Sailrocket 2 set to launch

No question that incredible kite sailing skill was required to remain in the narrow ditch during a gale. My reply was because I didn't like the snotty tone of Blue Leader's commentary rubbishing l'Hydropetere.

 

I think we all understand and sympathize with that!

I checked his posts and every single one of them boasts of the superior merits of kiteboarding; one-dimension. I suppose that’s why we have the ignore list.

 

Sailrocket-new foil

From the Sailrocket site- read whole article here: http://sailrocket.com/node/510


"20/20

Now that I can look back, I can see where some of our assumptions were just plain wrong... but that's ok as our understanding of the bigger picture is right on track. The boat is great and it is only the highly modular foil that is wrong. It was our first shot at a ventilated/cavitating foil and assumptions had to be made. If we were just doing 60 knots then it would have been a lot easier. If getting through low speed transitions just involved pouring more fuel down the carburetor then it also would have been easier... but this is a sailing boat that has to accelerate from standstill using only windpower so the problem is a whole lot more intersting. Now that we have a whole heap of real world experience we feel that we are much better placed to re-visit the problem. Of course we are. VESTAS Sailrocket 2 is currently sitting all nicely packed away down in Namibia and can be made ready to do 50 knots plus again within a couple of days. All our efforts now are focused on getting the foil right.

SOLUTION
After our design meeting hours after landing back in the UK,
the current thinking is that we will scrap the L-foil configuration and go to a T-foil arrangement.
This foil does seem to have many advantages. We had discounted it previously as we though that the end plate would need to be much bigger to enable us to generate the low speed side force to get started. This would have meant that we would have had structural issues around the junction... especially if the top tip of the foil popped out of the water as this would put large torsional loads on the junction. Now that we believe we can go a lot smaller (as demonstrated by our 'chopped down' runs towards the end of the last session), we believe that the 'T' configuration has many benefits...

-we can use the existing composite 'head' of the foils.

-we can machine the new components out of metal which is far cheaper, quicker and will allow us to try many variations.

-we can make the foils much thinner as the bending loads are greatly reduced

click on image:

 

I was a little surprised that more effort wasn't put into using T-foils in the beginning. I say because this is an outright speed machine and T-foils as I know give the best efficiency.

You can certainly see the advantage now of a simpler foil to construct and maybe they will be trialing some really alternative designs due to lower cost of construction.

It did always seem that there was a need for more of a thought out transition to the high speed foil due to the huge speed range. Perhaps a transition foil or perhaps some artificial ventilation of the foil until speed hump was over come.

 

================
I think Greg Ketterman would disagree with you-he considers inward pointing "J" foils to be far superior to "T" foils. At least on foilers with the speed potential of Long Shot and the Hobie Trifoiler......
PS- I can give you a link to Kettermans paper from which the following illustration was taken if you're interested....

click on image-

 

Maybe a “J” foil is theoretically more efficient than a flat foil but only if it is large enough to subtend a significant angle at the sail or wing. In SailRocket the foil is so far from the wing the angle is negligible. Foil size is more important at speed, at low speed the only important thing is not to stall it. The obsession with vortices seems to have caused the engineering problems of the J foil to be overlooked. There’s still a lot of evolving from theory to practice going on in the design of SailRocket.

 

--------------------
I think what Ketterman was showing was the reduction of effectiveness of the "T"-foil with high and low pressure on the same side. The same reduction in effectiveness occured in his tests with the "J" foil pointed outboard rather than inboard because there was high and low pressure on the same side and according to Ketterman, that reduced the effective aspect ratio of the foil.

click on image-

Ilustration: "T"-foil / Ketterman Theory

 

You guys are not even close in thinking through what the SR 2 team is doing.

The main strut on the main foil will NOT be used to generate any hydrodynamic forces. It will simply be a rigid member in tension that will optimized for structural needs and minimum drag.

The small foil on the end will be pulling down and away from the main hull. Since the central strut is not required to perform any hydraulic function, this application is more like an airplane with the wing mounted above the fuselage using a vertical airfoil shaped strut.

Since the junction between foils will be on the high pressure side of the lift producing foil on the end, and since the strut is not producing an "lift" there really should be minimal "bad" interaction between the strut an foil on the end.

A big advantage of this application is that the structural loads from the lift producing section are transmitted to the main strut in tension with close to the same amount of force pulling down and away on both sides of the strut. This allows the foil on the end to be a simple interchangeable "bolt on" item that is easy to swap out.

The construction of the end foil is also very easy. It can simply be machined from metal (stainless or aluminum depending on stresses). They can easily come up with a multitude of foils with different plan forms, aspect ratios, twist, wedge angles etc.

 

===========
Actually, we were talking about T-foils in general. I think you may be wrong about the Sailrocket set-up: it was clear thruout the discussion by Paul earlier that the rear foil develops lateral resistance and pulls down- but it appears that for high speed the strut won't generate much ,if any, lift.
As best I can tell the two foils in the first picture are both designed to develop lateral resistance and downforce; but based on the second picture I think you may be right that the strut generates no "lift" except maybe at relatively low speed:

click on image--

 

I assumed it pulls along a vector from the wings center of effort, with the central strut holding it at its operating depth; only thing that makes sense to me.

 

----------------
You raise a good point AK, and one Larsen brings up, as well, regarding the possible use of the T-foil:

" However, there are issues that we need to resolve. Some of the aspects that need to be considered are as follows...

-what dimensions will we need to satisfy all criteria?

-what are the expected loads for all scenarios?

-What are the realistic performance expectations?

-How do we best gain the understanding of how the foil will perform and make the necessary flow transitions?

-if we do still want to use ventilation as a means of creating the upper surface cavity on the foil, how do we ensure it can get down to the foil?

-how do we maintain good ride height?

-How do we reduce the drag at the junction of the T as these junctions are always messy, especially when highly loaded."

 

All those are way outside of my area of knowledge! However, I would note that the solutions for high-speed aircraft with extreme operating speed ranges have included variable sweepback wings and delta wings, with constant cross-sectional area fuselage/wing root designs. I don't know if these will translate to the water at the speeds applicable to SailRocket.

 

Thanks Doug pls send me that link (I think I've read it before though).
I thought that J foil had advantages over T foils for sailing yachts when you consider righting moment, lift and leeway all achieved by a set of foils, but that a higher L/D by a T foil can still be achieved.
I feel the foil acts against the vector sum of the wing and leeway motion and hence will still need a pivot as conditions change.
The strut is actually a tie bar and could possibly be slimmer using T-foil as the load is now balanced on both sides.
A lot of confusion about the foil trialed and it has not been shown to well, but I hear it being called various forms of cavitating foil. The foil sounds to me like a ventilated wedge /fin that relies on air being drawn down the back of the foil to release drag. was this foil meant to start cavitating at higher speeds 60+?

 

------
MechaNik,the link is in post 54 here: http://www.boatdesign.net/forums/sailboats/dinghy-design-open-60-influence-36401-4.html
In the posts prior to 54 I was almost convinced that a curved lifting foil(or "J" foil) pointing outboard would be superior because it added to RM. Greg Ketterman (and a couple of others and my own research) disuaded me of that notion. Ketterman is convinced that "J" foils are superior to "T" foils in most hydrofoil applications.
AK made a good point: the original foil used the strut as a sort of altitude control system-will that work with the T-foil + strut? Even Larsen doesn't quite know the answer to that yet....

 

Update from Paul Larsen: (read the whole thing here- http://sailrocket.com/node/517 )


"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!Cheers, Paul.