Hot off the press, fresh from design registration, we're down to a single support each side and gone back to a single wing as we are finding the drag is hard to overcome with 2 wings with the space limitation we have set ourselves. Though it is not a completely abandoned thought. We are trialling making it optimised for single direction (the front one will run in reverse) and taken many of the things we learnt previously into this design.
It's cold here in November, we persevered and took our board out, it has a lot less drag, there were no breakages and the single struts meant it was even easier to point the board. If you look carefully in the picture you can see we had a slight variant on one end, just to give us a direct feel between the 2. It's certainly given us some ideas on where we want to go next. We feel closer all the time to a solution with each iteration!
Assembly of parts
Where our latest designs were a single piece, our latest design is an assembly of parts. This allows us to make adjustments to the designs easier by changing individual components. It allows us to decrease the width of the support struts to reduce drag and act less as large fins.
We've taken this out on the water a number of times and the design is a lot more stable than the previous version. We found it easier to slide the board when we wanted to and we have had other riders test the board. Overall is a big step forward from the previous design, we want to work on reducing the drag and increasing the lift.
Removing the resistance
We've cored out the sides and middle 'fins' on the foil in order to reduce resistance, and relieve pressure off your knees. The large fins cut into the water and we need a way to allow the board to slide as per how normal twin tips ride. We looked further at how 2 sides could be joined with just screws.
We've been getting a couple of failures on the design at unexpected times. We suspect the material and how it is printed are partly at fault, so we want a way to address this, and also, how to make this designed for manufacturing.
When we have ridden it successfully, we were going upwind in lighter winds, though we still need to ensure this is the effect of the foils and not necessarily as a result from the effectively 3 large fins.
Balancing out the foils
This is the design we took to the Kitesurfing Armada, though the board did not quite arrive in time!
We reduced the angle of the fins, making them closer to horizontal. We added an upper foil to balance out the lower foil and swept them to reduce drag and create more of a platform to ride on. We've allowed for a spacer to change the rake of the foils.
We angled the rear foil up and the front foil down to get a lot smoother ride, the fin cuts through the water better at the front, though it is a slightly different riding style. The sides are effectively really large fins which cut into the water and put a lot of pressure on our knees. We want to reduce that effect and we had some issues with the prototype failing. At the moment we are unsure if this is due to additional stresses not being factored in when we ride, or because of how the prototype is printed and the material it uses. We suspect a bit of both. A few tweaks to the design and onwards we go!
no more carbon fibre
Straight from the 3D printer, this meets in the middle to give it the strength. The fin in the middle helps add the strength in a way a 'keystone' on a bridge does, and give it a good point for joining both sides. Twin tip boards have different distances between fins - something we will need to account for.
We first had the wing profile on the vertical sections, this makes the board continuously pull at the front, as one fin is more in the water than the other, so we tried again without the wing design there. The fins are designed to be parallel to each other, we got some churn at the front and it still felt like it was pulling to one side or the other. The angled wings help for strength and in theory will provide more vertical lift when you are leaning over, but we are not that convinced and feel they need to be balanced out more.
We changed the hydrofoil to point inwards. It's at this point you realise how narrow your kiteboard is, the fins seemed to become very short, so the loading on them would be a lot less but so is the lift. For the first time, we made 4 of them, whereas before we could only assess the effect of 1 fin, we really needed to know if the other fins caused a detrimental effect when riding in opposing directions.
It seems obvious now, but we never really accounted for the camber on the board. In effect, the front fins were angled up the same as the board curve and we got a lot of churn and drag at the front and the rear fins would have been pushing the board down into the water.
Also, in efforts to create a low cost design, using carbon fibre didn't seem to be the option, though the material was not a great expense, there was a long processing time involved. We might be able to simplify the load on us, though then we start needing more elaborate tools.
Onwards to the next design.
3D Printed Carbon Fibre Skin HYDROFOILs
Here's the starting point. We can 3D print our design and apply a carbon fibre skin to it. There were a variety of profiles we trialled, from straight horizontal to 30 degrees and a combination of the 2. A hydrofoil profile is put on the vertical section as well as the main wing, to still give you lift when you are leaning over. The main wing can have a slight curve which helps with control when you are riding.
We use Solidworks to create and analyse the design. This tells us the the expected load on the wing from a given speed through the water, which we can then put into the Finite Element Analysis tool, this allows us to set the parameters so the wing is not overloaded and the stresses are low enough for the material we choose to print it out from. We can see from the image on the right there is a fair amount of deflection, so we later chose to put a carbon fibre skin on our design to reduce this.
It is nice to see the foil slicing through the water. The foil was only trialled on the heel edges. As the speed picked up, the board would appear to lift and pull away. I later discovered this was an effect of the vertical hydrofoil profile. My weight could not react so directly into the foil because it was not under the board, so there was more of a pivot and I felt uncomfortable riding this. Plus, the fins sticking out either side of the board felt a little unsafe. Onto mark 2...
Making the dream
We entered a competition on Crowdfunder and became 1 of 6 finalists in July 2017.
We only had a week to prepare and we had not put much of it out on the public domain prior. Still, with little preparation we got £700 raised to help with the development, and we are super happy with that!