Forums > Stand Up Paddle Foiling

Mast position for sup downwind

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Created by Pacoo 1 month ago, 22 Jun 2022
Pacoo
33 posts
22 Jun 2022 12:51PM
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Hi all,
I have been experimenting with this for one year now, and I would like to get some input and confirm or refute my observations:
1. Mast forward, generate early popup. But there seems to be a limit if you have too much volume in the tail. Yesterday I could not pop up at 75 cm (mast leading edge from the tail), went back to 68cm and all was good.
2. Mast forward, generate more stability when paddling. The board feels a lot more stable to paddle. This one I'm pretty sure.
3. Mast forward, seems to generate more drag when paddling. I'm not sure about this but, that's the feeling I have when the mast is around 70cm.
4. Mast forward, the board reacts faster, more lively feeling as there is less weight in the front.
5. Others.

For me using a 6,2 by 23,5 custom (around 6,5 kg), the sweet spot is between 65cm and 70cm (again mast leading edge from the tail).
Best

JB
NSW, 2226 posts
Site Sponsor
28 Jun 2022 2:25PM
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It's an interesting tuning situation for DW. In my mind, ultimately you want to balance the board up as best possible for the bets results.

My board is 6'6" x 26" (198cm long).
Board alone balance point is 91cm from the tail (centre of gravity hull only).
Foil mounted all the way forward (Back of board to front of mast is 73cm) balance point on the board (foil down) is 80cm from the tail (centre of gravity complete)
Centre of lift with my down wind foil taken from 25% front wing chord is 87cm from the tail (centre of lift).

This essentially means my CoG is slightly behind my CL without taking my stance into account. Now, one might argue this is incorrect as ideally you would want at the most the CoG and CL to align, but it feels quite nice to me as my board is super positive front foot and eager to lift. I stand forward, any unweighting and my board goes up fast. at 90+kg I have no troubles holding it down.

Your points above.
1. depends on the board you're on. My board has a pin narrow tail which I can easily bounced under and it provides push back after.
2. technically more forward would increase yaw however having more mast under you body mass will help side to side.
3. no. The only thing that might change here depending on your board is AoA. but I find I paddle faster with the foil forward.
4. Like a fin on a surfboard, further forward is looser, further back is more tracking.

Try everything, every cm makes a big difference.

Ride safe,

JB

frenchfoiler
438 posts
28 Jun 2022 2:22PM
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Interesting, I usually mesure back of the mast (trailing edge) to tail of the board.

JB
NSW, 2226 posts
Site Sponsor
29 Jun 2022 11:53AM
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frenchfoiler said..
Interesting, I usually mesure back of the mast (trailing edge) to tail of the board.


So do I, but thought I'd keep it uniform with the original post. For my measures, just subtract 11.5cm to get back of mast, thus mast is 61.5cm back of mast to back of board.

frenchfoiler
438 posts
29 Jun 2022 8:13PM
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On my 6'0, I'm at 63 cm (front of the mast to tail board), I don't know if I would go much far forward, too much front pressure.

Pacoo
33 posts
30 Jun 2022 12:23AM
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Select to expand quote
JB said..
It's an interesting tuning situation for DW. In my mind, ultimately you want to balance the board up as best possible for the bets results.

My board is 6'6" x 26" (198cm long).
Board alone balance point is 91cm from the tail (centre of gravity hull only).
Foil mounted all the way forward (Back of board to front of mast is 73cm) balance point on the board (foil down) is 80cm from the tail (centre of gravity complete)
Centre of lift with my down wind foil taken from 25% front wing chord is 87cm from the tail (centre of lift).

This essentially means my CoG is slightly behind my CL without taking my stance into account. Now, one might argue this is incorrect as ideally you would want at the most the CoG and CL to align, but it feels quite nice to me as my board is super positive front foot and eager to lift. I stand forward, any unweighting and my board goes up fast. at 90+kg I have no troubles holding it down.

Your points above.
1. depends on the board you're on. My board has a pin narrow tail which I can easily bounced under and it provides push back after.
2. technically more forward would increase yaw however having more mast under you body mass will help side to side.
3. no. The only thing that might change here depending on your board is AoA. but I find I paddle faster with the foil forward.
4. Like a fin on a surfboard, further forward is looser, further back is more tracking.

Try everything, every cm makes a big difference.

Ride safe,

JB


Thanks, JB, that's very good input about the CoG/CL balance. How do you measure the board's center of gravity?
I'm not so sure about point 3, it looks like if the paddle ends near the foil there should be some interaction.
It looks like we are all finishing around the 66-73cm for a 6 to 6,7.
Looking at what JB says and last Kalama "stern barracuda" designs, it seems that the secret to moving the mast even more forward is to low volume very thin rears.


