t foil design

   I am going to make a t foil. However i have found examples of t foils with the 'fins' at the leading edge of the rudder blade, and others where they are at the trailing edge. Also there seems to be huge variation in how deep the 'fins are, with some just below the surface and others right at the bottom. What effect does this have , if any, or is it to do with constructing the foil?

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Reuben T Send Private Message Find Member's Posts Newbie Joined: 14 Dec 12 Online Status: Offline Posts: 17	Post Options Post Reply Quote Reuben T Report Post Quote Reply Topic: t foil design Posted: 01 Jan 13 at 6:47pm
	I am going to make a t foil. However i have found examples of t foils with the 'fins' at the leading edge of the rudder blade, and others where they are at the trailing edge. Also there seems to be huge variation in how deep the 'fins are, with some just below the surface and others right at the bottom. What effect does this have , if any, or is it to do with constructing the foil?

tgruitt Send Private Message Find Member's Posts Really should get out more Joined: 02 Dec 04 Online Status: Offline Posts: 2479	Post Options Post Reply Quote tgruitt Report Post Quote Reply Posted: 01 Jan 13 at 10:59pm
	It depends what boat it's for and what you need it for. If you want to hydrofoil clear of the water then they need to be at the end like an International Moth, or if you want them to 'take the weight' of the boat then you need to look at the International 14 or Cherub class. The Cherubs have a lot of useful information on their class website about building foils if I remember rightly.
	Needs to sail more...

Granite Send Private Message Find Member's Posts Visit Member's Homepage Far too distracted from work Joined: 12 May 04 Location: Scotland Online Status: Offline Posts: 476	Post Options Post Reply Quote Granite Report Post Quote Reply Posted: 02 Jan 13 at 9:50pm
	If you are making a T-foil rudder working on its own then it is good to have the lifting part forward, so it lifts, rather than levering the bow down, however structurally it is eiser to put the foil through at the back of the rudder. Some people say that there is a wake recovery benifit from the T if it is close to the surface, I am not sure if it is real or not, however if the tips of the T break the water it can lead to ventelation down the rudder, and a lack of steering. The stern wave does mean that the water will be at a different flow angle just behind the transom, what you don't want to do is have the foil both in and out of the stern wave, so if the T-foil is wider than the transom it will have a different angle of attack at the root than the tips, so is unlikely to be at optimum in either position. There should be a drag benefit from having the foil on the tip of the rudder, as it acts as an end plate to the rudder, and there are only two interfaces between T and rudder, rather than four with the T in the middle. However the joint is much more difficult to make right, and does need a torpedo fairing. With the correct rudder setup you can also have the lift quite far fwd.
	If it doesn't break it's too heavy; if it does it wasn't built right