Just got driftwood today

[pica_nuttalli]

 

[nc_nutcase]

I didn't go over any of your equations, mathematics or anything... I just glanced at it then looked at your final result... I hope there is a decimal in there I can't see... I see 367 lbf... I'm not exactly sure what lbf (pound feet?) means... is it pounds per foot (which I still don't get)... but pounds per foot would be lb/f and either way 367 is a big number when it is attached to pounds...

I'm not saying your wrong... but can you explain your process and abbreviations for us? Thanks...

Since I originally posted this I've gone over your equations and it makes no sense to me (again not saying your wrong...). A good part of my career includes engineering consulting... so I'm pretty good with maths/algebra (took Calc classes in college... but that was 10 yrs ago and I don’t use it... so I lost it...)... Anyway... my point is I should get it... just need some cluing in...

 

[pica_nuttalli]

yeah, sorry, lbf is pounds-force as opposed to lbm which is pounds-mass.

when i first finished it, i thought the bouyant force seemed absurd myself, so that's why i went on ahead and calculated the weight of the wood. since i seriously, seriously doubt that the driftwood weighs 300 lbs (as we would think of it), i'm betting that you'll just need the extra 12 lbs of gravel or so to get things to sink.

i'm in mechanical engineering and i keep forgetting that the sane portion of the world is unfamiliar with the concepts of pounds-force vs pounds-mass. i was actually hoping that you'd look at the numbers, nc, and i'm really glad you asked that question. i think Raechal would've just thrown the wood out otherwise!

 

[nc_nutcase]

Okay so I’m a geek… I had to…

I calculated:

Cone Volume (Height - 11”; Radius - 2”) = 138.2304 cubic inches

Cylinder Volume (Length - 25”; Radius 3”) = 706.86 cubic inches

Hollow Section (Length 5”; Radius 2.5”) = 98.175 cubic inches

I used a diameter of 5” instead of 6” on the hollow part. If you go with 6” then it is the same as if you ignored the last 5” of the 25” long cylinder, which is obviously not correct, as we can see a little piece of wood there… I don’t know how exact my approach is, but it has to be close. As none of this wood is truly a geometrical shape, all calculations are approx.

Anyway… add it all together and I came up with a volume of .4322 which is 92 cubic inches larger than yours. Some of this is accounted for in the different approaches to the 5” hollow part.

From here… you lost me… I’m not familiar with the equations your using and the abbreviations make no sense to me.

 

[nc_nutcase]

well I'm used to dealing with engineers... I'm not one... but my office is full of them... they figure stuff out... print out plans... then I have to read them and help regular people make sense of it all... I often say that I am a professional translator... I translate Engineer into English

 

[Raechal]

Lol you're right. I was thinking about just pitching it in the back yard. The guy that sold it to me e-mailed me back and told me to soak it and it would get waterlogged for a few days and it would sink or I can speed up the process by boiling it to open its "pores" Is this correct or do I have to weigh it down with 12 lbs?

I know I don't have a tub big enough to boil it in, but what if I got a large durable trash can, stuck the wood in there, boiled water on the stove and then just added the boiling water to the trash can one pot by one. And then repeat this a few times a day?

 

[OohFeeshy]

All I can say is if you have no idea what the engineer language means, ignore it until you do know But pouring boilding water on it might work.

 

[nc_nutcase]

Adding boiling water to it in a tub of sorts will help significantly... not as much as literally boiling it... but it will definitely help.


Water is 8.338 pounds per Gal...

1 Gallon is .134 Cubic Feet...

So your piece of wood displaces 2.23 Gallons of water which weights almost 27 lbs...

So my rough maths says that your drift wood and the gravel/slate you add to it needs to weigh a total of a little over 27 lbs (I added a little because the slate/gravel will displace additional water).

