Foiler Design

Trim Control Device

Hello John. Would you, or could you gather any more details on this vertical plate arrangement, and its operation?? I was gathering some info to eventually look at 'trim tab' usage on sailing cats under power in hopes of curbing some of the stern squat they experience. I'm working on some other unrelated projects at the time, but I would like to put this into the data bank.

 

The interceptor has always struck me as being a Gurney flap for hulls. I suspect the reason why it's efficient is because the downstream side is not wetted, so there's no turbulent wake. A flap adds wetted area of its own, while there's a separation bubble ahead of the interceptor that actually reduces the skin friction in that area. There's bound to be added drag due to the bubble itself, however. It's interesting that these effects all seem to add up to a net gain.

 

Interceptor Vs trim tabs

Yes, Interceptors of course do work- however there are key differences that do not mean they are always better than trim tabs, particularly for motion control. One key point is at lower speeds there is no added mass- so no control force, although this force must neccessarily be supplied by the actuation system. Certainly there is a lot less force/energy required to actuate them. The USN X-Craft employs large interceptors of our construction, capable of operation to beyond 50 Knots, dynamically controlling vessel motions in concert with our titanium bow T-foils.

 

Interceptor Foils

To be sure we are all on the same page, I reference two documents sent to me.

http://www.great-water.com/pages/product_pages/QL_Trim_Sys.htm

 

Interceptor Performance

Yes, that is an interceptor allright. We use a different approach for our large systems.

The interceptor is only a dynamic device- no speed, no force, since it is still dynamic head (1/2RhoV^2) that supplies the pressure. Near the interceptor blade itself, the pressure approaches dynamic head and tapers forward on the hull. It is actually an old idea the Russians used years ago.

Since we do motion control, not just static trim control, we are constantly looking for more force and less hydralic requirement, but if we need lower speed effectiveness, we go back to large tabs.

You can see our work at HTTP://www.islandengineering.com/ but I see we don't have a picture of the interceptors on there, I will have to work on that.

 

Brian, sorry, I don't have any information about the Austal interceptor system other than what I heard at a lecture the other day. I think subsequent posts here have clarified what this device looks like and how it works. You might like to look at the Austal website for pictures of their recent projects.

Rick, having taken a look at your interesting website, it struck me that your motion control systems seem to have much in common with systems described in the Austal lecture, so I just wondered whether your company is involved with any of the Austal projects? I did also wonder why the transom flap has an advantage over the interceptor at low speeds? Is is just that it is possible to make a very large transom flap? Or is it that moving a transom flap also moves a mass of water below the flap so you can get some inertial force, but I would have thought that this would be small unless you are moving the flap at high frequency, which I would not have expected to be desireable at low boat speed.

Perhaps this discussion is getting a little out of place on a sailing boat forum, the reason I mentioned it here was that Austal emphasised that the interceptor could give pitch control with minimal acutation forces. I just thought that might be useful if you are deriving your actuation force from one of those wand things.

Tom, a transom flap seems a bit like a supercavitating foil in that its lower surface deflects water downwards and the upper surface stays dry. So if it is true that the interceptor has has better lift to drag than a transom flap could we make a better supercavitating foil by using an interceptor. Perhaps this could be a flat plate inclined from horizontal just enough to keep the upper surface dry but no more, plus an interceptor/gurney flap under the trailing edge to generate the lift. One limitation I can see is that this would not be as strong structurally as the wedge section type of foil.

John

 

Interceptor

This is getting away from the sailing vessel subject.

Yes, I have been involved in proposals to Austal going way back. I designed ride control system effectors and performed sea trials for SES vessels, Wave piercing catamarans, and monohulls since 1991 and was involved in the design of the very first 4 sq meter T-foils ever used for ride control.

But I am also a sailor interested in high performance, answering this because I saw Tom Speer's name here. He is definately an expert and I have downloaded, studied and learned a lot from his work.

Incidentally, 'Sigma' or cavitation number also includes immersion pressure- I saw an earlier post concerning cavitation buckets. I use the formula (Patm+Rho*g*Depth-Pvap)/(1/2RhoV^2) to account for depth.

