There are many airfoils to choose from airfoiltools.com/ with lots of good data.
I'm interested in which foil profiles are used on the commercially available foils
I'm about 90% complete building my own using Rob Rock's Instructions.
This has NACA0010 (half) for main wing and spoiler (spoiler- thrust down 2 degrees).
I've been modelling up the NACA0010 half foil section as 6409 9% not being able to find any data on NACA0010 half.
Some interesting charts of below make me think about experimenting with symmetrical NACA0010 spoiler.
Later after I try this construction out - hopefully I can make it fly!

Interesting project. Couple of questions: Did you go with 2 degrees because of Rob Rock suggested it or another reason? Do you have an easy way to change that?
How long is your mast?

1. Mast length 45 cm - shorter mast meant to be better for beginners. I see Slingshot offering 38, 61, 76, 90 cm to this end. Also there are sand banks where I sail!
2. -2 degrees per the instructions. I think it is to put the main wing at a nice angle to generate lift and balance mast foot down force around the main wing up force fulcrum point. Achieved by hardwood wedge. I guess you could make a series of wedges or pack the bolts with varying numbers of washers to get some adjustment?

i think this is an older (superseded?) design. Are the commercial products converging on a similar 'optimum' design?

Are speed sailors designing foils for speed - can sailboarder start going significantly faster than wind speed like Paul Larson's Sailrocket = 65.45 in 27 knots wind. According to Zara Davis in Windsurf mag two years ago the top guys were hitting 52 in 60 knots wind. Last year she clocked 46.49 in 45 knots. Interesting times!

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Slim Jim said..
Are speed sailors designing foils for speed - can sailboarder start going significantly faster than wind speed like Paul Larson's Sailrocket = 65.45 in 27 knots wind. According to Zara Davis in Windsurf mag two years ago the top guys were hitting 52 in 60 knots wind. Last year she clocked 46.49 in 45 knots. Interesting times!

see http://mauiultrafins.com/technology/how-fast-can-we-go/

Apparently foil drag limits the application of foils for high-speed windsurfing, currently foiling is working very well at speeds below 30 kts. For higher speed, the foiling area must be reduced which shift the lifting speed to higher values. For very high speeds, the foil area has to be drag optimized for the required speed, for 50 kts e.g. it means about 100 cm2 in area. Such a small wing would lift the board out of the water, not before 20 kts.

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Slim Jim said..
1. Mast length 45 cm - shorter mast meant to be better for beginners. I see Slingshot offering 38, 61, 76, 90 cm to this end. Also there are sand banks where I sail!
2. -2 degrees per the instructions. I think it is to put the main wing at a nice angle to generate lift and balance mast foot down force around the main wing up force fulcrum point. Achieved by hardwood wedge. I guess you could make a series of wedges or pack the bolts with varying numbers of washers to get some adjustment?

i think this is an older (superseded?) design. Are the commercial products converging on a similar 'optimum' design?

Are speed sailors designing foils for speed - can sailboarder start going significantly faster than wind speed like Paul Larson's Sailrocket = 65.45 in 27 knots wind. According to Zara Davis in Windsurf mag two years ago the top guys were hitting 52 in 60 knots wind. Last year she clocked 46.49 in 45 knots. Interesting times!

The Slingshot works because the masts are easily interchangeable and come from a kiting background where a long mast is a bigger challenge when starting out. For your first couple of flights, you think, "why do they make masts so long?" Then, you realize that getting the foils to the surface is as big or bigger problem. The wings ventilate, stall and the board drops straight down. Not good. It's even worse if there's any sort of chop which can suddenly get the wings close to the surface in the troughs.

Understand about the sandbanks. You may end up at a happy medium around 60-75 cm. Hitting the bottom is no fun and potentially damaging. You may have to be more selective about your venues. I vacation at a very good windsurfing spot that is pretty much impossible for foils. I didn't even get mine out of the car for the week.

re: angle. 2 degrees is a fair amount. The amount you need varies by a number of factors including how the board balances out - your foot position vs the fin box and where the mast foot is as well as your size. The better balanced everything is, the less stab angle you'll probably require. Also, going faster tends to require less angle. Glad you have it adjustable and not "hard-wired".

For rec riding, 20 kts is plenty fast. Things go by in a blur when you are 50 cm off the water. If you are going to push the envelope - (afaik) currently in the low 30s, you'll probably want to at least ride a commercial wing to see what others have done before you. With planing boards so efficient at higher speeds and much more "pitch stable", foils aren't seen yet as straight line speed weapons. Their efficiency at lower wind and board speeds, though, makes them ideal for light wind competition.

