dawg, many fresh water inboard sleds are built of 5052 alloy which is softer, lower tensile (tears more easily) and is cheaper than 5086 or 5083 'saltwater' aluminum. 5052 will work fine in salt water but its not quite as corrosion resistant as 5086 or 5083 (the latter most common Down Under) but all aluminum will 'tear' when you reach the 'failure' yield point.
What isn't as obvious from the pictures (and might not be the case) is that aluminum welds that are uneven create as series of ragged edge notches or cold fusion points along the weld. This condition is very much present in the welds shown in the picture and they can contribute to tearing failure where better welds AND (or combined WITH) better alloys will 'bounce'.
What I'm discussing is the failure method of this collision impact tear. It appears the hull was moving in sliding turn ( I have no info on this- but that is what I'm inferring from the single photo) the sled was sliding at high speed to the port in a slipping turn and the rock was relatively sharp -not rounded very much.
[Quick note to everyone, salt water rocks are usually very rounded from all the time in waves, sand, surf but mountain river rocks may have been split by ice the previous year and be sharp]
Sharp isn't meant as in knife sharp its meant as in not rounded like a salt water rock where there is enough 'edge' for the entire wt or displacement of the hull PLUS the momentum of that moving hull to come to bear on a very small surface area. So if the tensile strength of the metal were say 35,000 psi (at failure) then the one ton boat moving at 40mph is a rough momentum of 50- 80 thousand pounds of potential force- if expressed on a 1 inch area. What makes this number higher or lower is the area of the immoveable object the boat hits- small area or sharp rock is higher where larger area or round rock is much lower impact force per unit area.
If the rock is rounded, say a foot round then the area of impact is 4 or 5 inches square or 15,-20,000 psi of impact at that point and the metal holds- it may well bend- but it holds. Next, if the rock had an area of impact of 3/8" x 3/8" or (0.365 x 0.365) or about 1/10th inch then the metal's failure strength could be reached and it would puncture or 'tear'.
OK lets complicate things a bit. What if the tensile strength of the weld lowered the metals' original tensile? Well welds to lower that figure in aluminum along a zone called the Heat Affected Zone or HAZ. Along this median boundary of the weld and the hull plates, the tensile is lowered say 20 -50% depending on the weld, speed, alloy of filler and size (cross section) of the welds.
Now.. lets really look closely, if the HAZ is not uniform along the welds' edges its because they are like a picket fence or notched because the welder didn't put the weld into the seam uniformly and that creates a ragged edge of HAZ and that ragged edge will not be uniformly degraded in tensile (tearing) strength, it will have weak spots and stronger spots all corresponding to the heat and cross section of the welds, and if the weld is not uniform in fusion line, that is the edges are not even- they imply the heat and weld cross section is not uniform.
If the HAZ is not uniform the metal's tensile zone is not uniform and therefore if such a hull were to hit a rock when.. it was:
#1 made of 5052
#2 poorly welded or inconsistently welded
#3 HAZ being ragged and not uniform
The tensile strength of the alloy would be subject to tearing beginning at the points of the weld with the worst case of weakening due to the conditions mentioned.
Now bringing all these factors together on the port chine of a poorly welded sled, perhaps in an area where a 6061 extrusion is welded (angle shown in photograph was likely 6061 T6 alloy) to a 5052 plate the weld joint is even weaker yet! And what if the welder used 4043 wire! with its high silicone content to make welding 'easier' instead of the correct alloy 5356?
I know that my take may be harsh for the builder of the boat, but the workmanship is not even acceptable for a JrHi shop class.
All welds should look uniform, the edges of the welds should smoothly fuse into the metal they are joining and the surface of the weld and adjoining metal should be relatively even and uniform, the boat shown was built by someone that was not well practiced.
cheers
Kevin Morin