Aluminum extrusion strength

Here's the deal. I needed to mount my kicker motor to my jon boat. But, being a short shaft, on a long shaft boat, I also need setback, about 10 inches worth, to tilt the motor up as well. When it comes to making something, I like getting raw materials, and making something from there, that fits my needs to a note. My dad, on the other hand, likes to get a preexisting object, and convert it to his needs. While looking for material to make my bracket, I decided to use what dad suggested. He thought I should use this big extrusion, which apparently was a piece from an overhead trolley system that the USPS scrapped. Now, because it has a little bit of a bend, I didn't like the idea of mounting it vertical. So, dad suggested I mount it horizontal, which I did. Now, I know that is not the direction that the thing was designed to be mounted, and am wondering what y'all think its strength would be.





The aluminum is about 7/32 thickness, all the way across the board.
Now, ignoring how the motor plate is attached (that was made quickly just for sizing reasons), how do sound do y'all think it is? It is just under 8 inches wide, and is about 8 pounds. The motor is a 4 horse, and weighs 50 pounds. When I put the trailer on a jackstands on the frame, to avoid suspension play, and bounce the motor, there is an infinitely small amount of play in the mount (a good bit in the motor plate, but that is irrelevant). I know from experience that this minor amount of play is usual in aluminum, but I am not sure if it is sound, seeing as the extrusion is facing the wrong way.

Thoughts?

 

Hi bassboy,

Without actually doing the calculations for the stress in that aluminum... thing:
It is evidently holding up in the static case, but I don't think it will hold up, as shown, for very long. What will kill it, I think, is the repeated loads it'll undergo while being trailered. You're putting a fairly heavy mass (the motor) cantilevered off a relatively flexible aluminum beam in its weakest direction. The vibrational period of this setup will probably be long (ie, slow) enough to get some nasty resonant behaviour on the road, that could cause failure at those channels in the main web. What you want is for the thing to be stiff enough that any vibrations that are set up in the mount/engine assembly are both small in amplitude, and nowhere close to the frequencies you get from the vibration of the trailer.

Do you know / are you someone who can weld aluminum? Welding a trapezoidal plate on each end of that beam, with suitable holes drilled to let water drain, would probably stiffen it up enough to work. As is, I think you're applying too big a moment on too weak a cross-section at those channels.

I should note that weaker mounts are commonly used with bigger engines on sailboats. But they don't have to deal with the vibrational frequencies that a trailer sets up. I have seen many an aluminum fishing skiff with 8, 10 or 15 hp, where the plywood-reinforced aluminum transom is falling apart because of the motor shaking up and down as the trailer goes over bumps.

 

I too have replaced many transoms on old aluminum rigs, because of a bouncing motor, and hence why I use a transom saver, in order to try to avoid the rocking that happens when bouncing.

Those were my thoughts as well. I had no doubt of it's ability to hold the weight of the motor, and likely the thrust of the motor in the water, but most every other motor mount or transom failure I have seen before happened on the trailer, which, in the case of the kicker, could be the boat bouncing down the lake, with the kicker tilted up.

When you say "stiffen it up enough to work," does that mean it will "probably be alright," or it should be just fine?

My thoughts were to go back to my original idea of forming a setback out of a thick plate, by basically bending a U that bolts to the transom, with a flange bent on either side to bolt through. I am thinking 3/16, if I take it to my buddies at the steel fabricators to bend, I should be good.

 

If the ally you're using is unwelded, then allow for a design stress of around 30MPa...if welded allow for 10MPa...then you'll be fine.

 

It is an ugly way to use this piece of extrusion.

Not only this section was not designed to support this kind of load - it is also not suitable for use with watercrafts because it has too many places where water can stagnate and cause crevice corrosion.

Even the structural analysis of this assembly would have to be performed with prudency, because common aluminum alloys have yield strengths anywhere from 50 to 300+ MPa, so if you don't know what alloy this is, any analysis would be just a guesswork. A prudent figure can be 60 MPa yield stress, assuming (guesswork again...) that it is made of 6063 alloy (commonly used for door & windows frames, pipes and other extrusions). It would then drop to 15-20 MPa in welded areas. A fat safety factor has to be used to account for fatigue due to cycle loads.

