Flotation Foam

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Chessie
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Flotation Foam

Post by Chessie »

I’m assuming this is the more appropriate place for this question.

I have a kayak that I recently installed an electric motor on the back plus a 12V battery. Because of that, it squats in the water. Not horrible, but enough that I’d like to balance it out.

Using the 2# flotation foam that would be used for the underside of a sole, will that provide additional flotation if put inside a kayak.

Does the kayak buoyancy come from the hull displacement and foam will have no added benefit? Does the foam only help a boat if it fills with water?

Trying to wrap my head around the concept of what does or doesn’t add buoyancy.

Any help appreciated.

Chris
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VT_Jeff
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Re: Flotation Foam

Post by VT_Jeff »

No benefit. You'd need to add volume to make it float higher. Put the foam outside the boat, then you're in business.
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Re: Flotation Foam

Post by Jaysen »

You can “reduce” the effect by putting some weight forward. While this lowers the bow it raises the transom by pivoting on the center of buoyancy.

The part you have to keep in mind is that it’s about volume not weight. Hope that makes sense.
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fallguy1000
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Re: Flotation Foam

Post by fallguy1000 »

Adding flotation foam to the arse of that yak will make it sink even more! Don't.

Jaysen's answer is pretty right on. If you add weight to the front of the boat; it would offset the aft weights; the problem is all you are going to do is sink the yak deeper in the water. And performance will decrease because the wetted surface area increases and so does drag.

On the off chance the bow is riding rather high and you have some available displacement; you might be okay adding weight. Typically, a kayak has varying degrees of sinkage and the only way to determine best performance is to add some weight forward and see what happens to your speed. I'd avoid adding the same amount, but say your battery and motor are 60 pounds; you might try putting 20-30 pounds forward.
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VT_Jeff
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Re: Flotation Foam

Post by VT_Jeff »

If you're pushing the kayak with a motor, adding external flotation is not a horrible idea. Something like this that would mount on the stern.

https://www.wavearmor.com/product/kayak ... gJuTPD_BwE

You could make a few cheap Ama's out of 3' long pieces of 6" PVC pipe and just strap them directly to either side of the stern, since it's the flotation and not stability you're after. Make sure they are low enough to actually provide lift. Crazier things have been done.
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TomW1
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Re: Flotation Foam

Post by TomW1 »

Adding both the motor and battery to the back end of your yak was your down fall. Can you move the battery forward? This would offset the weight of the motor at the stern and even out the weights some what. I would not worry about foam, it doesn't weigh enough to help in your kayak, nor would you even get a 2 gallon kit in it at 480cu.ft. :lol: at 20lbs, in and it would not lower your front end much. Stay with your flotation bags or what you have now.

Well good luck on your decision.

Tom
Last edited by TomW1 on Thu Mar 04, 2021 1:16 pm, edited 1 time in total.
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Chessie
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Re: Flotation Foam

Post by Chessie »

Thanks everybody. That’s what I thought.

In my mind two equal canoes (or kayaks) one filled with foam, one empty: you would think the foam filled would be more buoyant and be able to handle more weight.

In testing, I did move the battery to between my legs and it helped, but not an ideal setup for fishing. It’s the eternal give and take of a boat.

I may look at the additional external float option, just gets bulky. Maybe a foam, fiberglass experiment😂

Chris
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Re: Flotation Foam

Post by narfi »

For buoyancy, it is all about how much water is displaced.

***
Imagine a 1 gal paint can. We all know what it looks like and how big it is, so an easy thing to picture in our mind.
We know that it will displace 1 gal of water (thats the size the can is right?)
For example, you fill a bathtub exactly to the rim, then you push the gal can down into it. 1 gal of water will run over the edge of the tub and if you remove the can, you could add one gal to the tub to get it back up to the top again.
We know that water weighs 8lbs per gal.

So. If you push that 1 gal paint can down into the water, you are displacing 8lbs worth of water. That 8lbs of displacement works as a force on the can and you have a number you can picture and work with.

Imagine the can weighs nothing. You can put 8lbs of 'stuff' into the can before it sinks down to level with the top. Less than 8lbs and it will rise, more than 8lbs and it will sink.

Now imagine you filled the can with flotation foam. I wont math out the actual numbers but suppose the foam weighs 1/4 of a pound.
IF the can actually weighed 0lbs like we are supposing in this example, empty it would sit with the bottom right on the surface of the water. Even though the can can displace 8lbs, it is only displacing as much as it weighs and sinking into the water the appropriate amount for the weight. So for 0 pounds, it sits on top. if you add 8lbs it is fully submerged. If you add 1/4lb of foam it will sit with 1/32nd of the can submerged (8lbs divided by 1/4lbs).

Does that make sense?

To take this even further, you want your kayak to sit at the same level with 40lbs added weight. To do this you need to add 40lbs of displacement(under the waterline) 40lbs / 8 = 5gal worth of 'area' you need to add under the waterline. This can not be added inside your already existing vessel as its displacement wont change, so you would have to add outriggers, or hull extensions with that much volume below the waterline.

OR.... to make up that 40lbs you could eat at a 500 calorie deficit per day for 40 weeks or 1000 calorie deficit for 20 weeks. (provided you have in excess of the 40lbs in reserve to loose(I did))







***(All simplified numbers, estimated and rounded for the sake of simplifying the example.

fallguy1000
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Re: Flotation Foam

Post by fallguy1000 »

Chessie wrote: Thu Mar 04, 2021 1:35 pm In my mind two equal canoes (or kayaks) one filled with foam, one empty: you would think the foam filled would be more buoyant and be able to handle more weight.
Nope. Opposite. Foam adds weight.

I built amas based on Gary Dierking's work. Used 2" pink polystyrene with pvc tubes for cross member links and maple slats bonded together for the akas with a pumbing fitting.

The issue with the floats is they don't add buoyancy. I am sure I am going to sound like an arse. Sorry to all.

They only add a resistance to tipping. There is too much flex in the akas to really support much load and if the amas are down in the water deeper; they drag. Mine are two positions; unloaded and loaded. When I am loaded, they get moved up. The purpose of amas is to keep me from tipping in 39 degree water. They actually add weight to the boat!
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