Thursday, January 7, 2010


Foils and deck done. the laminates came up really nice. the araldite 3600 resin that Ive been using becomes so high modulus when you bake it (80 - 100 deg) that you'd almost say its prepreg. Foil came out at 1.8 kg and the deck at about the same, with a further 250 grams for the bulkheads. so the vacuum did the trick. The only problem is that the socket in the foil looks a bit too shallow, (by like 10mm, massively too shallow!) so if thats the case i might re-do just the horizontal with a much bigger bulb, and cut the tab on the strut a bit longer. Will try and make what ive got work first. I put 2 tubes down the strut this time, in case i want to do a swept horizontal with a split flap at some point. I really should have put some high build primer down in the mould first, as Andrew at CG composites suggested, because im going to have to go over everything with some black bog to patch up all the tiny holes in the weave that the vacuum leaves behind.

Anyway lots to do, tomorrow the bulkheads go in and I need to make some little angles so that the deck can stick to something. I really have no idea how people would normally stick a deck on a moth. If anyone has a suggestion let me know! also the gantry mould is ready for the CNC machine to cut out. opted for a box gantry over tubes because then I could avoid buying tube, and use the materials I have already that are leftover from the deck. quite like the design of it though so it should be fine. function over form!

ps. if anyone would like some free moth bits, i have some now redundant lowriding foils, need some care admittedly, some moulds for a NACA 66014 strut and an old repaired carbon strut that is about 1.3m long. and my old gantry.


MARKLA said...

re: bonding angle for the deck
I considered it but didn't bother in the end. I did however decore the hull side panels 4mm deep tho so a good (or adequate) bond was made between the inner/outer skins of the hull and the inner skin of the deck. Between this and one pass of taping on the outside of the join everything seems sufficient.

re: gantry.
My first one looked similar to what you have modelled. However ensure you still have pleanty of unis running along where you would otherwise use tubes. You still have to be mindful of the structure buckling so ensure it is stiff in all directions. My first gantry failed because the core sheared and the two skins buckled away from eachother.

You could consider boxing the top in to so it becomes very stable in torsion - however this is significantly heavier than tubes, and shouldn't be called a gantry.

Karl said...

Nice looking foils!

What did you put in there for core?

Assassin said...

Hey nick that main foil will more than likely break off,(it's similar to our 1st attempts) you need to be able to pull more than 80kg on one side.

nick flutter said...

Hey Karl, core is corecel T-foam. Not much in the foil, quite a bit in the strut. The bottom if the strut and middle of the foil are both solid.

I think that foil will most likely not be strong enough at the t. I'll have router look into a new one (with a huge bulb) I think.

Markla cheers for the deck / gantry advice. I'll take it on board. I like the de-coring idea

Karl said...

I posted something but it got lost apparently...basically saying the strut benefits greatly from a solid composite shear stringer in terms of stiffness for the same amt of carbon.

Respect to any home builder who can get a two piece T foil to hang in there!

What aspect ratio is that lifting foil Nick? Area? I am curious to see how far we can squeeze these things.

nick flutter said...


The aspect ratio i think, if i calculated it correctly is like 10.5 or 11. (average chord / span?)
area is 0.091m2 the area is less than normal because the section is a NACA 64610 which is much higher lift that I used before. i calculated everything based on data from my last foil. yet to see if its big enough. looks about right to me! haha The lift distribution is as close to elliptical as i could get without spending too much time. This is achieved by twisting the foil, the mid-span sections have the highest AOA. i took lift distribution at different flap deflections roughly into account as well.

the section from about mid half-span out is 9% going to 8% near the tip, and i would say at this point that the tips of this foil are well and truly stiff enough. a straighter taper results in lighter loads at the outer parts of the span so you can go a thinner section. overall im really impressed with the foil, it feels like a samurai sword in stiffness, but much lighter. the tips are only about 3mm thick and they are almost unbelievably solid. the section at the root is 12%, which is enough except if you want a good depth for the T-connection.

The socket needs to be 25mm deep, the deeper the better. mine is 13mm and its clearly not enough, i can tell that even before ive installed a bolt. I put a bladerider together for a friend once and i know how it should feel. I think ill ditch this foil and start again with a deeper socket, and modify the strut with the grinder!

