Aluminum Boat Transom Knee

[SlipperyOar]

Currently in the process of restoring/building an aluminum boat and have been converting it from an I/O, closed bow to an outboard, open bow for fishing. I've got my open bow bracing and structural stuff figured out. But im trying to get a sense of what i need to do for the transom knee brace. Boat is 18' and planning to throw a 75-90 HP motor on the back, would rather have it beefed up for a much heavier and more powerful motor and have the peace of mind knowing a 90HP is securely mounted without causing any future issues.

Does anyone have any experience on fabricating one themselves? I've taken measurements on what dimensions will be needed with the appropriate angles to rivet to the floor and be bolted through my transom.
- If i were to get an aluminum plate and cut it to the required size, what thickness of aluminum would be best for strength and still be adequate after being bent to shape?
- Is bending the best option knowing welding adds stress to the metal without a post weld heat treat?
- From what i see in other aluminum boats they all seem to have a longer edge running up the transom than the edge along the hull. Would my bottom edge being 8" up to the first rib provide adequate strength?

Link to my current build: https://forums.iboats.com/threads/1981-springbok-18-restoration-renovation.772380/

https://imgur.com/a/g1j71Z5

 

[Scott Danforth]

Any local fab shop can make you one

 

[racerone]

I would use 2X3" aluminum tubing to fabricate a structure or maybe 2 knees.----Along with some welding.-----Aluminum sheet does not always like being bent.

 

[SlipperyOar]

I would use 2X3" aluminum tubing to fabricate a structure or maybe 2 knees.----Along with some welding.-----Aluminum sheet does not always like being bent.

thank you, welding is very doable as i've got the TIG equipment but have read both good and bad about both but as ive seen with bending some thinner stuff it did not like to be bent past 60 degrees and this piece will have 8 90 degree bends in it which seems awful to do. I'll look into this advice!

 

[SlipperyOar]

I would use 2X3" aluminum tubing to fabricate a structure or maybe 2 knees.----Along with some welding.-----Aluminum sheet does not always like being bent.

one more thing... whats the optimal thickness for the triangle aluminum sheet i would weld to the tubing?

 

[racerone]

I would just use tubing and angle.-----Use 2 pieces of tubing for each knee and 3/16" angle to rivet to bottom and bolt to transom.

 

[Scott Danforth]

thank you, welding is very doable as i've got the TIG equipment but have read both good and bad about both but as ive seen with bending some thinner stuff it did not like to be bent past 60 degrees and this piece will have 8 90 degree bends in it which seems awful to do. I'll look into this advice!

Use 5051, it will bend just fine

 

[SlipperyOar]

Use 5051, it will bend just fine

What thickness is optimal?

I’m not sure if the required bends will be easily doable in a standard brake, would clamping to a table or steel and using a deadblow hammer be acceptable?

 

[Scott Danforth]

I would use 11 gauge (1/8")

 

[SlipperyOar]

I’ve got 7-1/2” of length on the base of my knee brace from where the transom will meet up to the first rib. Whats a general rule of thumb for the length extending along the transom? Would 14” be okay as it doesn’t interfere with my splashwell and provides a large surface area?

 

[SlipperyOar]

Which of these two designs would provide optimal strength for my transom. The first being between the rearmost rib and the transom, the second extending up to the 2nd rib from the back and going over the first rib. Would the cut around the first rib weaken this design in any way? I thought I would even rivet the knee brace to the rib using some small angled pieces.

https://imgur.com/a/SUOpRMS