Brainstorming help needed: Inflatable + polystyrene + polyurethane

[engineer137]

I would like to request some advice from those of you here with much more boat experience than me.

I have been searching this forum and doing a decent amount of research on my own but before I buy the final ingredients, I just wanted to double check and make sure I'm not about to do something foolish.

Abstract:
Make inflatable kayak rigid


Requirements:
Spend as little $$ as possible, preferably $35-$50


Preexisting materials:
-Two inflatable kayaks
-HH-66 vinyl cement, plus surplus vinyl from another raft
-Lots of single-compound polyurethane spray
-Foam gun


Background:
A few years ago I bought two inflatable kayaks at Walmart that were being clearanced out super cheap. My wife and I really enjoyed using them and we eventually bought nicer paddles to go with them. Then last year at a family reunion we rented some rigid kayaks and they just worked so much better. We have another family reunion next month but we really can’t justify buying rigid kayaks at this time. I looked into the sawfish and other DIY kayaks, but they were usually $100+ or so.

I built a tiny home out of a school bus (which we have been living in for the last two years) and I have several large cans of spray foam left as well as a nice spray foam gun. But my research shows that this foam is not suited for boating. It doesn’t fill enclosed cavities well (needs water to cure, plus the CO2 pressures it creates are not very high) and it can get water logged easily.


My idea:

1. Buy tiny polystyrene beads. I can get 75 gallons for $35. (they are similar but smaller than the ones in bean bag chairs)
   
   
2. Fill the inflatable kayak with as many of these beads as possible.
   
   
3. Accidently spill some beads which will get everywhere and static cling to everything.
   
   
4. Vacuum up the spilled beads, swearing we go them all, but somehow missing a surprising amount, which pop up over the years here and there and remind us of that fun, but somewhat crazy, weekend when we filled two boats with polystyrene beads.
   
   
5. Pour a small amount of water into the cavity and shake it around so it lightly coats the foam beads
   
   
6. Attach a long narrow tube onto the spray gun sprout and spray the polyurethane foam into the cavity
   

Positives I see:

• The only thing I should need to buy is the $35 of polystyrene.
  
• The vinyl from the inflatable kayak should protect the foam against UV light damage
  
• I’m hoping that I can get 90-95% of the volume to be polystyrene which will keep the amount of polyurethane to a minimum
  

Unknowns and potential problems:

• Will the polyurethane dissolve or adversely react to the polystyrene? My experiments so far show that there is minimal interaction between the two. But millions of tiny spheres have an unbelievably large surface area.
• The inflatable kayak has several independent chambers. Should I try to fill them all with beads and foam, or just focus on the larger ones? I suspect this is a law-of-diminishing-returns type situation.
• As the spray foam expands, and especially when it comes into contact with water, the outermost shell hardens and keeps the foam from expanding further. I suspect that I will need to make additional holes in the boat and not just rely of the valve opening. I was thinking of using a long thin metal tube kinda like an oversized medial syringe. One thought is to find a really long metal tube and poke a hole horizontally from the back. I would insert the metal tube to the front of the boat and slowly pull it out will simultaneously spraying. This could keep the number of holes to patch to a minimum. Or perhaps I get a shorter tube (I have some stainless steel drinking straws?) and achieve more precision at the cost of more holes. I can patch them with the HH-66 and it doesn’t have to be 100% airtight once it is rigid. Or maybe I install drain plugs on the underside of the boat. Then I wouldn’t need to poke holes, plus if it did get a little water logged, I could drain a lot of the water out later. But that would cost more and add drag.
• How much water do I need to add? Marine foam is two part and doesn’t need water to cure, but it is very expensive. My spray foam needs water to cure, which it normally gets from water vapor in the air. I have found that when I spray large gobs of spray foam, the outer shell hardens, but the inside stays gooey for a long time. It’s much better to do lots of small spaced out sessions than to just spray it all at once.

Anyways, thanks for your time and consideration reading this very long post. I would love to hear from you with any ideas, suggestions, or improvements.

 

[kapnD]

In my area, serviceable used plastic kayaks often sell for around $100, which will wind up being much cheaper than experimenting with unethical methods that will destroy your existing kayak.
I am a low budget kind of guy too, but have leaned the hard way that reinventing the wheel is always a bad idea.

