Thought I post a picture of last weekends sailing. I started half an hour before a thunderstorm started so the conditions got really gusty.
Right away the boat sailed really well. The gusts were light, and gave me a first idea of the speed potential of the boat. The tacking prolems were still there, but reduced so that I can live with it.
When the boat passed the shore and a gust fell in, you could hear cracking noises of the load boxes that are holding the unstayed masts. That was just the structure working thou and nothing to worry about, nevertheless somehow a cool sound. (The boxes are dimensioned to hold much bigger loads)
When the wind got stronger, the lee bow started to bury a bit, but it was still controllable.
Then it happened. A strong gust fell in and the boat was rushing towards the quay. I had little time to react and the only thing I could do - given the angle I approached the quay - was to gybe. Gybing in a gust is maybe a bad idea, if the gust gets even stronger that moment, it is a worse idea.
The boat accelerated, the bows submerged and within a blink of an eye, it tripped over its nose and capsized.
When the gusts picked up even stronger, I decided to reduce sail area. Since I havent got a B or C rigg yet, which would probably have been just right, I decided to take of the sails completly and try to sail with the masts alone.
The ship was a bit underpowered that way (0,25m2), but nevertheless quite an interesting experience. It worked better then I expected. When I sheeted the masts in, the boat immediatly picked up speed and sailed really well. It had a certain tendency to point away from the wind since the CoE is further forward in this configuration, but the faster it went , the smaller this effect was.
It turned out thou, that with slim mastprofiles like mine, the usable range for the angle of attack is so narrow that it needs a lot of experience to keep the boat sailing.
Since the profiles are so slim, even the change in apparent wind direction through a gust was enough to have the airflow around the foil collapse -> the boat stood still -> sheet out -> accelerate.
I think with a little more experience and better reflexes it should be possible to sail the boat quite fast. A small piece of cloth as a wingflap might improve the ease of handling dramatically.
The experience was so motivating after all, that I am thinking of using a solid wing rigg. Winter is coming, time to build.
I followd Tom Speer on this. I took a Clark Y foil and drew a line (the width of my resulting mast) from the tip of the profile to the outer edge. I then took the section that is ‘cut of’ by this line and mirrored it to get a symmetrical profile. Since I didn’t find any other resource for this on the web, this seamed to be a reasonable way to go.
What resources are you using for your profiles?
Bill, I would also like to know how you build your profiles. I usually build them by cutting half sections out of foam and glue them to an mdf board. I then glass the two halfs under vacuum. After that I take the two halfs off the board, glue them together and add another layer of glass around the whole thing.
I figured that there is a lot of room for inaccuracies when joining the two halfs. The best I could get after several tries was a twist of 3-4 millimeters over 180 cm length (which is too much in my eye). Did you find a method to build the masts that is really precise?
Marcus,
I’ve handcarved my foam cores in the past using NACA profiles derived from:http://www.pagendarm.de/trapp/programming/java/profiles/NACA4.html
But as you say, it’s very difficult to get them anywhere close to “perfect”. More recently, I’ve been paying a guy in California
(http://www.cncfoamfactory.com ) to cut them on his CNC hot wire cutter, which is VERY accurate. Then I vacuum bag them in 6oz glass using a technique developed by the model plane guys:
(http://www.pilotsguide.com/rc/vacbag.shtml ). After it’s cured & trimmed, I cut in a sail luff groove using a hot knife. I’ve been using NACA 0022 profiles, but may try a bit thicker. In my opinion, the most important thing is to get a fair curve at the transition from mast to sail on the leeward side. My 68" (1.7m) masts have a straight taper from 4" (10cm) at the base to 2.25" (5.5cm) at the tip, so I don’t know if this would work with your curved taper
When you glass and Vac-Bag the Wing, how to you secure, that it doesn’t get bent or twisted in the process? Do you have a special technic or are you just working carefully. I figured that I always introduce a small amount of twist and bend when i joined the mast halfs.
When you cut the sail luff grove, you cut through the glass into the foam, right? Aren’t you loosing a lot of stability by opening the profile, or are you sealing it again afterward with resin/glass?
I’ve read about cncfoamfactory on this forum quite few times, and they seem to do a good job, but I guess, it can get quite costly if you are going for several versions to test the best out.
I’ve actually thought of getting myself a small cnc mill that they sell here in Austria to mill female molds, but I guess my wife would kill me and my friends would send me to an asylum for being so multihull addicted. [:-crazy]
We used a computer driven wire cutter to cut wings for various projects over the years when I was back in school. It was very cool. Do you know the name of the guy that has the NC hot wire that you contract with? He might have been one of the guys who built the cutter we used in school…
Anyway, I would think you might be able to do a pretty decent job by just pasting a wood template on each end of the foam and then running the hot wire along the templates. If you want a tapered wing, you should be able to secure the wire at the “focal point” (the point where the projected taper results in a zero size airfoil) and then just run the wire along a teplate at the other end of the block of foam.
