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Author Topic: Floatation values  (Read 1856 times)
walknbob
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« on: April 21, 2009, 06:21:28 PM »

Okay this is for you engineer guys who love problems to solve. I know we have had similar discussoins in the past but I think it was back in the yahoo email group.

Here goes.

On each side of my cockpit I have 4 limbered, below deck spaces. 2 on each side are accesible via airtight (so to speak) hatches. One on each side is not accessible at all and one on each side has a large hatche and is used for storage of fenders and a couple other items that being wet is not a problem.

I just spent a couple hours filling the two chambers on each side, 4 total, with used plastic bottle that would normally be thrown away. I duct taped the caps to make them even more water/air tight. In each of the 2 smaller forward chambers I stuffed 14, 20 liquid ounce bottles (total 28). In the next 2 somewhat larger chambers I stuffed 14 each 1 litre bottles (total 28). In each of the chambers in which I store the fenders I have a 22"x8" dia and a 18"x6" dia dender so two of each when counting both sides.

My question is this. In saltwater, how much floatation have I provided for. Thats a two part question. Based on the displacement that should equal some amount of lbs of water. Then based on that lb factor how many pounds would that float given that a submerged item requires less than the same item sitting on dry land. What I am hoping is that I might be approaching the value that would float my DF140. if I were to somehow become awash. Because if that is true, and assuming that the rest of the boat is at least neutrally bouyant then all that is left is the gear on board to contend with... most of which is jetissonable if push comes to shove.

OOhhhh I can hear the brain cells burning already  Grin
« Last Edit: April 21, 2009, 06:58:09 PM by walknbob » Logged

WalknBob aka Bob Southwick - Anchor Point Alaska
The risk of collision became an issue the day the second boat was built.
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« Reply #1 on: April 21, 2009, 07:27:42 PM »

umm, i guess i'd be one of them..

'conversion' of weight on land vs weight in water is the difference of the objects volume times its density, less water.

for example,  consider a cubic foot of each-

doug fir is about 35 lbs per cubic foot, or pcf. in water, it 'weighs' 35-62.4 = (-)27.6 pcf, so it floats. it can float 27.6 lbs of weight as well and still be neutrally buoyant.   
ironwood on the other hand is like 70pfc, so in water, it 'weighs' about 8 pcf, it sinks.
a cubic foot of steel, is about 398pcf. in water, a cubic foot 'weighs' 335.6 lbs.

got it? its that easy. the hard part is figuring out what somethings volume and density are.

so, what does the 140 weigh? somewhere around 400 lbs? lets neglect the buoyant weight of the motor and just use gross, ok?  so, for your df140, i assume 200 kg = 440 lbs (yes, we are going to do this in metric because counting in units of 10 is easier than ounces and gallons)

seawater~ 3% salinity? 1.03 kg/liter  (i am a sweet water guy).

 => 200 kg /(1.03kg/liter) = 195 liters. to be approximately neutrally buoyant, you need to displace 195L of seawater.  that is about 100, 2 liter bottles to float the motor. count how many pop bottles you used, times their volume.

to put it in perspective, 9 cubic feet, (cubic yard) is about 250 liters, of 1/4 a cubic meter.  254.5 actually, but close enough for gob't work. DAMHIKT.

your 28, 20 oz bottles are 0.6L each, or 16.8L, 28- 1L bottles, 45L total, for 46kg of seawater or 102 lbs of NET buoyancy.
 
fenders, large are about 18L each, small about 8L each, total of 53L, or 55kg of seawater or 120 lbs of NET buoyancy.

so about 222 lbs of bouyancy.

your fenders are much more buoyant compared to the pop bottles.

don't forget the other 'heavy' stuff like batteries. lets guess a battery weighs 44 lbs, that is 20 kg, or about 20 liters of displaced volume of seawater. that is 10, 2 liter bottles.

you need more bottles.

where can i send my bill?  its due net 30.  Tongue

dennis
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walknbob
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« Reply #2 on: April 21, 2009, 09:11:15 PM »

umm, i guess i'd be one of them..  'conversion' of weight on land vs weight in water is the difference of the objects volume times its density, less water.
for example,  consider a cubic foot of each-

You engineers kill me! It would have taken me a week even with a calculator to figure all that out. So... essentially I am at about 1/3 the necessary flotation to float the motor assuming a total holy shit disaster swamping. Thats not bad... If I got real ambitious I could even open another hatch in the rear compartments and add another 50 lbs plus flotation then put hathes on them.... Hmmmm may be worth the effort.

