Mast Postion

[laurencet]

Just looking at building hydrofoiling sailing dingy but struggling slightly with the mast postion. the intial design has 80% of the lift from the front pair of vee surfacing pircing foils.. with a single rear conventional foil attached to the rudder.
All the skiffs i've sailed so far have had the have had the centreboard miles behind the centreboard. Is it possible to keep a neutral balance with this design. last thing i want is a boat with really bad lee helm
The Intial design has a conventional dingy mainsail. With the option of a aymmetric spinnaker when i'm feeling brave.

Thanks in advance

Laurence

[PAR]

You're kidding right?

I know this is your first post, but descriptions such as

conventional dingy mainsail

or

All the skiffs i've sailed so far have had the have had the centreboard miles behind the centreboard

just will not do. What are you building? Plans? What do the plans say? etc., etc., etc. If you don't have plans and are asking these questions, you need considerable help balancing out the design, as it's knowledge you current don't own.

[laurencet]

Sorry Par I should have explained.. I'm not looking for anyone to carry out a design review or too carry out the calculations for me. (I’ll post some pretty rendering at some point) The fun of this project has been to learning for myself. Re-arranging equations to calculate the height at which the hydrofoil will fly has been great fun. Especially as the front foils aren’t parallel and have a 3 degree washout.

I was looking for any basic rules, Or rules of thumb when calculating the mast position. from xfoil I can model the sail and come up forces and moments.


My understanding so far is that traditionally you place the centre of lift directly over the centreboard. (ignoring the effect of the hull shape). By placing the centreboard in front of the mast as soon as you get hit by a gust, I will suffer from shocking weather helm. So constantly be working the rudder just to keep the thing in a straight line.
Is this the case? Is there anyway to get around this?. But equally if I place the foils behind the mast as soon as I get hit by a gust, as the boat is short it will desperatly be trying to pitch pole....
How does Doug Halsey or the Rave get around this problem?

If I do build a test rig I’ll be using my old moth sail. But the design at the minute is far from that. Still got composite beams to break and FEA models to finish and that’s after the finishing the hand calcs.

Ps With regards to boats I sail. I currently sail a conventional moth and a rs700... Not sure why this helps? I can measure the distance between the mast and the centre board if that helps?

[sorenfdk]

Originally Posted by laurencet

from xfoil I can model the sail and come up forces and moments.

No you can't! XFoil can only carry out 2D-calculations, and a sail is 3D!

[laurencet]

From my 3d model of the sail I can generate a number of 2d sections half a meter apart. I can then input these 2d sections into xfoil this will give me approximate values of Cl, Cd and Cm for each section. As I know the appropriate area I can input all the values into excel. Using an Alfa lookup table I can now look at the force and moments at various angles of attack for the total sail. The fun part is adding prandlls tip loss equations and trying to take into account mast bend.

[laurencet]

Still got a long way to go.....

[sorenfdk]

Originally Posted by laurencet

From my 3d model of the sail I can generate a number of 2d sections half a meter apart. I can then input these 2d sections into xfoil this will give me approximate values of Cl, Cd and Cm for each section. As I know the appropriate area I can input all the values into excel. Using an Alfa lookup table I can now look at the force and moments at various angles of attack for the total sail. The fun part is adding prandlls tip loss equations and trying to take into account mast bend.

And how precise will your solution be

Have you tried xflr5? I think that it does more or less what you're trying to do!

Good luck!

[laurencet]

I've never used it.. looks really good. Similar interface to xfoil? I've struggled with Avl in the past.

[laurencet]

ps. All cfd work needs taking with a large pinch of salt....good for pretty pictures and presentations. I was just trying to get my head around how a windsurfer is neutrally balanced last night. With most of the board out the water and only a skeg in the water at the back of the board, I’m surprised they don’t swing leeward when hit by a gust of wind.

[sorenfdk]

Originally Posted by laurencet

I've never used it.. looks really good. Similar interface to xfoil? I've struggled with Avl in the past.

Xflr5 uses XFoil as "machine", but it's much more user-friendly!

[laurencet]

Will Xflr calculate the centre of lateral resistance (CLR)? I'm not going to hold up much hope as I doubt xfoil will converge with the flow set to 90 degrees to the foils... I've read a few examples of people cutting the shapes from card and finding the centre with a pin. Doesn't seem like the most accurate method. Does anyone have any ideas before I start cutting?