As far as getting the shape right, the Naca 4 series thickness distribution is defined by an equation so it is fairly simple to make a template for a 4 series airfoil using Excel. Coordinate files for many other airfoil shapes are available from Mike Selig’s site: http://www.aae.uiuc.edu/m-selig/ads/coord_database.html These datafiles can be downloaded electronically and read into excel for plotting…
Marcus,
I just lay the bag on a very flat table & it comes out straight. I suspect part of your problem is when you glue up the 2 halves. Perhaps some sort of jig would be able to maintain alignment.However, I think that a twist of only 3-4 mm is very small over that span, so you must be doing something right.
The luff groove doesn’t seem to weaken the mast at all, as the strength is mainly in the skin, especially at the thicker areas of the profile. The foam core is acting as a shear web, but in the narrow area at the trailing edge luff groove it has little significance. The only problem I’ve had in this area is some delamination of the glass skin after severe use, but this is easily repaired with some epoxy & a syringe.
The cores cost me about $18 USD delivered, which I consider very reasonable considering the time it would have taken me to carve an inferior result. Of course, shipping would be more to Austria, but I would guess someone in Europe is doing a similar thing. As Will points out, it’s also possible to set up a hot wire cutter using profile templates at the ends, but that seemed like a bit of a distraction. There’s all sorts of info on the web as to how to do this if you’re interested, mainly from the model airplane guys (do a google search under hot wire cutter).
Will, the CNC guy’s name is Mark Mirada. My thought about running the wire out to the zero point is that it would be quite long & might have problems with uniform temperature control & deflection of the wire. BTW, do you have any suggestions as to profile shape considerations?
18$ is a good price. I get it for around 30$.
Since half of the fun for me is the design of the ship, I am somewhat stuck to bent, round shapes of the wing. That means at least twenty sections and a way more complicated handling…and thus higher costs for cutting the foam. [:-banghead] I probably should stick to a straight tapered wing…I know…[:D]
I think building in foam is a good way to go, but I also believe, that a foam wing carries a lot of unnecessary weight that doesn’t add to stability. The foam core I was using was around 90gr heavy.
Since the weight in the mast is twice as bad as weight down in the boat, I buildt my last two masts in a female mould (that I got from my foam wing).
I made the mistake to have the skin of the mast to be the load carrying structure. I should have buildt it up like a plane wing, with a load bar in the middle and carbon bands running along, the skin made of thin glass just mounted on a few crossections. - The glider guys show how to make it, its just sooo much work…[:-boring]
Allthough not even as professional as your boat, i once tried with hard profiles.
the performance wasn’t was i expecetd, but there is abig potential. the sails would have to be narrower to inuce less drag.
It only was i try and i gave up too soon. mybee i’ll get it out of the cellar again an try with some other wings.
check our at www.n.ethz.ch/student/grassm then sailing then CAT70
btw. the boat also nose dived as i tried it with two huge normal sails. so i gave that one up aswell.
you sure got a hell of a cool boat there. but you’ll probably need hydrofoils at the bow to prevent it from pitch poleing!
there might be a porblem with the server.
i have got my own webserver and sometimes other people just switch it off, or trip over the cable or whatever
The reynolds number on our wings is ridiculously low. So airfoil shapes are pretty critical. When I was working on the HPVs back at school, we used Xfoil to evaluate a bunch of shapes for strut fairings. We took the Eppler 387 profile and took out the camber and that seemed to do a really good job. We also developed a few airfoil shapes of our own.
For a wing mast, I would look at the cambered Eppler 387 profile and then do the reflection trick at the 30% chord (or whatever chord percentage you want to use for the mast. The leading edge of the Eppler foils are excellent for low reynolds numbers. But the real trick is getting the sail portion to match the suction surface profile. With low Re you need to worry about the pressure recovery region. If you try to recover pressure too quickly you will seperate. A nice gentile pressure recovery region is needed to keep the drag low…
Most of the other Eppler profiles are also designed for low Re so take a look at a few of them.
I have been fooling with foils in the NACA 0015 family. My biggest problem has been finding a good shear web. Glass over foam wasn’t strong enough - and adding diamonds seemed to defeat the purpose of a wingmast. Is it possible to use a carbon core of maybe 3/16 thick unidirectional - foam over the carbon for shape and capped off by one layer of 1/2 oz. glass?
