T foils, a steep learning curve

[ifoxwell]

Hi Michael

Thanks for your answer, and yes we should meet up. We've been pretty busy recently which is why I havent been back in touch and are on Holiday soon so unfortunatly will be missing Burton Week (again  )

We always pencil in the round Sheppy race each year which we are planning on taking the 12 to this time. More of an adventure than a race but great fun all the same.

Ian

[Marcus]

Having now seen some T foiling boats in action at the Inlands and read the discussion threads relating to T foils I'm interested in building one for my boat. However, before commiting to building one I want to make sure I understand what the foil is doing and I also have some questions relating to aspects that I'm not clear on yet.
 
As I understand it the foil is normally trimmed to produce a positive lifting force that assists in raising the back of the boat out of the water thereby reducing wetted area and the associated drag. The resultant lifting force acts upwards from the foils so the further back from the boat they are located then the more effective they will be because they will provide more leverage (I assume this is why boats like the 14 put the rudder and foils on a gantry that extents behind the boat). However, because the lifting force acts aft of the rudder support points not only will these points be sheared upwards, the lower support will be pulled aft and the upper one pushed forward with these pulling and pushing forces increasing as the foils are located further back. In addition, the supports are loaded in this manner by the drag force acting on the rudder and foils. At present there appear to be two approaches to trimming the foils: in the DCB/Foolish system the rudder pivots about the fixed upper support point and the lower support is pulled forward by a purchase system to overcome the aftwards pulling effect that the lift and drag forces from the foils and rudder create. In the Paradigm type system the setup is reversed, the rudder pivots about a lower fixed point and the upper support is pulled aftwards by a purchase system to overcome the forward push.
 
Now for my first question: if the rudder were located in a conventional stock that attached to fixed rather than pivoting pintles would it be possible to control foil trim simply by rotating the rudder relative to the stock in the usual way? If the vertical line of action of the lifting force were directed approximately through the pivot point then the purchase required to trim the foils would surely be reduced and the control system could possibly be contained entirely within the rudder/stock /tiller assembly itself. Infact, if the line of action of the lifting force were ahead of the pivot point then it would tend to rotate the bottom of the rudder forward as required when generating positive lift although the drag force would still oppose this.
 
My second question concerns the positioning of the foils on the rudder itself: is there a concensus on how far down the blade they should be located to maximize their effect? At the bottom of the rudder the water probably only flows over the foils at the speed of the boat but in the wake region near the top of the rudder the flow speed relative to the foils may be faster and therefore more lift generated. There are probably other aspects to consider in positioning the foils such as the practicalities of tacking and gybing and launch and recovery.
 
Sorry for the rather long and technical posting but I'd like to hear what others think or have discovered.
 
Thanks,
 
Marcus 

[John Meadowcroft (Guest)]

Hi Marcus

Great to see you at the weekend.

Re Q1.  i dont think anyone has tried exactly what you are suggesting.  My first T-Foil had fixed gudgeons on the transom like a Moth or an I14 has.  The stock was fixed to the boat by a single vertical pin.  This pin also went through a rod inside the tiller which could be moved by rotating the tiller extension.  It was mechanically sound except that (a) it needed quite a lot of turns to switch it off and (b) when hanging over the back of the transom it did not like being rotated.  This was important as being able to switch it off is rather crucial at times

Re Q2.  everyone seems to have them in roughly the same vertical place about 6 inches below the waterline.  Aft sweep is very good for clearing weed and the fashionable set up seems to be to be also swept upwards.  the tips are almost breaking the water when at speed.  Why?  no idea?

John

[Jeremy C]

Dare Barry's boat has fixed pintles on it with adjustment being in the rudder stock. It is very neat for retro fitting as it does not require playing with the back end of the boat. a single line comes out of the rudder stock and runs down a couple of pulleys on the transom. If I remember correctly this line then runs off to the back of the centreboard case and has a simple 2:1 purchase. The stock itself is quite exotic as the top pintle mount in it slides inside the stock. Does require a single bar gudgeon pin arrangement as opposed to two separate ones. Contact Dare for more info, would be good to see a pic of it here to explain, but I don't have one.....

[ifoxwell]

I made ours as just a straight foil, no tapper, sweep or dihedral. But all of that was just for ease of manufacture. I cant comment on any performance advantage/disadvantage as we have no experiance of any thing else, but we have snagged a plastic bag on the T foil once which stopped us pretty quickly so on the mk3 I will probably include some sweep.

I would imagin that as soon you add sweep you also need to include some dihedral as well or as you rotate the foil for some lift you will be moving the blade tips down, away from there set position.

Ian

[Jeremy C]

Dare very kindly sent me through these pics of his rudder to help explain it.. If you look carefully you can see the adjustment line coming out of the rudder, back to a block on the transom. It then goes down to the floor and up to the thwart through the kivk bar.

[Kevin]

Marcus, in response to your first question: I believe that "rotating the rudder relative to the stock" is a recipe for disaster. If I understand you properly, you are suggesting pivoting the blade in the stock using the normal pivot bolt. All this does is load up the blade as happens with a conventional (unwinged) rudder when you don't pull it right down. All the winged systems keep the stock and blade fixed relative to each other for this reason. In terms of vertical force, virtually all of it is transmitted through the bottom fittings on the boat and rudder. There is very little affecting the top fittings, which are basically there to stop the rudder falling over sideways. For this reason all stocks for winged blades attach below (or around) the lower fitting on the boat.
 
Kevin N3527

[Marcus]

Thanks for the comments and the photos. However, I still don't understand why a control system that rotates the rudder relative to the stock rather than the stock relative to the boat won't work. I realize that with a conventional rudder that pivots about a bolt through the stock there's a significant sideways load unless the rudder is close to vertical. But surely the point is that when trimming a foiled rudder it's rotating relative to the transom anyway but only by a small amount about the vertical position. Why does it matter whether this small rotation is achieved by rotating the rudder relative to the stock or the stock relative to the boat? I was thinking of developing a trimming system based on a Topper type rudder stock and tiller. The idea would be to introduce some 'play' between the stock and tiller with the rudder in the down position and then move the tiller relative to the stock using a control system like the one Dare has developed. As the tiller moves relative to the stock the rudder would be rotated and the foils trimmed. Please could someone explain the error in my logic before I make a mistake?

Thanks again,

Marcus

[ifoxwell]

Hi Marcus

 

<font style="font-size: x-small;" size="2">It will work I guess, but will be far from ideal. Not because of the
affect on the foil just because, as stated earlier, of its affect on the feel
of the rudder. Its kind of how I started out but the one big lesson that I had
at the beginning was that what ever system you choose needs to be simple to
adjust, reliable and you need to be able to effectively turn off the foil at
any time in an instant. You choose when to pull it on but it’s the wind that
dictates when to turn it off so it needs to be full proof. </font>

 

<font style="font-size: x-small;" size="2">All that said if you look at this as an iterative learning
process, like I did, then what your suggesting will get you afloat and experiencing
the effects of a foil, and what it can do for you… its just that we can all
pretty much agree you'll be changing again it at some point soon after.</font>

 

Have fun

Ian

3453