Gundog, 5052's tensile is lower than 5086 or 5083' so it will "tear" (or fail) with less force. If a panel of one size, say a 12"x 12" panel, of the hull of a boat that weighs 2,240 lb. all were made of each of the three metals each would resist puncture from the same exact rock/pinnacle/steel stake/pointed object based solely on the tensile strength or (first yield) then 'ultimate' or failure point of the material.
What usually happens is we compare boat A with boat C or E so the result of the 'tests' are not really an apples-to-apples comparison. If you've spent any time on the BoatDesign.net and read some of the threads discussing these types of issues (?) you'll see these discussions ( of material properties) can be "
non trivial events"; as a close friend reminds me constantly.
I suggest- the only way to make this comparison is with a hand gun.
No one should take my word for this, but everyone should set up a Myth Buster's type of experiment- using hand guns!! 
Take equal thickness pieces of exactly equal dimensions, of each alloy in question; and support them on a 2x4 frame. (for years at the old shop what we used were 2x4's at 12" inside clear area and we'd cut 15" squares to screw onto the frame flat but fully supported at the edges ) then fire the same exact handgun and loads into the three pieces.
I think this is a "
fair" comparison (?
but that may not be true? and I'm willing to discuss the construction of a 'good test') as the bottom of a boat hitting the rock? Here is the fast traveling object and the 'boat' still- but we still get to see how the metal bends or stretches or resists the same exact forces and can therefore compare the metal not using different tests and then trying to discuss the tests.
A boat is a ton or two moving at 45 50 mph and the bullets are a few grains and moving a thousands of feet per second, agreed, but the idea is to see which metal will 'take the bullet' and how it will behave.
Which alloy(s) hold and stop the bullets? Which alloy(s) doesn't? This is the same boat bottom that will do the same thing as for the boat made of that material. My advice is to use the .38 and modulate the loads from some wad cutters to something hot and jacketed. .357 or .40 cal and up will take some 1/4" plate samples to test so things get 'scaled' up and at .44 you'll need 3/8" and 1/2" (or more in 5052) to do the same experiments. (at least that has been my experience)
I don't say 5052 has
no place, and is not a great product..... but its less expensive for a reason. It offers less performance. A 5052 boat and a 5086/83 boat of the exact same wt and shape and all other conditions being exact except alloy- (that means framing, welding schedule, and all other details) the stronger alloy is a strong overall boat.
Elongation before failure is not going to stop the softer material from failing sooner. If that were true then sheet lead, which will really stretch, should be good for impacts, and its not that good overall. The bullet test should demonstrate this for anyone who'd like to spend a hour setting it up. The only time elongation would be enough to absorb an impact would be where the impact is below the tensile anyway... therefore elongation is just a dent that the '86 and 83 alloys wouldn't have experienced or not as deep.
5086 and 5083 have different bend radii' because they are so much stronger and stiffer. 4T=R is what I use, and that excludes most of all presses or leaf folds not using a 1" nose bar! I know that sounds like a huge radius but its what is needed to keep from strain hardening the '86/83 alloys and '83 will fracture at that radius some times as well.
Not only are the '86 and 83 alloys stronger but as all 50series alloys
gain both tensile and hardness when cold worked to the elastic limits (press braked, press formed, punched with rims, rolled) they gain more than the 5052 does, disproportionally more.
So a hull longitudinal of 8" width bent to a 6" x 2" angle in '52 will harden and increase its tensile along the bend zone and therefore that member can be reduced to 7-1/2" and the flange can be say 1-1/2" instead of 2" and the increased strain hardening and tensile shift will compensate for the slightly reduced scantlings.
But by the same token as the '86 and83 alloys being tougher in all aspects originally they gain more tensile and hardness for the same plastic deformation. It goes without saying you have to accept bends in a radius sufficiently large to avoid fracturing in order for this to hold true. So you can't bend the alloys with a steel knife edge and expect to gain much except more scrap.