There isn't really "a" design. Different foils address different objectives. Slingshot is intended as a stable free-ride platform, which they do well.

I enjoy Rob Rocks' videos a lot. Glad others are getting inspiration from them.

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Slim Jim said..
1. Mast length 45 cm - shorter mast meant to be better for beginners. I see Slingshot offering 38, 61, 76, 90 cm to this end. Also there are sand banks where I sail!
2. -2 degrees per the instructions. I think it is to put the main wing at a nice angle to generate lift and balance mast foot down force around the main wing up force fulcrum point. Achieved by hardwood wedge. I guess you could make a series of wedges or pack the bolts with varying numbers of washers to get some adjustment?

i think this is an older (superseded?) design. Are the commercial products converging on a similar 'optimum' design?

Are speed sailors designing foils for speed - can sailboarder start going significantly faster than wind speed like Paul Larson's Sailrocket = 65.45 in 27 knots wind. According to Zara Davis in Windsurf mag two years ago the top guys were hitting 52 in 60 knots wind. Last year she clocked 46.49 in 45 knots. Interesting times!

Re.......Are the commercial products converging on a similar 'optimum' design?
I've just been on Maui for a month. The shops have all sorts of foils for sale and there is a huge variety of foil section types and sizes. Looks to be a long way from any 'optimum design' yet.
Talked to a home builder who was using MH32 for the front (8.7% x 2.3% camber) and NACA 2408 (8%x 2% camber) upside down for the stabiliser.

Wanted To Fly, but had No Foiling Idea....

2nd sail with the DIY foil and surprisingly it actually (sort of) works!
It lifts the board out of the water when the nearest weather station was showing 20 - 24 knots and I had a no cam 9.0 on a 155L Starboard Go, but I could only keep it aloft for ~5 s. Even so it was great fun and I think I'm hooked!

I need to do some more work on it /suggestions for anyone thinking of making their own:
1) I kept flying into wind - the mast stalls and then I dropped. I think I need to add the vertical stabilisers to the main wing and spoiler (per the Rob Rock Windsurfing instructional vid (RRW)).
2) I made flat foils because it was easier than trying to bend 12 mm marine ply like Rob Rock does in the video. Next foil will have some curvature which should help with the stability issue. I'm thinking of a bird wing shape with a WACO Cootie section.
3) Rob Rock stressed to reinforce the aluminium fuselage with stainless steel square section... I didn't - the up thrust is considerable!:



I'll probably buy a commercial one once I get through the short mast learning phase.....
(I'm pretty sure I can't make a longer mast using these construction materials - I had to add plenty of extra carbon layers just to get it so I couldn't twist it by hand and it looks more like a tree trunk than a fin!)

Cool project but I have to ask are you doing this just for interests sake, cost reasons or the challenge of it? I only say this because you'll find it a lot easier to learn on a commercial foil as the hard work has been done design wise. Also in my opinion, don't buy a short mast, go straight to as long as you can, there is much more room for error.

If you're going to build another one or modify it, build adjustability into the design, this is more important than worrying about NACA profiles at this stage. The two main things are; you need to be able to adjust your rear stabiliser trim and you need to be able to adjust the position that the mast mounts to the fuselage. This lets you balance out the moments of the front wing and rear stab which is very difficult to calculate accurately, IE you have to try it. You might also need to adjust front wing trim but this is more to get board attitude correct once you have it flying sweet. Also look and benchmark your design against commercial designs, they do things for a reason and yes there are imperfect commercial designs out there but all of them are infinitely better than anything Rob Rock has produced as harsh as that sounds.

I actually built my own foil too initially but it was a huge effort as I went 'all out'. I used in no particular order; Vac pump, lots of carbon and resin, a lathe, a CNC mill, CFD software, CAD software, a 3D printer, a welder, lots of other tools and a ****load of time. It was what I would consider successful, as it worked and was fast straight away. It was competitive with what other guys were using and what was available at the time (a year or so ago), pre Starboard race foils and the Pryde F4's etc. I iterated the design and was up to version 3 rear stab and version 2 front wing but that takes a lot of time and effort, something a home gamer doesn't have compared to the big players.

