DIY rudder build.

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

  1. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    Dennis,

    What you did should work fine, I would make sure the rudder mount is strengthened since you will transfer more bending moment to the transom because of the deeper rudder. I think for a cruising boat too much is made of foil shape. Any shape with a rounded leading edge and pointy trailing edge will make a perfectly good foil, you obviously are not going for an optimised rudder design.

    The benefits of specific foil shapes are only incremental, most of the drag is actually from plan form shape, by increasing the aspect ratio you may have actually reduced the trim drag anyway.

    The most important thing for a cruising rudder is that the leading edge have a fairly large radius, and you have a reasonably sharp TE to reduce drag. The large LE radius is to prevent stalling the rudder at larger tiller angles. Symmetry in a sail boat is way overrated, except for going strait down wind, the water never "sees" a symmetrical rudder, keel or hull anyway, it is trimmed out by the tiller (usually unnoticed by the helmsman). Just get the final profile shape reasonably smooth to contour, using filler if you have to.

    To use a router to make perfectly symmetrical rudders, even to specific foil shapes, you have to make a simple jig with a profile template that the router jig will travel over, following the profile as it cuts away the wood on the rudder (than you flip your rudder stock to cut the other side). There are a number of simple ways make a fixture, or you can buy expensive fixture. It is made from some plywood panels, door hinges and some kind of moving table that allows the router to slide back and forth over the face of the whole rudder stock. Some people also make several plywood profiles that index over the rudder stock, and you slide the router from LE to TE to get the shape (this takes much hand smoothing between the router cuts).

    Good luck with it.
     
  2. DennisRB
    Joined: Sep 2004
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    Location: Brisbane

    DennisRB Senior Member

    Beautiful boat PAR. I love the classic wood look.

    Thanks Petros. I think I took too much wood off the leading edge and its a bit too sharp. You can see the profile in one of my pics. But as you say, it a cruising boat so who cares? All I want for it is to work and not snap. But I am little concerned now. I would feel terrible if something happened to my dad because the rudder broke! The old rudder was made of a dark brown heavy hardwood with what looks like bits of ply in areas surrounding the pivot pin. Possible repairs? I put my new rudder on a ledge and stood in the middle and it took my 95kg/210lb no probs with minimal deflection.

    I would love to see some pics of these router jigs.
     
  3. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    It is not just the strength of the rudder, I think it is strong enough, but with a deeper rudder, you put much larger loads on the mounting hardware. The side force on the mounting hardware is further way, so the leverage on the mounting hardware is larger.

    Say your upper and lower rudder pivots on the rudder are 20 cm apart, and with a 1 meter long rudder, with the lower 30 cm in the water. Now you make the rudder 30 cm longer, so there is now 60 cm in the water. The side force you generate in similar sailing conditions would be the same, but is now effectively deeper, generating a side force about 15 percent higher on the hinge pins.

    And of course your risk of dragging the rudder in the shallows is now higher as well.

    Just make sure your rudder mounts are up to the task.
     
  4. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member


  5. hoytedow
    Joined: Sep 2009
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    Location: North of Cuba

    hoytedow Carbon Based Life Form

    Thanks for the photos. They are a perfect example of the way to set up a rudder on a small boat.
     
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