Two plates on rudder - curious

D

djyacht

Well-Known Member
Joined
9 Apr 2009
Messages
107
Visit site
I`ve just lifted my boat out for the first time to renew anodes and anti fouling. The rudder has a plate on each side, fitted so they stand off. I`m curious as to what they might be for - I`ve googled and have seen the term rudder "cheeks" but no explanations.

Any ideas?


David
 
Might they be steps? Like the Itchenor Mirror-dinghy chap, who put little wings at the bottom of his rudder, to enable easy boarding from the water.
 
Steps ?
Edit: sorry Dancrane you beat me too it. Great minds......
 
Last edited:
Sorry, didn`t take a photo before I removed the plates to bring home for cleaning etc.. The boat is an IP24 motor sailor. The plates are bolted onto the rudder, 1 each side, parallel, with 4 spacing bolts so that they stand off by about 3 inches. Each plate is rectangular about 18 inches by 9 inches.

Cheers
 
It's a device for one of the following;

A) To increase rudder effectiveness by delaying stall at low speed and high deflections.

B) To improve rudder balance - where are they mounted rudder chord-wise? Close to the hinge-line axis or further back?

I had a picture in my head of your boat being a motor sailer from your first post which you've now confirmed. My guess would be its to improve rudder authority in astern.

Does the boat handle OK steering-wise?
 
Last edited:
They are further back from the hinge line. I think you may be right about going astern - there is a lot of prop walk.
 
I have found a photo, which I`ve cropped to show the rudder with one of the plates showing. I took this whilst it was being jet washed. I can see that the plates are directly in line with the prop.
 
If (like my rudder) the blade is made from two symmetrical halves, bonded around the stock, perhaps those plates are there to secure the halves tightly together?
 
Think that is an attempt to clean up the waterflow over the rudder to help straight line running. Flat plate rudders are very inefficient and can slam over as soon as they are turned. The extra balance area probably added to try and reduce heavy steering.
 
djyacht said:
I`ve just lifted my boat out for the first time to renew anodes and anti fouling. The rudder has a plate on each side, fitted so they stand off. I`m curious as to what they might be for - I`ve googled and have seen the term rudder "cheeks" but no explanations.

Any ideas?


David
Click to expand...

The plates seem to make the rudder into a kind of triplane. However I suspect the low aspect ratio of the planes and the closeness of the planes to the rudder would be quite poor at improving rudder performance.
Radical but I would suggest leave the plates off. Clean up the rudder itself and add epoxy foam and fibreglass to make the rudder a decent airfoil section. My guessing being that the good profile will improve rudder performance and certainly minimise drag.
However I don't know if it will improve reverse control. good luck olewill
 
You probably have a rudder which is just a flat surface rather than an aerofoil shape
As a result the water breaks away from the "lee" side of the rudder causing mild cavitation of the water thus stalling the rudder & it looses effectiveness
In other words it looses the laminar flow
Adding plates is done to make the water " stick" to the side of the rudder
They are set aft from the hinge line because the laminar flow is not immediately lost until a quarter or third of the way back( varies, depends on lots of influences)
The plates themselves will loose the flow but being narrower it is less so
In theory the plates could have plates, & so on, but then drag becomes a factor
Some people will try a bigger rudder but it just increases drag
If it were specifically to aid reversing the plates would not be at the back but nearer the front in a reverse configuration to the above however, they probably do help if not reversing then for catching plastic bags
 
Last edited:
interesting stuff which I have not seen before. Perhaps the aeronautical theory that more wings add more lift has been translated to the marine industry. However, you don't see many bi- and tri-planes being made nowadays.

OK I know that airplane wings do not have to be symmetrical, but a proper chord section for the rudder must be a better proposition than multi-plates. As William H suggests. :)
 
When you get a "new to you" boat it is probably natural to assume that the "person or persons unknown" who set up the rigging or made other changes or modifications to the boat actually knew what they were doing!
I made that assumption with my own boat, but eventually I found that this was a false assumption. It seems that there are a lot of people out there who wont let ignorance stop them pursuing their own ideas.
Your rather interesting rudder additions might well have been done by someone who enthusiastically fancied themselves to be the natural heir to Bernoulli - but sadly lacked the ability.
The science of rudder profiles is well established as noted by above posts. Certainly seems to me that your extra bits are adding drag for no benefit.
I second leaving the plates off and seeing what happens.
The last comment in the link (http://www.boatdesign.net/forums/boa...odl-23542.html). supports this idea.

Good luck.
 
You must log in or register to reply here.

Share this page

Top