# Thread: wood core rudder question

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## Re: wood core rudder question

Originally Posted by Alan H
I figured out the Excel thing...take the Jig columns, make them the X and Y axes, and then do a scatter plot. Now I'm stuck on how to make the two axes the same scale. Grr.
Hi Alan,
Look for my PM with some instructions. You shouldn't need to do a scatter plot, the spreadsheet draws it out nicely for you already, it's on the second tab.

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## Re: wood core rudder question

Good idea

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## Re: wood core rudder question

Ah the tab at the bottom....

Lookie what I did. Sweeeet. Thanks, galleywench.

rudder_foil_template.jpg

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## Re: wood core rudder question

Excellent, that spreadsheet really saves a lot of time although for fun I spot checked the calculations to see how the math was done.

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## Re: wood core rudder question

There are foil generators onloine, several of them. If I was doing this manually, I would have used one of them to print out my foil and then done random expansions/contractions on the copier until I got it exact.

Then I could have taken a small T-square and marked off maybe 20 points at the router-bit-depth, perpendicular to the foil surface. Then, I'd have linked them all with one of those flexible artists curve-drawing things. It wouldn't have been perfect, but it would have been reasonably close. This is probably more accurate and a heck of a lot less time consuming!

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## Re: wood core rudder question

OK this weekend saw me do the rudder blank glue-up.

rudder-glue1.jpg

rudder-glue2.jpg

Rudder is 7 feet, 10 inches long, overall. Essentially, 50 inches of that will be submerged. The bit on the left is the trailing edge, which is thinner pieces than most of the rudder. I epoxied that on, tonight (sunday) along with two strips that will be the leading edge. This rudder will be about 12% balanced. The chord is 15.5 inches. That's close enough to 16. I figure that when I glass it, I will probably get another 1/4 inch of chord out of it, so it'll be awfully close to "the plan".

My shortest piece was 7 feet long. I cut everything to 7 feet, which was blitheringly stupid, as I wanted a 7' 6" + rudder. Me=dummy. So I wound up offsetting some joints. The ones you see in the rudder closest to the camera don't worry me even though they line up a bit too closely, as that's the absolute bottom of the blade and the loads are minimal down there.

I wouldn't make a mast like that! Also, there are NO joints anywhere near the point of maximum load, which is right near where the bottom gudgeon will be.
Last edited by Alan H; 11-05-2017 at 11:06 PM.

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## Re: wood core rudder question

Final glue up..leading edge and trailing edges are on. I ran out of wide clamps so I'm resorting to taping...yes, taping on a couple of small pieces that are really only there for shaping and fairing. There are a few bit of wood in there to support things while they dry. They're covered with wax paper so I don't glue them to the rudder.

rudder-glue 3.jpg

I cut off a few bits that I epoxied together...these are squares roughly 2 x 2... and I can't tear them apart. So that seems pretty strong. The leading edge is to the upper right in this picture, trailing edge is lower left. There's a little strip in there between the main body of the rudder and the piece that will form most of the leading edge. I needed that to make the 12% balance. Without it, it would have been 10% and I wanted just a scooch more than that, but not 15%. It might be a little bit thin, but there's tons of gluing surface in there so it's going to be plenty strong and if I have to fair it with epoxy and some fairing compound, no big deal.
Last edited by Alan H; 11-05-2017 at 11:02 PM.

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## Re: wood core rudder question

OK, it's all dry and solid. Now to cut out the router templates for the foil. I took the paper printout from the Excel spreadsheet and laid it out on a piece of 3/8th plywood. I used a thumbtack to prick indentations in the wood through the paper, and then "connected the dots" left in the wood with a pen. Hmmm. I might need to acquire a scrollsaw for this. I don't think a handheld jigsaw is going to be accurate enough. Off to Craigslist.

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## Re: wood core rudder question

I mucked up the first router foil template, moron mistake, so re-did it. Second one came out fine, but that meant a visit to Orchard Supply to get a cheap 2 x 2 piece of plywood for the other template. Since they had no 3/8th ply, I bought a sheet of 1/4 inch, cut two 5-inch widths to size and I'm glu'ing them together with Weldwood to make a 5" by 24" x 1/2" piece.. OK, so one side will be 3/8 and the other side will be 1/2. It's just the router template. While I was at OSH I bought a couple of new jigsaw blades, "many teeth" and narrow profile for cutting curves in wood. That worked like a charm and I now have one remarkably accurate foil template. Handholding the jigsaw and going slow was just fine, I didn't need a scrollsaw.