Note: I'm using gofoil, so measures are 13,8 cm less if you want to use the trailing edge.

frenchfoiler
438 posts
30 Jun 2022 12:34AM
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It also depends on the foil you use. For exemple Axis, Gofoil, Takuma don't need to be super far forward. But Armstrong, Lift, Unifoil/Singature need to be far forward (expecially the smaller size).

Axis, you have the front wing further than the mast compare to other brand.

frenchfoiler
438 posts
30 Jun 2022 3:38AM
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Also depends on the width of the tail. I think on a wide tail, the foil can be more at the back.

On a super narrow tail especially pin tail, the foil can be far forward. It is the same for prone actually. For exemple Adam has his foil reallt far forward.

Pacoo
33 posts
30 Jun 2022 11:06AM
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frenchfoiler said..
Also depends on the width of the tail. I think on a wide tail, the foil can be more at the back.

On a super narrow tail especially pin tail, the foil can be far forward. It is the same for prone actually. For exemple Adam has his foil reallt far forward.


I fully agree with your observations. I'm ordering a new custom, 6,9 by 21, and I'm trying to figure out where the box should go. Perhaps s generate some kind of formula based in length but it looks like volume and shape are major factors to consider.

Scotty Mac
SA, 2004 posts
30 Jun 2022 10:12PM
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Yer that's why the decent boards are really the end of a series of trial and error to get the box position right! Assuming you have a board which is well designed and proven the way I find the best mast position is to have a really good guess then go ride and test and keep adjusting to its right. Perm texta and mark the bottom of the board for the final mast position. When you catch a bump get some speed then steer off the runner and paddle pump 10-20m and get a feel. Normally that's how I set my mast position. Too far back and it feels like you need to stand on the tail
of the board but be trying to have weight over the nose which is really not comfortable. Too far forward and it just feels weird. So it's paddle pumping on the flat between runners is where I decide the position and confirm if the balance is right. But maybe that just me.

frenchfoiler
438 posts
30 Jun 2022 10:26PM
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Select to expand quote
Pacoo said..

frenchfoiler said..
Also depends on the width of the tail. I think on a wide tail, the foil can be more at the back.

On a super narrow tail especially pin tail, the foil can be far forward. It is the same for prone actually. For exemple Adam has his foil reallt far forward.



I fully agree with your observations. I'm ordering a new custom, 6,9 by 21, and I'm trying to figure out where the box should go. Perhaps s generate some kind of formula based in length but it looks like volume and shape are major factors to consider.


Also depends on the foil. Big foil around 1700/2000 need to be further back while some ha front wing around 900/1200 need to be far forward.

Pacoo
33 posts
1 Jul 2022 12:48PM
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Select to expand quote
frenchfoiler said..

Pacoo said..


frenchfoiler said..
Also depends on the width of the tail. I think on a wide tail, the foil can be more at the back.

On a super narrow tail especially pin tail, the foil can be far forward. It is the same for prone actually. For exemple Adam has his foil reallt far forward.




I fully agree with your observations. I'm ordering a new custom, 6,9 by 21, and I'm trying to figure out where the box should go. Perhaps s generate some kind of formula based in length but it looks like volume and shape are major factors to consider.



Also depends on the foil. Big foil around 1700/2000 need to be further back while some ha front wing around 900/1200 need to be far forward.


Or you can also move your feet forward in relation to the mast. Isn't it?

frenchfoiler
438 posts
1 Jul 2022 2:08PM
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Select to expand quote
Pacoo said..

frenchfoiler said..


Pacoo said..



frenchfoiler said..
Also depends on the width of the tail. I think on a wide tail, the foil can be more at the back.

On a super narrow tail especially pin tail, the foil can be far forward. It is the same for prone actually. For exemple Adam has his foil reallt far forward.





I fully agree with your observations. I'm ordering a new custom, 6,9 by 21, and I'm trying to figure out where the box should go. Perhaps s generate some kind of formula based in length but it looks like volume and shape are major factors to consider.




Also depends on the foil. Big foil around 1700/2000 need to be further back while some ha front wing around 900/1200 need to be far forward.



Or you can also move your feet forward in relation to the mast. Isn't it?


Yes but when you stand too much forward it can tricky going down full speed.

Windgenuity
NSW, 537 posts
Site Sponsor
7 Jul 2022 4:30PM
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Select to expand quote
Pacoo said..

JB said..
It's an interesting tuning situation for DW. In my mind, ultimately you want to balance the board up as best possible for the bets results.

My board is 6'6" x 26" (198cm long).
Board alone balance point is 91cm from the tail (centre of gravity hull only).
Foil mounted all the way forward (Back of board to front of mast is 73cm) balance point on the board (foil down) is 80cm from the tail (centre of gravity complete)
Centre of lift with my down wind foil taken from 25% front wing chord is 87cm from the tail (centre of lift).

