Using coroplast for a mold and "marine grade" silicone

Before you start reading, know that I know ZERO about building boats, methods and terminology. But I am extremely artistic, good with my hands and mechanically inclined. I can build just about anything well with some direction.

That being said, I am going to build some unique SUP prototypes. My plan, just because I have never heard of it being done, is to "laminate" 4mm coroplast sheets together, shape it and glass or epoxy it.

My question is about "laminating" it. Because of it's structure, laminating so that the corrugation is vertical makes a board or, in this case, small catamaran hulls very sturdy. (It's also environmentally friendly, cheap, and easy to shape with woodworking tools, rasps, etc. My experience with coroplast has shown that normal silicone holds up really well when attaching two pieces of pvc. What I don't know is what will epoxy do to the silicone? What is the difference between standard silicone and "marine grade?" Is there a better alternative?

 

Silicone is about impossible to adhere to. Polypropylene is very difficult to adhere to. My point is that you are making a core that will de-laminate from from the FGRP skin.

If you reply is you don't care, there is no long term I suggest you just shrink wrap your core.

Plain old contact cement is cheaper and would work better to laminate.

Standard practice is to use a foam surf board blank, shape and glass. I don't see how your method is better.

 

It's an experiment. As for my reasoning, coroplast is cheaper and more readily available. It's also more rigid than foam (by a long shot) and I can make pieces as large or small as I need them. It's also polyvinyl chloride, (PVC) not polypropylene. While I appreciate the guidance, I'm not looking for approval of my material or methods. I'm just asking what will adhere well to PVC.

 

All the sheets I have seen are PP or PE

Contact cement still works for PVC. PVC also can be chemically welded. Plain PVC 'dope' used for pipes works on about all PVC. The positive aspect of welding is the result is still one recyclable material until you put another material on it.

If you are doing small CAT hulls in PVC, why not thermoform pipe?

 

I'm making something functional and beautiful. Pipe ain't gonna cut it. Once I make a one-off 12 footer, I'm going to work on making an 11-footer with (3) 6 foot hulls making the deck detachable for better portability.

 

Thermoform can do any shape but it is not easy to get right the first time. Foam and glass is easy for one-off.

By the way, you do know that speed characteristics are highly dependent on waterline length -6ft hulls will be slow ~2.5 knots.

 

That's why I'm here. You guys know these things. I should probably google "fluid dynamics" and do some research.

As for Thermoform, I thought you were referring to just plain pipe. (I'm an idiot.) I'm not familiar with "thermoforming."

 

BTW, I just realized I posted this in the "DESIGN" forum instead of the "building" forum. Sorry Mr. Moderator.

 

Being clear about what you know and don't helps us comment effectively.

If you study hydrodynamics you may never return to designing watercraft. The rule of thumb I will give you is that you are unlikely to SUP faster (in knots) than the square root of the waterline length of your board (in feet). You may think you don't care too much about speed, but it relates to safety when the water is moving -tides and currents. The other thing to consider is that resistance*speed=power, the power you will need to propel the SUP. If you don't have the power you will not reach hull speed. So the right length depends on how much power you are capable of. So the 12ft you mention has a hull speed around 3.5 knots and when we see your hulls we can estimate the drag and power required.

 

Doesn't the surface area of the craft in the water have a bearing on potential speed? Isn't that what catamarans are designed to do? Or does that not come into play at lower speeds?

As for power, I'm athletic, but more of a cardio athlete, 6-1, 180 and lift regularly and play multiple sports. (My last build project was a weightlifting power rack to complete the gym in my garage.

 

BTW, I'm gluing together corrugated PVC sheets, not pipe, to use as a more rigid base to glass over. This stuff:


The top is 5mm. I'll use white because it's cheaper. It's pretty strong when glued together. Like 1000 small plastic I-beams in a row.

 

Surface area dominates low speed drag and rises fairly linearly. Wave drag grows quickly as the wave from the aft reaches the one made by the bow and it soon equals and outgrows surface friction. Narrow and long has more surface than short and fat but has lower drag at higher speeds.

If you want to try and estimate the optimum length I think we could do it based on a known hull and the rate you can paddle it. If you are strong maybe you can beat the square root by 10 or 20% even 30 on a short board. But the optimal length still depends on your power.

 

I am sure a stack will be plenty strong but I am less sure of the weight. I tend to think it will be much heavier and harder to shape than the typical PVC foam used.

 

Why do you think corrugated sheets will be better than foam? What will they do better in your design than foam would do?

My experience with corrugated sheet is that is an absolute pain to shape, I can't see how you will get smooth rails without using some other material around the edges that will add quite a bit of weight and complexity. Shaping rocker will also be really tough, you will have to bend the sheets or else have lots of large open spaces from the end grain of the material, this will mean using heavier glass to span the open spots than would be needed for foam.

You say coroplast is more rigid than foam, maybe by thickness but I doubt it is more rigid than foam by weight, especially once the foam has a glass skin. And rigidity by weight is what matters in SUP or surf boards since you can't really make the board any thinner and still float.

 

I say go ahead and try it out! Just epoxy the sheets together and just shape the edges. I've seen paddleboards that are straight and not curved with rounded edges. Start with that basic design and see how it goes. BPW is correct about the weight though. A sheet of 4'x8' 6mm coroplast sheet does weigh more than foam would so your overall board without glass would be heavier.

Without a doubt your board will float and be able to withstand a decent amount of weight. As with the other posters though, I dunno how durable and strong it will eventually be when you're done getting the thickness you're going after. If I remember correctly, just standing on a sheet of coroplast by itself will collapse the material. But maybe if you cut 1/2" strips and lay them on top of each other with the open ends facing up and then alternating them 90" with each layer, I'm sure that'll be plenty strong to stand on without collapsing.

Either way, it'll be neat to see someone try. Love seeing people learning by trial and error, without it we wouldn't be here today and as advanced as we are in all aspects of life and not just in the boating world.