Progress!

I'm contemplating whether I need to run my whole rudder blank through a planer to take down the high spots. Hmmm. Who's got a 16 inch wide planer, I wonder.
Last edited by Alan H; 11-10-2017 at 01:16 PM.

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## Re: wood core rudder question

It occurred to me that this would be a great way for someone to knock out a simple emergency rudder for a smaller boat. You could "go simple".... Three dry and straight 5.5 - 6 foot 2 x 4's plus a 2 x 2 would give you a 12 inch chord with a rudder thickness of 1.75 inches. That's very roughly J-24 size, and significantly thicker. Glue all the pieces edge-to-edge with epoxy. Make a foil template like what I'm doing if you're hardcore, or even rough one out with the table saw, a surform plane and a bunch of sanding. Just shape the leading edge to be nice and round, and taper the back half of the blade to about a 1/8 inch wide at the back. It won't be perfect, but it's easy to do, just time consuming. So, ihn summary, shape the bottom half into some sort of foil, leave the top half un-shaped...just rectangular. Front and back should be streight up and down...parallel.

Cut the leading edge, in the unshaped top half of the rudder back about 20%. That's right, 20%, which will be about 2.5 inches. That's a lot. Why so much? Read on. Put a layer of 8oz glass in epoxy over the whole thing. Sand, and sand some more. Now you probably have a rudder blade with a 12.25 inch chord and almost 2 inches thick. That's pretty bluidy strong. Epoxy/bolt something to the top so the rudder doesn't slip all the way through the cassette. Paint, and admire.

Now build a cassette that will accept a rudder that's the thickness of the upper half of your blade, but with an fore-and -aft dimension big enough that you can slide the bottom half down inside. When it's all the way in, it will wobble fore and aft because the top half of the rudder is 2.5 inches smaller in chord than the bottom half. Now, make a 2.5 inch wedge/spacer out of some 2 x 4. Tap that into the cassette behind the rudder so that it pushes the rudder forward in the cassette and jams it into place. No more wobble.

Upshot...semi-balanced emergency rudder in cassette, U-Bild-It-At-Home-4-Cheep

The major problem with my e-rudder from 2008 was that while it steered the Santa Cruz 27 just fine, the leading edge was a good 3-4 inches behind the rotational axis because of how I made the cassette. It was horribly UN-balanced and loaded up immediately. This system I outline above would make a somewhat balanced rudder, depend on exactly how you fit the cassette to the pintles on the back of the boat.
Last edited by Alan H; 11-10-2017 at 01:43 PM.

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## Re: wood core rudder question

Today I made the router carrier/foil template for the project. I cut the foil shape in one piece of plywood the other day with the jigsaw, as explained a couple of posts, above. I then used the printed pattern from the Excel spreadsheet to mark the cutouts where the plywood foil template has to fit over the unsawn board. Then, fortunately before I cut that out, I went and compared it to the rudder. It was a good 3/4 of an inch too big. Not only that, but the offset for the router bit depth was 4-5 mm too big. What the heck?

Come to find out that apparently the Excel program works great, but just because it generates an accurate foil and router-offset pattern does NOT mean that your printer will print the exact dimensions. BE WARNED....print your pattern and compare it before cutting! I had to go back and enter new numbers in the Excel spreadsheet to generate an exactly-sized paper pattern to work from. This is trial and error, but I guessed well, and the second time I taped the printed pieces of paper together, I hit the right sizes.

Once I did that, I re-marked the plywood I'd already cut, which was fine as the new template is smaller than the earlier, wrong one. So I cut out two plywood foil templates, and two blocks, exactly half the thickness of the rudder blank for the foil templates to ride on. Add Weldwood glue and clamps and....

Here's the router carrier/foil template...the dark wood was cut from a lap tray I made for Joan when she got her hips replaced a couple of years ago. I didn't need the plywood or the tray, so now it's been re-used. The other stuff is junky plywood and scrap, cut-up 2 x 4's. It bugs me that the two templates are different thicknesses, but it totally doesn't matter.

template carrier-table.jpg

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## Re: wood core rudder question

So this rudder will be comparable to a J-29 or J-30 rudder except that this one will be balanced and the One Design rudders for those boats are not balanced. The entirety of the One Design rudder blades for those boats are behind the axis of rotation of the rudders; the entire rudder is aft of the gudgeon/pintle pin axis. Balanced rudders are not class-legal...though maybe the classes should make them legal as those guys all talk about how their rudders load up something fierce on a long, heavy-air reach.