I always meant to laminate a sheer web into the strut. i was planning on using about 4 layers of 300g biaxial cloth, and squeezing it in a slot in the foam. i didnt do it because i forgot, and when i remembered i was half way through putting the thing together, and although the resin wasnt going off yet i just didnt do it. It would have been sticky, and i try not to get sticky these days! I reasoned that because the vacuum had put more carbon at the outsides of the structure Id probably not need a strong web, (which is clearly bullshit if you think about it) but in the end the foil is more flexible marginally than the old one. (which was nearly solid carbon)In its proper load case it will probably be the same stiffness because of the extra structural depth i gained by making it 140mm chord where it exits the hull. I think having a 20mm wide section and 20mm deep stock at that point is a good move, although youre right, if i did it again I would remember a sheer web. I dont like using pultrusion for it though because I try and avoid grinding carbon where i can. i hate the itch and id really like to live a long and healthy life. haha.

the foil for 15mm or so either side of the socket is solid carbon uni and biaxials, and i reckon thats enough. there isnt much foam in the foil (90ccs maybe?) because there was quite a lot of carbon and the section is mostly very thin. the strut has lots of foam and it came out really light, about 1.2kg. the tab at the bottom to go into the foil is solid carbon for the bottom 80mm.

anyway having had a couple of days to digest the info thats about all I know. I have no doubt though that if i put a bigger, mach 2 style bulb on like Bruce designed,the connection will be extremely solid. the horizontal feels stiffer than those of my previous 1 piece foils. anyway ill keep going with the boat and when I feel up to it ill modify the foil in cad, CNC it and have another go. no great loss because i think the horizontal cost maybe 80 or 100 AUD all up to build, and I easilly have enough material leftover to make another one or 2 or 3. next job is centreboard case and stem post. better get back to it!

Karl said...

Lately I'm pondering 18:1 at 0.078, ellipse, constant section. Will lay up a beam of similar proportions and test it to failure. No flaps or T joint.

The rudder is now windsurfer fins, but I am wondering where to run it depth-wise. It will be a hell of a lot deeper than what came with the Prowler, as I am tired of spinning out uphill, downhill, etc.

I cut shear webs from plate on a table saw. Messy but precise and efficient.

Thanks for the info.

nick flutter said...

cool. i wouldnt be suprised if that works. I think it will be possible to go with a much greater span. flex might be an issue. im not sure how solid a foil needs to be. maybe its possible to design it with some downward curvature, so that it straightens out when loaded up like an aircraft wing?

ive never had the spinning out problem, but i suspect forward rake would go some way toward solving it.
ive also been thinking about the much discussed tubercles along the bottom of the rudder strut, to act as fences using the high/low pressure regions to stop air moving down the span. dont know if that will work but its something that I will probably try when i get a chance.
anyway good luck with it, im keen to know how the super-wide span goes. if it is promising i might knock one out

Karl said...

It isn't that super-spanny but the area is down a lot so it ends up pretty thin. I had the same thought re: pre-bend but maintaining constant AOA is probably a concern when talking about that much flex. I'll make something and get out the lead shot to see how it behaves under load.

How deep are you making your rudder? Cooke/Ninja looks to be running his rudder at the same depth as the mainfoil, as is Mach2 from what I can see. Perhaps the extra strut allows one to wind it up more offwind without ventilating the strut. My current boat generally spins out because I fly too high and the rudder lifting foil comes out of the water.

MARKLA said...

QUOTE: The lift distribution is as close to elliptical as i could get without spending too much time. This is achieved by twisting the foil, the mid-span sections have the highest AOA....

Using this twisting, is the lift distribution still elliptical when the foil is operating off the design angle of attack? Gut feeling says no but I haven't proven it. Twisting is cool, but I can't justify it compared to running a constant section and running an elliptical shape.

