How I figured flotation

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aeviaanah

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First I figured sq inches of under seats. I left half of the back seat open for storage.
The first equation represents the rectangular portion of the seat, the second is the slanted area on edges of seat near side of boat. Each side is roughly half of a rectangular box. This helped get a better estimate of total sq inches.

Front seat -
10 x 9.5 x 34= 3230 sq in
7.5 x 4 x 9.5 = 285 sq in

Middle seat
10 x 9.5 x 42 = 3990 sq in
9.5 x 6 x 9.5 = 541 sq in

Half of back seat
9.5 x 9.5 x 20.5 = 1850 sq in
(9 x 6 x 9.5)/2 = 256 sq in

After adding all together I come up with 10, 152 sq inches. I divide by 1728 to convert to sq ft. I come up with 5.87 sq ft. Since the foam is not a perfect fit I will go down to 5.5 cubic ft of foam.
__

Now on to the total weight of boat. I have a 12' aluminum mod V. Similar models were between 140 and 160 lbs.

Boat weight empty
160lbs x (alum. factor .63) = 100 lbs.

Weight of motor 115lbs
Swamped = 105lbs

Weight of battery 45lbs
Submerged = 25lbs

Gear etc 100lbs

*I didnt figure weight of 6.6gal gas tank as they say gasoline is lighter than water so it will float.

Add all this up we get a total of 330lbs. Now i factored in the wood on boat.

I have 50 lbs of plywood.
50 (factor -.81)= -40lbs

330 - 40 = 290lbs.

To figure amount of flotation in cubic ft I took 290 and divided by 60.3. I come up with 4.80 cubic ft of foam required for boat weight.
I didnt figure in weight of persons as I just dont have enough room for foam...anyone who I take will need to be good at swimming and we do have life jackets. What do you guys think did I figure this properly?

5.5 cubic ft of foam in boat.
4.8 cubic ft of required to float boat.
 
Are the seats bolted in? If it sinks, the could pop out and not hold up the boat.

KRS
 
No offense but your math is off by a whole dimension. When figuring volume as opposed to an area, its cubic inches, or feet....not square anything. Square is for an area.

Also, your plywood, battery, and motor aren't neutral in water. Set your battery in water and see how it will act. My guess is that it will be second to an anchor! Consider any added goods, added weight to the boat itself then you'll have a realistic sinking figure.

I don't want to piss on your parade but I also don't want you thinking your boat can simply float because you have enough wood and cubic inches of foam.
 
clarkbre said:
No offense but your math is off by a whole dimension. When figuring volume as opposed to an area, its cubic inches, or feet....not square anything. Square is for an area.

Also, your plywood, battery, and motor aren't neutral in water. Set your battery in water and see how it will act. My guess is that it will be second to an anchor! Consider any added goods, added weight to the boat itself then you'll have a realistic sinking figure.

I don't want to piss on your parade but I also don't want you thinking your boat can simply float because you have enough wood and cubic inches of foam.
My math is correct, I'd say my diction is incorrect. I meant cubic inch when I put square inch.
 
Looks good, although I've found that the weight of water per ft^3 is 62.5 lbs instead of 60.3. Not a huge difference, and shouldn't really affect your calculations dramatically. However the one thing to consider is the location of your foam. While you may have enough to keep the boat from completely sinking, the distribution of your weight vs the foam can mean that you have a boat that stands up under water, meaning all of your gear goes under.

You will typically have the majority of your weight at the back, Motor, battery, yourself, etc. If possible, focus your foam in the back. Maybe a false floor with foam underneath?? That will help you in the case that it ever does go under.
 
KRS62 said:
Are the seats bolted in? If it sinks, the could pop out and not hold up the boat.

KRS
Seats are 3/4 pin mount. They arent bolted in but arent easy to take out.
nomowork said:
Impressive, but the only way to actually see is to put it in the water, slowly.
Yea Id hate to get everything wet on the inside of the boat, with new carpet and decking its just extra wear and tear I dont need. I do understand that is the only way to check how everything will react.
 
My mistake....I thought you were calculating the foam IN the seat cushions and were expecting that to (help) hold up the boat. I see now that you are talking about the place under where the seats are mounted.

KRS
 
KRS62 said:
My mistake....I thought you were calculating the foam IN the seat cushions and were expecting that to (help) hold up the boat. I see now that you are talking about the place under where the seats are mounted.

KRS
No i didnt calculate foam in seat. I actually used the weight of the seats against flotation.
 

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