why does this 1986 steel Junk have holes in rudder?

Maybe the fenestration was mainly meant to make the steering under sail alone* lighter . . ? ?

* main purpose of this boat

Cheers,
Angel

 

Anything to make a flat rudder work better in an advance, they are pretty useless.

 

Trim Tabs ( that only take a small portion of the trailing edge) can be operated quite easily by hand (manually) , and move the largest rudder easily in the direction required. http://en.wikipedia.org/wiki/Servo_tab

 

I think I've missed your point here Watson, this rudder has no trim tab but rather diamond shaped holes cut out of it.

-Tom

 

I suspect the 1986 was copying "traditional" look of ancient junks, hence the fenestrated rudder. Unfortunately, they apparently neglected more laudable features of the junk. Such as staggering the masts, stepping them on the gunwales not the centerline. The bulwarks served as one side of the mast tabernacle, the deck was open and clear for cargo handling and carrying deck cargo, and the sails didn't blanket each other. Junks were/are fascinating to study. Smart cookies, those ancient chinese.

 

The original purpose of fenestrated rudders on junks was lightening them. The rudder can be hoisted considerable distance up and down by block and tackle Top of rudder post had multiple mortices for insertion of tiller when rudder was deep or shallow. Purpose of adjusting immersion of rudder was balance under sail. Ancient chinese autopilot.

 

Best resource for information on junks is: Junks and Sampans of the Yangste River, by W. R. R. G. Worcester.

 

Not if you follow my train of thought - ..choo choo

1) Squiddly was surprised that a modern boat would have to revert to such an antiquated method of making steering lighter.

"Seems like a 1986 steel boat should be able to turn the rudder without trouble or drag inducing holes."

2) I pointed out that there was another better option

"Its just a bit of inefficient nostalgia since the trim tab "


3) Angelique seemed to think (probably correctly) that the appropriate gear for a boat like this should be purely manual

"Maybe the fenestration was mainly meant to make the steering under sail alone* lighter . . ? ? * main purpose of this boat"

4) I pointed out that a trim tab could indeed be an effective option to use manually

"Trim Tabs ( that only take a small portion of the trailing edge) can be operated quite easily by hand (manually) , and move the largest rudder easily in the direction required."

5) Tom mistook my comments and thought that I thought, it was using trim tabs

"I think I've missed your point here Watson, this rudder has no trim tab but rather diamond shaped holes cut out of it."

If you look carefully Tom - I wasn't accusing the junk of having trim tabs at all.


Ok. back to the main conversation - as YoB points out, they dont utilise all the traditional features of junks..........

Carry on everybody !

 

"The rudder, as is common with Chinese rudders, has five diamond-shaped holes in the blade, and the blade itself is of very thin wood. The purpose of these diamond-shaped holes is undoubtedly to permit dead water to run through to the back of the rudder blade and prevent the accumulation of dead water along the after edge of the rudder. As already stated, the rudder can be completely housed in the hull, simply by setting it straight fore-and-aft, and then drawing it up through a slot, which is cut for the purpose in the deck. This construction is shown in detail at the stern of the hull in the sail plan."
http://www.thecheappages.com/junk/sampan.html with some more junk info like a rudder in a slow draining stern bun
divebombers had buffeting problems: http://stevenwhowardart.blogspot.com/2010/11/helldiver-fixed.html
"Some of the Chinese junks did have holes in their large rudders but the reason is up for argument."
Tom Lathrop: http://forum.woodenboat.com/archive/index.php/t-11447.html
opening books on north and south china seas old junks couse i belive i've seen a better explanation, yet there must be reasons why most boats have foil rudders

 

The reason is simple, these holes shed large vortexs that improve rudder control. With the rudder behind a large bluff body, and behind the prop, there is a lot of turbulance and likelyu a very large boundary layer, making the rudder rather ineffective. The vortexs shed out of the holes help keep the flow attached better, and improve rudder effectivness.

Yes it adds drag, but it is the simplest and least costly way to gain better rudder authority.

 

Ummm, I'm confused a bit here Petros, .

How do you get an attached flow with "lot of turbulence behind the prop and bluff body" ?

I was under the illusion that you had to have a nice clean flow to get it to attach, and the same with the boundary layer.

 

Petros has the right idea if not spot on but I prefer to view the holes as vents that should keep the rudder functioning well at high angles a bit like a slotted wing on an airplane that increases lift at slow speeds and reduces stall speed. I thought of putting some holes near the LE of my rudder and now I'm more inclined to. With heavy following sea conditions rudder response may be improved w a few holes and I already use my rudder at 45 degrees while docking with great success.

 

The chinese built to last. When W R R G Worcester wrote his book on junks, he had concluded a 30 year career as a british customs officer in china assisting the chinese customs service.
He reported that many of the largest wooden junks still operating were over 100 years old when he inspected them.
They usually had topsides of fuchien pine, a resinous strong wood. The lower parts and sumerged hull were teak and iron wood. Rudders were iron wood framed, but had slats of lighter wood to fill in the blade, to reduce weight when hoisting.
The rudder could lower untill the blade was entirely below bottom of hull.

 

Interesting bow dagger . . . . .

Are there uses for bow boards nowadays . . ? ?

Cheers,
Angel

 

Some Wharram Pahi have bow dagger / trim boards . . . .


click pic for source

Cheers,
Angel