Light weight flotation

[FE Wannabe]

Better go back and take high school physics again! To float, an object must weigh less than the water it displaces. Most of your body is water so it has close to neutral density/buoyancy. Your bones are the densest part and are why you would sink at all. Your body will float without any flotation if your lungs are full. That is why a small pool noodle will help to keep your head high above water.

A boat is far far different. Nothing it is made of is close to neutral density, all its parts have densities far higher than that of water. Plastic, metal, lithium...all are far denser than water. It is the density that is the difference between how a boat and a body float. The flotation we add is not to keep the boat floating high above the water, it is to maintain the buoyancy just high enough to keep the boat awash at the surface.

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Jay,

I have seen the error of my ways and I do realize that my using the human body as an example was wrong for my explanation.

However, any object submerged in water, even dense metals, have a bouyancy effect that is exerted upward against the object making weigh less than it would if weighed above the water.
This can be seen by anyone who has ever had to pull up a boat's anchor by hand. As soon as the anchor breaks the surface of the water its weight increases noticeably.
Granted the relative density to water of the object being submerged may make the bouyancy seem to have more or less of an effect.

Maybe your 29ci/lb calculations for pool noodle bouancy take this into account, but I could not find any specific density information for pool noodles to verify.

By my calculations, the below boat making materials are a percentage lighter when submerged in fresh water (based on relative desity to fresh water):
Fiberglass 65% lighter (depends on content of resin)
Aluminum 37% lighter
Steel 13% lighter

All I am saying is that a boat that is 6 pounds on dry land will weigh less when submerged and should need less floatation than a boat that weighs 6 pounds submerged.
Brad

[Fluid]

Brad, you are making things too difficult. Every boat will have a slightly different density depending on the balance of materials used in the build - but all will be pretty similar. A rule of thumb of 29 cubic inches covers all boats with a safety margin. Not many boaters want to measure the actual density of each component in their boat, then figure out the exact amount of flotatation needed!

In my example above, a 2.5" x 60" noodle weighs 3 ounces. It has a volume of ~270 cubic inches, so it will displace almost 10 pounds of water. That is ten pounds of lifting force - enough to keep a ten pound boat on the surface. If you want to split hairs, it will keep a boat which weighs ten pounds under water at the surface. You go ahead and split hairs, I'll just keep things simple where possible. There is enough about this hobby which is difficult for the average guy.


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[FE Wannabe]

Jay,

I understand that it appears that I am splitting hairs, but bouyancy has been a subject I have been looking into while I was building a lifting platform for my PVC rescue boat. I wanted to better approximate the amount of lift I needed to be able to raise a partially submerged boat to a level where it could be brought back to shore. Even if I was not able raise the boat completely out of the water, I wanted to at least be able to lift it enough to contain it and bring it back.

I do appreciate every bit of wisdom you impart to us relative newbies!
Brad

[martin]

As long as part of my boat is still sticking out of the water with flotation so i can see where it is im happy & not on the bottom. I use pool noodles as i found it difficult to get enough air bags down into the boat, Where as with the noodles you can just cut odd pieces & slide in any wear. Thanks.

[mm123521]

Fluid:
FE Wanabe is correct. Every object in the boat is bouyed up by its volume.
If you have a battery that is 10 cubic inches, it counts toward the total.
If you have a motor thats 4 cubic inches it counts toward the total.
The volume of the stringers counts.
All the volume of the fiberglass hull counts (not the internal volume of the hull!).


Each of these items will be bouyed up by its volume when submerged, so the boat will actually weigh less when submerged.

I have a 15 pound boat and it needed 15 X 29 cubic inches = 435 cubic inches.
I found it almost impossible to find this volume in a 34 inch cat.
Then I realised that the volume of all the items in the boat count toward the total.

1 Weigh your boat.
2 multiply the weight by 29 to get a total volume.
3 subtract the volumes of batteries, motors, RX battery, etc. from the total volume in step 2. the result is the required volume of flotation (to be added) to just keep the boat floating.
4 Add a little more flotation as a safety margin.

I understand that you want to keep it simple, but in some hulls it may be very hard to find the "rule of thumb", 29 cubic inches per pound without considering the displacement of the objects in the boat.

A 6S 5000MAH battery is worth about 20 cubic inches of volume. Two of those are 40 cubic inches toward the total, that is 40 cubic inches of flotation you dont need to add.

Mark


Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object--->
Archimedes' principle

29 cubic inches of water weighs 1 pound.
If you had a 29 cubic inch block of aluminum (weighs 2.8 pounds in air) in your boat and your boat sunk, then that aluminum block would only weigh 2.8 - 1 = 1.8 pounds underwater. Thats because it displaces 1 pound of water(see Archimedes' principle above). So you only need 1.8 X 29 = 52 cubic inches of pool noodle to just keep that block of aluminum up (with no safety margin), not 2.8 X 29 = 81cubic inches.

You see, 29 cubic inches of anything (lead, air, batteries, plutonium, fiberglass, gold, cat poop, etc) submerged under water will weigh 1 pound less, resulting in 29 cubic inches of flotation you dont need to add to your boat.

[bbdave]

Hi i have read this with interest and i'll just add my two penneth worth from my full size boat experience

Expanding foam in time soaks up water and becomes useless as it is not a closed cell foam (pool noodle) it becomes a sponge.

Air bags are fine i use them in sailing dingies but they are made tough not sure i'd trust a poly bag in a model boat a tad vunerable

[HOTWATER]

2 part expanding foam IS a closed cell foam and does not hold water. The spray in foam, however is not closed cell foam....