This essentially means my CoG is slightly behind my CL without taking my stance into account. Now, one might argue this is incorrect as ideally you would want at the most the CoG and CL to align, but it feels quite nice to me as my board is super positive front foot and eager to lift. I stand forward, any unweighting and my board goes up fast. at 90+kg I have no troubles holding it down.

Your points above.
1. depends on the board you're on. My board has a pin narrow tail which I can easily bounced under and it provides push back after.
2. technically more forward would increase yaw however having more mast under you body mass will help side to side.
3. no. The only thing that might change here depending on your board is AoA. but I find I paddle faster with the foil forward.
4. Like a fin on a surfboard, further forward is looser, further back is more tracking.

Try everything, every cm makes a big difference.

Ride safe,

JB



Thanks, JB, that's very good input about the CoG/CL balance. How do you measure the board's center of gravity?
I'm not so sure about point 3, it looks like if the paddle ends near the foil there should be some interaction.
It looks like we are all finishing around the 66-73cm for a 6 to 6,7.
Looking at what JB says and last Kalama "stern barracuda" designs, it seems that the secret to moving the mast even more forward is to low volume very thin rears.


Note: I'm using gofoil, so measures are 13,8 cm less if you want to use the trailing edge.


I used a windsurfing mast between two board stands. I measure the CoL on the foil (roughly 25% of the chord) and the distance from the this point to the front of the mast. Then I balance the board (foil down) on the windsurfing mast until the fuselage is level (I use a spirit level) and the windsurfing mast is exactly at that that CoL distance from the mast. You may need to move it a cm or two here and there to get it right, but once you get it all set up like this. The feel is amazing. The further apart your forces are, the higher the forces need to be to obtain balance.

There is a cool tool on this website that allows you to see what the forces do when the are separated and when they are aligned. The further your CoG is from the CoL the more thrust, AoA and Rear wing down force is required to balance things up, and this results in more drag. Here'e the tool - www.boldmethod.com/cfi-tools/cg-effects/

As everything basically pivots round your CoG, the further your forces are away, the larger the force acting become. Because everything needs to make an equal equation.

Here's a video explanation -

cornwallis
26 posts
10 Jul 2022 7:44PM
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Windgenuity said..
As everything basically pivots round your CoG, the further your forces are away, the larger the force acting become. Because everything needs to make an equal equation.



Thanks for the explanation, quite useful in tying the tweaks to something concrete..

I drew some pictures to try and wrap my head around the dynamics. Would you disagree with any of these?




Windgenuity
NSW, 537 posts
Site Sponsor
11 Jul 2022 11:08AM
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cornwallis said..

Windgenuity said..
As everything basically pivots round your CoG, the further your forces are away, the larger the force acting become. Because everything needs to make an equal equation.




Thanks for the explanation, quite useful in tying the tweaks to something concrete..

I drew some pictures to try and wrap my head around the dynamics. Would you disagree with any of these?





Very cool pictures. One thing to understand is that the Centre of Gravity includes your board and foil too, not just the rider. You are looking at a combined mass balance point vs the CoL.

Essentially what I am trying to achieve is get the CoG and CoL of my board and foil aligned, then I base my stance equally either side of this.

Here's a slightly lessor quality drawing.


The Red line represents the aligned CoL force going up and the CoG force going down. Then in Green I align my stance according.

This has worked really well in eliminating pitching moment.

Regards,

JB

Windgenuity
NSW, 537 posts
Site Sponsor
11 Jul 2022 11:43AM
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Select to expand quote
Windgenuity said..

cornwallis said..


Windgenuity said..
As everything basically pivots round your CoG, the further your forces are away, the larger the force acting become. Because everything needs to make an equal equation.





Thanks for the explanation, quite useful in tying the tweaks to something concrete..

I drew some pictures to try and wrap my head around the dynamics. Would you disagree with any of these?





Very cool pictures. One thing to understand is that the Centre of Gravity includes your board and foil too, not just the rider. You are looking at a combined mass balance point vs the CoL.

Essentially what I am trying to achieve is get the CoG and CoL of my board and foil aligned, then I base my stance equally either side of this.

Here's a slightly lessor quality drawing.


The Red line represents the aligned CoL force going up and the CoG force going down. Then in Green I align my stance according.

This has worked really well in eliminating pitching moment.

Regards,

JB


Here's a bit of a representation of the forces. I couldn't get the exact weights to make it perfect, but you'll get the idea.


Imagine this is your setup. The pivot is the CoL. The up force that is equal to the sum of the down forces to achieve sustained flight.

If we use this as a "good" balance (as it will not let be align the weights, so slightly offset works). The 20kg represents the combined CoG of board, foil and human. The 5kg is the down force of the rear wing to stabilize. The torque or moment is 198Nm each.