Anyway, Competition Composites..aka "Phils Foils" makes a balanced rudder for the J-29/J-30. It's a beauty.

http://www.fastcomposites.ca/site/wp...-gallery-2.jpg

...It's also \$2400 (Canadian dollars) cha-chiiinnnnggggg....though honestly probably worth every loonie you pay for it.

Rudder Craft also makes a balanced J-30/J-29 wood-core rudder. It's \$2100 USA. Waterline Systems makes a class rudder theoretically for \$1400, but it's not balanced and they didn't return either of my e-mails. Larry Tuttle at Waterrat quoted me \$2K for a much "hotter" elliptical rudder, and I thought about that but two thousand dollars is a lot of moolah for this guy.

Rudder Craft makes rudders which are somewhat smaller than these which would probably work for my boat, but the largest one they make and market for 24-26 footers is made of machined HDPE plastic. That stuff bends under load. If I was just cruising around I might not care, but.... They also make an \$895 wood core rudder, but it was just a scooch smaller and thinner- aka "less robust" than I wanted. It probably would work, but it would suck to find out that it didn't on the LongPac Race, 185 miles out from the California coast. On the other hand that \$895 plus shipping includes custom rudder gudgeons for 5/8th inch pins, and those are about \$350, right there. But still, their wood core rudder was just a bit too small and thin for what I wanted. I wanted ridiculous BEEF.

So here I am, making a rudder.

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## Re: wood core rudder question

Originally Posted by Alan H
... Then I could have taken a small T-square and marked off maybe 20 points at the router-bit-depth, perpendicular to the foil surface. Then, I'd have linked them all with one of those flexible artists curve-drawing things. It wouldn't have been perfect, but it would have been reasonably close. This is probably more accurate and a heck of a lot less time consuming!
Here's a clever trick for getting a second curve that's exactly the right distance away from the foil surface plot.

The geometric basis for this is that, when rolling a circle along any shaped master pattern, the center of the circle can only follow a path that is exactly the radius distance away from the pattern, and always perfectly normal/perpendicular the the master pattern. Actually cannot physically make an error using this method.

Lets say you wanted a pattern to use with a router depth of prox 1.5 inches ((see PS note below for a trick))
(1) acquire a round disk of 3.0 inches across (twice the depth you want. examples: metal lids on jars of stawberry rhubarb jam is what I use
(2) fix a pencil/pen exactly in the center - drilling a hole dead center, of a suitable size for you pencil/pen
(3) lay your foil master template on the stock that will become the guide for the router base
(4) using your magic circle pen/holder, trace the outline.
(5) cut on the line
(*) probably need two of these that you can keep perfectly aligned, so pay attention to how the routing jig will be positioned with respect to the rudder blank.
Shazam!

P.S. I found that the exact size of the circular pen holder is not critical, you just want it close to what your router can do. Once you have the pen holder built, just make a test trace using a straight line pattern and then simply adjust the router plunge to match that exactly. Bingo.

Cheers,
Last edited by tom151; 11-12-2017 at 01:36 PM.

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## Re: wood core rudder question

Brilliant....and of course. Why didn't I think of that!

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## Re: wood core rudder question

Today I made the *Flat* surface/table that the rudder and router guide rest on. That was pretty straightforward, glued-together particle board and 2 x 4's, with outdoor deck screws. it rests on the sawhorses that I use for working on cabers.

I then "microadjusted" the router guide/template with the table saw so it fit to close tolerances over the rudder. It's great at the top and to about 2/3rds of the way down the submerged part. I'll have to shim it below that. This photo is taken just where it's starting to get a bit loose. I also had to plane down some of the epoxy dribbles, since this was the underside, while I glued it. That wasn't bad, though.

Then I started to set up the router. I'd bought a nice 3/8 round router blade at Home Deport, so after futzing around, trying to figure out how to change bits, I finally got it in the chuck, only to discover that it's not long enough. Come to find out that there are "router bits" and there are "extra long router bits"..... GRRRRR. That's \$20 shot, as I destroyed the packaging getting it open. Oh well, now I have a router, and I expect that I'll need some bits someday. So no foil shaping today. Instead, I trimmed off the top and cut the angle in the back edge, above the waterline.

