I believe most boats will need to be modified to Loop with ease.
In the huge commercial locks nothing more than a hefty cleat , midships that is easy to pop outside and work will be required . 15 inch would be a reasonable size.
Ride the pipe float , easy with one line.
The hassle would be the small locks in the Hudson and the Erie barge canal or in Canada..
Nothing but ropes hang from the top of the lock, no sliding Pig to tie up to.
Usually 2 folks (one fore and one aft) will pull or release lines to keep the boat on the lock wall.
I would contemplate a powered windlass , or powered old sail boat winch , so with good fendering a single line could be held taught enough to keep the hull on the wall.
The windlass would be on the "natural" docking side as required by the boats prop direction.
Might be useful after the loop in docking
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A couple of comments. First, many people make locking out to be far more difficult than it is. Now, if that moves them to be better prepared then great.
As to preparing fenders and lines, you must do so on both sides of your boat. You'll often be directed to a position and occasionally even if you've chosen a side, during the process you'll be asked to move. So be prepared on both sides of your boat. The other reason to do so is that sometimes you will raft to another boat or one will raft to you.
The majority of locks have pipes or cables or floating mooring posts. In these cases the easiest way for most boats is to have a line affixed to a forward cleat, loop it over or around the post, pipe or cable, pull tight from the rear and just loop it around a cleat. Do not knot it. You must always be in a position that if there is a malfunction you can quickly release. For that reason a windlass would probably not be usable or allowed as it could malfunction when you needed to release. I've never seen one used for that purpose.
In the small locks where lines are dropped down to you, then you just hold them. As said you do generally want two people, although not impossible for one to do it.
The small locks are simple as the turbulence is generally mild. Only difficult in high wind and that's simply then a matter of getting and staying in position. As to locks such as those on the Mississippi and Tennessee and Tenn Tom, the turbulence can be greater and require some effort. The biggest thing there is not to untie until the signal is given. Then on those locks there is one more potential factor and that's the turbulence from tows in front of you. Let them clear, don't try to rush and be too close to them. Now, often you'll be allowed to lock through rafted to one of them. That is absolutely the easiest locking. They'll then have you go out first. Tows are generally very cooperative and helpful if you're polite. The two captain always has the right to say yes or no to you locking through with them. The exception of course is a red flag tow (flammables or dangerous chemicals) and you can never lock through with them.
For the most part lockmasters are extremely helpful as well.
Now the only other type lock one might face doing the loop is if one chooses to take the Welland Canal. It's very unique and largely commercial and a pilot to take you through is strongly encouraged. Plus right now they're redoing the mechanisms and there are delays for pleasure vessels as they work on the locks between commercial boats.
Then there is the Panama Canal where you use line handlers and if you're large enough they even use mules. Not the animal kind. Equipment along tracks they have set up designed to move you at the right pace.
We don't ever mind the locking itself. We do mind the delays waiting sometimes.