Whale fin Rudder Modification

[finding41]

I think you are right Walt. It is a bit blunt looking. I was trying to eyeball the old rudders leading edge which isn't much different. I was working from simple measurements coming back from the leading edge while trying to get the waive shape. (Note the marks on the foam in black marker.) I was aiming to get the thickest part of the chord between 30ish % back from the leading edge.
I'm going to make this mold anyway the way it is. I have to much time into it to re-do it at a different less bluntness. I plan in the future to cut the 0012 shape first next time and work from the front of it. (This all depends on the length of the chord decided on...) I'll measure the rudder under way tomorrow and let you know what it is coming in at for length.
The tiller finally dried today. I wasn't too happy with the feel if it so I coated it in new spar varnish this evening. If this product dries I'll put on a few more coats before I call it done.

 

[Chris & Lenore]

So her is an idea to add a "comparison" to the project - something I tried a few years ago that made a noticeable difference.

Turbulators were mentioned earlier and these are used by most gliders and jets in order to control the laminar/turbulent flow point (separation bubble).

These are typically built into the wing shape itself but can be added by a strip of tape. On an aircraft wing it doesn't take much - a spanwise strip of 1/32 thick by 1/4 inch wide tape at the right place between the leading edge and mid-chord has a profound impact. Drag is a little lower but the wing tolerates radical changes in angle of attack much better plus stall characteristics are predictable.

I tried this on my original Mac rudder with a strip of thick pin striping tape (from CTC) on each side of the rudder and played around with placement. At about 3 inches back from the leading edge there was a noticeable difference with rapid and extreme rudder deflections at very low speed. The rudder did not stall as easily and stall speed was about a knot lower.

I only saw/felt a difference at very slow speed and very mild wind. Anything else and it felt the same.

Unfortunately, with the Ida rudder - nothing sticks to it...

All fun stuff to play with. I was thinking of turbulating the centerboard for something to do in my spare time

Chris

 

[finding41]

Chris I have been trolling the glider forums and seen what you are referencing. They have special ones for the wing tips too. I have even seen it in some of the car forums too. They put it on the windshield posts and it keeps the water from riding up the side windows. Also just before the windshield on the hood to keep the wipers from lifting. In both cases they were using that hard tape you can punch words out on.
Here is a neat link. http://www.aerospaceweb.org/question/aerodynamics/q0228.shtml
I love the "ask a rocket scientist" idea!

I was reading about sail shape in reference to racing 1M class RC boats. They can stick those into a wind tunnel and get things tweaked pretty quickly. Anyway way I saw that the most efficient the head sail (jib, genoa) could be is with a bubble of air ending about 6" back from the luff. ( I have short 3" tell tails 3" back from my luff. 3 on each side of the sail.) When the windward one spinning and the leeward one is strait back that's the zone.
I never thought of adding anything to get the bubble made for me. Either on the rudder or DB. This could even work for the mast. (Us guys without the rotating masts will gain an advantage.) Why not even on the furler extrusion...
I'll try some 1/4" or thinner on my old modified rudder this weekend.
Thanks for the tips!
Picture is the tiller handle with 1 coat of this stuff. I gave up on the old can of Spar varnish. It finally did dry. 3 days later.
I put the 2nd coat on tonight.

 

[Chris & Lenore]

I have used the Helmsman urethane as well as several (expensive) marine products. The Helmsman held up just as well as any of the other stuff.

One tip about the turbulator strips - the difference is slight to non-existent in terms of drag reduction. The main difference is in slow speed where you need a lot of rudder deflection or a sudden deflection. If you adjust the position based on improving low speed this will make a difference when you are racing in low wind. I really ticked off another 26S sailor when we were racing (as in two boats going in the same direction) in about 3 kts of wind. Needed a lot of rudder deflection due to the low speed and his rudder kept stalling and mine never did. Beat him by a few feet but he felt he was the "better sailor" and came up with all kinds of reasons why he "lost".

