My head hurts. An Overdue update from Speed School!
Mon, 14 May 12 13:17
Hi all, I've tried to write this update a few times now but have either written a 'novel' that even had me confused or, things were in such a state of flux that the key points were hard to pin-point for the purpose of a summary.
Still, it's not right not to let you know where we are at.
The design of the new foils is a massive hurdle. There are just so many critical areas that need to be considered before we go forward. Things get weird from here on in and there is very little precedent to go on for our application. We can only reference other craft and situations i.e. high speed propellors, Naval tests done on high speed hydrofoils etc. but each instance is distinctly different from ours. We have all had to go back to school. A lot of CFD is being done but it has its limits and you have to be very careful that it doesn't lead you down the pretty pictured garden path.
The fact is that from here on in there is a lot going on around the foils. It's no longer just water down there. We have to deal with vapour cavities that can vary in pressure from ambient (normal atmospheric) to near on a full vacuum (approaching 14 psi or 10 tons per square meter). There is a world that exists between these two pressures and somewhere in there may lie the difference between a world record or another 'also ran'.
We have looked and are still looking at all the areas that matter trying desperately to find the best solution. We have re-visited our first foil to see where it went wrong. The belief is that it was just too big. At high speed it was travelling at such a small incidence to the water that the sharp nose may have actually been at a negative angle and thus cavitating on the pressure side. It's the hydrofoil equivelent of carrying too big a sail upwind and having to 'luff' or backwind the front of the sail.
THE FOIL IS A BIT FATTER THAN REALITY HERE BUT YOU CAN SEE THE CAVITATION FORMING ON THE WRONG SIDE AT THE NOSE. ALL THE ACTUAL LIFT IS BEING GENERATED BY THE SUCTION ON THE UPPER SURFACE AND THE CAMBER AT THE BACK OF THE LOWER PRESSURE SURFACE.THIS LEADING EDGE CAVITATION IS HYDRODYNAMICALLY UGLY. VERY DRAGGY!
It seems that we had very little chance of generating the angles of attack necessary to get this foil fully side ventilating as we had hoped. So the problem now comes that in order to make it generate those high angles, we have to make the foil so small that it becomes dangerous as there is so little in the water.
That's just one aspect of cavitation... then there's the even more random minefield of ventilation where ambient pressure air gets sucked down onto the foil from the surface. That's all well and good provided there is a nice 'path' for it to get down. The reality is that water is flying all over the place both at the surface and down inside the ventilated cavity and the path is constantly getting 'choked'. Once it gets choked the pressure drops and the sides of the cavity get sucked in harder. Ventilation can be both hard to start and hard to get rid of. In the picture above, if we could introduce ambient air into that blue cavity, the difference in performance would be massive. The base area (blunt face at back of wedge) for our first attempt at a wedge was around 0.04 square meters. If it is properly, fully ventilated then there is no drag here... just like on the transom of a cleanly planing yacht... but if it is cavitating, then the suction pressure on that face is around 400kg of pure, speed sucking drag. It's crippling. Think about it. No wonder bullets don't go far underwater! So, if we can add ambient air into that cavity to reduce the pressure then it makes a big difference. By the rules we are not allowed to force it down there. There are options.
See... here I go again and this is why I have struggled to write this. I am only lightly touching on one of the aspects of the design and yet as we get deeper into the problem it all gets slightly 'grey' and oh so inter related. Ventilation can blend into Cavitation. We want them thin but we want them strong. We want them small but we want them deep. We want to get air to them... here but not there. Definitely not there etc etc.
We have been around many design loops. We're on the M25 of design loops. "Can't get off there, too much drag". "Can't pull off there either, too weak". We have to consider how we maintain ride height, choppy water, safety margins, low speed performance... and so on.
It really is hard to say if we aren't over analysing the whole problem. Maybe we are a lot closer to a solution than we think and simply aiming too high. We don't have to go that much quicker. Maybe some slightly smaller foils than what we had will do the trick. Just small enough to keep enough angle to avoid the pretty picture above. WE now have models for 'T' foils, 'L' shaped foils, Straight foils and have looked at all means of supporting them and reconfiguring the boat. We have looked at stepped profiles, wedge profiles, cambered wedges, pinched parabolas, stepped pinched parabolas, parabolic wedges. Everything is considered and there is still no clear favourite. The final total package has to tick all the boxes.
This is the problem at the frontier. The path forward is not illuminated. We look back at the problems confronting the first through the Sound Barrier and think "Hey, that was no real problem in the end. What were they worried about". We think that with hindsight. The fact is that until you have done it... or someone has done it... you just don't know for sure. These projects are often fragile on many fronts and we simply can't afford to get it wrong too many times. It can all come to an end in many ways.
Right here I will stop myself. If I keep going on this blog will get out of control and I won't send it... again.
SO HERE"S A SUMMARY...
-VESTAS Sailrocket 2 remains on location in Namibia. She has proven herself to be able to reliably take us up to the limit. Getting to this stage alone is a big result and not to be underestimated. We now need to plug the right foil into the boat. We believe that we will be able to work out if it is good or not quite quickly.
-We are working flat out on solving this foil problem. The project is by no means standing still. CFD and complex VPP's (velocity prediction programs) are running all the time.
-VESTAS are standing by us all the way here. We all know that times are tight and thus we are being as tight with our budget as is practical. Everything is being poured into solving the foil issue. No extras, no retainers. All hands at the coal face.
-We have obviously missed this early weather window so we don't expect to be back down in Namibia until some time in August.
-The new foils will be made out of steel and we are looking for help here as it is outside our expertise. It just seems right that these foils should be made somewhere in the UK Midlands. If we can, we will build a few foils but machine time gets expensive. The design budget has already over-run.
So here we are. Our heads hurt. It's a complex problem that we can's stand not to understand. I personally think about it constantly. It feels like the last piece of the puzzle that we desperately want to complete. Chris, his team at AEROTROPE and Malcolm are putting a lot of work into this. We will apply the methodology that has brought us this far. Some times the battles are in the build shed with material processing, some times they are on the water with the elements. This one is in the understanding of a theoretical world.
I can't wait to plug all this back into reality down in Namibia.
We are getting there... and IT WILL BE WORTH IT!