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## Re: wood core rudder question

My "long" router bits arrived. Now, if it just stops raining....

So, question... should I sheathe this rudder in a layer of 10 oz fiberglass cloth in epoxy, or just paint with epoxy and call it good? I was going to add the 'glass, was even thinking about a layer of light mat and then the heavier glass over that.

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## Re: wood core rudder question

I was going to post a video of myself, happily routering-away, removing wood at a great pace and forming a perfect foil this afternoon. Haha! From things I've read in this thread I was expecting an hour or so per side and done....

NOT.

After 4 hours of work I have about 3/5ths of one side done.You know how it is, about 1/3rd of the way through the job you figure out the best way to do the job.

Gaaaa.... I discovered that router bits drift in the chuck after a while. You have to check them every 10-15 minutes. Imagine how I figured that out. Except for the godawful area where the bit moved, which will have to be faired, bigtime, most of the body of the thing is tolerable. "Precise, I'm not so sure, but tolerable. The trailing edge is OK. The leading edge is disgraceful. Now I know why the guy in the YouTube video used an elliptical edging bit for his leading edge. I'll be able to fill with epoxy and wood dough and it will be fine, but yeeeecccchhhh.

I refuse to show you all photographs of this mess.

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## Re: wood core rudder question

Sorry to learn of your tribulations.The wandering router cutter problem might have a very simple solution.The usual root cause is that the cutter is inserted too far prior to the collet nut being tightened and when the nut is rotated the collet moves a little and the end of the cutter bottoms out before the full clamping force is applied.Then with use things free up a touch and the cutter moves in the only direction in which it has any freedom.The cure is to retract the cutter a little-say 1/8th of an inch before the final nut tightening goes ahead.From the description you are likely to be painting the rudder,rather than varnishing and I would do the same even with a successful shaping concluded.Mainly because a light coloured rudder allows you to see if you have picked up a few strands of seaweed.

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## Re: wood core rudder question

Originally Posted by Alan H
Gaaaa.... I discovered that router bits drift in the chuck after a while. You have to check them every 10-15 minutes. Imagine how I figured that out. Except for the godawful area where the bit moved, which will have to be faired, bigtime, most of the body of the thing is tolerable. "Precise, I'm not so sure, but tolerable. The trailing edge is OK. The leading edge is disgraceful. Now I know why the guy in the YouTube video used an elliptical edging bit for his leading edge. I'll be able to fill with epoxy and wood dough and it will be fine, but yeeeecccchhhh.
Consider doing some 'rough' fairing now then using the router/jig setup to bring the high spots of fairing compound down to the perfectly correct profile. Better to have the perfect foil shaoe BEFORE doing any poxy/glassing

Make sure to apply epoxy neat (no additives at all) and allowing enough time for it to soak in -- really don't want any starvation when you add the filler material.

Everything will be fine.

Cheers,

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## Re: wood core rudder question

Thre's an Aussie lad over on Sailing Anarchy who is recommending that I coat with gel-coat rather than paint. I assume I'd be rolling it on. I just might do this.

As for fairing, I absolutely will fair BEFORE the glass goes on. No question about that!

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## Re: wood core rudder question

Little late to the party here.

I learned from the boatbuilder Larry Ligget – who has videos on making the non-constant chord Thistle foils – to use a hand-plane for almost all the shaping. It is laborious but reasonably fast, and less dusty than some other methods. It allows getting the near razor sharp trailing edge that would otherwise be challenging. It is fairly straightforward to hand-plane a NACA 30 or 40 section with just a ruler and pencil line. Western red cedar can be had with beautifully straight grain and it holds epoxy well, for a clear-coat rudder, it is an excellent choice. It is also very light. One great trick for thicker boards is to make the glue-up in halves so that it is easy to find the trailing edge, at least on the ends. It should be absolutely finish faired before a composite layer is added.

