Metal stress/fatigue question

[u/redheadedcanadian97]

I'm looking to put together a setback bracket for an outboard motor.

I need 16-18" of total set back. The motor weighs about 480lbs

My current plan is to use 12"×4"×1/2" aluminum C channel as an offset bracket and then mount a 6" or 8" hydraulic jackplate to that.

My question is 6061 or 5083 alloy.

Or, does anyone have a better idea.

Thanks!

[u/Extra_Intro_Version]

Need a diagram of your bracket set up to get a sense of where it might want to develop fatigue cracks.

Will the transom be able to handle the increased bending moment?

Note- welding will reduce parent metal strength of these aluminums by a significant amount. Moreso for 6061.

[u/JackpineSauvage → u/Extra_Intro_Version]

Great point. You will want to add oversized, heavy gauge aluminum backing plates to your transom with any kind of jack plate assy.

[u/JackpineSauvage]

Honestly, based on my experience with my designers and engineers, our dealership networks, and the general boat owning public; I personally would not DIY this project if it was my first time. There is a fair amount of voodoo involved in any hull/transom modification that is only learned through experience. You would probably be time, money, and overall boat performance ahead to take it to an experienced marine fabricator or dealership that specializes in performance boats. Just my $0.02 based on what I've seen over the years.

On a side note, when I worked C/S and Tech Support, my co workers and I all agreed that the two worst retail demographics to sell new boats to were engineers and lawyers. True story. Lol!

[u/JackpineSauvage]

5083 all day! It's the marine standard. Better corrosion resistance and way tougher.

Source: 20 yrs in the mainstream boatbuilding industry. Over the yrs was C/S, tech support, purchasing, and sales. 2 different companies, one fiberglass, one aluminum.

[u/No-swimming-pool → u/JackpineSauvage]

Isn't the yield stress of 6061 higher than 5083 though?

@OP: I suppose your bracket will see varying loads? Look into fatigue.

[u/JackpineSauvage]

Two things to consider. First, you want a very small amount of flex in your base metal. Prevents stress cracks. Second, 5xxx series alloys have very high magnesium content as far as aluminum goes. Higher the magnesium, the lower the crack sensitivity in and around welds.

[u/45t3r15k]

For a boat, you DEFINITELY want to go with the 5083, ESPECIALLY if it is salt water. Also, how are you connecting it to the boat? Welding, bolting...? If you are attaching it to a dissimilar metal in a salt water environment, you need to take appropriate precautions against galvanic corrosion.

[u/UIUCTalkshow]

I'd go with 5083 aluminum for the setback bracket—it's tougher for marine use, especially when it comes to corrosion and handling the stress from the motor over time.

[u/sibilischtic]

Im not a boat guy but. here are some things to keep in mind.

it wont matter if the channel fatigues over time if the motor rips its self off the boat.

its not just the weight of the motor, thrust and rotor depth from the connection matter.

if you cant buy the offset you are looking for there might be a reason its difficult to find. often when boat things go over a certain size it becomes a custom job for boat builders.

sometimes its better to commit to changing out a part every year or so instead of letting it fatigue or corrode out.

More information would be needed to give a good answer.

[u/JackpineSauvage]

6061 is harder. Equals more brittle. Nice to machine, not ideal to weld...