One thing I am considering is making the "bulb" a symetrical section which generates no lift at design AoA to avoid pressure spike near centreboard - lift will be geenrated further along the foil. At low speeds when the AoA is higher it will still generate some lift, but I haven't found ventilation a problem at low speeds.

nick flutter said...

yeh, after a few degrees + or - the lift distribution is all over the place. but really, the AOA stays pretty constant in my experience + or - 2 degrees or so, and in that range it seems to be ok as far as i could work out.

im using my old rudder for the time being, which is square and sits at the same level as the main foil or maybe 20mm below.

that small bulb i had at around 0 angle of atack i think and a cambered section, -generating proportionally little lift for its chord - but im looking at a much more symmetrical section for the large one for that reason Markla - also i guess you want to make sure the max velocities dont coincide between the bulb and the strut. ive been trying to work out how to do that because for the sectons im using the velocities seem to plateau for a while. i think i might use a 0014 or smoething (or slightly cambered version) for the bulb that has a pressure peak near the front, but still some thickness near the back.

i think avoiding a pressure spike is a good idea for lots of reasons. tom speer had a 'dumbell' fairing that might be interesting too, but i have no idea how to design it.

Karl said...

The Assasin guys did a nice job with the rudder I thought. Extreme rearward strut offset and even the join point itself is well aft of the leading edge with a fairing. Without a flap or a two-part foil you can basically do the same thing with the mainfoil and make it all monolithic to reduce the strut footprint.

I don't know anything about bulbs. My thought would be forget the bulb and make a solid T joint, moving the joint up the strut a fair distance where it is not so highly loaded and does not have negative drag implications for the lifting foil. We all sail with two part masts so a two part strut can't be that hard. Then make a proper fairing at the T.

Karl said...

Oh yes I forgot you are running flaps. Forget the last bit about the fairing then.

nick flutter said...

mm intersting ideas.

ive had trouble in the past with the T joint doing it the 'monolythic' way, thats why i went for this arrangement. its much easier this way to get continuour fibres running across the top and the bommom of the foil horizontal. that said thats really the only problem with the other way and im sure thats pretty easy to overcome with a clever layup.

i do like the assasin rudder. i bet that forward kick at the bottom helps with ventillation too.

how do you make a flapless foil work anyway? i sailed with one of the very early foils at one point, which was flapless. it was a lot of work and at my skill level at the time i couldnt really control it. do you need to have a wand connected to the rudder pitch or something?

Karl said...

I've been flying flapless for a couple of years now. The kinks are not all worked out, but through some clever design I think I will have a less tweaky system in the boat soon. Current version is a bit too much trouble to bother with, though it is fun when it is up and going. Marginal conditions are painful due to wand drag mostly and making adjustments for that; disengaging and reengaging the wand while taking off is tough.

MARKLA said...

what about cutting the bottom out of the socket on the lifter so the strut ends flush with the bottom surface of the lifter.

nick flutter said...

Yeh, considered doing that, what would you do about the bolt? Also put most of the lower surface structure straight under the socket. Probably worth a try though, though it's not that much work to just do it again propperly.

Anonymous said...

I just saw this video
of some french ideas after wandering through the german site.

You might find it of interest, given the stage you are at. There is a degree of modularization and separation of concerns that is evident and interesting in the video.
I like the bulkhead plate that separates mast from control console, and minimizes violations of the integrity of the bulkhead. The gantry mount is neatly done too.

nick flutter said...

cheers, cool link
lots of ideas there. very nice model too. some thoughts: the gantry is very similar to what im building, except not adjustable. (so why pivot?) actually the only reason im building a gantry like that is because its very cheap! tubes are stiffer. the rest looks quite standard - bladerider, mach2. kingpost design is exactly as per bladerider, wing sockets are similar, same as mine but bladerider/mach 2 have arguably a better system, less leaks? maybe? (rx was still quite leaky) i had to go with something like that because my wingbars were already glued together at too obtuse angle. geometry is possibly a bit behind state of the art - foredeck is quite long and the wings are too big (long). actually, its maybe ok, the centreboard is pretty far forward. the trend seems to be to close up the distance between the foil and the mast step. mesh tramps dont really work, beacuse theyre too slippery and there is pretty much no aerodynamic advantage. bolt-on centreboard case top plate is interesting, assuming it allows for interchangeable foil sections. like could i buy a mach 2 foil and install it in this boat? that said, its never really been a problem in the past, or has it? foil T joint connection is not strong enough (see assasin, needs to be wider) rudder box is bladerider / Andrew Stevenson, works well. vang / cunningham cleat setup is very steve clarke IC style, im sure it was done before him too. but would definietly look nice in clear pre-preg carbon. vang saddle not strong enough. wing joints are nice but hungry tigers and prowlers maybe do it better. anyway, its pretty classy. would like to see it built. theres disproportionately few moths coming out of France, considering they're all such pros!

