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Thread: Low-tech E-rudder build

  1. #1
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    Default Low-tech E-rudder build

    I gave George McKay the emergency rudder that I build as a trial run for the Wildcats new primary rudder last year. He's using it on his Capo 30 (modified to 33 feet) "Skye", so I need to make another rudder. The E-rudder I made then was a bit of overkill for an S2 7.9, though according to George, it steered Skye just fine in 10-13 knots of breeze and some swell. It'll go to Hawaii with Skye on the next Pac Cup. That confirms that the basic method works, so here we go again. Skye's rudder had a 15 inch chord and was almost 7 feet tall.

    I'm not a big fiberglass guy. I mean...I'm not high-tech. With advice from Greg Nelson, and after borrowing some pumps etc. from him, I vacuum-bagged the Wildcat's primary rudder, and it's tight. Skye's rudder was a dry-run for the primary rudder and I bagged that. It didn't go as well but what I learned from doing that, made the primary rudder better, so it's all good. However, this time, I'm going to do it low tech, no vacuum bagging.

    The core of the rudder is 2 x 3 redwood. Why? Because there's a huge pile of 12 and 16 foot pieces of this stuff lying in an empty lot down the street and it's free. I used it for Skye's rudder...it works. this will be smaller than Skye's rudder, so seems good to me. The pictures are deceiving, as I was trying to glue on a piece in the bottom of the rudder, the leading edge to make it balanced. I was using TiteBond II, which has zilch gap-filling ability and it just didn't work. So I knocked it off with two whacks from a hammer and I'm back to a 12 inch-chord, straight blade. That's the four "Main" edge-glued boards in the picture. That will be plenty big enough.

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    Here's the rudder core, clamped on my driveway, where is where I do all my low-tech work. The glue holding the four remaining pieces, glued up edge-wise, is PL Premium polyurethane construction adhesive. PL Premium is cheap. It gap-fills just great. It's strong as #$%^&*...lots of the DIY boat guy use it for quickie plywood-panel boats. I used to use it to repair cabers for the Highland Games. the wood failed before the PL Premium, ever did. I make up some test joints when I built Skyes e-rudder and tested them. The redwood failed before the joint did.

    This rudder is 6 feet tall and will have a 12-inch chord, maybe a scooch more, like 12.5, when I'm done with the fiberglassing.
    Attached Images Attached Images  
    S-2 7.9: "Wildcat of Loch Awe"
    1968 Selmer Series 9 B-flat and A clarinets
    Piper One Design 24, Hull #35; "Alpha"

  2. #2
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    Dimensions are...

    Overall length, 6 feet...will be 5' 10" when I trim off the ends...
    Chord .. 12 inches
    Thickness.... the boards are 2 inches thick... a REAL two inches, not the 1 3/4 of standard "2 x 4"'s. However, it'll be planed down to about 1 3/4 inches thick. Add in the thickness of the tri-axial fiberglass, and we're back to about 2 inches thick.

    Above waterline, 2 1/2 feet, below the waterline, 3 1/2 feet. Let's focus on the below-waterline area.
    The 12 inch chord , converted to metric is 30.5 cm.
    The 3 1/2 feet depth, converted to metric is 106.7 cm

    So my emergency rudder will have about 3254 sq. cm of underwater area.


    A J-24 rudder, which would be a touch small for the S-2 7.9, but not by much, has a submerged chord length of 30 cm and a submerged depth of either about 89.0 or 95.5 cm depending on which picture you look at on the class rules...I can't figure it out. Would a J-24 rudder control an S2 7.9? Sure. Is it a "keep racing" rudder or a "get home" rudder? I'm thinking, somewhere in between.

    Those numbers give a submerged area of 2670 sq. cm or 2865 sq. cm for the J-24 rudder.

    So my e-rudder has significantly more surface area, underwater, than the J-24 rudder.

    The maximum thickness of the J-24 rudder is 3.9 cm thick.
    If I say my rudder will have a final maximum thickness of 1.8 inches, which is about right, that's 4.6 cm

    So my rudder is a bit thicker than the J-24 rudder...should be a little bit "grippier" if not as fair and fast.

    CONCLUSION...if the J-24 rudder would steer the S2 7.9, and I think it will, then this should be fine.

    How does it compare to the rudder I built as the Wildcat's primary rudder?

    which is actually a tich larger....but nowhere near as thick, as the One Design rudder.

    The Wildcats primary rudder has an underwater chord of 13.5 inches / 34.4 cm. The submerged blade is 44 inches / 111.7 deep. That gives a total submerged area of 3842 sq. cm.

    compare the e-rudder area to the primary rudder area... 3254 sq. cm / 3842 sq. cm = .85...so my e-rudder is 85% of the area of the primary rudder.

    I'm good with that!
    S-2 7.9: "Wildcat of Loch Awe"
    1968 Selmer Series 9 B-flat and A clarinets
    Piper One Design 24, Hull #35; "Alpha"

  3. #3
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    OK, today I spent about 90 minutes getting some shape into the rudder blank. I started with my favorite crude woodworking tool, the drawknife. This redwood has some knots, and therefore some grain distortion, so I accidentally knocked out some chunks that will need fairing, later. That's all right.

