DIY rudder build.

Discussion in 'Fiberglass and Composite Boat Building' started by DennisRB, Sep 28, 2010.

  1. DennisRB
    Joined: Sep 2004
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    DennisRB Senior Member

    Hey guys. Here is my attempt at making a rudder for my dads 25 foot trailer sailer. My dad snapped it off by reversing up the driveway with it down. The boat is 25 foot long and about 1500kg the keel is 500kg on its own. The original rudder was only 2.5 foot long (water depth) and with some weather helm you would be sailing with it at 30 degrees which cant be good for speed or anything for that matter. So I added an extra foot to the new rudder. Obviously there are heaps of variables to how long the rudder should be but, surely 2.5 foot is way too small for a 25 foot 1500kg boat?

    Not sure if I am in the right section but being plywood and glass I guess that is more composite than wood right? I realize this is going to make me look pretty clueless but thats OK :)

    I have never used fiberglass before, or micro balloons or epoxy and in general just went at it to have a go. So please feel free to tell me all the things I done wrong so next time I can do em better. I sort of figured out a few things I done wrong all on my own anyway. Like glassing the whole rudder in one go! That ended up with a lot of wrinkled glass and way too much resin. I went and got a new orbital sander and that made short work of roughly smoothing it to get rid of the high spots even if I did sand through glass.

    How much could I have expected a pro to build a rudder replacement for this?

    My mate Wes and I done this over a few day and many beers. We started by screwing 2 pieces of 15mm marine ply and cutting out the shape of the old rudder and added 1 foot to the length. We then undone the screws and epoxied the 2 pieces together,holding it together with screws again.

    We removed the screws and power planed it to approximate the foil shape and finished the shaping it with my new electrical orbital sander. The foil shape wont win any races as its just by eye. How important is this? This boat is an old clunker anyhow. :D

    Nice work shed.

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    Nice work boots.

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    We then coated the whole lot in glass and epoxy in a very unprofessional manner (many beers had been consumed by this stage). I drilled a larger hole than required for the pivot pin and filled that with micro spheres so that when the whole is re drilled no wood is exposed to keep water out. I then sanded off the high spots and used west system micro spheres to fair it and sanded again. For the last few little imperfections I just used builders bog as I was sick of waiting 24 hours before I could sand the epoxy.

    Nice work bench

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    Nice glassing job :p

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    Nothing a sander cant fix.. at least aesthetically. The glass is just for abrasion and water protection anyway not strength.

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    Roughly foil shaped. I did try to make it even in the sanding and planing stage by looking at the layers of ply as I removed material.

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    You can see the size difference.

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    Now its ready to undercoat and paint.
     
  2. susho
    Joined: Dec 2006
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    susho Composite builder

    too much resin, use a squeegee, and maybe some peelply to, could save you some sanding.
    you could use a router to shape it, you can work very symmetrical with it, and very precise.
    It looks nice though, better than my first one :)
     
  3. DennisRB
    Joined: Sep 2004
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    DennisRB Senior Member

    Thanks. I think the results so far are actually pretty good despite the somewhat substandard intermediate steps.

    Fully agree on the too much resin. We were just pouring it on to try and use up what we mixed up. I think because we done the whole lot in one go it became impossible to roll the excess out with the fiberglassing roller tool. I think I will be able to do it right next time by just doing the edges first so I can roll any excess resin out. Peel ply might work then. I don't think it would have helped a lot this time given how bad we laid it up :p

    How do you mean a router would help to make it even and shape it? Would I set it to pre determined different depths and scribe grooves at even spacings then plane down to the grooves or something?
     
  4. Landlubber
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    Landlubber Senior Member

    ...bit late, but have a look at the NACA site for different profiles for different outcomes, it will teach you a few good lessons anyhow.....

    this one will do....

    http://en.wikipedia.org/wiki/NACA_airfoil
     
  5. DennisRB
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    DennisRB Senior Member

    Nice link. I know a little bit about airfoils. I have been into RC planes since I was a kid. But I don't really know how much it matters for a rudder on a boat? Do foil changes really make a big or even noticeable difference for a cruising boat for a rudder? I know on my RC planes the main wing needed a good airfoil but when it came to the rudder and elevator a flat board was good enough.

