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Introducing the Hoff - a transparent acrylic board

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rynhardt
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Re: Introducing the Hoff - a transparent acrylic board

Postby rynhardt » Wed Mar 12, 2014 7:28 pm

I am aware of several plexiglass boards from a few years back. One was CNC'd from a solid piece of acrylic if memory serves, and was helluva expensive.

Here's another one:
http://surfforum.oase.com/showthread.php?t=80200

No new movies yet. It's been raining solidly for the last two weeks :(
I'll post as soon as I get a chance to get on the water again.

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rynhardt
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Re: Introducing the Hoff - a transparent acrylic board

Postby rynhardt » Sun Mar 16, 2014 9:17 pm

After some further testing I managed to buckle one of the spars slightly.

The expectation was that the board would experience the most strain at the points where your feet contact the board. And this is exactly where one of the spars buckled, as can be seen by the slight wave in the aluminium upright.

As this is essentially a beam under load, I suppose buckling would be one of the major failure modes. Prevent the beam from buckling and you get to rely more on the tensile strength of the next weakest component which might just be the rivets. Time will tell.

So I riveted some aluminium square tubing 12x12x1.2 (mm) to the existing spars. This resulted in a much stiffer and stronger structure, with an additional weight penalty of 400g, bringing the total weight to 3.35kg. This is still 400g less than my other board. Overall the stiffness of the board is now in line with my other boards as well.

Took it out for a spin at the cable park today - the first day it has not rained in the last two weeks. This time I had much more confidence in the board and did a couple of kickers, sliders and ollies.

https://vimeo.com/89247155

Nothing broke. :thumb:

The acrylic got some scratches from the sliders as expected but overall I'm pretty impressed with how well it has held up.

Next steps are to recess the rivets into the acrylic to provide a flat bottom. The cable park owner had some concerns about my rivets scratching his sliders. And then to mount some fins.

Some pictures below:
1. Buckled spar
2. Reinforced with square tubing
3. Size comparison with some other boards
Attachments
Buckled spar.png
Reinforced spars.png
Boards compare.png

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Laughingman
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Re: Introducing the Hoff - a transparent acrylic board

Postby Laughingman » Sun Mar 16, 2014 10:34 pm

Do you have a tube bender?

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rynhardt
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Re: Introducing the Hoff - a transparent acrylic board

Postby rynhardt » Sun Oct 12, 2014 4:22 am

I've been lying awake since 3am and it doesn't feel like I'm going to be getting any more sleep.

As good a time as any to start documenting the project I've been working on for the last six months..

My original transparent board was an unexpected success, but it had a few shortcomings that I've been trying to address through various design iterations. The two main areas of concern was:
1. buoyancy - it had none
2. durability - I've managed to destroy the acrylic by applying too much stress on a drilled hole - not good on a notch sensitive material.

So the design spec for the next board was something along the lines of the following:
1. It still has to be transparent
2. Needs to float
3. Needs to withstand abuse, i.e. sliders and kickers
4. Needs to be light and efficient

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rynhardt
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Re: Introducing the Hoff - a transparent acrylic board

Postby rynhardt » Sun Oct 12, 2014 4:32 am

Starting with the efficiency requirement, I thought I'd try a hydrodynamic rocker profile along the lines of the Nobile 666.
So for better or for worse, I picked a NACA 0006 profile and mirrored it to get a rocker line..
I'll use the same curve for the board profile.
Attachments
naca 0006 mirrored.png

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rynhardt
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Re: Introducing the Hoff - a transparent acrylic board

Postby rynhardt » Sun Oct 12, 2014 4:42 am

Imported the resulting SVG from Inkscape into Blender, and started playing around until I got a reasonably nice shape.
As you can already see, the board is geared towards intermediate to light wind use, with minimal rocker and fairly square outline.
Length is 140cm. Width will be between 45cm and 48cm - I haven't finalised that yet.
For the record, I'm 183cm, 85kg and ride boots, so I prefer a bigger board.
Attachments
Board profile.png

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rynhardt
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Re: Introducing the Hoff - a transparent acrylic board

Postby rynhardt » Sun Oct 12, 2014 5:00 am

Now I had a board shape, I had to find a way to support the transparent sheet material I intended to use.
Acrylic, PETG and Polycarbonate are all candidates. Acrylic has the most colour options, including fluorescent :-).
PETG and Polycarb are more robust.

My previous board cracked where the boots were bolted through the acrylic.
I only have this picture of before, but where the bolts were recessed into the acrylic (red circle), is where the crack started.

I had also riveted the aluminium supports through holes drilled into the acrylic, but these seemed to have held up.
Anyway, I decided against drilling into the sheet material, which meant finding an alternative bonding technique.
Attachments
recessed acrylic.png

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rynhardt
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Re: Introducing the Hoff - a transparent acrylic board

Postby rynhardt » Sun Oct 12, 2014 5:43 am

The plastic sheet I intend to use for the majority of the board surface will be between 3mm and 5mm thick.
Any thicker and the board gets too heavy. In these thicknesses the plastic is neither strong nor rigid enough to maintain a specific shape (rocker) by itself.
This means supporting it with a beam of strong, rigid and light material. Having had success with aluminium alloy previously, I decided to stick with this material for the time being.

But bonding aluminium with plastic is not a trivial matter. Especially if you want structurally sound bonds that do not include mechanical fastening like rivets or screws.

After much research into various bonding mechanisms, I eventually decided on using double sided tape.
Not just any tape, mind you, but 3M VHB tape.
Given the tape's performance characteristics, this would be an ideal method to join the supporting beams to the sheet, as well as base plates for mounting the boots.
All without drilling any holes. Which meant the fluorescent acrylic was still a viable option - the redneck side of me rejoiced!

The double sided tape however needs two flat surfaces to be bonded - the bigger the surface area the better.
So this means designing a curved beam with a flat bottom area.
I also wanted the beam to be stiff in the middle, flexible at the ends, hollow to provide buoyancy, strong and light.
Ultimately I came up with the following: a triangular, tapered beam. One one each side of the board, running down the length.
Attachments
beam design.png

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rynhardt
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Re: Introducing the Hoff - a transparent acrylic board

Postby rynhardt » Sun Oct 12, 2014 6:05 am

The next challenge is how to manufacture this thing. It is curved in 3 dimensions and needs to be manufactured pretty accurately to give me the desired profile.
One way is to take 3 flat pieces and bond them on the edges. With aluminium this amounts to an outside corner weld.
But the 3 pieces need to be supported in some kind of jig.
So I came up with the following idea of using plates with cross-sectional cutouts to support the beam pieces at 6 points along their length.
These pieces can be cut from the same plate as the beam pieces, which makes manufacturing cheaper. The pieces are also keyed so that I can orient them correctly during assembly, and have flat areas so I can rotate and place the beam for welding each seam.
Attachments
beam jig.png

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rynhardt
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Re: Introducing the Hoff - a transparent acrylic board

Postby rynhardt » Sun Oct 12, 2014 6:10 am

Having finalised the design, I exported the cutting diagram from Blender using Pepakura designer.
The cutting diagram for the beam and jig looks as follows - there are enough pieces for 4 beams and 2 jigs.
Attachments
beam cutting diagram.png


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