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    S-2 7.9: "Wildcat of Loch Awe"
    1968 Selmer Series 9 B-flat and A clarinets
    Piper One Design 24, Hull #35; "Alpha"

  4. #4
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    After spending some time with the drawknife, I moved onto more wood-hackery tools...namely a surform plane and a really coarse disk on my drill. Between judicious use of the surform plane, with occasional intervention from the drawknife, and grinding down the harder wood of the knots with the disk, I managed to get a halfway decent shape in the leading edge of the blade, and smooth the junction between the first and second boards.
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    Not-so-gentle ministrations of 100 grit sandpaper on a long block faired the leading edge to "round-ish", though it's not by any means elliptical. If I can get a really nice trailing edge I might go back and thin out some of the leading edge again.

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    S-2 7.9: "Wildcat of Loch Awe"
    1968 Selmer Series 9 B-flat and A clarinets
    Piper One Design 24, Hull #35; "Alpha"

  5. #5
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    The way I glue up the boards, the trailing edge board had a huge knot in it at the bottom. That was going to be a pain to fair, so I decided to just round off the bottom of the trailing edge and tell everybody that it's an "elliptical" shape, which will reduce turbulence as the attached flow leaves the rudder surface. This is garbage, but what the heck.

    I spent about 45 minutes with the belt sander, thinning the trailing edge and working a taper through the back two boards. There's actually quite a lot of that left to do, and the trailing edge is nowhere near straight yet, but it's progress!

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    S-2 7.9: "Wildcat of Loch Awe"
    1968 Selmer Series 9 B-flat and A clarinets
    Piper One Design 24, Hull #35; "Alpha"

  6. #6
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    Todays three hours of rudder work began with more shaping with the surform plane and belt sander. I spent the majority of the time thinning the aft 1/3rd of the blade. I also sanded off all the old weathered wood up above the waterline and got down to clean wood, which will bond with the fiberglass much better.

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    The underwater part of the blade is looking smooth and the aft 1/3rd of the chord is a reasonable taper down to a roughly 3/16th inch trailing edge. That edge is not perfectly straight, but it's not bad and this IS an emergency rudder, after all. Putzing with the belt sander had produced a happier-looking leading edge as well. It's not really elliptical, but it's "more" elliptical than it was yesterday evening.

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    S-2 7.9: "Wildcat of Loch Awe"
    1968 Selmer Series 9 B-flat and A clarinets
    Piper One Design 24, Hull #35; "Alpha"

  7. #7
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    I've been toying with the idea of not using a cassette at all. This e-rudder weighs about 18 pounds right now. Add the fiberglass sheathing and it's pretty surely going to be around 30 pounds. That's a lot lighter than any other e-rudder I've built. I know how I'll make the gudgeons..wood, fiberglass, 2-inch glass tape and carbon fiber, but the question is....put that stuff straight on the rudder and just have a blade, or build a low-tech cassette?

    If I go with the "just a blade" route, it's less to carry on the boat. I will cut the aft edge of the upper half of the rudder at an angle, so the top of the rudder is a bit smaller than the underwater chord, which will save a bit of weight and space. Of course, then I won't have to deal with storing a cassette.
    If I DO need it, if it takes me an hour or 90 minutes of cursing to line up the pins and gudgeons, does it REALLY matter?

    On the other hand, a low-tech plywood cassette is braindead easy to build. You store the whole thing on the boat with the blade IN the cassette, so it doesn't take up that much room.

    Whichever route I go, there's nothing to do quite yet, as any cutting of the upper half of the rudder will happen after it's fiberglassed
    S-2 7.9: "Wildcat of Loch Awe"
    1968 Selmer Series 9 B-flat and A clarinets
    Piper One Design 24, Hull #35; "Alpha"

  8. #8
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    Two of the boards had some significant warpage, so they didn't glue completely flat. There's plenty of glue contact, so it's strong, but there are some big "dips". That's irrelevant in the blade, because SO much material gets removed, but up in the above-water part of the rudder, it'll make for some funky-looking fiberglassing. So I glue'd in some cheater bits, which I'll belt sand down flush with the rest of the rudder.

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    S-2 7.9: "Wildcat of Loch Awe"
    1968 Selmer Series 9 B-flat and A clarinets
    Piper One Design 24, Hull #35; "Alpha"

  9. #9
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    And finally, I mixed up some epoxy with redwood sanding "dust"...which is about the same size as the wood dough you can buy, and made some wood spooge/putty. That went on/into the two "oops's" that I labelled yesterday, that got hacked out a bit too deep with the drawknife. They will kick off overnight and maybe tomorrow I can hit 'em with the belt sander before work, we'll see.

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    All in all, this won't win any awards for super-fast rudder design...it's not as accurate as the primary rudder, where I routed out the foil shape using an elliptical template. However, it's reasonably smooth, plenty strong, and a reasonable shape.
    S-2 7.9: "Wildcat of Loch Awe"
    1968 Selmer Series 9 B-flat and A clarinets
    Piper One Design 24, Hull #35; "Alpha"

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