A hand layup with leading edge up allows wrapping the leading edge with your composite of choice and you can get good adhesion and a perfect leading edge wrap. Make the section smaller than final so that you can use the glass wrap on the trailing edge to "connect" (about a guarter inch) and provide a waterproof seam. When possible (most of the time) I vacuum bag and confess to have never gotten a perfect leading edge wrap no matter how hard I try. It always "folds" in the bag. But, if the foil is pre-coated with epoxy, and you're using peel/bleeder you can get a virtually perfect adhesion and glass/resin mix with a great secondary coat surface. Skip the pre-coat and you'll get tiny air pockets. I then wrap the leading edge with an abrasion guard (1000 weight carbon tape or 4/6oz s-glass or kevlar) which provides abrasion resistance and waterproofing with minimal shape disruption.

Looks like you are getting the project done ... that's the important thing.

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## Re: wood core rudder question

On the home front, I smoothed over the worst of the part that I gouged up pretty badly with the router, when I first started. The leading edge is now acceptable for this stage. I built up the worst gouges with epoxy and sawdust...not even wood dough, sawdust. They're back to acceptable now, but for one dollar-sized high spot and a ragged inch on the trailing edge. I'm going to need another box of wood dough for fairing, later.

Tomorrow, back to the router. I should finish off routing this side, tomorrow. I was going to go sailing on Saturday, but my boat-host has strep throat so I'll either go back at it with the router or take the skerry out, as I haven't sailed the skerry in months. It's supposed to rain, Saturday night, so everything has to come back inside.

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## Re: wood core rudder question

A friend who is a professional composites guy has offered me the use of his vacuum pump and some fittings. I scored some pretty heavy duty plastic tubing..think double-weight professional lawn disposal bags...that sort of weight... from some mold-remediation guys that are doing a job at work. With bag and vacuum pump, I am seriously thinking about vacuum-bagging the biaxial cloth onto the rudder. I've never done vacuum-bagging before but I've watched some video's and how hard can it be, right? Right?

...............Right.

Also, while I was at TAP plastics getting more epoxy, I saw about 5 1/2 feet of leftover unidirectional carbon fiber cloth in the "2/3rds off" bin. For \$7 I can add some 4-inch wide carbon reinforcement strips to this thing. It's going to be bombproof.

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## Re: wood core rudder question

Originally Posted by squeeze1333
Little late to the party here.

It should be absolutely finish faired before a composite layer is added.
I had figured on getting about 85-90% of the way towards a perfectly faired finish before 'glassing.

Make the section smaller than final so that you can use the glass wrap on the trailing edge to "connect" (about a guarter inch) and provide a waterproof seam.
One of the guys on Sailing Anarchi is suggesting that I trim the trailing edge back about 1/4 inch, cut a groove in the trailing edge and then glue in a fiberglass battern. Then glass/clamp to the batten. Trim off the flash and sand to whatever edge I want. That way he says I'll get a completely watertight seam. Seems reasonable.

When possible (most of the time) I vacuum bag and confess to have never gotten a perfect leading edge wrap no matter how hard I try. It always "folds" in the bag. But, if the foil is pre-coated with epoxy, and you're using peel/bleeder you can get a virtually perfect adhesion and glass/resin mix with a great secondary coat surface. Skip the pre-coat and you'll get tiny air pockets. I then wrap the leading edge with an abrasion guard (1000 weight carbon tape or 4/6oz s-glass or kevlar) which provides abrasion resistance and waterproofing with minimal shape disruption.
I've never vacuum-bagged before, but am thinking about doing it. I would absolutely use peel ply, release layer, and bleeder. After all, it's only money Thanks for the tip about the pre-coat. Will do.

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## Re: wood core rudder question

Originally Posted by Alan H
I had figured on getting about 85-90% of the way towards a perfectly faired finish before 'glassing.
If you need to fair after glassing, you will risk cutting the warp. Best approach (especially if vacuum bagging) is to fair completely before glassing. If you use a peel ply providing good secondary bond you will not need to fair again (significantly) until filler/paint layers ... so the risk of cutting fabric is truly minimized.

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## Re: wood core rudder question

Today I finished off routing one side of the blade, flipped it over and did the leading edge of the other side. It's not turning out as thick as I'd wanted, so I'm scabbing on some"cheater" 3/16ths inch fir strips, which I will rout and belt sand. I also got the first "fairing" coat of epoxy and sawdust on the leading edge side that I did today.

Tomorrow I'll use the surform plane to even out the leading edge on that side, and then fair some more with epoxy and wood dough. If I can, I'll route out the trailing edge. If that's the case, then the basic forming of the overall shape will be done.

