basic engineering help/input needed please

[Wayne nicol]

hear hear!!
hear you on the two piece bracket.
if i rounded the flange corners, and made the bottom bracket smaller- it wouldnt look so bad.
any other suggestions/ideas
thanks
wayne

[Highlander]

Heres a link from a SS outfitters near me maybe u can get some idea's from their pics
http://www.stainlessoutfitters.com/prod ... arine.html

u can seach various sites to get some ideas & designs that u maybe able to adapt for ur mac
http://garhauermarine.com/catalog_process.cfm?cid=82

J

[Wayne nicol]

thanks mate- some good ideas-

my needs are a little different, cos of the need for a cooler with #200 of fish in it, that needs to be supported( toatal about #250)
but the basic principles are the same!!

[Wayne nicol]

was talking to another friend of mine on island here- and he has built a bunch of boats(a 70' steel schooner- amounst others )and barges etc and is a ticketed welder- he suggested that i go with aluminum for this davit system- says it will be easier and more durable. go a touch bigger and heavier wall, and it will do fine - he said
so was gonna go with a 1.5" with 1/4" walls
says he has had more problems with stainless than ali
the s/s steel is more prone to cracking and fatiguing
S/S is also prone to something he called "crevasse corrosion"- where the weld corrodes from the inside

what do ya'll think???

[mastreb]

was talking to another friend of mine on island here- and he has built a bunch of boats(a 70' steel schooner- amounst others )and barges etc and is a ticketed welder- he suggested that i go with aluminum for this davit system- says it will be easier and more durable. go a touch bigger and heavier wall, and it will do fine - he said
so was gonna go with a 1.5" with 1/4" walls
says he has had more problems with stainless than ali
the s/s steel is more prone to cracking and fatiguing
S/S is also prone to something he called "crevasse corrosion"- where the weld corrodes from the inside

what do ya'll think???

He's right. If you don't have to have the strength of stainless, AND you don't have any other metal that it needs to touch, then aluminum has fewer pitting and crevasse corrosion issues. But you should go all aluminum for your build, as aluminum corrodes wherever it touches other metals. For example our masts corrode underneath the stainless hounds, which you'll see as a white oxide when you take them off.

[Highlander]

Well when I got quotes for my 4ft bowsprit double anchor roller I was quoted $4600.00 for S/S & $3700.00 I beleive for Alum , so I bought the allum & designed & made my own with allum & no issues
Cost me $500.00
J

[Tomfoolery]

. . . so was gonna go with a 1.5" with 1/4" walls

1/4" is pretty thick. To save weight, use larger tube and thinner wall, as a general rule, up to a point.

. . . the s/s steel is more prone to cracking and fatiguing
S/S is also prone to something he called "crevasse corrosion"- where the weld corrodes from the inside

That's two of the reasons I suggested staying away from welding if you can, but if you can't, then it has to be done right. But do be aware that aluminium has no endurance limit (or fatigue limit) like steel and titanium alloys, so no matter how light the load cycle, it fatigues. The upshot being, you still have to be at least a little concerned about fatigue, especially around welds, where the sudden change in material cross section (sharp corners, crevices, bumps) can result in cracks starting.

Also, be aware that there are many different aluminium alloys, with many being far stronger than common steel types, though weldability may suffer. I don't design in aluminium (except for helping our local HS's robotics team ), so I can't give any specific advice on welding and fabbing the stuff, but there are folks here who do this work with great success, so you should be able to get what you need.

You might want to consider round tube, bent into the shape you need with no welding or reinforcement other than the mounting plates. I don't remember if you said you had access to a tubing bender, but there are other ways to do this (that I haven't tried personally), like sand-filling and bending over a form.

I don't know if it's necessary, but you could also add X-bracing between the two davit arms to brace it against side-to-side torsion, but I doubt that it's necessary if you tie the dinghy to the boat when it's on the davit.

Just thinking out loud here.