Another thing to consider/play with is turbulating the centre board. Coming about is a radical change in angle of attack with a "low speed" segment - keeping the CB from stalling (plus if it does, regaining laminar flow sooner) will also improve your racing.

The only issue I see with our boats is when the CB is not fully down. Angled back, the turbulator probably needs to be in a different position to be most effective.
I tend to really use the swing feature of Teliki. Not just to lower drag but to move the clr and balance the boat.

I'm also anxious to see how your whale flipper rudder words. Nature tends to way ahead of us in terms of design.

Some days there is just too much "stuff" to play with...

Chris

 

[walt]

I was very much into windsurfing between about 1985 and 2005 and windsurfers had a different problem of a high speed stall. You can imagine what happens on a windsurfer when pretty much almost the only thing holding you in the water stalls going fast.. About the time I stopped windsurfing, the fin guys had pretty good high speed stall resistant foils. I took some pictures of the last one I had bought. No turbulators or that sort of thing, just a CNC milled foil shape. Interesting how sharp the leading edge is (attached pictures).

http://wright.nasa.gov/airplane/lifteq.html This discussion has all been about low speed stall and I think this is explained by the lift equation. Lift is determined by some constant times a coefficient of lift times the speed squared. And the coefficient of lift is directly proportional to the angle of attack.

The "square law" of the speed makes this interesting. It is reasonable to say that we need the same lift force from the rudder to turn at 2 knots or 1 knot. However, because the speed is squared in the equation, going from 2 kn to 1 kn is 1/2 the speed but to get the same lift, we need the angle of attack to increase by four times. I.e., if 3 degrees angle attack worked at 2 kn, you would need 12 degrees angle of attack at 1 kn - and these foils stall at not much higher than this.

Must be a racing thing mostly where your trying to get good position at the start or rounding’s in very light wind.

 

[walt]

I guess this high angle of attack needed at low speeds (and chance of stall) is also whey I like my outboard to be able to rotate.. When you’re going slow trying to get into a slip, an easily stalled rudder (because it needs such high angle of attack) doesn’t do much. Maybe the "Find-a-matic" new rudder will make it easier to get into a slip at slow speeds without needing to rotate the outboard as much?

 

[Chris & Lenore]

I can't imagine trying to get Into my slip on rudder alone! Need 2 kts minimum to make the sharp turns required. Moving the outboard lets me halve the turning radius - which is "just enough" for our tight marina.

But under sail at 1 to 2 kts and being able to effectively tack (nothing stalled) and beat the other guy - that's priceless

Chris

 

[finding41]

I have seen most of the stall I encounter at the starting line then at the dock too. It's the tacking back and forth just before the line and trying to slow down that really gets me. The slowing down turns into parking and being in the way of the other guys. (I left 2 other boats stalled at the line last wed. I was stalled and didn't have my genoa out yet. Just before the horn I unfurled the genoa and got turned onto a Starboard tack and went across about 2/3 of the distance of the cans before I crossed. That messed them up... They had there head sails out and were stalled then I took all the room.)

Chris, I don't know how the bumps would work on a 26S with the center board swung back and able to be set at different angles. Maybe the bumps would have to be shaped to the most common SB angle.

Didn't get time to work on the rudder today. I did pop in and sand and put another coat of Spar Varnish on the tiller handle.
"Some days there is just too much "stuff" to play with... Chris" Wow are you ever right! I have so much on the go at once right now. Not just 2 boats to finish working on but 2 new to me Saab 9-5 Wagons. I love these cars! Both just needed a bit of loving. (Time) Now it's get rid of the truck... The old work van left last weekend... And then there is Squeaky.... Push me daddy...
Tomorrow I sail and try tape on my original modified rudder. Hopefully get a coat or more of PVA on the rudder.