    How do people usually get their rudders to match the foil shape? With RC planes its easy as you just print the foil shape out then use a hot wire bow to cut the foam core. I guess this could work with some types of foam for boat rudders too but what about wood?
     
  6. Landlubber
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    Landlubber Senior Member

    ..as you can see in the plywood "grain" it is very easy to set parallels and thickness of course because the veneers are the same thickness, therefore you can "see" the shape, that is how I know that you have not done that...have a look at your own pics.
     
  7. DennisRB
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    DennisRB Senior Member

    I did use the veneer lines. I thought the lines were parallel enough that sanding the fairing compound would lead to a pretty straight result. I can see that the lines are not totally straight but pic one shows a (failed?) attempt to get it fairly straight. I took too much material away in some areas and elected to fill those areas later rather than taking more material away to make it even. Given my glassing job I doubt much more accuracy here would have made a differnce anyway :p The pencil lines were made so I could trace the veneer lines on paper then draw them on the other side to get the same shape on both sides. The thing was I didn't actually have any particular foil shape, just whatever looked OK. But I hear you that my job was pretty shithouse :) I wont argue there.

    Maybe I next time I can do what I do for a RC plane wing and draw the airfoil on both ends then shave it to that profile on the ends, and from there I can make parallels down the whole rudder? Given I will actually have a real foil shape then it might pay to put more effort into keeping the lines straight and doing a decent glassing job. Thanks for the tips :)

    Does anyone have any input on the new and old rudder sizes?
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    On small, light weight boats, a plywood rudder is acceptable, as the loads are fairly light, but once you get over 18', the loads increase enough to make plywood a less desirable rudder material. This is because plywood is about 2/3's as strong as solid wood in this longitudinal flexural role and failure from "rolling shear" is a real possibility.

    A better wooden approach for a boat of this size is a strip planked blade. The pictures aren't great, but you can see the foil sections are being cut. These pictures where taken as I switched foil templates, which was routed across the blade. You can see the quarter sawn strips (about 1.25" x 1.5") edge glued. What can't be seen is the alternating grain orientation to eliminate internal stresses.
     

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  9. hoytedow
    Joined: Sep 2009
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    hoytedow Wood Butcher

    Great reference thread!
     
  10. DennisRB
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    DennisRB Senior Member

    Thanks PAR. I really should have made this thread before I started the build. Do you think my rudder is strong enough realistically? The ply is about 27mm thick at the top where it joins the stock since some material had to be removed so it would fit in. The old rudder didn't snap from normal use. But I assume the extra length will only put more stress on it.

    I still don't understand how you use a router to accurately cut the foil shape?
     
  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    27 mm is a thin blade for your boat, unless you plan on racing and looking to reduce drag as much as possible.

    The break in the picture looks just like what a break from a rolling shear failure looks like. You may have had some internal fiber failures, but the blade was still whole and the drive way was the straw that broke it's back. In other words it broke at it's weakest point when it hit the driveway. Why did you add length to the blade?

    Thee are a few methods that I know of for using a router jig to cut foil sections. Naturally each has good and bad points about them. The blade I show was for a light weight, 17' daysailor and finished about 1" thick. Because the blade changed shaped pretty dramatically along it's length I elected to make several templates and adjust the foil type along the length. The top portion of the blade was a surface piercing shape, the middle portions transitioned from 0012 to 0016 and the end of the blade was a 63 series section. This caused me to have to cut in 6" sections at a time, but this isn't unusual.

    The setup can be tedious for these router jigs, but you end up with precise sectional shapes and more importantly symmetrical foils.
     
  12. DennisRB
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    DennisRB Senior Member

    The bade is over 32mm thick in total for most of the span. Only the section that goes into the stock was shaved to 27mm so the glass could pack it out to 30mm so it would fit in. The stock slot is 30mm. That just happens to be the most stressed part of the rudder too. This boat was used for racing and was apparently one of the fastest trailer boats in Moreton bay 40 years ago. It was owned by the Commodore of the Manly Yacht Club. Now its performance would be regarded as slow with all the cruising "improvements" my dad made to it. Its much slower than my Binks 25.