These were the specs that I intended to try this winter after the snow arrives.
Refering to my mast size, I would do the following:
I if you build in foam I would recoment a solid foam core, 2 1" wide bands of unidir Carbon (i.e. 2 layers of 80gr/m2 unidir) running along the sides to take up the bend loads and a web of single Carbon rovings put around the mast diagonally.
The plane guys do that for adding torsional stability. You can take the strands from a standard carbon weave that you take apart. You then lay it around the mast diagonally like a fisher web with a spacing of 2-3 Inches between the strands. Little weight, lot of strength. Then you cover the whole with a very thin layer of glass. Works for me.
Marcus
PS: Sorry for the metric measurements, but I have a hard time converting that, since US standard sizes might be different.
Dick,
I’m surprised you can’t get enough strength with glass over foam. What type of foam & weight of glass are you using? I’m using 6oz glass over Dow blue foam. I had tried 3 oz glass in an attempt to test the limits of lateral mast bend, but it proved to be not strong enough under severe conditions, although the failures occured in compression, not on the tension side. Marcus’s suggestions will work well, as will a wrap of unidirectional carbon around the leading edge. Another thing that would help is to increase the thickness to NACA 0020 or larger, which will give a higher section modulus.
Regards,
Bill
ps to Will: Thanks for the suggestion about the Eppler foils, I’ll look into it. The hard thing about any of this is figuring out the mechanics of getting the hard & soft portions of the airfoil to work together in a way that closely approximates any idealized profile. There will always be spanwise variations with twist. Sometimes I wonder if Herreshof’s “TLAR” approach isn’t a good starting point. That stands for “That looks about right”
Same here… The problem is the compression side. The Unidir Carbon bands will help with that. In my view it all depends on the bonding between Glass and foam. As soon as it seperates under load, the glass on the compression side will wrinkle and fold.
You get a good connection only if you use Vacuum and a foam that bonds well.
I used 2 layers of 1.5 oz. glass - not carbon - over “Dow” foam with a 3/16 inch wide of uni-directional carbon tows running vertical on each side - from base to top of mast. I routed a groove into foam and filled with carbon tows and epoxy. When tried I had virtually no side bend, but when it failed - it broke from front to trailing edge and just below the hounds. Almost as if I over-tightened the backstay - but I wasn’t using a backstay - just fore stay and one side shroud on each side. Didn’t go much further on development (last winter) so was giving thoughts to winter projects for this year.
What dimensions did you use for your profile cross section?
A carbon band along the leading edge should be enough.
I wouldnt make a groove and ‘fill’ it with epoxy thou. A thin vacuum bagged layer is just fine. Epoxy doesn’t add to strenght. Too much weight in the mast will kill your ships performance. It will start to pitchpole badly when going upwind in small waves.
the groove was routed on each “side” of the mast and filled with epoxy <u>AND</u> unidirectional carbon tows. (not just epoxy) Epoxy was used in quantity only to “glue” carbon strands in place - and then also to fair off the exposed side of the inserts beforee glass covering was added.
Total width (chord) of the mast section was 5/8 inch (9,54 mm) and total length (front to rear) of mast was approximately 3 1/2 inches (88,89 mm) at maximum. Mast height was 72 inches (1828,8 mm) - so it was a pretty big section. It was set up for a uni-rig for an F-48/Mini40 catamaran platform.
Here’s an idea for covering solid wing trailing sections.
I stopped at our local big-boat marina and asked for a few feet of the boat storage shrink wrap that they use to cover boats for winter storage. They cover and then use a large torch to shrink-wrap the boats until spring.
I tried a small piece over part of the broken mast section. Required a heat gun to get it hot enough to shrink - but is thicker than monocoat - and thus less likely to get a puncture by mistake. It is also heavier - but will wait to see how much for the area needed. I am contemplating use on a sold wing that I have plans for - either on leading endge - or perhaps as double sided trailing edge.
If you try it - a hair dryer may not have enough heat to shrink!
I have to clarify:
I of course didn’t assume that you would just use Epoxy. I just forgot to write Epoxy <u>and</u> Carbon… My mistake.[:-banghead]
My mast was pretty much the same size, just wider. I wonder if a slim profile like yours would be able to keep a stable airflow over a wider range of angles of attack. Otherwise it will be hard to sail. (Is this the reynolds number Will is refering to?)
Regarding the shrinkwrap… I am really curious to hear how your solid wing turns out.
Any idea what kind of material do they use for the wings on cogito and the other c-class cats?
Wild Turkey - our 18 Square Meter solid wing used full size airplane shrink covering for the leading edge and trailing edge was regular old Dacron sail material.