 

[finding41]

"A coat of PVA" is what I said.... Well 20 coats later! I gave up for the night. I did get allot of stuff cleaned up in the shop while I waited for PVA to dry.
Tiller handle is done! No picture because it was raining.
Here is a few pictures of the rudder.The last PVA shot has about 20 thin sprayed coats. Very thin coats. That stuff is like water to spray.
The last picture, I started cutting my glass before hand. I figure 3 layers should do it. It is a heavy weave. I've no idea how the weight is calculated with fiberglass.

 

[Bob McL]

That is some thick weave. I would suggest a fine weave if you can take the time to find it. Heavier weave is not as strong as multiple layers of a tighter, lighter weave. This is a neat project.

 

[finding41]

I have some thinner weave stuff but was planning to use it on the part not the mold. I plan to use 2 layers of thin weave and a layer of thicker kevlar and glass. Ill get a picture of the kevlar stuff tonight.
I'm hoping to use a vacuum type process to get allot of the resin out for more strength.

 

[finding41]

Here is a picture of the Kevlar and glass weave stuff I have. It is kind of neat as it has the glass on one side and the Kevlar on the other. So I imagine it is triple thick. It is hard stuff to cut with scissors!
I put 3 more cats of PVA on tonight. I'm calling it good enough. I'll spray on the first few coats of black gelcoat and wax. Then give it a brush coat and off to laying up glass. Hopefully tomorrow....

 

[finding41]

Got some time this afternoon to fiberglass the rudder plug. I used Black gelcoat because I have a bunch of it and planed to make the molds with it. I mixed liquid wax into the black when I mixed the hardener in. I decided to brush it on instead of cleaning my spray gun for an hour after each sprayed coat.
The cloth didn't go as well as I had hoped. In tests before hand it fit very nicely on the rudder plug and didn't look like it would be a problem with fitting to the bumpy shape. In practice the first layer went on well. The second layer was a pain in the but! It didn't want to sit in the hollows like the first... I did use one big sheet for each layup. In hind-sight I probably should have let the first layer dry then attempt the second. I did let the gelcaot kick before I went at it with the cloth.
Here are some pictures of the process. I forgot to take one when I finished...
Last picture is a tool I made.... I just cut the foam off a wheene roller refill to use to squish the air out of the cloth.

 

[finding41]

So her is an idea to add a "comparison" to the project - something I tried a few years ago that made a noticeable difference.

Turbulators were mentioned earlier and these are used by most gliders and jets in order to control the laminar/turbulent flow point (separation bubble).

These are typically built into the wing shape itself but can be added by a strip of tape. On an aircraft wing it doesn't take much - a spanwise strip of 1/32 thick by 1/4 inch wide tape at the right place between the leading edge and mid-chord has a profound impact. Drag is a little lower but the wing tolerates radical changes in angle of attack much better plus stall characteristics are predictable.
I tried this on my original Mac rudder with a strip of thick pin striping tape (from CTC) on each side of the rudder and played around with placement. At about 3 inches back from the leading edge there was a noticeable difference with rapid and extreme rudder deflections at very low speed. The rudder did not stall as easily and stall speed was about a knot lower.

I only saw/felt a difference at very slow speed and very mild wind. Anything else and it felt the same.

Unfortunately, with the Ida rudder - nothing sticks to it...

All fun stuff to play with. I was thinking of turbulating the centerboard for something to do in my spare time

Chris

Chris I tried this on Wed night for the race. (1st)
It removed the vibration from my rudder! I used 3/4" painters tape.... along the curve of the foil. It was very light sailing conditions. About 1-2k and that was pushing it.
I will try this again with a plastic tape that wont get soggy and fall off like painters tape.
Here is a picture. Not to perfect in my attempt to get it on strait. I was hanging off the back and I didn't want to fall in. Not enough time to get a club tender boat to apply it before the race.