Ultimately it had a fatigue related load failure and I went out and bought a commercial one (Pryde F4). I enjoyed the challenge of the process and maybe that's what you're seeking but yeah, there's something to be said for buying one. Just sayin'

You should post pics!.

Also wasn't the bit that broke the bit you bought :) Well apart from all the wings that detonated haha.

CJW - to DIY definitely satisfies some need for creativity and the process is certainly exercising my mind and body.
It also has the potential to provide amusement for the other water users - I gather from Swoosh's comments!
You're most certainly correct - buying one is the way to go!

Good stuff Slim, as you say nothing like sailing something you built yourself As Swoosh eluded to and as you have found out that it does have its perils in terms of teething problems I had a rear stabiliser failure due to the 3D printed core of the carbon foil going soft (printed with too little infill) and eventually the mast snapped....oops. I did reinforce it but it was a kite mast (manta foils) and wasn't designed for the windsurfing loads, which are also tricky to work out. As I added bigger sails and started putting racing style loads on the foil it couldn't cope.

If you've got any questions though just let me know.

I stuck it out and here's a bit of an update:

Hydrodynamic - wise it seems to work - the 900 cm2 wing flies in 18 - 20 knots and it feels most at home just under the surface:


.....but as you've told me getting it structurally correct seems problematic:

#1 fuselage bent and I fixed that by reinforcing the 1" aluminium tube with 16swg 7/8" SS316 tube between the main wing and the mast connection. Tasmanian hardwood dowel filled the tube and glass/resin cemented it all together.

#2 fuselage to mast bolted joint failed. I fixed that on #3 by switching from 9 to 12 mm cross nuts and adding a horizontal bolt to supplement the 3 vertical ones.

#3 had mast had glass core/Kevlar/carbon construction with the 5 deg forward rake angle on the mast. The MDF wooden mould makes a great profile. 750 mm mast::


The silicone mould for the deep tuttle fin box saved on sanding: I hand sanded with wet sand paper because it cuts down the mess/dust dramatically. Getting the mast angles - 5 deg forward rake and vertical in the fin box required a bit of concentration. Make sure you jig the mast up and down in the carbon/resin filled mould to clear any air bubbles which could lead to a catastrophic failure later (see below):; filling them later will not be strong enough.


Getting the profile for the load plate was tricky because Starboard Go has a double concave thru the fin. I made a wax impression/plaster cast and did lost wax method to form a really stiff carbon fibre plate.

That still didn't get the shape so for final fit up I used chopped strand glass to get a moulded fit to the board (plenty of PE film and double sided tape to protect the board!):


#3 didn't have any taper on the wings - this was to increase area and for ease of construction. Glass fibre wrapped wooden core in the mouid:

The final product before and after painting:







I was getting quite confident controlling altitude in a straight line and started trying for some gybes. This was tricky and definitely a work in progress. Crashing appears to be an art form - to jump clear or slide across the deck in this situation?



Falling on the foil - is not a good option! Key learning point - Tuttle boxes are designed so the fin rips out to save the board. I'm not sure this logic applies to foils!!!!



I'm picking up material for #4 tomorrow - I'm planning to cast/wrap M6 SS threaded rod in the mast to connect fuselage to board. The second bolt will also be threaded rod with at least two cross nuts set deep in the Kevlar sections of the tuttle fin head.

Hopefully then I'll have a stable platform and can make a larger main wing to explore lighter wind foiling.

I've really enjoyed this project both in the garage and on the water. Thanks Rob Rock for the inspiration and the helpful comments you've all posted on this forum.

Well, we told you it would be a journey! Well done.

Pro (well, overconfident amateur at least) tips:
1) hold on to the boom when falling. 99.9% of the time, it will flip the bottom of the board away from you as you go in.
2) If you want, get some black epoxy colorant so that when you do your glassing and filling, it wonderfully matches your expensive CF and you don't have to paint if you want to show the world that cool CF fabric.
3) A thin layer of EVA foam sometimes is enough to work between the bottom plate and the board to conform to the gaps, etc.

Non pro tip: I retract the goofy things I said about stab angle above. More experience and more reading has modified that a bit. 2-3 degrees seems fine and don't worry about flattening it out for speed - doesnt really work that way like I thought then. There are enough current posts on the topic that I won't belabor the point other than the cringe factor of reading that.

Thanks for the update!

I think the radical tips may be contributing to your difficulty in jibing.... that design looks like it only wants to head in 1 direction and that is straight forward. I would rather just stick to the curve you have and round off the tips without the tip dihedral.