Anonymous said...

re gantry side bolts , I don't think the designer was thinking in terms of pivoting vertically, so much as keeping the gantry loads through the bolts in sheer ( i.e. compression or tension from side to side). If the bolts had been set in vertically, all side to side play gets (exponentially, over time)worse as the bolts act as pivots, and a failure would have instant catastrophic effects.

As designed, a gantry mount or compression failure might not be so instantaneously messy.

The design is pretty smart about putting through hull bolts/screws into the structure at points with a lot of natural "build" in the laminate; it wouldn't take much extra effort to avoid complete penetration of any fastening.

I like the chines on the deck to give rigidity to the dance floor, simple idea to implement with great probable results.

nick flutter said...

ahh yeah, the deck chines were pretty hard to see, i like them. something to stand on too. the gantry thing makes sense. didnt realise there was a turned down join at the hull/deck (dont know what thats called) but the way it fits into that is nice. and the bolts through the thick laminate are a good idea, although ive seen those type of things used in the past, for the purpose of minimising holes, and they can just glue on the outside.

anyway, cheers for the link, im always open to new ideas! admitting that your wrong is sometimes hard, but if this whole process has taught me anything its that i have been wrong plenty of times, despite my best efforts to be right! still pissed off about forgetting the sheer web in my foil strut! im sure some of those ideas we will see again.

The Flip Side said...

Hey Nick, hi from France. I'm working on Kitesurf foils however, we not as yet up to the same level of technology and calculations as you Moth guys. I have lots of respect for all of the interesting calculations and development work that you and other Moth riders are carrying out. You mentioned in this post that you had a number of foil sections and bits and pieces kicking about - I just wondered if you still had them and if yes it would be possible to get hold of some foil sections??

nick flutter said...

hey mate
yep, check out the latest post, i put the hydrofoil 3d rhino model there so you can download it. its all based on a NACA section that i cant remember the number of!.

i cant guarentee that the foil is actually good, but is is quite similar to the state of the art foils in terms of its basic parametres.

otherwise contact me flutter (dot) n (at) gmail (dot) com and i can sift through and see what I have accumulated.

where do you kite in france? im heading over soon with no real agenda, so if there are foil experiments going on id be keen to come and have a look on the beach one day!

Luke said...

Do you still have any extra moth parts

nick flutter said...

ive got a carbon gantry, and a foil strut that can be had for free. are you in brisbane? i could possibly lend you a set of foils for the moment to get a lowrider project going.

Luke said...

Im in the states. Im starting a moth project and was looking at some different peoples blogs on them and yours caught my attention. Ive bought a used older hull and it needs wings and blades and was wondering if you had any advise for me about how i should go about building those parts. Any input is much appreciated.

nick flutter said...

hey luke, just email me flutter dot n at gmail dot com

ok so a hull is good, theres a lot in building a hull, more than you would think, so thats a big chunk sorted out. you may at some point want to think about cutting it down to save weight, as i did with mine and plenty of other people have.

wings are quite easy, have a look round the blogs you'll find the jigs people have set up and the way you can join tubes. for tubes you can either buy new ones, or get an old set of wings and make do. carbon tubes last much longer than a moth design generally, so you might find something around the traps. my wings are made predominantly from a pair of old sailboard masts, youll find thats probably the cheapest way to do it.

for foils, my suggeestion (of course) would be to download the rhino files available in some of my earlier posts and then beg borrow or steal some CNC time and get some female tools cut out. its not really possibly to make an adequate foil mould by hand. maybe it is but you will have problems. ill post again pretty soon with some new info because ive finished the file complete with almost cnc ready 3d files for a centreboard and rudder, am just trialling the foils at the moment.

also, if you dont have it already, get rhino and learn how to use it. to start with, model your existing hull with a reasonable degree of accuracy so that you can model the new wing bars and make all the decisions in 3d, so you don't risk stuffing up expensive materials.