How to calculate forces from an object bouncing
How to calculate forces from an object bouncing
(OP)
Hello all,
I am done with the 1st half of my BSME so I have touched on this type of thing but as for the actual application I am having trouble. I am trying to design a bracket to hold a small (110 lb) outboard motor on the back of my boat. Right now I have it modeled with .125'' thk 6061-T6 mostly because we have a large sheet of it at my work I can use for free, but I want to make sure it will hold up. I want to figure out what forces will be acting on the bracket as the motor bounces around while going over waves or while on the trailer.
Thanks in advance for your help!
I am done with the 1st half of my BSME so I have touched on this type of thing but as for the actual application I am having trouble. I am trying to design a bracket to hold a small (110 lb) outboard motor on the back of my boat. Right now I have it modeled with .125'' thk 6061-T6 mostly because we have a large sheet of it at my work I can use for free, but I want to make sure it will hold up. I want to figure out what forces will be acting on the bracket as the motor bounces around while going over waves or while on the trailer.
Thanks in advance for your help!





RE: How to calculate forces from an object bouncing
This will also introduce you to the mandatory lifetime engineering requirement of spending large sums for codes and standards that you will have to replace every three years.
ps: Your question is too broad.
RE: How to calculate forces from an object bouncing
As it is, what you are looking at is making an estimate of how long the bracket will last and what effect its deformation will have on performance. And that's going into fatigue analysis, which requires a lot more than can be covered here.
The alternative is to just build it and see what happens. It's not exactly engineering discipline to do so, but it can be valuable to build a mental model of what works and what fails in a way that calculations don't do well. One big example of this type of effort was Paul MacReady in pursuing the man-powered flight prize. While a great deal of calculation went into the plane, they also used actual use to guide the design - there were two rules. If it didn't break, make it lighter. If it did break, make it stronger. Most others just tried to make a one-shot effort and were typically too heavy to make it.
RE: How to calculate forces from an object bouncing
RE: How to calculate forces from an object bouncing
This is like one of those tests in school where the prof gives you all this extraneous information intentionally to lead down the garden path to an incorrect answer. In my opinion the actual stationary weight of the motor is among the least important factors. The main forces (and resulting damage) will come from the forces and vibration generated by the motor and by the boat as it encounters waves.
My suggestion? Ignore that sheet of 1/8" aluminum in your shop. Use it for a cover or decoration if you like. Widen your scope to find much heavier material, much heavier. That will probably also mean you can't fabricate it as a sheet metal component (with bends and flanges). You're probably looking at a weldment.
My two cents worth...
RE: How to calculate forces from an object bouncing
Most aluminium sheet metal fabrication is from 5052-H32. Aluminium 6061-T6 requires a large bend radius, otherwise, it cracks.
--
JHG
RE: How to calculate forces from an object bouncing
build one, bolt it down, wack on it (in a carefully controlled way), see what it does ... return to step 1 if required.
if you've got 6061T6 for free, use it ! (but be aware of the 3t bend radius required).
another day in paradise, or is paradise one day closer ?
RE: How to calculate forces from an object bouncing
2. Try to identify the 'worst case' - or failing that the easiest to analyze out of the top 3 worst cases or similar. For instance maybe you don't look too hard at vibration and fatigue on the assumption you'll evaluate these by testing & continued inspection of the part in operation.
3. Research that case. For instance if bouncing over waves is your perceived worst case then research this to try and find the peak acceleration loads. A quick google of "acceleration 'g force' speed boat on waves" got me: http://www.sname.org/HigherLogic/System/DownloadDo...
4. Use a reasonably generous Margin or Factor of Safety especially on ultimate load but also on yield and do the appropriate analysis.
5. Static test the bracket before you go out on the water - easier to pick up a few sacks of sand or what have you from the ground than a 110 lb motor from the bottom of a lake/ocean.
Given the bend radius issues folks above mention, you may want to consider other manufacturing methods rather than/as well as sheet metal forming.
