Welding Trailing Arms of a Frame Work

I would have 2 ea plate C, above and below square pipe D, weld to pipe B one at a time C D C...Replace A with ss rod 3/4"od, insert sealed bearings, 2 ea, on rod w/od measurement to match id of B...Insert pipe 'A' into pipe B of a length 2" shorter to allow 1" either side for bearing insert...Once pipe 'A' is in place, drill hole through both pipes, and weld pipes together...Put bearing stop collars on either side of each assy...

For connecting A and B through 3/4" holes in 2" tubing you are losing nearly half of your parent material to accommodate fasteners. These seem a bit big. 1/2" is plenty big. 3/8 or even 1/4" would work too with the config shown.

Your design relies too much on plate C's rigidity and its weldments to the collar. That 7' (cantilevered) boom is either going to bend and possibly fail long before connection ABC gets to feel any action.

With a moment like this on the boom I would be bracing it with either a rigid stay or a guy between the end of the boom and somewhere forward and above.

Basic rigging practices really.

Assuming you want to stick with using C then professionally executed welding would work fine. If it has to come apart then mechanical fasteners would work. (where the fastener dia should be the same as the boom thickness ie 5/16")

Have you considered dispensing with A, B and C altogether? If, in addition to the net fixing eyes, you had an eye at each end of the 7' boom you could tie and lash to the craft at the top (big cable ties are really strong and disposable) and have a guy as described above to keep the boom vertical when loaded. This is easy to deploy and furl as well.

Is there any special reason why you selected aluminium and not stainless steel as your preferred material?

Metric would be nice too.......

Boom "A" will be supported by two cables running to the bow of the boat.

Trailing arms need to be at angles of 35° at their lowest to 45° at their highest.

You are correct, I was looking at .25 ( not .75") This rig will have some movement as those trailing arms rub on the river bottom and bump into uneven sand riffles.

We need some flexibility.

Stainless is out of the cost range for this project.

We have tried a similar proto-type on even larger trailing arms ( made of steel ) and it was highly successful. The welding was augmented with extra bracing top and bottom....very heavy and extremely bulky but it was on a much larger vessel.

This is a down sized project and we came to this arrangement.

I just wanted some opinions on different strategies in securing these arms.

Thank you both.

I'll bite. What are you doing this for? Catching fish? Dragging for bodies......

Taking fish samples of Young of the Year hatchlings.

The rig is fitted with a specially designed net that skims just over snags and rocks.

A small electro current is also used to nudge them from the rocks and put them in the net's path.