieve the anchor and get underway. There is not any single anchor highly effective on all type of seafloor, and most anchors work better in some sediment that in others. The effectiveness and versatility of an anchor depends on the selection of an anchoring area with sediment that allows an anchor and rode to resist the influence of external forces. Generally, though sand and mud (but not clay muds) are the best choices for anchoring. 10