high speed rudder

[stupidbaker57]

While my shaft and tube are being machined, I will be building my rudder. The style is what I would call a dagger style that is normally found on inboard hydos in the Y and T classes.
I haven't been able to get up close enought to see one, so I was wondering about the shape as it slices the water.
Is the rudders' leading edge sharp and trailing edge rounded over? (like an airplane wing in reverse, sorta) or would it be the other way?

I have recieved my plans today and the boat choise is a Hal Kelly design called the Ben Hur.
I chose the cab over design since the last 24" of the boat will have the bike engine in it. The design will allow me to use my weight to get it on plane as I'll be further forward than a conventional hydro.

[CatBuilder]

Originally Posted by stupidbaker57

While my shaft and tube are being machined, I will be building my rudder. The style is what I would call a dagger style that is normally found on inboard hydos in the Y and T classes.
I haven't been able to get up close enought to see one, so I was wondering about the shape as it slices the water.
Is the rudders' leading edge sharp and trailing edge rounded over? (like an airplane wing in reverse, sorta) or would it be the other way?

I have recieved my plans today and the boat choise is a Hal Kelly design called the Ben Hur.
I chose the cab over design since the last 24" of the boat will have the bike engine in it. The design will allow me to use my weight to get it on plane as I'll be further forward than a conventional hydro.

Not sure about your exact design's specifications, but Google up NACA foils.

It's an airplane wing (well, two of them- there is no flat side) going the same way they do on the airplane.

The rudder below would be traveling to the left of the page if it were moving in the correct direction.



You might find this link very handy for visualizing the process...

http://www.fastcomposites.ca/publica...padeRudder.pdf

[baeckmo]

Your rudder will operate in a ventilated state. Here, the NACA profiles are not suitable, you should use a wedge profile instead.

[DMacPherson]

I concur with baeckmo. There are two issues here. First is that at high speed, a foil shape will likely have separation at (or near) its thickest point. You will want something that promotes clean separation at its aft end (like a wedge). This is analogous to a transom stern on a planing craft. The other issue is that if the rudder is aft of the hull (versus being under the hull), it is "ventilated" (as baeckmo points out). So, again you will want something that works with this "base ventilation" (the base being the flat "transom" of the rudder), and again a wedge-type shape is suggested. However, a true wedge is often not ideal, and a "parabolic wedge" can provide a better overall performance across a wider range of speeds. It still has the thick base, but its section shape is somewhat rounder at the nose, and gets thicker more quickly before reaching the base.

[gonzo]

A wedge is the standard for high speed rudders. Naca foils will cavitate and the boat will loose steering.

[DMacPherson]

Cavitation actually has been shown to have very little effect on a rudder's lift-drag performance (even of a foil type) until you get well beyond about 10 degrees angle of attack. It is bad for lots of other things, of course, and would have consequences for applications requiring high maneuverability, but probably not so much here.

[baeckmo]

Principally, the parabolic profile has a lower resistance at zero aoa, as DMacP mentions. However, it is more sensitive to shape imperfections close to the nose, which may result in sudden changes of side force. The classical wedge gives a better "predictability" in rudder response, and of course it is easier to produce than a parabola.

[CatBuilder]

Of course, I did not understand the op's boat design and concur with everything above as well.

[stupidbaker57]

Jeezzzzzzzz, You guys are getting kinda technical, huh? I was just thinking of making the rudder out of a piece of 1/8 inch aluminum (maybe thicker is needed) and the profile would be that of the inboard hydros I've seen (but not real close up). I was just wondering if I should round the leading edge and sharpen the trailing edge.
Remember, this is a trial and error experimental boat. Very low cost jst to see if it can be done.

[CatBuilder]

Originally Posted by stupidbaker57

Jeezzzzzzzz, You guys are getting kinda technical, huh? I was just thinking of making the rudder out of a piece of 1/8 inch aluminum (maybe thicker is needed) and the profile would be that of the inboard hydros I've seen (but not real close up). I was just wondering if I should round the leading edge and sharpen the trailing edge.
Remember, this is a trial and error experimental boat. Very low cost jst to see if it can be done.

Depends a lot on your application. Take a look at this link for pictures:

http://www.fastelectrics.net/rudders.php

There are a bunch of schools of thought on the subject, which is why people are getting technical. You just have to pick one.

[baeckmo]

Originally Posted by stupidbaker57

Jeezzzzzzzz, ........Remember, this is a trial and error experimental boat. Very low cost jst to see if it can be done.

What's the point in repeating other peoples trials and errors? Your rudder has to have a specific bending strength, which is in fact the primary dimensioning factor regarding thickness. In order to get the required strength, the wedge (two straight sides and a base) is far better than a flat plate.

To get equal strength with a flat plate, it has to be so thick, that there is a problem shaping the nose correctly, and even with a "good" shape the steering is highly non-linear. So if simplicity WITH function is what you seek, go for a wedge!

[Frosty]

A flat alluminium plate will do you fine. If it doesnt keep a straight course and wanders use a wedge.

[daiquiri]

What are the planform dimensions of the rudder, and what is the max. boat speed?

[PetterM]

Do you have any design papers/rules for designing ventilated wedge and parabolic rudders?

Originally Posted by baeckmo

Principally, the parabolic profile has a lower resistance at zero aoa, as DMacP mentions. However, it is more sensitive to shape imperfections close to the nose, which may result in sudden changes of side force. The classical wedge gives a better "predictability" in rudder response, and of course it is easier to produce than a parabola.

[daiquiri]

Originally Posted by PetterM

Do you have any design papers/rules for designing ventilated wedge and parabolic rudders?

Check this paper:
D.L. Gregory - "Force and Moment Characteristics of Six High-Speed Rudders for Use on High-Performance Craft" : http://www.dtic.mil/cgi-bin/GetTRDoc...f&AD=AD0772098

With respect to the data given in the previous reference, the following paper implies that blade aeration can decrease rudder's lift coefficient by up to (approximately) 75% - V. T. Shavchenko, "Hydrodynamic Characteristics of Rudders Operating in Air-Sea Interface": http://ftp.rta.nato.int/public//PubF...$MP-015-17.PDF