 

[engineer137]

In my area, serviceable used plastic kayaks often sell for around $100, which will wind up being much cheaper than experimenting with unethical methods that will destroy your existing kayak.

I am a low budget kind of guy too, but have leaned the hard way that reinventing the wheel is always a bad idea.

I'm a little confused by your reply. Did you mean to say "unethical"? I don't think there is anything morally wrong with this approach but I would acknowledge that this idea is definitely unusual and atypical.

As for the opportunity cost of destroying the existing kayaks, I am not really concerned too much about that for the following reasons:

• One of the kayaks (I'm not sure which one) has a pinhole leak so it is already unusable in its current state
• Because I got the kayaks so cheap, I feel like I already got my money's worth out of them
• They are an older model, so I don't think I could sell them for much
• The hard kayaks are so much superior, unless I modify these inflatables I doubt we will ever use them again.

I agree with you that I could find some used kayaks for around $100 each, but $200 would be too far out of budget for this project.

Perhaps I could explain my objective more clearly with a 1-10 scale (10 being high). Brand new kayaks would be a 10 on the fun scale, but considering they would cost a few hundred dollars each, the ROI would be low. Used kayaks would be almost equally as fun as new ones, so I would give them a 9 on the fun scale, and because they cost half as much, the ROI would be twice as high. But I think that for $35 out of pocket, I could build two kayaks that would be 8 out of 10 on the fun scale. And since I love making stuff, I would get bonus points for the build and the whole experience.



As a side note, the last time I went kayaking I built a “drill paddle”. I got a 24V drill from the thrift store then 3D printed a propeller. They were connected by a 3ft section of threaded rod. I got some metal tubing from Home Depot as a sheath for the threaded rod and then I covered that with a pool noodle for an easier grip and so the whole thing would float if dropped. The total cost was under $10 and it was super fun.

 

[kapnD]

Ok, you got me on the word game, poor choice, but the gist of what I’m saying is that if you really like kayaking, you won’t let the money stand in the way.
Get some decent equipment and develop your skills and it can buy you a lifetime of enjoyment!

 

[fallguy]

Styrofoam will absorb water. You must fully encapsulate it in waterproof materials. Once wet, it is game over.

The idea is nuts.

I say you will successfully retain your man card by making one heluva mess that goes in the can.

My suggestion is to save your money until you can afford used kayak.

If you love the shape of the inflatable and want to make a mold of it; that is another story.

 

[Blueknarr]

Welcome Engineer137

I've seen several attempts to fill an inflatable with foam. They don't turn out as well as planned. They result in missshaped waterlogged lumps.

The lucky ones were overfilled and burst. The excess foam was cut away but water progressed under the remaining skin, saturating the foam while the flappy skin added alot of drag.

The less fortunate under filled and the foam hardened without the boat achieving proper shape and couldn't be rectified.

The few who got the fill right, still failed to achieve longevity or increased performance. Inflatables need to flex, otherwise they burst seams instead of safely absorbing impact energy.

Inflatables have notoriously bad performance compared to rigid construction. Partly from their squishyness but mostly from their inability to form efficient hull shapes. Their ability to bounce off blunt objects and small storage requirements are their counter-benefits to poor performance.

Filling with foam makes them less squishy but does not alter or improve their fundamental inherent performance flaws.

Enjoy the inflatables you have.
Go rigid when you can.
Don't try to turn a sow's ear into a silk purse.

 

[Mr Efficiency]

Consult a butcher, they put all sorts of indeterminate materials into flexible skins, the results are probably only marginally more nutritious than polystyrene !

 

[BayBoater]

I agree that with the inflatable kayak only part of the "low fun factor" is due to its squishiness and that most of the low performance is due to the bad shape required by the need for inflatability on the cheap. I doubt that you will get a fun factor of 8 from making that shape rigid, you might end up with a 2 or 3 (assuming that the inflatable before the hole was a 1). And there is always the risk that you spend some money and end up with a 0, but if the fun of the experimentation is worth it then just try it.

Building a kayak out of thin plywood or skin on frame might provide the best return on investment. For example, recently I picked up some old interior doors at the spring clean up where people put their big junk by the road. I cut off the frames, now have some 3mm plywood to play with. I haven't tested yet for waterproofness of the glue; the plywood might not be rated marine but it looks better quality than any thin plywood available at a big box store around my neck of the woods. You can spread the cost over a time period.

