Some great points GeoRon. I think attaching the haul line to the frame is also a good option, especially if you have a good strong frame. Makes a good case for using 6 tie down straps on the frame if you're boating in a place with high pin potential, like low water Selway, etc.
As for attaching a haul line to the grab line, I would clip it with a carabiner rather than use a Prusik knot. A Prusik has a tendency to really concentrate force, which generates lots of heat that can melt through the rope its attached to.
When I taught swiftwater rescue, we always used to have a tug of war type exercise with z-drags or other hauling systems, and the Prusik knot was almost always the first point of failure when tension built up, either burning through the haul rope or the Prusik loop itself. The Prusik knot can also slip under high tension.
The effect you describe of the grab line slipping can definitely happen. I think the key is to find the exact right spot to clip into, with the exact right angle of pull. Granted, that's not always possible and you kind of have to improvise and work with what you can get a hold of. I've tied into two different places on the grab line with a sort of sling before. I've also had to tie the rope around a tube going through holes in the self-bailing floor. Unpinning boats is both art and science.
Like you say, there is a lot of personal preference involved on rigging. There is definitely a lot variation in how outfitters rig grab lines on boats on all kinds of rivers. Many different approaches can work just fine. Some general principles on grab lines for me would be:
1. They should not be too loose. You don't ever want a swimmer getting worked in between the boat and the grab line. A tight grab line also makes it easier for a swimmer to self-rescue back into the boat.
2. Use non-stretch material for the same reasons above, and in case you ever have to haul on it to unpin.
3. The material used should be strong enough to allow it to be used to haul on if the boat is pinned.
4. Avoid using any kind of fasteners or knots that can get hung up or create any kind of snag hazard for swimmers.
5. Inspect and replace your line as needed. A sun faded line that has been exposed to the elements loses strength.
6. Also don't make it so tight that people can't get a hand around it. I've seen this done before when people secure gear loads by tying into the grab line.
Originally Posted by GeoRon
I've never thought to put a z-drag on a perimeter line. I usually go to the frame and have it distribute the load to as many d-rings as possible. But I guess if you don't have a frame such as a paddle boat then Pine's recommendation makes total sense(and therefore possibly makes sense for a framed rig also).
Some people seem to prefer the suppleness of tubular webbing but for sure you have to take precautions to make it tight. When using tubular I thought to tie to each d-ring when wet was best therefore localizing potential slack/stretch to the distance between d-rings. Depending on the direction of pull, tying to each d-ring would distribute pulling forces to adjacent d-rings I thought.
Pine, do you tie to perimeter line using a Prusik?
I'd think that at most directions of pull the perimeter line if slung through through d-rings without knots at each ring or even a single tie off somewhere in the system would rotate until the tie point transfers to a d-ring thus focusing most of the load to one d-ring. Maybe not.
Someone needs to pull out a slide rule to figure this rocket science out. Probably in the end it boils down to personal preference with a few general guidelines. But what are those general guidelines? We can't even agree on the type of material to use?