Oh yeah your check is in the mail... via sled dog but I am sure it will get there... probably with an Iditarod stamp on it  Grin
« Last Edit: April 21, 2009, 10:29:11 PM by walknbob » Logged

WalknBob aka Bob Southwick - Anchor Point Alaska
The risk of collision became an issue the day the second boat was built.
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« Reply #3 on: April 21, 2009, 11:21:14 PM »

bob-

the difference of actual weight and volume is what i was trying to get across. to be conservative, you can use the actual weights of things and ignore the land weight and water weight.

i am guessing that the DF140 weighs 440 lbs, which works out to 200 kilos. so GROSS motor weight equates to a volume of seawater of about 195L. since the bottles are graduated in fluid ounces and liters, liters are easier to keep track of.
 
your empty bottles are worth about  45L. the fenders are worth 53L. total is 98L. you are about half way there.

44lb batteries are about 20L each.

here are some other constants to write down and figure with.

a 16 fluid ounce bottle displaces enough seawater to balance about 1 lb of land weight. that is 0.5L
a 1L bottle will displace about 2lbs of land weight.
a 2L bottle will displace enough seawater to balance out about 4.4 lbs of land weight.

a cubic foot, or 1728 cubic inches will displace enough seawater to balance out 64.3 lbs of land weight.

so for all the gear you decide to try and balance out with flotation, use those numbers.

example- the 10lbs of crap, stuffed into a 5lb bag needs almost 3 2L bottles. 

now consider how many cubic inches of sealed volume you have under the decks and your reserve buoyancy is HUGE. iirc, this is why renn does not like using foam below deck. keep nice airtight volumes under the floors and you will be good.

dennis
ps: i'll feed the dog sled team when they get here.
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larspa
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« Reply #4 on: April 22, 2009, 09:48:44 AM »

Sinned,

Did you neglect to factor in the bouncy of the hull itself?   Given the amount of wood in the craft, its non-trivial.   Of course, there's the FG.  What does it weight per unit volume?

<sigh>  Wouldn't be easier to design a quick release mechanism for the motor mount with a button or lanyard near the wheel?  Grin Grin

TE
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Jumbo 24 / Skiffkits
Build start: Sept '09
Loc:  Hood Canal
larspa
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« Reply #5 on: April 22, 2009, 09:55:00 AM »

Bob,

Discussion on pool noodles:  http://www.fishyfish.com/boards/index.php?topic=201.0

Also one I started:  http://www.fishyfish.com/boards/index.php?topic=358.0

But I think the one you may be referring to  involved Brian...and I think you're right, it was on the Yahoo group.  Try a search there.

TE
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Jumbo 24 / Skiffkits
Build start: Sept '09
Loc:  Hood Canal
sinned
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Posts: 245


« Reply #6 on: April 22, 2009, 12:30:34 PM »

larspa-

in only calc'd the buoyancy of the pop bottles that  walkingbob posted about. i understood that he wanted to consider floating only the motor.

reserve buoyancy of the hull itself is a good question. i ran some numbers and have been surprised by the results.

generally, we estimate the weight of plywood at 3 psf/inch of thickness, or about 36 pcf. its the same basically as the unit weigh of the wood okume is probably a little lighter, DF or SP is going to be a little heavier. so, for 1/2", that is about 1.5 psf of weight.

assuming 6 oz cloth, epoxy coverage is about 80 sqft/gal. assuming the specific gravity of epoxy is 1.15, a gallon of mixed epoxy is about 10 lbs/gallon. so 0.125 lbs/sq ft of epoxy. the FG is 6 oz/9ft^2, so about 0.042 lbs/sq ft per layer. i would guess that 10oz fabric would be 50% more.

so for 1/2" ply, covered in 6 oz of cloth on each side, total weight would be 1.5+0.25+0.084= 1.83 psf. the weight of the fiberglass and epoxy is minimal.
 
seawater, at 64.3 pcf, works out to 5.36 psf/inch, or 2.7 psf / per 1/2". so the difference is about 1 lb of reserve buoyancy per square foot of plywood.

another thing to point out, while i give these numbers to 2 places, the precision is not really there. we are ESTIMATING things.

another way of looking at it is to use the gross material quantity. iirc, 1 sheet for the tansom, 6 for the bottom and 6 sheets for the sides. so 13 sheets of plywood, 32 sq ft each is 416 sq ft, at 1 lb of reserve buoyancy each is about 416 lbs of reserve buoyancy. that is not bad.

there are MANY things that we are neglecting, things like the rest of the framing materials, stringers, trapped volumes of air like the crash bulkhead, etc.. so the more precise you get when you are counting everything, each little bit adds up. i think all this figuring basically supports Renn's position that there is enough reserve flotation in the boat.

dennis
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gmclain
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« Reply #7 on: April 22, 2009, 12:37:59 PM »

If it gets that bad the hell with the motor where is my life jacket!
Glenn
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JMB
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« Reply #8 on: April 23, 2009, 01:59:00 AM »

Thank you for all those baffleing #'s.  Why not add an enocuous "crash bag", or in this case "sinking bag" to the top of O.B. so when it ............it will float the stern of the boat....or, stand the boat on its nose.
Just my $.02.
JMB
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