have been tested and good anchors are better than the 70% I quote above (but the Bruce anchor agrees with my 70%). Fortress has been extensively tested (at different times under different conditions, and an analysis of the results develops a figure of 83% - I'm cautious and think our anchors are more like a Bruce than like a Fortress.
Comparing a 6kg Delta with a 6 kg Kobra - then the Kobra is a better choice.
Its not only about surface area but also the ease with which they engage and set - and again a Kobra is a better choice (2 anchors of the same size).
The only way to answer your question is to do it and I don't have a 6kg Delta and a 4kg Kobra (and Kobra are not available in Oz).
If you could photograph the 2 of them side by side, a shot taken vertically down - you could compare surface area. Its the area of the shadow they make that you need to compare (if that makes sense). If the area of the shadows are similar or only slightly in favour of the Delta I would not hesitate - but buy the Kobra. I'd tend to favour the Kobra at 4kg over the Delta at 6kg as the Delta might develop more hold (but frankly it will be marginal) - but that is irrelevant if it does not engage and set easily.
I'd throw the Epsilon into this specific choice and would opt for the Epsilon over a bigger Delta and the same sized Kobra - but then it might come down also to cost - and does it fit where you want to store it..
Just to make it clear an anchor of twice the weight will NOT have twice the hold. Hold might increase by 70% NOT double. There is not enough work done on the effect of scaling of anchors plotted against increase in weight - but all the evidence points to about a 70% increase to doubling weight. So your choice of a 6kg and 4kg would not be reflected in the same variation in hold - the hold will be a lower increase than the increase in weight. The 2kg increase in size, from 4kg, would not result in a 50% increase in hold - of the same anchor.
Jonathan
edit
Analysis was conducted by Knox of Knox anchor and he came up with results that totally contradict what I say above. But he was comparing anchors tested under totally different conditions, at different times with different rodes. Oil rig anchors, which tend to be a bit bigger than our anchors
The only certainty - double weight - you do not double hold.
One reason that the simple idea of double weight should double hold is that the stress on the shank increases by the 'square' so to maintain strength the shank has to increase in strength by that same square function - which means more weight has to be incorporated in the shank (this then alters the balance etc demanding another modification).
). If you move to different anchorages then the Kobra will set more reliably in harder sea beds and seabeds with a bit of weed. Initially I'd stick with the Delta - but keep the Fortress handy and assembled. , the confidence of your wife) and you will laugh together over your current concerns. Anchoring will be simple and second nature to you by then.A secret with deploying any anchor is to allow it to engage, bite, before you become aggressive with the throttle. Simply let the yacht drift back and let the wind play its part in straightening out the rode and then slowly increase the revs. If you sit at the bow and touch the stretched out rode you will feel, instantly, if the anchor is skiing over the sea bed. You can also feel when the anchor has engaged as the rode starts to tremble but you cannot feel the anchor bouncing about. Just touch the tight rode with your fingers and you will soon learn what is right or wrong. So next time you set the anchor have your wife touch the rope as the rode is straightening out, gently with 2 fingers, and she will 'feel' the differences as the anchor engages, holds and the rode goes taut.
I hate cutting rope or chain with a passion but with such small anchors you could save a few kgs by reducing the chain length to 5m (of 6mm chain).
I'm all for encouraging wives and partners to find tasks on yachts as easy as possible and running the rope portion of the rode to a sheet winch, if the rode is thin enough and the winches big enough (so the rope actually fits) then all of retrieval can be conducted by the you and your wife only needs to be at the bow to tell you when the anchor is coming to the bow roller. But you could equally retrieve the rope hand over hand and pull the boat forward until the rode is vertical - your wife does not need to do anything except watch. You can continue to retrieve, break out there anchor - she simply needs to tell you when she sees the chain and then the anchor itself.
I cannot recall but I think you want to remove the anchor from the bow. I'd find an easy way to secure the anchor temporarily once at the bow, lash or have a short strop with a carabiner, snap link to hold - then you can motor out of the way of anyone else and you can go and lift the anchor off the bow roller and store. I'd invest in a decent pair of gloves for your wife as the chain may be muddy - but she can then disengage the rode (anchor has been secured) without getting covered in mud - and you can rinse the gloves later.
Personally I'd use the Delta, 6kg, with the Fortress as back up. You suggests it sets reliably and holds the yacht in full reverse. The weight savings you will achieve in moving to a 4kg Kobra are not that relevant (to me
If any anchor comes up carrying mud, dangle it off the bow and let the boat movement, gently wash the mud off (using the outboard). Again don't get to aggressive or it will chip the stem. It depends how deep is the water but you can wash the chain the same way.
In a few weeks time, or after a number of nights at anchor, your confidence will increase (and much more important, you don't matter
Enjoy
Jonathan
. We only live on our cat for around 3 months in the summer - we went down in weight 
But do they reset? IMHO Fortress are amazing when the wind is from one direction.
Do you have the Kobra 1 or 2?
All I ever see, especially around the Solent area, is people chucking the Anchor out and motoring back at full tilt, ploughing a lovely furrow in the seabed.
Fair enough. I think the Kobra 1 is up to 12kg and the Kobra 2 is anything above that. The larger ones have a bolt arrangement I believe and the smaller the funny T slot thingies.
