Holes in rudder?

Historicly old chinese rudders were made of strong woven wood sheets in a frame
stands to reason than diamond holes occured

probably the holes worked well and were kept in metal rudders
and again used on fast torpedo boats last century, its in the google book links

ok, as a little child i might have been reading to many popular mechanic magazines
http://books.google.nl/books?id=Q-M...Bg#v=onepage&q=perforations in rudder&f=false
.

 

If I did not have so much other work to do to complete my project I would build another rudder with diamonds to compare. Would be easy to do on a small craft with a plywood rudder - I'm thinking someone here with such a boat should take it on and enlighten us

 

One last point before the thread finally dies, we know the Chinese had more advanced rudder systems long before the west. What if fenestration had nothing to do with steering. The deep rudder was an essential part of the craft's lateral resistance. Perhaps we are looking in the wrong place and should be cutting holes in our keels and daggerboards.
Just a thought! Have fun guys.

 

Nothing wrong with Popular Mechanics. More support for actual performance benefits. Based on common sense, I would bet that they actually work, and not just for decoration.

 

Rwatson,

Sorry, but I think there is a lot wrong with Popular Mechanics. I have reviewed issues that I remember from the 60's. All the flying car articles, etc which proclaimed great changes have a common thread with today.

Unwarranted exaggeration of the benefits or promise of a new technology or idea leads to a belief in things that are just wrong.

 

I got it ---I was thinking about this all day yesterday. Maybe the holes are for fish to swim from one side of the rudder to the other.

Popular mechanics !!! tongue in cheek at best.

 

question nags, drew a quick crude rudder and downloaded autodesk falcon
its free and now works standalone, needs a bit gettin aquented, cant find lift or hydro yet but calculates drag
than drag translates to torque and more rudder design on the net, attached an article by Eric Sponberg
bleeding thrue holes shows a better attachment but for now its just frivolous chineese decoration

 

Surprise yourself and Google 'flying cars'

There are over half a dozen successful prototypes, and 3 or 4 models granted airworthy certificates.

Sure, there are lot's of ideas that weren't 'goers', but there are heaps that were 'leading edge'.

 

attachment isn't the thing to look for according to the available info. If junks ever moved fast enough to develop any kind of hydrodynamic flow, I would be surprised.

Its looking for relief of pressure. How does one calculate helm pressure at various angles and different flow directions ? There should be software that can do that.

 

google'd what to look for in rudders and there are formules but quickly i was kicking myself not to be a NA nor ME
falcon is a basic free cfd program giving insight in flows f.e. how sensitive flows are on velocity and rudder angle
while a rudders longtitudal drag is very important as well and would not benefit from fenestration

 

Thats why we all got one today.

 

From the images presented in this thread it would appear that junk rudders are relatively small and I would guess that they operate at high angles of attack. They are also flat plates i.e. not aerofoil shaped. I would suggest that the diamond shaped holes have evolved as vortex generators and that the junk rudder is a vortex lift device rather than an aerofoil. The advantage of vortex lift (used in delta wing aircraft for instance) is that you can maintain attached flow and generate large lift coefficients at high angles of attack and vortex lift surfaces are highly resistant to stall. This allows you to use a smaller planform area for the same maximum lift as an aerofoil surface. The disadvantage of vortex lift is the low L/D ratio which translates to high drag when large lift forces are being generated, but as long as the rudder is not being used at a high angle of attack all the time, that may be a reasonable trade off. The Junk usually has a split rig so most of the steering is probably done by sail trim, and if you are broaching in a following sea, who cares how much drag there is?

It is likely to be more ecconomical to build a smallish simple flat plate rudder with a few holes in it than to build a larger aerofoil rudder to close tolerances. I doubt that the holes are there just for decoration, they take time and cost money to put there and they spend most of their time under the waterline where they can't be seen.

I'm guessing that these are the reasons why the junk rudder has evolved the way it has. Furthermore, if something has evolved a certian way and has survived that way for hundreds of years or more, it's unlikely to be wrong.