[Tomfoolery]

Regarding tube size, a 1.5" x .25" wall round tube has an area moment of inertia I = 0.199 in^4 and a section modulus of S = 0.266 in^3, while a 2" x .125" wall tube has I = 0.325 in^4 and S = 0.325 in^3. That makes it 1.6 times stiffer in bending, and 1.2 times stronger in bending with only about 3/4 the weight. Square and rectangular tube works the same way.

The downside is when the wall gets too thin, local stresses from attachments and such can be problematic, so through bolts should be sleeved inside the tube to prevent crushing (for example).

Just some more stuff for you to think about.

[Wayne nicol]

thats great info thanks-

do you think that the aluminum would be superior to the s/s in this application?

[Tomfoolery]

As I said much earlier, my experience is in structural steel plates and shapes (and castings), not non-ferrous materials, so I don't have a feel for the peculiarities of those materials other than a little hobby work. Hopefully some others chime in, as there are folks here who do a lot of this sort of work (Highlander included).

I do think you'll find aluminium cheaper, though, and I suspect the finished product will be lighter (for the same strength, all other things being equal), which probably has some value here. I also suspect it would be easier to find someone to weld aluminium over SS, but that depends on the local talent in your area. I think you also need to treat SS welds somehow to prevent corrosion and make them look nice, but again, that's out of my sphere of experience.

[Tomfoolery]

Since I already have the Mathcad sheet that I use for this, here are the values for square tube.

1.5" sq x .0.25" wall
I = .284 in^4
S = .379 in^3
A = 1.25 in^2

2" sq x .125" wall
I = .463 in^4
S = .463 in^3
A = .938 in^2

You can see that the square tube is stronger, though a little heavier, than the round of the same nominal size, and the 2" weighs less than the 1.5" but has more strength and stiffness. I don't know where you'd get 1/4" wall in such a small tube, so you may be into the 1/8" wall, or 3/16", both being common sizes.

2" sq x .188" wall
I = 1.333 in^4
S = .632 in^3
A = 1.359 in^2

The 2" x 3/16" wall tube is just a little heavier (per unit length, as determined by the area A) as the 1.5" x 1/4" wall, but the strength is double, and the stiffness is almost 5 times as much. It's usually better to go with a larger cross section and lighter wall, as long as local considerations don't make that difficult, but even then, local reinforcement is usually how it's handled. Like bosses around holes to thicken it up, or reinforce against local tearing/cracking. That sort of thing.

Edit: You can tell I'm bored at work.

[Wayne nicol]

thats all perfect info thanks,
think i will go the ali, going to place my order now for the tube
many thanks all.

i do plan to put a lighter brace across at the bend and maybe one out at the point of the davitts
would be nice to get some nylon bushes for the stainless steel bolts

[Nautek]

For less corrosion problems with alloy use galvanised bolts

Allan

[Tomfoolery]

For a general idea how resistant this tube is to bending, multiply the force applied to the arm member by the distance from the vertical to the point of load application (250 lb and 26" for me hanging off the end of one davit arm), then divide by the "S" value to get the bending stress. The yield strength of 6061 is around 40,000 lb per square inch.

The 2" x 3/16" wall square tube would work out to 250 * 26 / .632 = 10,284 psi, for a factor of safety on bending of around 4:1. Someone falling onto that arm can easily load it to 2 or 3 times that much, so it still shouldn't bend under those nasty conditions. The 1/8" wall tube would have a FoS of 3:1, which should still be acceptable, though the thicker stuff shouldn't need any reinforcement at the bend if it's bent properly without kinking it.

You might want to consult whomever is going to weld it as to tube thickness. He/she might prefer the thicker wall tube. The 3/16" wall is only 1.6 lb/ft, so this whole rig won't weigh much even with the heavier stuff, but the 1/8" wall is just a shade over 1 lb/ft, so if you're concerned about weight, it might be a better choice assuming welding won't be an issue.

Just a few more data points.

[Wayne nicol]

So... If the chap can weld the 1/8 x 2" then it would be adequate?
But for the tiny bit extra weight the 3/16 would be whole bunch better!

Thanks for all the info mate