Obviously, the surfer uses body weight and posture to achieve the effect of stern down and bow up.
A vessel in a large following sea must replicate this posture so that on reaching the trough at the bottom of the wave face, its bow does not dig in.
My findings are, over several deployments, to achieve this requires pull on the stern sufficient that the stern becomes depressed and the bow bouyant (in comparison).
If the towline and its rig (The Seabrake) is too horizontal, the restraint on the crest (at a point in time and position when it is most needed) will not be effective. It will only succeed in slowing down the vessel but not in holding down the stern as well at the same time.
This can not be allowed to develop as a defect.
In order to correct this potential defect the stern must be held down beginning when the vessel is on top of the crest and continuing all the way down the face of the wave until the trough is reached, mirroring the result achieved by the surfer.
Therefore the horizontal angle formed by an imaginary line extending from the the deck and past the stern and the towline (the towline brace) ought not to be too acute, otherwise the effect is for the towline to slow the vessel down, but not to hold the stern down as well when the advancing wave approaches from astern lifting the stern (and trying to get under it and to roll the boat over, worst case scenario, if not on that attempt then with the wave that follows).
Therefore the same horizontal angle and the angle formed by the towline (the towline brace) should be less acute.
The consequence of this is that the pull exerted on the stern is more positive
and introduces the oblique element helping to exert pull AND keep the stern down.
The angle at which the towline(the towline brace) enters the surface of the sea shoutd therefore not be shallow. This can be immewdiately recognised by the distance of the entry point from the stern.
As the end of the towline before the connection with the Seabrake is the chain. The towline....does not descend from the brace to the Seabrake in a straight line.
It descends in a curve.
This ensures the Seabrake is properly embedded in the sea that follows and when extra pull is exerted on it by the effect of the vessel being lifted to a crest, this towline is stretched taut, in a straighter line.
This tautness serves to pull the stern down and lift the bow up on the crest of a crest. The boat then surfs / slides down the face of the wave with its bow UP like the surfer on his surfboard. The stern remains down. The possibility of knocking the vessel off course is avoided. The vessel now maintains directional stability as well.
Guidelines for the length of chain and its calibre for different vessels is privided by the manufactures.
In heavier seas I personally prefer extra weight, so I have extra lengths I can shackle on to ensure depth of the Seabrake is maintained.
In a previous post on this thread there is an attachment to a report of a test carried out on the Seabrake with the unsatisfactory result of it being towed behind a vessel and it breaking surface.
The test was carried out in Chesapeake bay.
Chesapeake Bay is a shallow inland sea in the Untied States.
It is not suitable for testing this type of rig because it is too shallow.
The rig should be deployed in water DEEPER than the length of the towline at least, and then it should be WEIGHTED PROPERLY.
Notwithstanding my immediate comment above, it can be used in shallower water, but with care that it does not foul the bottom, and, in any event, that it is weighted properly.