 

[finding41]

I got it off. Kind of. The plaster and foam remained in the mold. So I picked out a bunch of foam and filled the mold with water and rags to moisten the plaster. I'll spray it clean with the hose tomorrow.
I couldn't find the plastic wedges I have to pop the mold off so I used a 9 in 1 tool to get it started and a plastic putty knife to get it the rest of the way.
I'm not too happy with my job on the edges. They didn't turn out right. (I think I went at the fiberglass mat part too early after the gelcoat was done. About 20 min...? Or I played around too much at the edges trying to get the glass to sit in there nice...)
The green in the pictures is plasticine and the red is the foam. I painted the foam red to be able to see the edge of it when I was sanding the plaster and removing it. I kind of expected it to stick in the mold.
The chunk I am holding is supposed to be a 90* edge! Fail! It's not too important.

 

[finding41]

I'm not sure how it's going to come out... I cleaned a bunch of the plaster out today. (I only had about 15 min to work on it.)
The edges are not what I wanted but that's ok. (Learned something there! Don't touch the gelcoat until the next day. Or until its very dry. I tested it on the side of the rudder... It was thicker at the bottom....)
The edge is just there to act as a lip. It will be cut off anyway. I'll just have to make it smaller or do some fancy gelcoating to fill and sand...
I can't tell yet what the rest looks like. I couldn't get all the plaster or PVA out yet. So it is soaking with rags and water again. Here are some pictures.

 

[finding41]

Well I tried to get the PVA and plaster out tonight. I got most of it out.
Unfortunately I don't think the mold is useable. I really made a mess of the gelcoat when I added the fiberglass. Too soon! There are spots that are rough and uneven. I may be able to sand them down and fill the low spots.
I'm not sure it's worth it at this point.
I can probably knock off another faster.
I have a busy week and plan to do some gelcoat work on Topiary.
I have a laz hatch to make (copy) and a bunch of gelcoat and hole filling to do on the topside.
Here are some pictures of the light coming through the mold ware I was a "little too aggressive" when I was adding the glass. (They are smooth but you shouldn't see through it. I did put the gelcoat on pretty thick.) A couple of shots of the PVA stuck on very well.

I might lube the mold up with Vaseline and make a plaster cast to work from again. That way I can make the new mold a bit thinner and more hydrodynamic. Or just less blunt.

So all is not lost.

 

[finding41]

I decided to give the mold a go at making a plaster male that I can re-shape and start again from.
So I got out the Vaseline and gave it a good lube up. Then mixed plaster (CCG 45 drywall mud. Very hard stuff to sand.) I mixed a thinner than usual batch and coated the sides and the bumps of the mold. This thin coat is thin so it doesn't have bubbles close to the surface.
I will add more plaster and add some burlap to it to help hold it together. Probably fill the mold in 3 pours so it doesn't crack. The plaster shrinks a bit when it drys.
I mentioned before I want to make the mold thinner or less blunt. I will add to the male plug after I get it out of the mold.

 

[finding41]

Mmm Vaseline... Messy stuff. I forgot to put gloves on too.
Well not for the plaster. Here are some pictures of how it is going.
I coated the sides and let it dry. (thin coat0 Next I mixed more plaster and cut my burlap. I coated the sides and inserted the burlap, then re-coated inside and folded the burlap in on it's self. Then re-coated again to get all the air out and the burlap evenly distributed along the sides with a gap in the center.
next step is fill the gap. ( I may put a piece of wood (1x) in so I can screw it down to work on instead of clamping it like the last one.) Probably add a few screw heads sticking out inside the plaster to get an extra grip.

 

[finding41]

This thread is on the second page. I'm working too slow!
Here is what I did today.
I used a 1x2 to act as a attachment point when the piece comes out of the mold.
I will screw blocks to it and be able to pull on the blocks to help get it out of the mold too.
This will also give me something to attach the plaster piece to the work bench so I don't need to clamp it and risk breaking it. I'll just screw up through the bottom of one of my workmate benches.
Family from out of Provence will be here this week so....I'll pop it out as soon as I get time.