 

Back to the Moth Foiler

I need to get up to speed on the history of this thread, but I am very interested in the Moth foiler and can help verify with detailed 3D vortex lattice work using vortex lattice, if anyone is interested (Tom...). I can only simulate the free surface with an image foil, but generally it has worked quite well for our ride control foils.

Also, I have worked with Walter Shurtenburger of Multihull Technologies in Key West, FLA. ( http://www.constellationyachts.com/ ) who built Sam Bradfield's hydrofoil boats. I've followed Sam's work forever and would love to put a servo flight control system on a bigger sailing tri maybe. Our website, by the way, also has our own 1/4 scale MM56CX hybrid hydrofoil demonstrator, which has incredible seakeeping performance. (it's 44 ft long...can do 32+ Knots although the foil system is sized for weight not speed, and it will do it in 4 Ft seas while you can simultaneously have a bottle of Coke on the dash and not spill a drop)

If you can understand 1/3 octave vertical acceleration plots, check the Data page. The curves to the left are O'Hanlon and McCauley seasickness incidence curves, the ones to the right are fatigue decreased proficiency.

 

Foiler jumping

I've enjoyed finding out about the intercepter and all your discussion related to it .
But Rick, what is your opinion of the idea of designing a foiler similar to a Moth with the intention of facillitating skipper controlled jumps? I can see the relatively inefficient foil shapes used on the Neil Pryde foilboards(see previous post for url) and am wondering if that's the shape we're going to have to wind up with if jumping is important-seems like too big a tradeoff.. I am experimenting with a set of foils similar to what is used on the Moth for use on a sailboard at low and moderate speed and jumping but tests are a few weeks off. The Moth type foil is a 63412 section at about 169lb.'s per sq. ft. loading and the Neil Pryde jumping foilboards use a very low aspect ratio "delta" type shape with small turned down tips- and probably substantially higher loading..
I've sailed a Rave and had the pleasure of working with Dr. Sam one summer when he put foils on one of my big RC trimarans as a test for the Skat which Multihull Technologies built. Subsequent to that and with lots of help from Dr. Sam I designed the F3 RC hydrofoil which was the first production RC sailing hydrofoil.
I would appreciate any comments or thoughts regarding the potential "jumping" aspect of a foiler similar to a Moth.

 

Jumping

Really it sounds wild to me, but if you think you can jump successfully I'm not going to try to stop you.

I am afraid I speak from little or no experience in this area other than to jump off the tops of waves in an M-20 scow which was pretty exciting, but it seems like as soon as you decouple from the surface you lose all lateral forces at the bottom of the centerboard and rudder. But, you still have the aero forces on the rig then coupled with the ballistics of the craft until landing. They do it on sailboards, but with different results and they sometimes do aerobatic stunts, but it is not a fixed rig. There was a sailboard called the 'Wind Weapon', that had an inclining rig on a club mast, they used to fly all the time.

 

I found a picture of something that might be the Wind Weapon flying;

http://www.actionsportsmaui.com/graphics/large-pics/category_windsurfer.jpg

 

jumping

Rick, can you speculate at all on the the design of a foil that might be more suitable for a safe re-entry and yet efficient for "normal" sailing like the Moth foil? The foilboard using an adaptation of the "air chair" hydrofoil jumps and re-enters safely most of the time but it's no where near as efficient in producing lift as is the Moth foil.

 

Hydrofoil motions and jumping

I tried to read this entire thread this evening so I would not come in totally half baked, although I must admit I skimmed a bit here and there. Tom Speer has supplied suggestions galore, and without a lot of further study I would not hazard to suggest a foil section for jumping, save to say it should not have any tendency to dive, maybe it probably should have a zero lift pitching moment as you would use for a flying wing.