 

[engineer137]

Styrofoam will absorb water. You must fully encapsulate it in waterproof materials. Once wet, it is game over.

If you love the shape of the inflatable and want to make a mold of it; that is another story.

Thank you for your response fallguy, I don't have much experience with Styrofoam in water. In fact, I've read conflicting reports about Styrofoam in general. I understand that part of this is because some people are referring to the white stuff that crumbs, while others are thinking of the blue hard foam.

So I have a follow up question. The white little balls that have been compressed into a block - I've read that they get water logged. Is it because the water seeps in between the balls and resides in the cracks? Or is it because the water actually absorbs into the little balls themselves?

I also liked your use the inflatable as a mold comment. This was something I thought of after reading the sawfish instructable:
Sawfish, an Unsinkable, Lightweight, Foam Kayak (23 Lbs). Free DIY Kayak Plans, the Hardware Store Boat http://www.instructables.com/id/Sawfish-foam-kayak-build-a-funtional-light-wieght-/

I'm going to think about the mold idea some more...

 

[engineer137]

Thanks Blueknarr for your comments. I have seen a few youtube videos that documented the beginnings of these types of projects, but I haven't heard much about how well they stood up over time. I appreciate you sharing what you've seen.

 

[fallguy]

The foam takes on water within the cellular structure; not just between the balls; afaik. There have been studies done mostly on residential homes as noone much uses the stuff in boat building, with a few exceptions.

It will take on water at up to 20% per cuft or about 13# which would render the craft unable to pick up at saturation

Once inside the cells; there is nothing except perhaps vacuum that would remove it and that might be tough to do. How do I know? Hot tub covers that get water or steam impregnated over time are junk as they weigh too much to lift.

Take some good advice and save some money and forget the project. It is a bad idea.

If you consider styrofoam packing shapes; if it worked so well; they'd just pump an inflatable full of them and next on down the line, but most are molded. Once in awhile they are inflated in place when there is a hard object that can't move in a bag, but those never really look perfect.

 

[Skyak]

you said brainstorming, here are my thoughts;

-as other have said, the weakness of your current inflatables is the shape. Inserting foam does not fix the problem, it only eliminates the one redeeming quality -the ability to pack smaller. What I think you are not seeing is that what you are proposing looks less like a pursuit of a good boat and more like a way of putting your chemical garbage on our waters.

Your list of stuff has a purpose. You don't need it, but somebody does.

You stated you desire better performing kayaks, and you are willing to work but not pay. I see two alternatives, skin on frame, and stitch and glue. Petro has shown success with SOF using split 2x4s, cotton canvas, and quality but leftover latex paint. There are loads of public domain designs. Stitch and glue is around the top of your budget if you use good 4mm marine grade ply and epoxy. You can knock the price down with chine logs and Tight Bond III. I have a good 12ft single sheet butterfly design (Flo-mo).

You mentioned you have 3D print capabilities. That is what it takes to make a good kayak from garbage.

I will leave you with wisdom from my Granfather "There is nothing so expensive as that which doesn't work!"

 

[engineer137]

I have a question about boat shape for anyone who may know the answer. I think I may have started off this whole idea with a bad assumption. I was comparing my experiences with two kayaks, the inflatables I owned and the rigid ones we rented. For the same amount of arm energy expended paddling, I felt like the rigid kayaks went almost twice as far as the inflatables. Also, the rigid ones seems to be much easier to maneuver with.

I thought that there were two main reasons for the difference in performance. First, I thought that I was loosing a lot of energy in the inflatables because of deflections through the vinyl. (Newtons 3rd law and all) The water was pushed by the paddle, with pushed against my arms, which pushed against the rest of my body, which pushed against the floor and sides of the inflatable. It felt to me like a lot of energy was being wasted between my body and the inflatable. The boat would deflect a lot, and move forward a little. In the rigid kayak, I felt a much stiffer connection with the boat, like it was an extension of my body.

Second, I thought that the rigid kayak worked better because it had a flatter bottom. On dry land, my inflatable has a flat bottom, similar to the common rectangular pool floats seen everywhere. But when I sit in it in the water, my 200lbs didn't seem to be efficiently distributed over the entire bottom of the kayak. Instead there was a deeper bulge directly below me while the stern and aft raised up slightly.

