[Public-List] Rudder Shape

Tue Feb 21 07:05:25 PST 2017

Another interesting thing…  in discussion with Steve Killing, the yacht designer who lives near us, and is the fellow who figured out how to make those huge America’s Cup catamarans fly by making their centre boards curved, I learned that the way to make certain a dinghy rudder or centerboard doesn’t vibrate is to make the trailing edge straight and squared with sharp corners.  We make the servo blades of Voyager self steering vanes that way.

Along the same lines…  I’ve got an original Hasler self steering vane in our workshop.  I bought it years ago for the Folkboat but never used it…  I keep it to show where Voyagers came from.  The Hasler is a fair bird cage of tubes, tiny blocks and lines, but it shows evidence of Hasler’s brilliant mind all over it.

Now here’s the point of all this…  Hasler’s servo blades, the part of the vane that’s in the water, is NOT foil shaped.  It’s wedge shaped with a small radius parabolic leading edge, then steadily widens to a squared trailing edge about 1” thick.  This is the same shape used by surface piercing hydrofoils of the 1960’s.    interesting, isn’t it?  At low speeds it likely makes more drag resistance than a pure foil… but at high speed much less, and especially at angles of incidence.  Whatever turbulence it ‘feels’ is swept off the wedge and tumbles in futile frustration aft of the squared edge where there’s no blade.

As a maker of vanes I was also interested to note that Hasler resolved the issue of making the vane fit various freeboard heights by simply making the blade way too long.  The spruce is cheap, and the owner can trim it to whatever is required.  Genius.

Gord #426 Surprise

On 2017-02-21, 9:24 AM, "Public-List on behalf of Rod Symmes via Public-List" <public-list-bounces at lists.alberg30.org on behalf of public-list at lists.alberg30.org> wrote:

    What I remember from aerodynamics in that time period was that European car rally drivers prefered the Volvo station wagon over the sedan because,  in spite of the extra pounds of glass, they were 2 or 3 mph faster with that square back end.
    Go figger ?

    Cheers,  Rod

    -------- Original message --------
    From: Michael via Public-List
    Date:21-02-2017 7:37 AM (GMT-05:00)
    To: Gordon Laco , Greg Roberts , Alberg 30 Public List -- open to all
    Subject: Re: [Public-List] Rudder Shape

    I always think of Kamm back cars of the late '70's on this subject. Kamm
    was an aerodynamic scientist who calculated the drag of building in a
    tear drop shape was more than just cutting off the back of the car
    square. Made for a bunch of ugly little cars.

    Michael Grosh
    #220

    ------ Original Message ------
    From: "Gordon Laco via Public-List" <public-list at lists.alberg30.org>
    To: "Greg Roberts" <greg at midnight-oil.us>; "Alberg 30 Public List --
    open to all" <public-list at lists.alberg30.org>
    Sent: 2/21/2017 3:27:50 AM
    Subject: Re: [Public-List] Rudder Shape

    >Good day Greg,
    >
    >A foil shaped rudder would do you no good at all… one must consider,
    >with our sorts of boats, that the ‘foil’ has it’s leading edge at the
    >forward part of the keel and the rudder is a sort of flap or trim tab
    >on that structure, not something on it’s own.
    >
    >The only improvements one could give our rudders would be to minimize
    >the ‘leaking’ of flow from the leeward side to the windward between the
    >rudder post and the after part of the keel (already a pretty good fit
    >in Alberg 30’s except of course for the troublesome propeller aperture)
    >and perhaps a narrower and more squared trailing edge.   The thicker
    >rounded trailing edge (when considered in cross section) is not so good
    >for minimizing turbulence at the trailing edge.
    >
    >Gord #426 Surprise
    >
    >On 2017-02-21, 12:32 AM, "Public-List on behalf of Greg Roberts via
    >Public-List" <public-list-bounces at lists.alberg30.org on behalf of
    >public-list at lists.alberg30.org> wrote:
    >
    >     Did any of our boats come with a foil shaped rudder?My wood rudder
    >has
    >     flat sides and a gentle taper..I'm not sure how much one would gain
    >     with a foil given the 3/4 keel,I'm having a hard time picturing how
    >     you would handle the tilt when defining the profile.
    >     Regards,
    >     Greg
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