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## Re: wood core rudder question

Originally Posted by squeeze1333
If you need to fair after glassing, you will risk cutting the warp. Best approach (especially if vacuum bagging) is to fair completely before glassing. If you use a peel ply providing good secondary bond you will not need to fair again (significantly) until filler/paint layers ... so the risk of cutting fabric is truly minimized.
True. OK, this is good to know. Thanks!

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## Re: wood core rudder question

http://www.fiberglasssupply.com/Prod...d_fabrics.html

I am pondering how to 'glass this thing. It's been suggested that I use "600 gram biaxial". Hmmm. Over here the glass is sold in ounces per sq. yard rather than grams/sq. meter, but I can probably figure it out. The question is, should I use 12 or 17 ounce biaxial, or 19 ounce triaxial, aligning the 0-deg layer straight up and down the board?

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## Re: wood core rudder question

Foils break, from sailing loads, at the place where they exit the case - at least for me that is
So that means
(1) the area where the board exits the case is the only place that is worth any structural glassing - taper to zero structural glass within a foot or two of the bottom (and top)
(2) lots of failures are due to the shape of the mating surface at the exit from the case. It may be more important (after getting a perfectly failr strong board) to shape that contact patch between the board and the case. The wider the mating contact patch the better - the less the board is at risk from point loading at the highest loads.
Not much else matters all that much.

PS - if not vac bagging the glass... either weigh or calculate the weight of glass to be used -- then mix only enough resin ( same weight in total) to give you a 50/50 proportion of epoxy to glass.
Unless you force yourself to stretch out the epoxy, you WILL use too much, use squeegie or hard/solid roller to work the epoxy

Cheers,
Last edited by tom151; 11-25-2017 at 08:18 PM.

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## Re: wood core rudder question

Tom, in this case, there is no cassette. There is, however, a lower rudder gudgeon! I'll soon be building an emergency rudder for peace of mind as I use this thing and there will be structural glass and carbon helping out where that blade exits it cassette.

=====

OK, here's my routering setup. I finished the last trailing edge today and got one side leading edge semi-faired. Like my "workshop"? ...aka, the driveway.

IMG_2059.jpg

and some close-ups to show you the degree of smoothness.... bleecccch.... and accuracy... middlin'.. that I got.
IMG_2060.jpg

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## Re: wood core rudder question

close-ups

IMG_2061.jpg

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## Re: wood core rudder question

As you can see, the side I just finished is quite a bit more accurate than the flip side. You know how it is, by the time you finish the job, you've figured out how to do it, right.

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## Re: wood core rudder question

Originally Posted by The Bigfella
I don't know that particular glue - is it clear, or does it already have a thickener added? I use West System or Botecote - and for that application, it has to have fibre added to it.

Here's the glueline test from a centreboard we made - the glueline is stronger than the wood

I see you mentioned making profiles. We used a mate's profiles from a World Championship winning boat that he'd built - back in the 60's

Thread on the centreboard is here http://forum.woodenboat.com/showthre...ht=centreboard
hey, I finally read through that thread. I see that you vacuum-bagged your rudder flat on your workbench. I was thinking of suspending mine an some bits of string between two a-frames of scrap wood with some throwaway lagbolts in the ends of the wood as suspension points. I'd have to be careful to seal tightly around the bolts. Anyway, do you feel that you get a good symmetrical board, when vacuum-bagging on the bench, or does the pressure of the centreboard on the flat bench surface induce some irregularities?

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## Re: wood core rudder question

I simply envelope bag foils and have never had a real problem.I have used both commercial bagging materials and improvised with a bag made from damp proof membrane and an old towel as breather cloth.I wouldn't want to risk a problem with a hard won rudder as a first project though.Is there anything else you could use as a trial run?

35. Designated Loose Cannon
Join Date
Jan 2009
Location
Narragansett Bay and Approaches
Posts
151

## Re: wood core rudder question

A stunt for making the leading and trailing edges robust is to cut a 1/8 wide by 1' deep on the center of edges of the blank and bond in a strip of G10 or equivalent. You could also cut the two staves in half before bonding them and reassemble them with a few plies of glass and epoxy. Aluminum or brass will work for the traditionalist, neither will kill the router when it nicks the metal. Point is that you will have a killer trailing edge you can drive nails with.
1/8" is a good enough trailing edge for bigger boats, but for anything under 20' you would like to be half that. 0.05% of the chord length is the number I remember, I always try to be as sharp as I can get.
SHC

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