Based on these two assumptions, I thought that a significant improvement in performance could be achieved with a relatively small amount of work and cost. I thought that I could fill the inflatable with cheap foam beads, then lock them into a rigid shape with the spray foam. Kinda like how cheap rock aggregate is added to more expensive cement to make concrete. I thought that once the outer compartments were rigid, the bottom of the inflatable would deform much less and instead float much flatter on the water. I thought that the forces applied would transfer with less loss from my body to the boat.

But what I'm hearing on this forum is that the shape of the boat (long and narrow) in more tightly connected to performance than rigidity. Is this a correct statement?

 

[Skyak]

I have a question about boat shape for anyone who may know the answer. I think I may have started off this whole idea with a bad assumption. I was comparing my experiences with two kayaks, the inflatables I owned and the rigid ones we rented. For the same amount of arm energy expended paddling, I felt like the rigid kayaks went almost twice as far as the inflatables. Also, the rigid ones seems to be much easier to maneuver with.

I thought that there were two main reasons for the difference in performance. First, I thought that I was loosing a lot of energy in the inflatables because of deflections through the vinyl. (Newtons 3rd law and all) The water was pushed by the paddle, with pushed against my arms, which pushed against the rest of my body, which pushed against the floor and sides of the inflatable. It felt to me like a lot of energy was being wasted between my body and the inflatable. The boat would deflect a lot, and move forward a little. In the rigid kayak, I felt a much stiffer connection with the boat, like it was an extension of my body.

Second, I thought that the rigid kayak worked better because it had a flatter bottom. On dry land, my inflatable has a flat bottom, similar to the common rectangular pool floats seen everywhere. But when I sit in it in the water, my 200lbs didn't seem to be efficiently distributed over the entire bottom of the kayak. Instead there was a deeper bulge directly below me while the stern and aft raised up slightly.

Based on these two assumptions, I thought that a significant improvement in performance could be achieved with a relatively small amount of work and cost. I thought that I could fill the inflatable with cheap foam beads, then lock them into a rigid shape with the spray foam. Kinda like how cheap rock aggregate is added to more expensive cement to make concrete. I thought that once the outer compartments were rigid, the bottom of the inflatable would deform much less and instead float much flatter on the water. I thought that the forces applied would transfer with less loss from my body to the boat.

But what I'm hearing on this forum is that the shape of the boat (long and narrow) in more tightly connected to performance than rigidity. Is this a correct statement?

NOTHING about your inflatable is optimized. The good news is that it is a safe bet that the kayaks you rented were as far from the performance you can achieve in a home build as they are from your inflatables. If your infls are a 1 and the rentals are a 2, you should be able to build at least a 3, maybe a 4 to 10.

The top inflatable flaws -too much wetted surface area, bad entry angle, short waterline length, lack of directional stability, poor paddle entry position...

If stiffness was the problem, you could just inflate to a higher pressure.

 

[engineer137]

I was at NPS the other day and they had two paddle boards for sale. One had a plastic exterior, but it said it had an EPS core (EPS = expanded polystyrene), which I understand is completely standard for paddle boards. Almost all of them have the white crumbly Styrofoam inside. The other paddle board was just plain white styrofoam with a rough (nonskid?) surface glued to the top.

So I think that using Styrofoam inside the inflatable should be fine, the trick will be to keep it away from contact with water.

I don't think any manufactures would build a boat this way because it combines all the costs associated with inflatables and combines them with all the costs of foam. Usually, I try to emulate industrial processes because they have spent a long time perfecting them and reducing costs. Like it was stated above, why recreate the wheel? But in this very unique case, I have an unusual amount and quantity of surplus building materials that makes this weird process potentially viable and cost effective.

If this foam idea doesn't work, I have several sheets of 4ft x 8ft 1/8" thick aluminum. (The stuff with the diamond pattern on it) Maybe I could bend it around a wooden 2x4 frame. Or perhaps make two pontoons and do a catamaran style boat. I'm not really attached any particular shape. I build stuff all the time, and I often go through multiple iterations. Because I use so much reclaimed material, and because I never invest a lot of money in any one project, I am able to build new projects constantly.