I think you'd better substitute it with some extrusion which will have continuous, smooth surfaces and possibly made of a corrosion-resistant, marine-grade aluminum, like 5083 or 5086. And, of course, safely dimensioned for bending loads to which it is subject.

 

My original thought was to make something like this.
CLICK HERE
Only, out of 3/16 aluminum, instead of that stainless foil that one seems to be made of.

Only thing is, the piece would cost me about 115 bucks, at least from the place I can get an online quote from. Plus, I will have to pay my buddies at the steelyard to bend it for me, as I can't bend that thickness at the house. I will see what I can come up with this weekend at the scrapyard, especially now that the scrap aluminum price was cut in half 2 weeks ago . Still, I need to make sure it is of a bendable alloy.

Say I made a nice gusset on the bottom of this. Say, out of 1/8 or 3/16. I know I will be adding crap to fix a mistake, but in this case, I think I can make it look ergonomic. Plus, provided the gusset would solve the other problem, it the combined strength of the two would certainly make the static strength fairly high, unless I am way off base.

As far as the construction is concerned, I know about the alloy issues, but for a freshwater boat, that sees salt maybe once a year, and is hosed off thoroughly after each dunking, how big of an issue would it be? It is trailered, so water contact is minimal.

Am I way off base with the gusset idea? (Figure I probably am, but there is a slight possibility in probably, that isn't there in definitely).

One other thing, has anyone any experience with those kicker mounts that raise up? I do kinda like the idea of being able to lift it up, as opposed to tilting it up, but those things have always seemed flimsy.

 

For your small kicker bracket, you're on the right track, though that extrusion is clearly not up to the task.

Picture a box with angled front (to match the transom rake) and back side (to provide rake for the outboard). It can be open on the bottom which will drain well. You could incorporate an internal web on the centerline to help stiffen it up a bit. Toss in a gusset or two if you want.

This could be bent to shape or welded up from cut stock. For that little engine (I'll assume 10 HP at most), 3/16" plate would be just fine. Use a T-6 alloy for corrosion resistance and stiffness.

Bolt a couple of pieces of 1/2" plywood (sandwiched) to the back side to help bear the engine clamps. Insure you bed the bracket to transom holes very well. Coat the plywood with epoxy, followed with paint, for maximum protection. If using just paint for wood protection, a good quality single part polyurethane will do.

No need to over engineer this thing.

For what it's worth, there are two types of those lifting brackets, the flimsy ones that are rated for low HP and a heftier one rated for more power. The bigger ones are built much better and they work very well. I have a 120 pound outboard on one and it's flawless in operation for the last 4 years. I got it at West Marine and it was a lot more costly then the smaller one, but one look at how it's made and you'll see why. I can still tilt the motor up. I had to install a spacer block (a big *** piece of teak) to get the bracket far enough off the transom to tilt, but it works. The advantage is I can drop the motor lower in rough conditions, so she doesn't free spin when the boat is hobby horsing.

 

Alright, unless I find a piece of 3/16 this weekend at scrap price, I don't wanna spend 115 on something I would need to pay to bend on top of that. And, that was my original idea before even the extrusion, but when trying to find such piece of scrap, I was having trouble.

However, I did have a better idea in the night. More to come when I have time later on......

 

It doesn't need to be bent. It can be a purely welded affair. Just make sure you use T-6 so it's stiff enough and has reasonable corrosion resistance.

 

I would be careful about an arrangement like that with aluminum. From what I've been told, aluminum will weaken over time without showing evidence, and finally break suddenly. It looks like there is a lot of leverage with that piece sticking out so far with a weight on it, let along the additional force of the motor pushing on it. Securing your motor with a cable is probably a good idea, regardless of the mounting setup.

 

Venomousbird
What you are describing is "fatigue".
Aluminium is prone to fatigue, or 'low' stress repeating-cyclic loading. Aluminium has no fatigue limit (a number of cycles after which no more fatigue occurs). Unlike steel, which does have a fatigue limit.

 

Yep, that idea is in the scrap pile. Had a couple other things slow me down (parent in the hospital) but I should be able to put something together by new years.

 

This is what I finally came up with.

Click HERE