But for some more ruminations on hydrofoils and stability, our own 44 Ft hybrid hydrofoil DAT testcraft (Dynamically Assisted Trimaran), The MM56CX, has a classic 'airplane' layout with a fully submerged forward foil and an inverted pi foil aft. the aft foil is lifting. Foil section is my own custom section. it was difficult to fly manually, the 'man in the loop' wasn't fast enough, however with our fast flight control system and hot servos, using a bow height sensor and motion reference unit, it becomes rock steady. Groups of people may roam about at will, pretty much, and you don't even notice the minor adjustments to the flap positions on the LED displays. It can be selectively told to 'platform', ignoring small waves (up to 4-5 Ft, of the short period we get here in the Chesapeake!) but can be told to 'Contour' in swells, athough we've never had the testcraft anywhere but in the Bay. We did fly it down to the MACC conference at Little Creek Virginia a few years back. This system can be 'downsized' somewhat but the earlier comments by Tom Speer about servo mechanisms is right on the money- that is the tricky part. We are working on it though.......that's our business.

A note on cavitation- Our inverted titanium 3.75m^2 bow T-foil system, mounted on the bows of the same LSC X-Craft we have our interceptors on, has operated at speeds in excess of 50 Knots without appreciable cavitation- although we find we can 'dip' into the cavitating region slightly for motion control on extreme conditions. It is pretty deep, the T foil depth is 4.4m, helping with cavitation by increasing the local pressure considerably.

I also have had a design for a hydrofoil assisted catamaran I call the 'Longbow 22' that I did over 15 years ago. I never built it but have been thinking about it more lately. Earlier some discussion about flying A class cats cropped up here, with assistence only, no full flying apparently written into the rules.

Although I am definately a proponent of fully flying the boat, our MM56CX is a partailly flying concept, although we freely fly it at 100% when sporting about not taking data. When we took John Myer, President of the IHS, for a ride his comment was 'it'll be great when you fly it completely'!

Rick

 

Foil Selections and tools

I have been mucking about with XFoil and my other foil tools today, and thought I would share a few tools I have created to aid my design efforts, hopefully for the purpose of furthering the cause here.

I prefer DesignFoil from Dreescode software (http://www.dreesecode.com/ ) for it's wide range of NACA coordinate calculation routines, but also have used VisualFoil from Hanley Innovations http://www.Hanleyinnovations.com/) because they both have good user interfaces. XFOIL is extremely good but the command line interface slows me down, and I find I must get to the point since I do much more than hydrodynamic design. I also have a seat of Dr Hanley's MultiSurface Aerodynamics I use for VLM, although I have also a copy of Ian Kroo's Linair from Desktop Aero.

Fortunately, the UIUC database of foils is a consistent format compatible with most of these tools. Linair is only a camber line VLM tool, you must scale the cross section information to it which I used to do, but Hanley has integrated this into MSurf.

if you would are interested in the history of Dr Eppler's work, and man is his stuff good especially for hydrofoils, you can play around with Michael Selig's WWW implementation of the Eppler codes at http://www.profoil.org/ where you can design it yourself. There is also a great history lesson there.

Now to the point- I have written two very useful scripts for Rhino I thought I would share with the world for free- one which imports foil shapes as interpolated splines for design work, and one which 'walks' a NURBS curve and exports the foil shape so you can analyze it in the tool of your choice- it will also write camber line files for XFOIL, since the format is the same. I have uploaded the VBScript files to BoatDesign.net. I have used them successfully for many years now. If you receive an arbitrary surface file from someone else, you can slice the surfaces and export for foil analysis.

Usage Notes;
Importing foil shapes requires you have a point at the leading edge and the trailing edge in space, and works in the Front view (Z up), typically with the leading edge to the left. You can however place the points during the running of the script if you forgot.

The foil export script requires that you make the 'closed curve seam' start from the trailing edge and point forward along the upper surface. It too likes the Front view, and on export 'normalizes' the coordinates to a unit chord.

Rick

 

Attachments

Sorry, I am new to the forum and have to figure out the attachment schemes.

Apparently .rvb (Rhino VBScript) is not in the list, so they are in the attached .zip file. Here's another try...

Rick