    I lengthened the rudder to the same length as my Binks 25 (which is the same length but lighter). It now appears to the same size as most other boats of this length where before it was much smaller than the average. The rudder just seemed too small and always seemed to drive though the water at a overly large angle of attack. I hope it can now sail with the rudder in a more centered position to reduce drag broaching.

    The way the rudder hit on the driveway would have snapped any rudder. Something had to break with it wedged into the ground and a car reversing up with force. The driveway is steep and the stern lifts high into the air is it comes up from the road. So the rudder would have almost dropped right down before the full weight of the boat would have pushed back down on it while going backwards. Thats not to say it wasn't damaged already though. In fact this was the second rudder he snapped like this. I think a better method of holding it up is required or at least checking its up. :p

    If it snaps I now know how to make a stronger version with strip planking :D But I really don't want it snapping and causing a dangerous situation.

    Sounds like you really tried to make your rudder as good as possible with all those foil changes. Did it work as you hoped? Was it for a new boat or an upgrade?
     
  13. PAR
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    PAR Yacht Designer/Builder

    This rudder blade (shown above) was for one of my designs. It was a new build, but the first to have a full foil shape allotment it's full length. Previous versions had flat sided foil sections which are easy to cut by the novice, but not quite as efficient as the "full up" version. Both the centerboard and rudder received this treatment and I had an opportunity to sail it against another of the same design, shortly after launching. The foil section equipped boat, which was identical other wise, right down to the sails. I made both boats (well made one, bare hulled the other) and the sails came from the same supplier. The new foil equipped boat was able to point slightly higher, had less turbulence at the top of the rudder, handled beam sea conditions better, the rudder seemed to stay "in the groove" longer, etc. So, yes the improvements where worth the extra work. The slab sided centerboard would "hum" a touch when up on plane, but the fully shaped centerboard didn't.

    If you where experiencing a lot of helm deflection with the short rudder, you may have other issues then rudder length. I would have had to access the boat a little before making this change, like what condition are the sails, which is a common cause of excessive helm deflection (weather helm). How about the rig, what is the mast rake and do you know what it's supposed to be? Is the boat sitting on her lines or a little down at the stern? This is also a common problem, usually because the boat has become a storage box and gained a considerable amount of weight, not unusually under the cockpit out of sight and mind until it affects trim.
     
  14. DennisRB
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    DennisRB Senior Member

    Thanks for the info on your boat design. It seems real foil shapes are well worth the effort. Do you have a pic of the boat?

    The 25 foot trailer sailer in question has been molested from its original specification. (no pics sorry, I will get some eventually) From day one it had 100mm more freeboard. The swing keel was extended 30cm and with more weight. The keel is pretty much a hollow rectangular steel plate construction with lead filled in the bottom. Only the leading and trailing edges are sharpened so its has a very crude cross section. However in this state it was successful in racing when it was made 40 years ago.

    From here the boat has had a few mods which detract from performance and aid cruising. It now has a electric anchor winch and carries 50m or chain up high in the bow. It has a heavy copper 80L fuel tank in the stern. The mast snapped from a rigging failure and my dad decided to get a taller mast so that the mainsail and boom could be higher up to fit a canopy under. Despite the extra mast hight the new mainsail that had to be cut is still smaller in area than the original but with a higher center of effort. The new mast was of a significantly heavier section so with the extra length and size it is probably twice the weight. The rigger who set this up was a ***** and the mast has a slight forward rake. It was much worse but we got it to almost vertical. The taller mast would have changed the angle of the forestay but the boat does have adjustable points for the jib sheeting. The hull shape has fairly straight runs aft, which make it good downwind in a strong breeze (planes) but the transom sits about 100mm under water which causes drag in most situations.

    I'm sure there was a noticeable performance decrease with the new rig. But the boat always had weather helm and this only made it worse.
     

  15. PAR
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    PAR Yacht Designer/Builder

    This is her just coming together. Not all the coming is in place, but most everything else is. The other picture is the only rudder shot and the blade isn't there, just the rudderhead. The blade was also painted, but the tiller was a lovely vertical lamination (instead of the typical horizontal) of mahogany and white fir. Instead of tapering the outside of tiller, I tapered the fir portions of the laminate which made it especially interesting to look at.
     

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