Now back to the hull longitudinal. So the 8" 5052 piece was reduced by pressing to (say) 7-1/2" but the same member in '86 or 83 can originally be 6" total depth and bent to 1-1/4" x 4-3/4" and deliver the same 'as welded' strength to the longitudinal hull stiffening. Now... adding the increased tensile along the bend line and the extra hardening of these alloys and that same member can be reduced to a 1" x 4" member and perform equal to the 6" tall 5052 long.
(all my dimensions in these hull long examples are for the point of illustration;I have not taken time to do the comparative math of these longs but the relative depth vs alloy vs cold forming differentials are shown.)
Last item about impact and damage. If we take three exactly identical boat, moving at exactly identical speeds and were some how able to run them over exactly identical objects, we'd get results based on different reactions to the impact- those results would be dependent on the alloy's performance in impact.
Time is a very big deal here, but its happening too fast to 'see'/understand- easily. Each alloy will have identical forces at the same place at the same time. All the alloys will remain at or near the exact original location (no denting) and will continue to move forward (unless the impact at at the bow stem ! then it move upward)
for some tiny fraction of a second. As the materials at the location of the impact are strained (that word is kind of important) each one will react based on its chemical composition and heat treatment. (remember for our discussion we're ignoring differences in support, framing, welding, and 'back up')
Each one will react in a time period based on the rate of approach of the first stage of bending. (yield/bend/elastic deformation) So if a rate of change is 1000 lb of force per 0.10 second then the alloy with the lowest/lower yield/failure is going too begin deforming or failing sooner in time than the harder/tougher/stronger alloys.
If the forces continue beyond the yield or 'elastic' force levels for that material, then... the forces may reach up to ultimate or failure levels. (plastic or deforming force levels) That is when the material will tear, puncture, or otherwise part. (rent asunder/FAIL)
To say the softer material will stretch or elongate and therefore
absorb the impact without tearing seems likely it looks like an intuitive response to the impact. But the fact is the tougher stronger material will last better-as it will last longer in time. That time span means the stronger yield material will be less bent/gouged/dented when the boat move past the impact.
What happens when a blow hits a panel below the bullet speed impact? Depending on the material the forces are usually transferred into the overall structure and the location of the impact is not at or above the yield or ultimate strength force values. So the boat bounces.
Bouncing without dents, then bouncing with a dent, then not bouncing but puncturing or tearing are relationships to that particular alloy's ultimate tensile strength. 5052 is just lower than 5086 and that is lower than 5083.
I'm not an engineer, but I've shot a
few pieces of aluminum and this is how I understand the Miracle Metal to behave.
netman, I hope no one reading my remarks thinks I have any
fears of building in 5052? I have built some very nice boats of 5052 and will again (Lord willing and the creek don't rise) I was just trying to give a comparison to the tougher, stronger, and much more expensive alloys in reply to the first post.
gundog, this is not a simple topic for online discussion, and I hope you understand that I'm not authority just someone that has used aluminum for a while and these are my impressions, which may not be accurate?
Cheers,
Kevin Morin
I will be keeping the boat on a salt water mooring for 6-8 months a year.
As Jetty alluded to, you have re-taught me Metallurgy 1001 with your well worded explanations. Many freshwater alloys like jons and the smaller skiffs are mostly 5052 I have discovered, as I seek a small one to fish the seldom fished upper reaches of some tributaries here. Beaching here is one of the Lakes is simply running up on Georgia Red Clay, a substance foreign to Alaskans or if in a creek, a few rocks but primarily fallen timber debris. And you will NOT be moving fast either, so impacts aren't a worry unless you are the $6 Billion Man and can paddle like 30 mph. Now the work boat side is entirely different with 5086 standard and the Mudboat guys use it exclusively in 3/16-1/4 plate as smooth as a baby's bottom for their swamp piercing tank-like skiffs.