E.G. welded assy with tabs & slots? https://www.google.com/search?q=welded+assembly+lo...
If you do weld remember that the heat treated condition around the welds will change - i.e. it will no longer be 'T6' material. Do some research on this topic and allow for it in your analysis.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: How to calculate forces from an object bouncing
TTFN
I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: How to calculate forces from an object bouncing
RE: How to calculate forces from an object bouncing
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: How to calculate forces from an object bouncing
Comments from Jimbob?
RE: How to calculate forces from an object bouncing
TTFN
I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: How to calculate forces from an object bouncing
another day in paradise, or is paradise one day closer ?
RE: How to calculate forces from an object bouncing
Is the transom not already set up to receive an outboard?
What HP rating on the boat ID plate?
Looks like 2 strokes may (still) enjoy a pretty (non) hefty weight advantage.
https://www.mercurymarine.com/en-gb/au/engines/out...
https://www.mercurymarine.com/en/us/engines/outboa...
You could use a 30 HP 2 stroke but only a 20 HP 4 smoker with a 110 lb weight limit.
RE: How to calculate forces from an object bouncing
Initially I was planning to mount it to the swim platform as it supports my weight fine but then I thought about it more and gave consideration that the motor will be on it 24/7. Now I am leaning towards just cutting the swim platform and making a mount that bolts to the fiberglass transom with a backer plate on the other side of the glass.
From the comments above it looks like I should look into a weldment. Maybe use 2X 1/2" thk 6061 plates, one to mount to the boat and one to hold the motor and then buy a square tube to span the gap between them and have it welded. I can buy a 1' section of 6" x 4" x .25" wall 6061-T6 tube for around $40.
It seems as though any calculations I do wont be accurate so I wont rely on them. My first two years have been more generals than engineering classes, although I have taken statics and dynamics we didn't cover fatigue. I think I could figure out how much force it would take in one shot to make something fail but I don't expect there to be enough force from 1 bump in the road to break my mount/boat so that leads me to fatigue and how to factor that into my analysis.
drawoh, I would have to disagree with your comment. Most fabrication is not done with 5052. The company I work for does fabrication primarily in the aerospace and we hardly ever do anything from 5052, and never anything structural. If the part needs a tighter bend radius than the material allows we either start with O and then heat treat to the desired temper or bring it to W, form and let the parts age harden.
Thanks to everyone for your comments. Although I am not much further along in figuring out how to do a failure analysis for this problem (which I understand was vauge especially not knowing where I am in my own knowledge) I feel more confident in the bracket. This was my 1st post on this forum and I am surprised by the helpful feedback. Hopefully someday I can contribute.
RE: How to calculate forces from an object bouncing
quote: "...making a mount that bolts to the fiberglass transom with a backer plate on the other side of the glass..."
Yes that is basically what you want to do. It is unfortunate that you must cut the swim platform to do it, but I don't think that it would be sturdy enough alone.
This kind of job should be solved with "stiffness" rather than "strength". What I mean is that you should be looking at thick sections and stiff components; once you do that, the strength you need will follow.
Have a walk along the dock at the marina and look at the outboards on the boats nearby, maybe get ideas. Perhaps before deciding on the exact materials you want to use, ask yourself a few questions about the implications:
Are you choosing a structure that requires drilling holes through the transom, or will it be clamped in a way that leaves the hull intact?
Are you introducing pockets where water cannot drain?
Can the motor be accessed for maintenance? Is there a place for the fuel tank?
My grandmother bumbles up and down the lake from her cottage all summer in a boat with an old 15HP motor attached to the transom with two C-clamps. The transom is "stiffened" by a piece of plywood. Annual maintenance involves cutting another piece of plywood. Admittedly, it's an old 2-stroke, and you probably have a 4-stroke (heavier), but you may be over-thinking the strength issue.
STF
RE: How to calculate forces from an object bouncing
http://adventuremarine.ca/3-boat-accessories
RE: How to calculate forces from an object bouncing