Here's we see what would happen if the combines CoG of the Board, Foil and Human was further forward just one increment. The Torque increases to 392Nm and therefore the rear wing force needs to increased to achieve equalibrium. As a flying state requires a balanced equation this also means that the foil would need to produce the equal and opposite force to achieve stable flight.

If the CoG of the board, foil and Human was way out of wack, then the down force required by the rear wing would be significantly larger. And the torque forces acting are all much bigger.

Back to our original idea of of just using minimal down force from our rear wing, if our CoG of the board, foil and human was behind the CoL we would not be able to achieve sustained flight. So next image....

So here the Human portion being moved forward (the stance going forward) to create an equilibrium. Whilst this is not too bad on the forces, it is still more than the better setup at the top.

Again, this is only showing the concept with this program to understand the ideal of lessening the forces involved to achieve equilibrium. The lessor the forces wasted to achieve stable flight, the more the forces becomes effective for use.

Again ,obviously the person is more the 10kg, the board and human is likely close to 10kg, but, but it is the concept.

So the idea is the more aligned all the forces can get, the more efficient your foil can be. You will also eliminate pitching moment which I proved over the weekend riding seriously over foiled on a 2140 in big swell exceeding 30kmh staying perfectly balanced according this methodology.

Pretty cool, and worth playing with.

Ride safe,

JB

kobo
NSW, 872 posts
11 Jul 2022 5:36PM
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To simplify. the less downforce you need from the tail the more efficient your rig is.(Smallest tail and least amount of shim ) That's why boards are coming out with boxes that go further forward so you can balance your ride with an efficient foil setup like the Armstrong FG boards.

cornwallis
26 posts
12 Jul 2022 4:14AM
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Select to expand quote
Windgenuity said..

Windgenuity said..


cornwallis said..



Windgenuity said..
As everything basically pivots round your CoG, the further your forces are away, the larger the force acting become. Because everything needs to make an equal equation.






Thanks for the explanation, quite useful in tying the tweaks to something concrete..

I drew some pictures to try and wrap my head around the dynamics. Would you disagree with any of these?






Very cool pictures. One thing to understand is that the Centre of Gravity includes your board and foil too, not just the rider. You are looking at a combined mass balance point vs the CoL.

Essentially what I am trying to achieve is get the CoG and CoL of my board and foil aligned, then I base my stance equally either side of this.

Here's a slightly lessor quality drawing.


The Red line represents the aligned CoL force going up and the CoG force going down. Then in Green I align my stance according.

This has worked really well in eliminating pitching moment.

Regards,

JB



Here's a bit of a representation of the forces. I couldn't get the exact weights to make it perfect, but you'll get the idea.


Imagine this is your setup. The pivot is the CoL. The up force that is equal to the sum of the down forces to achieve sustained flight.

If we use this as a "good" balance (as it will not let be align the weights, so slightly offset works). The 20kg represents the combined CoG of board, foil and human. The 5kg is the down force of the rear wing to stabilize. The torque or moment is 198Nm each.

Here's we see what would happen if the combines CoG of the Board, Foil and Human was further forward just one increment. The Torque increases to 392Nm and therefore the rear wing force needs to increased to achieve equalibrium. As a flying state requires a balanced equation this also means that the foil would need to produce the equal and opposite force to achieve stable flight.

If the CoG of the board, foil and Human was way out of wack, then the down force required by the rear wing would be significantly larger. And the torque forces acting are all much bigger.

Back to our original idea of of just using minimal down force from our rear wing, if our CoG of the board, foil and human was behind the CoL we would not be able to achieve sustained flight. So next image....

So here the Human portion being moved forward (the stance going forward) to create an equilibrium. Whilst this is not too bad on the forces, it is still more than the better setup at the top.

Again, this is only showing the concept with this program to understand the ideal of lessening the forces involved to achieve equilibrium. The lessor the forces wasted to achieve stable flight, the more the forces becomes effective for use.

Again ,obviously the person is more the 10kg, the board and human is likely close to 10kg, but, but it is the concept.

So the idea is the more aligned all the forces can get, the more efficient your foil can be. You will also eliminate pitching moment which I proved over the weekend riding seriously over foiled on a 2140 in big swell exceeding 30kmh staying perfectly balanced according this methodology.

Pretty cool, and worth playing with.

Ride safe,

JB


Thanks for those, will try update the diagrams incorporating that concept of the lever moment around the combined boat+rider "pivot", as the extra weight in front needs to be balanced with extra tail downforce, and therefor drag.

It really drives it home that in order to minimise the size tail you use, you need to first minimise the amount of force that it needs to counteract!

I feel this most clearly with dockstarting in flat water - my feet are both nearly right above the wing CoL, and then it just flies!



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"Mast position for sup downwind" started by Pacoo