Flat (not wedge) rudder blade?

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Terry Keeley

Terry Keeley

Well-Known Member
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Joined
Jul 24, 2002
Messages
7,124
I know someone tried this in the past, just can't remember the outcome.

Anyone?
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I have a copy of Brian Callahan's paper on ventilated rudders. Basically says that the wedge has less drag than the straight. If I recall correctly, at high speeds the straight blade cavitates a short distance back from the leading edge. The wedge stops the cavitation due to constant pressure rise and reduces drag by ventilating the rear.
 
tnrcboatracer said:
I have a copy of Brian Callahan's paper on ventilated rudders. Basically says that the wedge has less drag than the straight. If I recall correctly, at high speeds the straight blade cavitates a short distance back from the leading edge. The wedge stops the cavitation due to constant pressure rise and reduces drag by ventilating the rear.
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Yup, I tried the flat blades and found the wedge rudders are better.
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I have a knife blade rudder on my outboard powered Eagle SGX. I had to make it extend almost 2" below the propeller arc, in order to have enough steering. With the blade at 1" below the prop the boat would torque steer bad during acceleration.
 
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This knife rudder was good for 110 mph. Changing to it from a standard aluminum wedge added 6 mph at around 100 mph. Knife rudders on several slower gas boats were always faster with good control. The strut needs to be either angled or offset to counteract propwalk.

Lohring Miller
 
My experience with knife blade rudders was boat wouldn't track straight- switch back to wedge type made boat do so-

The thickest (rear) edge of the taper was controlled by what diameter hole I drilled thru the length of the rudder for water pickup-

(never did get prop blast water pickup working at all speeds so couldn't use that for proper motor runs)

Looking at pic. of the boat shows it has tapered leading/trailing edges- I felt with testing that parallel leading/trailing edges was what the boat liked & seemed to

be more settled @ WOT- also shorter rudder length could be used & boat ran freer-

Last idea: photo shows rudder pivot ctr line is ahead of leading edge- I tried various pivot ctr line placements to find what the boat liked-

I settled on a 25% offset finally & the boat was happy with that everywhere on the course-

your endless testing has prob. guided you to set up the boat as shown; my spoken of ideas were pretty valid some year ago when speeds were lower then you are running today-

Fun sruff-

Hope this helps-

Dave Richardson

RPM Inc. (Retired)
 
I experimented with the flat and knife edge rudder blades on my SAW boats and did not like them. The boats tended to hunt and I had to run the blades much deeper to get the boat to track remotely straight. Have run just shy of 120 with wedge rudder using parallel leading and trailing edges and was rock solid.
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In my experience with SAW boats yes you need the wedge profile, but if you find the right blade shape both thin and wedge profile are possible. I have no problems tracking straight in SAW boats with fairly thin knife blades set at the correct depth. I will say it is extremely hard finding the right kitchen knife that has the strength, ductility, length and preferably straight edge that will last on a SAW boat. Especially when those pesky turtles decide to surprise you.

TG
 
Thanks for all the input guys, just confirms what I thought I saw. Mikey Ross made a nice rudder blade from a knife but it's flat, the transom "oscillated" back and forth rapidly.
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Lohring: The blade in your photo looks like it's a true wedge, is that so? Care to share where it came from?
 
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Anyone ever tried an eliptical foil shape? It might have to be a little wider from front to back but the shape is known to prevent flow seperation.
 
Terry Keeley said:
Thanks for all the input guys, just confirms what I thought I saw. Mikey Ross made a nice rudder blade from a knife but it's flat, the transom "oscillated" back and forth rapidly.
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Lohring: The blade in your photo looks like it's a true wedge, is that so? Care to share where it came from?
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We used hammer knife blades. I'm not sure McMaster Carr still carries the same one we used, but this is similar. The advantage is they are hardened and wedge shaped. We cut them down to get the shape we need. The rudder is a huge part of the drag in a SAW boat. We took 1/8 inch off the rudder length and added 2 mph to a 108 mph pass.

Lohring Miller
 
lohring said:
Terry Keeley said:
Thanks for all the input guys, just confirms what I thought I saw. Mikey Ross made a nice rudder blade from a knife but it's flat, the transom "oscillated" back and forth rapidly.
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Lohring: The blade in your photo looks like it's a true wedge, is that so? Care to share where it came from?
Click to expand...
We used hammer knife blades. I'm not sure McMaster Carr still carries the same one we used, but this is similar. The advantage is they are hardened and wedge shaped. We cut them down to get the shape we need. The rudder is a huge part of the drag in a SAW boat. We took 1/8 inch off the rudder length and added 2 mph to a 108 mph pass.

Lohring Miller
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Great, thanks a bunch!
 
Terry,

I think Mike was using the same Hammer knife from McM, although Lohring and Mike B have shaped it quite a bit. I tried the same hammer knife on several gas SAW boats and did not have as good results compared to kitchen knifes. I would find a good 6" to 8" pairing knife with reasonable heft and try cutting that down to fit.

TG

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John Finch said:
Terry,

Your rudder blades are awesome. You just need a larger boat.

John
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Thanks John, yup, figuring our how to drill long holes in 6Al4V is the trick. Built a 6" longer boat 2 years ago, might see ya in ECity this fall.
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It's for one of my FAST buds Mikey Ross (Suicidal Engines). He's running his Nowicki hydro with a flat knife blade rudder, thought I saw the transom doing some weird stuff, think I was right for a change...
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