By the way… my maths quota for the day has been met…

 

[StrontiumDog]

Hi Raechal,

When you put any piece of wood in a pan of boiling waterand ,keep it boiling. It does this "heats the wood up, also heating the air contained in the wood. As the air heats it expands. It has then only one way to go out, therefore after it cools down water is drawn it to replace the air"

As you dont have anything big enough to boil it in. I would put it in your back yard in a large container, wiegh it down fill it with water and and wait. Changing the water every so often to get rid of the tannings.

HTH

 

[pica_nuttalli]

I used a diameter of 5” instead of 6” on the hollow part. If you go with 6” then it is the same as if you ignored the last 5” of the 25” long cylinder, which is obviously not correct, as we can see a little piece of wood there… I don’t know how exact my approach is, but it has to be close. As none of this wood is truly a geometrical shape, all calculations are approx.

ditto for me on the hollow bit. but you used a 4" inch radius on the cone-bit, whereas i used a 2". i did neglect that spar on the end since i didn't have any hard measurements on it. but since i'm betting the 25 inches is tip-to-tip, that portion is actually overestimated and included in the solid cylinder portion of the log.

nowadays we're taught to use pounds-mass (lbm) and pounds-force (lbf) as English units to distinguish between the concepts of force and mass (otherwise you're left with a sick feeling when asked to give the force exerted on a 5 pound mass accelerated 2 ft/s/s).

my fluids book does all the English measures in feet, so i went on ahead and did my work in cubic feet. the funky p is rho, or density (bet you knew that). the book gives the density of water at 80F as 62.1 lbm/cubic foot & the density of a soft wood is 32 lbm/cubic foot.

gravitaional acceleration is 32.17 ft/s/s, but since it is just operating as a conversion factor between force and mass, i notated it as lbf per lbm. when i was doing this, i was still uncertain about whether the typical scale measures force or mass, so i went on ahead and included it (which in retrospect, was unecessary ).

given that the displaced volume of water is equal to the volume of wood:

• Bouyant Force = density of water * gravity * displaced volume of water
  and Weight = density of wood * gravity * volume of wood
  Required Force = Bouyant Force - Weight

since 400 lbs seemed excessive for a peice of driftwood, i decided that scales measure mass. therefore i converted the required additional weight from pounds-force back to pounds-mass by dividing by gravity. this then gave me the 11.5 lbm i drew a box around. if you eliminate gravity from my original calculations you get:

• Bouyant Force = (62.1 lbs/cubic foot) * (.3789 cubic feet) = 23.53 lbs
  and Weight = (32 lbs/cubic foot) * (.3789 cubic feet) = 12.12 lbs
  Required Force = Bouyant Force - Weight = 11.4 lbs

feel better?

 

[littlefishie]

Holy crap. and I thought me knowing how to do derivatives in calculus was complicated!
Nice pink pen though.

 

[pica_nuttalli]

the only complicated thing about this particular process is figuring out how to best approximate the volume from 200 miles away!


really, anyone can use these equations so long as they've gotten a pretty good estimate on their driftwood:

• Bouyant Force = (62.1 lbs/cubic foot) * (Cubic Feet of Wood)
  Weight = (32 lbs/cubic foot) * (Cubic Feet of Wood)
  Required Force = Bouyant Force - Weight

i just have a habit of giving more steps than i really should... except for all those places where i don't give enough!

--EDIT--
Actually, the best way to find the weight of your wood is to just use a friggin' scale (me<-- pure maths addict)

 

[littlefishie]

No, you lost me after the wizard smiley!

Though the words aren't confusing.. it's just the order they're placed in.

 

[nc_nutcase]

ditto for me on the hollow bit. but you used a 4" inch radius on the cone-bit, whereas i used a 2". i did neglect that spar on the end since i didn't have any hard measurements on it. but since i'm betting the 25 inches is tip-to-tip, that portion is actually overestimated and included in the solid cylinder portion of the log.

Actually I used a radius of 2, the 4 was a typo. I'm better with numbers than I am letters

* Note : Original post with the typo has been edited

 

[pica_nuttalli]

then it must've been how we read/incorporated the hollow bit.