MathiasW
Active member
The general advice for anchoring is to set the anchor, then drift backwards due to wind or current, and slowly pay out more chain until the desired length is reached. Then slowly crank up the engine in reverse to get the anchor dug in. „Slowly“ is important here, as the anchor needs time to set, otherwise it would only get dragged across the sea bed. The interesting question then is how much load have I tested the anchor with in this approach? To get an approximate answer, I did the following calculation: As physics dictates, work is force times distance, and so power is force times velocity. So I do the following measurement of my vessel: Whilst not at anchor and without any sails set, I put the engine in straight reverse and measure the velocity v at full engine power p. Ideally, this can be done with an apparent wind speed of zero, and no current or waves. The force f which is pushing the vessel through water is then simply given by f = p / v, and this should be the anchor load with which the anchor is dug in when using full reverse throttle when setting the anchor. The relevant power is not the nominal engine power, but the effective horse power after accounting for all losses in the system (within the transmission line from the engine to the propeller as well as the propeller itself). A simple way of dealing with this is to introduce an efficiency factor ?. In my case, this all amounts to – very roughly – 60% * 75 HP / 8 kn = 8042 kp, which is clearly way too much (being of the order of the chain breaking load for many chains), despite of an assumed efficiency of only 60%. So, something is wrong with this calculation, but I yet fail to understand where it has gone wrong. Perhaps the efficiency for reverse throttle is even much less than 60%? Some more food for thoughts can be found here and here.
Any ideas?
My thoughts on anchor loads and chains can be found here: Catenary Anchor Chain Length - Die Kettenkurve - Fun Facts - SAN (in English, no worries )
Cheers, Mathias
Any ideas?
My thoughts on anchor loads and chains can be found here: Catenary Anchor Chain Length - Die Kettenkurve - Fun Facts - SAN (in English, no worries )
Cheers, Mathias