Subject: Re: [harryproa] Re: Design your proa
From: Rick Willoughby
Date: 5/25/2011, 6:54 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

Ben

Any blade that breaks the surface has potential to ventilate because it will have a negative pressure on at least one face.  It requires an air path to get air down the blade not only connection to the surface.  I expect the AoA of the board at high speed would not be enough to get flow separation to open up an air channel unless it has some odd section.  A rudder typically works through a wider range and flow separation is more likely.  Anything that prevents air from getting down the case will stop ventilation.

If the trailing edge is blunt (wide or radiused rather than sharp) then there is a good chance it will be vortex shedding.  That sounds more like what you are experiencing. The different "gears" is the different modes of twisting of the board.  First gear is the whole immersed part of the board twisting about the top inside the case.  Second gear would be immersed top and bottom out of phase and third gear will be centre section out of phase with the top and bottom.  There is good data around for frequency of vortex shedding as it has been the cause of large structures falling down.   If you measured the torsional stiffness of the blade you could predict the speed that the different modes will occur.

An alternative to taking to the board with a sander is to do a simple test next time you are going fast.  Stop lift the board and use duct tape down the trailing edge to improve the sharpness.  A strip down each side joined to each other behind the blunt edge should provide a faired edge.  It should stay attached long enough to confirm the cause.  Then you have to decide a permanent fix or just live with the problem.  

Any vibration like this is bad from a fatigue life perspective so best to find the source and eliminate it.    

Vortex shedding is one of the problems I envisaged during my discussion with Dennis Cox on the bi-directional rudders.  The large radius leading edge is good for extending the range with regard to cavitation but the vortex shedding would create high vibrating forces getting there.  I actually thought about using a thin flexible strip down the edge that folds back on the blade when that edge is leading.   

Rick
On 26/05/2011, at 4:54 AM, bjarthur123 wrote:

 

thanks rick. i still can't say i understand everything, but at least i understand a bit more.

thanks also for keeping the discussion with gardner public. it's been quite educational following along.

one question regarding ventilating foils: on boats whose daggerboard sticks up through a trunk, do you think they ventilate? usually there's not that tight of a seal. air could get sucked down i suppose.

when my weta gets going 10+ knots, i can hear the daggerboard humming. the pitch jumps discretely with speed, like gears on a car, it is not continuous. third gear is pretty exciting, and i usually try to tone things down at that point.

i've been told the von karman vortices shedding off the trailing edge excite different modes of vibration. makes sense. but maybe the driving force is related to ventilation instead? should i try to seal it up?

ben arthur
weta #358
ithaca, new york

> The majority of information you read regarding "hull speed" is based
> on beamier hulls than ever contemplated for catamarans, tris and
> proas. Around the time Froude was doing his thing his frame of
> reference was probably British Navy sailing vessels and sailing cargo
> carriers. I expect he had not seen the fast light weight sailing
> boats on the other side of the world.
>
> The wave drag for a slender hull is a relatively low proportion of
> the overall drag for the speeds you have indicated and anything
> higher. Key word is slender. Hence the viscous drag dominates so
> wetted surface area is critically important.
>
> This is why the charts are useful in getting you in the ball park.
> Longer is not necessarily faster for a given power and displacement.
> These charts only consider displacement mode however at some point
> there is actually less drag to plane and the hull shape gets short
> and wide.
>
> When you are looking at the basic dimensions don't just look at how
> they are changing, also think of the actual values. A 14.5m long
> hull is quite long. A 600mm wide hull is not very wide and draft to
> beam ratio of about 40% is still quite deep. Note the increasing
> block coefficient as the speed increases.
>
> Typically when you get into these high speed displacement hulls the
> block coefficient goes up. The ends get fuller with wider entry
> angle and less rocker.
>
> A factor that might encourage a longer hull is the bow down trim
> resulting from a tall rig. So being able to pre-determine trim is a
> useful part of the analysis.


Rick Willoughby
03 9796 2415
0419 104 821

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