Short Scope Continued - Crash Test
14/02/15 18:48 Filed in: Anchoring | Rocna Anchor
By Paul Shard, Copyright 2015. All rights reserved.
Testing continues... In this case we are anchored in 12.5 meters of water (plus 1.5 meters to the bow) for a total of 14 meters bow-to-bottom. We have our Rocna 33kg anchor (73 pounds) plus 45 meters of rode... 45 / 14 = 3.2:1 scope...
We wonder just how much force the anchor will get if we load it right up with this short scope. With our previous tests we believe our engine will simulate 30 knots of wind - but what about stronger gusts or lulls that allow the boat to build up momentum and jerk back harder? We decide to simulate this by jerking back on the anchor from a position somewhat forward then putting the engine in hard reverse. Sheryl will be on board and running the engine. This is not a precise test since the speed we attain in reverse will vary with each test. But it will make much greater forces than just running slowly up to full throttle astern. Sheryl will also try to estimate how this approximates to forces we have felt on the boat in violent squalls at anchor.
We are anchored and the boat resting - chain hanging down - as Sheryl puts the engine in full astern. We develop some sternway and momentum before the chain comes up taut, bouncing our 36,000 lbs of boat back forward. The force on the boat is quite impressive - Sheryl describes it: “The bow immediately snapped around to line up with the anchor and then the bow got pulled down from the force on the chain due to the anchor holding so well. It felt like when gusts of 40-50 knots whip the bow around”.
Underwater I watch the chain lift up off the bottom. It is coming up at an angle that actually raises the shank a little. The anchor moves about 12 inches back and digs in deeper. I wonder if this would have been the same with our old CQR Anchor? It might have possibly pulled out and dragged - but the 73 lb Rocna held on well.
Now Sheryl extends our scope to 52 meters - 52/14 = 3.7:1 so we are still less than 4:1 scope. We retest with a similar full power fast reverse and the bow snaps around again. The extra scope makes a big difference here as the shank doesn’t quite come up off the bottom. The anchor barely moves this time.
Conclusions on Deep Water Anchoring
In normal conditions (not storm force forecast) an all chain rode will allow reduced scope in deep anchorages. We find 3:1 scope to be OK for anchoring in 50 feet of water. Increasing scope to 4:1 or 5:1 will be better and would be a good idea if strong squalls were forecast. Even an increase to 3:5:1 will make the anchor more effective.
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