Just because you can do something, doesn't mean you should.
If you're a recreational climber, you can do anything you like. But if you're doing this for a living, or hoping to, then for better or worse you've got to conform to industry standards if for nothing other than reducing your liability. By and large, industry standards come about as a direct result of feedback from people who have been in the industry their whole lives, careful analysis of injuries, and long term studies of safety. Not saying that there's no room for innovation, but a lot of the time things are the way they are for a reason.
Should you use a bit of rope in place of a carabiner or shackle? Not really. It works fine on your boat, but for different reasons it just isn't practical or safe in the tree. The minimum standard for life support equipment is 23kn, or 5,000 lbs or thereabouts. You might think that your soft shackles are going to stand up to that, and maybe they will... but do you have an in house testing program? Carabiners are generally tested to the 3 sigma standard. A sigma is a standard variation in breaking strain between the strongest and weakest unit in the batch. So, say you take a bunch of carabiners and break them. The strongest one breaks at 6,000lbs, the weakest breaks at 5,800. One sigma is then 200lbs. The better your manufacturing tolerances are, the less deviation you'll get. Poorly manufactured equipment can have a very large deviation. The sigma is then multiplied by 3, and taken off the breaking point of the weakest unit, so in this case 3 sigmas = 600lbs, and the weakest unit broke at 5,800 then the breaking strain will be rated at 5,200lbs. Do you have the time/money to do repetitive destructive testing of large batches, and keep records of where your raw materials came from and the certification they came with, then develop an in house quality assurance and quality control program, rate and tag your gear, and get certification for your splicing? If not, then your home rigging has no place in the work place. It's fine on your boat though.
In reality, even if you did all the above, the soft shackles you make are not fit for purpose. They have no double/triple locking mechanism, and single braid degrades rapidly. Single braids and webbing slings will lose as much as 60% of their rated strength within 12 months. The primary reason for it is that in a single braid there is no cover/core, the load is carried 100% by the exterior. Because the load bearing part of the rope is the exterior, it abrades and suffers UV degradation. Dirt, grit and nature abrade it at a microscopic level, weakening it. Not so with double braids or kernmantle ropes which have a protective cover with more of the load carried by the core. Not the case also with rated crane slings where the entire sling is wrapped in a non load bearing sheath. Destructive testing of single braids (and I'm including nylon webbing there!) have lost as much as 60% of their strength within 12 months.
The biggest reason to not use a soft shackle in place of a carabiner or shackle though, is that there is no good reason to do so. It's neither more economical, safer, more convenient, more durable or more practical. It also opens up no new possibilities in using that piece of equipment in a new way (unlike in sailing where it has done many of those things). It exposes you to new liabilities, and puts your life at greater risk for no benefit. It's a step backwards, or maybe 4 or 5 steps backwards from just using a carabiner. The only reason you might do it is just out of curiosity.
It's probably worth mentioning that I'm not against splicing yourself. I've been sailing off and on for about 20 years now, yachts and catamarans and also served a stint in the coastguard where i got my boat masters ticket. I've always rigged my own boats, and done my own splicing. I occasionally splice my own climbing ropes but I really shouldn't because of the previously mentioned liability issues. My splice might be every bit as good as a professionally done on, or even better. I've read all the manufacturers instructions, have several books and videos on splicing and own all kinds of fids, wire fids, pushers and a toss wand. But at the end of the day I'm not a certified splicer, and I don't have QA/QC or records or testing and tagging. I'd certainly never sell a splice.
Shaun
