[MHml] Ventilation of surface piercing daggers.

[Rob Denney]

G'day,

We use oversized rudders (no daggerboards) on harryproas.  They are
hung off the inside of the hulls and have had no ventilation problems
up to 18 knots boat speed.  The safety and ease of use aspects of this
set up make the possibility of high speed ventilation an insignificant
problem.

regards,

Rob

On Dec 27, 2007 9:19 PM, Allied <allied at clear.net.nz> wrote:
>
> Very interesting for me at present, as I am going to try my rudders inboard
> of the two hulls, as part of a lengthening and lifting of the stern's
> exercise. I used Derek Kelsall's, single central rudder, as my "why not", to
> do this. I am going to put in a little leading angle, but probably not as
> much as 7º.
>
> I am not a racing speed Cat, but in the right conditions can be in the mid
> to high teens. What sort of speed are we talking about ventilation starting?
>
> As I need to be out on the water soon, I am going sailing with my old centre
> boards and rudders, but in the future I will use my big machining centre, to
> shape some better casings and boards. Always some new things to try :o).
>
> Colin
>
> I was discussing this with a friend who thought he remembered Malcolm
> Tennant having once said that inclining a surface piercing foil forwards by
> 7 or more degrees would stop ventilation. I did not rexcall it, but thinking
> about it one can visualise that a forward raked foil would tend to have
> water rushing up its forward surfaces, and if water was rushing up could air
> fight its way down????
>
>     Did Kelsall have significant problems or not when he  used one many
> years ago? I know that the Cherokee cruiser from Sailcraft had one but it
> was very much a cruiser so perhaps it did not matter.
>
>         It does not seem to worry very fast trimarans flying the hull, but
> they are relying then on a curved foil to leeward for both lift and leeway
> resistance.
>
>     Comments, experience or advice on this topic would be appreciated.
>
>         Roy Mills
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